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Related research on refrigerant optimization based on the dynamic BOG has yet to be proposed. This study offers a new dynamic optimization method for boil-off gas (BOG) re-liquefaction system to improve the system's energy efficiency. The performance of C3MR (the propane precooling mixed refrigerant cycle) is analyzed based on the steady state model of leading liquefaction equipment in the process simulation in Aspen HYSYS software. And the principle of the energy consumption variation law of propane precooled compressor and mixed refrigerant compressor, based on the actual possible disturbance range obtained in the natural gas liquefaction process, is applied to construct the multi-objective optimization model with minimum energy consumption and maximum heat exchanger efficiency. Finally, the dynamic responses of disturbances were achieved and discussed with the aid of GA (Genetic Algorithm) and PSO (particle swarm optimization). The optimized performance of the dynamic liquefaction cycle is 34.8% better than the steady condition system in terms of energy efficiency, and the maximum energy consumption can be reduced by 50.60%. The advantage of the proposed optimization framework is its adaptability to other dynamic liquefaction processes. It has practical reference significance for the cost control of the BOG re-liquefaction recovery project. • A new dynamic optimization method for C3MR liquefaction. • A multi-objective optimization model with minimal energy consumption and high thermal efficiency is constructed. • Provides a theoretical basis for the optimization of dynamic liquefaction systems. [ABSTRACT FROM AUTHOR]

In this paper, design strategies for improving electrical efficiency, thermal design and fuel utilization of an ammonia-fed SOFC are investigated. Three strategies are presented to improve system performances: (i) the introduction of an additional stack to distribute the power i.e. power rating, (ii) the evaluation of the anode off gasses recirculation and (iii) the use of the off gasses to operate a cascade stack (re-powering), where the anode flue gas is recuperated. A system design that integrates these new features is modelled with zero-dimension thermodynamic equations. The three strategies were evaluated for net system efficiency and the heat exchanger area as main design parameters. The power rating allows to reduce the heat exchanger surface while the recirculation and repowering are suitable to increase system efficiency. With an integration of the three solutions, it is possible to achieve an increase in net efficiency from 52.1% to 66% and a reduction in heat exchanger surface area of 67% compared to the reference design that does not consider any of the proposed design strategies. • Efficiency can be improved using multiple stacks and by recycling the anode gases. • Heat exchanger area can be reduced by introducing additional stacks to the system. • Afterburner can be avoided by optimizing the air flow management. [ABSTRACT FROM AUTHOR]

To solve the intermittent problem of solar drying systems, this study proposes a solar vacuum tube collector coupled heat pump drying (HPD) system that can adopt three operating modes as different climatic conditions. The performance of the system under different working modes and drying characteristics was analyzed; moreover, the system was also based on the relationship between energy supply and consumption. The results show that under load conditions, the system runs in the solar drying (SD) mode and the drying chamber temperature can reach >50 °C. In the HPD mode, the average heating power of the system is 11.88 kW, the heating coefficient is 2.26 and the average thermal efficiency of the heat exchanger is 39.3%. In the solar-assisted heat pump drying (SAHPD) mode, the average coefficient of performance of the system is 3.26, compared to HPD model, a 44.2% increase. The heating ratios of the SD, HPD and SAHPD modes were 37.9%, 58.5% and 3.6%, respectively. Furthermore, the Two-term models with R 2 value of 0.9986 and RMSE value of 0.01038 was considered the best drying kinetics model for the vacuum tube collector coupled heat pump drying grapes. This study guides the application of SAHPD systems in agricultural products drying. [ABSTRACT FROM AUTHOR]

The fouling factor is a complex non-linear function of several feature variables. Accurate estimation of this measurable factor is applicable for controlling the heat transfer efficiency of heat exchangers. Artificial neural network (ANN) and adaptive neural-based fuzzy inference system (ANFIS) techniques are accurate approaches for the understanding treatment of the most complicated systems. The cascade-feedforward (CFF), multi-layer perceptron (MLP), radial basis function (RBF), and generalized regression (GR) neural networks and ANFIS were applied for predicting the fouling factor of the hot wire probe from some measured variables using a huge databank, including 1870 empirical dataset. Pearson's and Spearman's techniques confirmed the highest relation between considered features and the first order of fouling factor. The results demonstrate that the cascade feed-forward neural network containing eight hidden neurons was the best artificial intelligence (AI) model with excellent overall AARE = 3.44 %, MSD = 0.0000315, RMSD = 0.0056, and R2 = 0.9982. This work's significance lies in presenting an applicable and accurate tool for estimating fouling factors on hot wire probes to the research community and industry. This model can be considered a reliable replacement for empirical analyses that are often expensive and time-consuming. [Display omitted] • Hot wire probe has used in academic studies of fouling in heat transfer surfaces. • Five powerful artificial intelligence models are used for fouling factor estimation. • The Levenberg-Marquardt is found as the best training algorithm. • An accurate CFF neural network with eight neurons in one hidden layer developed. • Effects of less studied and various independent variables on fouling is simulated. [ABSTRACT FROM AUTHOR]

• Introduces ultra-compact TPMS heat exchangers with record efficiency up to 16.5 W/(cm³·K). • Successfully 3D prints complex TPMS structures, enabling new design possibilities. • Develops experimental correlations in heat transfer and pressure drop. • Achieves up to 200 % improvement in PEC over traditional heat exchangers. With the rapid growth in energy efficiency demands, compact heat exchangers are gaining increasing attention. This study proposes a novel heat exchanger design based on Triply Periodic Minimal Surface (TPMS) structures, leveraging their unique three-dimensional porous structure to enhance heat transfer efficiency and compactness. This study employs computational fluid dynamics (CFD) simulations to analyze the thermal performance of three TPMS structures (Gyroid, Diamond, and Fischer-Koch S) heat exchangers. Two optimized structures are selected for additive manufacturing based on the simulation results. The manufactured heat exchangers are inspected using CT scans, and a dual-fluid experimental performance testing system is constructed to assess the potential application of the two heat exchangers in thermal management systems. Using the least squares method, correlations for heat transfer and pressure drop are separately fitted for the two TPMS heat exchangers. Dimensionless Performance Evaluation Coefficient (PEC) values are utilized to analyze the comprehensive performance of the two TPMS heat exchangers. The heat transfer rate per unit volume has increased by approximately 50 times compared to traditional heat exchangers reported in the literature, which reaches an impressive 717.7 W/cm3 and 16.5 W/(cm³·K). This study will provide design guidance for enhancing the compactness and efficiency of TPMS heat exchangers. [ABSTRACT FROM AUTHOR]

The performance and thermal efficiency of a heat exchanger are critical in various industrial applications, necessitating the continuous improvement of the thermal management system. In the present study, a novel heat exchanger which comprises of delta-nabla-trapezoidal (DNT) configurations of flow channels, is investigated and compared with commercially available concentric tube heat exchangers (CTHEs), to address the limitations of conventional design in terms of thermal efficiency and pressure drop. The current study is the first of its kind to explore the thermal energy exchange potential of the delta-nabla-trapezoidal heat exchangers (DNTHEs) with numerical simulations and experimentally validated them using an in-house designed experimental setup. The proposed heat exchanger comprises of triangular and trapezoidal cross-section channels that are arranged such that the hot fluid flows through the delta and nabla channels while cold fluid flows through the trapezoidal channels. The energy exchange between the two fluid streams is experimentally measured, and Nusselt number correlations are developed for various inlet mass flow rates. The numerical simulations are then used to predict local fluid and heat flow patterns with velocity and temperature profiles. Experimental results indicate a reasonable energy balance, revealing a maximum loss of 15% across all sets of experiments. Furthermore, the cold-side temperature increased to its maximum of 22 K, while the hot-side temperature decreased by 25 K across all tested conditions. The simulation results demonstrated an enhanced energy balance, characterized by a maximum loss of 10%. The thermal efficiency of the proposed DNTHE was experimentally assessed and compared to that of a CTHE in parallel and counter flow configurations. Under similar operating conditions, the maximum heat flux in DNTHE and CTHE was 18.35 and 14.26 kW/m2, respectively. Furthermore, the largest difference in temperature drops and gains between DNTHE and CTHE are 15 and 9 K, respectively. These findings confirmed the efficient thermal energy exchange potential of DNTHE in comparison to CTHE, establishing a new industrial standard for heat exchangers. [ABSTRACT FROM AUTHOR]

• The mechanism of heat conduction of printed circuit heat exchanger with anisotropic thermal conductivity was numerically investigated. • The synergistic effect of thermal resistance in three directions of the solid region on the performance of the heat exchanger was revealed. • A solid region thermal resistance ratio optimization method was proposed. • The efficiency of the heat exchanger is improved by approximately 23% through solid region thermal resistance ratio manipulation. Printed Circuit Heat Exchangers are compact and efficient heat exchangers, widely used in nuclear engineering, very high-temperature reactors, and aerospace systems. This study investigates the heat transfer performance of a heat exchanger with anisotropic thermal conductivity, such as fiber reinforced composites. Numerical simulations were conducted to examine the synergistic effect of three-dimensional thermal resistance on heat exchanger performance. The most significant impact on performance is the z-direction thermal resistance, followed by the y-direction, while the x-direction has the least impact. Contrary to traditional design thinking, increasing the overall heat exchanger thermal resistance under the same thermal resistance ratio improves heat transfer efficiency at the studied conditions. The results suggest that it is necessary to design the lowest thermal conductivity direction as the z-direction and increase the y-direction thermal conductivity to enhance heat exchanger performance. In the numerical investigation presented in this study, the efficiency of the heat exchanger was improved by approximately 23 % under specific operating conditions by adjusting the thermal conductivity of anisotropic materials to control the thermal resistance in the x, y and z directions. It is evident that the manipulation of anisotropic material properties has a substantial influence on the performance of heat exchangers. [ABSTRACT FROM AUTHOR]

The pursuit of sustainable district heating solutions has driven a growing interest in ultra-low temperature district heating (ULTDH) systems, where booster heat pumps (BHPs) play a pivotal role despite challenges posed by their efficiency limitations under large temperature glide conditions. This paper investigates the potential of drop-in R-1234yf/R-32 zeotropic mixtures in BHPs compared to a baseline R-134a system, within the context of a ULTDH framework. This study focused on the viability of the mixtures of R-1234yf/R-32 with the composition ratio of 80 %/20 % and 90 %/10 %. The investigation reveals disparities in compressor efficiency and heat exchanger pressure drop at the component level. Device-level analysis unveils increased COP for R-1234yf/R-32 mixtures, alongside with maximum second-law efficiencies reaching 0.32. A remarkable enhancement in heating capacity up to 58 % was found. System-level analysis demonstrated exergetic efficiencies and identified preferable district heating temperatures. Exergetic efficiencies of 0.47, 0.55, and 0.59 were achieved for domestic hot water preparation at district heating supply temperatures of 30 °C, 35 °C, and 40 °C, with a subsequent shift in optimal district heating temperatures as central heating station efficiency decreased. Temperature profile analysis underscored challenges stemming from excessive subcooling, highlighting the need for configuration refinements. [Display omitted] • Drop-in application of zeotropic mixtures in booster heat pumps are studied. • Coupling effects between district heating and heat pump parameters are analyzed. • Heating capacity and COP improvements with mixture refrigerants are demonstrated. • Temperature profile analysis identified the challenges for efficiency improvement. [ABSTRACT FROM AUTHOR]

Airfoil-finned printed circuit heat exchanger (PCHE) is a new type of microchannel heat exchanger with high efficiency and stable compact features for high temperature and high pressure supercritical carbon dioxide Brayton cycle system. Usually, only one kind of fin structure is used in the microchannel heat exchangers, and little research has been done on flow heat transfer of hybrid fins structure. In this work, a new model of heat exchanger with hybrid fins structure is developed with supercritical carbon dioxide (S–CO 2) as the flow medium. Specifically, three-dimensional numerical simulations of PCHE are performed to study the flow heat transfer characteristics of hybrid fins. Besides, the effect of the different spacing of the airfoils on the flow heat transfer performance is studied. The results show that when the left and right adjacent spacing of the fins L b increases, the heat transfer performance of the heat exchanger improves while the pressure drop decreases. With the same pressure drop, the heat transfer of microchannel heat exchanger increases with the increase of the upper and lower adjacent spacing of the fins L c. Besides, the heat transfer performance of the new hybrid microchannel heat exchanger is much better than that of the conventional zigzag microchannel heat exchanger. The new hybrid fin microchannel heat exchanger shows better heat transfer performance at high Reynolds number, compared with that of the swordfish fin, airfoil fin and rhombic fin microchannel heat exchangers. [ABSTRACT FROM AUTHOR]

Here, to resolve the open problem of the second-law efficiency evaluation for heat exchangers, an efficiency-effectiveness method is developed from pure convection theory, where the entropy transfer efficiency defined as the outlet temperature ratio of cold and hot fluids exchanging entropy and local entropy generation number can measure the global and local irreversibilities of heat transfer. The efficiency and effectiveness are related by the inlet temperature ratio τ and capacity rate ratio φ , and their maxima subject to the local entropy generation number are determined for different flow arrangements. When τ = φ 2, the effectiveness maxima are 0.5/(1 + φ) and 1/(1 + φ) while the highest efficiencies are φ and unity for parallelflow and counterflow arrangements, respectively. The optimum operating points of counterflow exchangers, at which the equal outlet temperature τ T 1 ∞ (T 1∞ represents the hot inlet temperature) of two fluids with a critical capacity rate ratio τ induces uniform irreversibility, the effectiveness limit of 1 / (1 + τ) , and the efficiency limit of unity, are obtained. A linear temperature difference method is also developed based on the redefined convective and overall heat transfer coefficients, invariably yielding an effectiveness equal to the number of heat transfer units. This method has potentials to be extended to wet heat exchangers with phase-change flows. • The open question of second-law efficiency evaluation of a heat exchanger is resolved. • The heat transfer effectiveness and entropy transfer efficiency limits are presented. • The global and local irreversibilities of exchangers are measured by N S and N S,l. • Two optimum design criteria of exchangers are obtained subject to N S,l. [ABSTRACT FROM AUTHOR]

• Chemical equilibrium was introduced and a novel two-stage in-cylinder steam thermodynamic model was proposed. • A theoretical thermodynamic model considering in-cylinder steam assist and exhaust gases waste heat recovery was established. • The effects of the different working fluids on the in-cylinder steam assist cycle were revealed. • The effects of in-cylinder steam assist on thermal efficiency optimization under different working fluids were evaluated. Improving efficiency and reducing emissions are essential to achieving carbon peaking and carbon neutrality in transportation sector. As a carbon neutral fuel, hydrogen is widely considered the most promising fuel for future zero-carbon internal combustion power, while iso -octane, represented by gasoline, symbolizes the current hydrocarbon fuel utilized in modern internal combustion engines. In this study, the maximum indicated thermal efficiency of both hydrogen and iso -octane utilizing air (blending O 2 and N 2), oxy-fuel (blending O 2 and CO 2) and argon power cycle (blending O 2 and Ar) were investigated theoretically based on in-cylinder steam assist cycle. A theoretical thermodynamic model combining in-cylinder steam assist and exhaust gases waste heat recovery was established to explore the thermodynamic process of in-cylinder steam assist and systematically analyze the effects of different working fluids. Results show that the utilization of in-cylinder steam assist improves thermal efficiency under O 2 , N 2, and CO 2 working fluids, but limits the improvement of thermal efficiency under Ar. In addition, the maximum achievable steam temperature and mass are limited by the exhaust gases temperature and heat exchanger efficiency. When running with H 2 under 400 K steam temperature, the optimized water-to-gas ratios were 1.58, 1.13, 1.62, and 1.13 for O 2 , Ar, N 2 , and CO 2 , respectively, the maximum thermal efficiencies increased by 35.80 %, 0.28 %, 28.21 %, and 62.79 % compared with baseline condition. Meanwhile, the competition of overall specific heat ratio and working fluid mass under different working fluids leads to different thermal efficiency enhancement, such as Ar, the decrement of overall specific heat ratio under limited steam mass leads to thermal efficiency deterioration, which can be further improved as steam mass increased. [ABSTRACT FROM AUTHOR]

• A novel open-loop air cycle heat pump drying system is proposed. • The mathematical model built is verified to be both accurate and feasible. • Compared with electric heating drying system, the energy-saving rate of air cycle heat pump water heater system is 15∼27%. • The impact of different working conditions on system performance was determined. Different from conventional drying technology, the novel technology of an air cycle heat pump drying (ACHPD) system, using air as its working medium, is proposed to solve the problems of low energy efficiency, HCFC refrigerant use, and so on. A mathematical simulation model is constructed to evaluate system performance under various working conditions, and its verification via experimental results from a constructed test bench produced errors of temperature within ±1.6%, of drying rate within ±0.01%, and of system power within ±7%. Compared with a conventional electric heater drying (EHD) system, the energy-saving rate of the ACHPD system reaches about 15–27% when the mass evaporation rate increases from 0.75 kg h −1 to 3.45 kg h −1. Under set working conditions, an increase of inlet air flow rate (150–400 kg h −1) improved the dehumidification energy efficiency ratio (MER) by about 1%. However, the increase of inlet air temperature (10–40 °C) and relative humidity (30–80%) reduced MER by about 7% and 21% respectively. The efficiency improvements of heat exchanger (0.3–0.8) can improve the energy efficiency of system by 20%; however, the efficiency improvements of expander (0.55–0.8) and compressor (0.4–0.9) can deteriorate the energy efficiency of system by 17% and 36% respectively. These results would provide new technical references for the application of heat pump technology in drying field from a different perspective. [ABSTRACT FROM AUTHOR]

In industry, the thermal flow reverse reactor (TFRR) is considered to be an effective means to reduce ventilation air methane (VAM) from fossil fuel operations. The purpose of VAM combustion is either to reduce the emission of greenhouse gas, to recover energy, or both these subjects. The heat exchanger tubes have been embedded on both sides of high-temperature zone of the regenerative oxidizer for heat recovery in this study to improve economy. The mathematical model is established, as well as the effects of inlet methane concentration and air flow rate on dynamic heat extraction characteristics of heat exchanger tubes under periodic conditions are investigated. The results show that sustainable and stable heat recovery can be achieved when the concentration is greater than 0.6 vol.% and the air flow rate is less than 596 m3/h. Heat extraction mainly depends on downstream heat exchange tubes. The asymmetry of heat transfer process between upstream and downstream heat exchanger tubes is analyzed. It is worth noting that the asymmetry is improved at high inlet methane concentration and low air flow rate. The heat recovery efficiency by the bilateral heat exchanger tubes is 61.72% at most, which provides a reliable theoretical basis for the heat extraction mechanism of the heat exchanger embedded in TFRR. [ABSTRACT FROM AUTHOR]

• First and second law analyses of newly oval-CGHEs are introduced. • Oval CGHEs exhibit superior heat transfer with a trading-off in irreversibility. • The irreversibility distribution pinpoints the irreversibility of the studied CGHEs. • The results show reduced irreversibility at the oval-CGHEs sidewalls. • Sensitivity analysis highlights key factors influencing the CGHE's irreversibility. Optimizing the heat transfer and thermodynamic efficiency of Ground Heat Exchangers (GHE) is crucial for maximizing the energy efficiency of Ground-coupled Heat Pump (GCHP) systems utilized in building heating and cooling applications. This study pioneers an effective approach of both the first and second laws of thermodynamics of the newly oval-shaped coaxial GHEs (oval-CGHEs), assessing heat transfer, thermohydraulic performance, and local entropy generation rate to explore the optimum design and operational conditions for heat exchange maximization and entropy generation minimization. Utilizing a three-dimensional numerical model validated by experimental results, this study examines the trade-offs between heat transfer improvements and entropy generation rate intensification of the oval-CGHEs in comparison to the typical circular-shaped CGHE (circle-CGHE) further than exploring the effect of key operational and design parameters and performing sensitivity analysis. The results reveal that oval-CGHEs exhibit higher irreversibility as a penalty for their superior heat transfer capabilities compared to traditional circular-shaped CGHEs (circle-CGHE), which can be mitigated by optimizing the position of the inner tube. Moreover, the study underscores the criticality of maintaining a balance between enhanced heat transfer and the associated rise in irreversibility, particularly concerning the factors of mass flow rates and installation depths. [ABSTRACT FROM AUTHOR]

• A thermo-hydraulic modeling of the plate-fin heat exchanger was performed. • Variations of physical properties for streams were considered in the model. • Investigation of longitudinal conduction heat transfer through walls was performed. • Phase change for natural gas occurs in the last segment of the heat exchanger. • Geometrical parameters mainly affect the high-pressure mixed refrigerant stream. Accurate modeling of a multi-stream plate-fin heat exchanger as a vital component in various industries is crucial in dominating its design. This study considered the main heat exchanger of a single mixed refrigerant process for modeling and simulation. First, the process was simulated with an efficient specific energy consumption. Then, a robust model was developed for the thermo-hydraulic performance of the heat exchanger. The model accuracy is enhanced by considering essential features such as multiple streams, multi-component flows, variation in physical properties, phase change phenomena, and longitudinal heat conduction. The temperature and pressure drop distribution for all streams in the multi-component two-phase parallel flow heat exchanger length are evaluated. The effect of geometrical parameters of fins is investigated on the heat exchanger thermal performance. Phase change occurs at the end of the heat exchanger due to the maximum temperature driving force between hot and cold streams. The results show a good agreement between the results of the proposed model and the commercial software, as well as numerical and experimental case studies. Then, the process simulation was linked to the PFHE model to simultaneously consider the effect of PFHE geometrical parameters on the simulation results. Also, the sensitivity analysis was performed to investigate the effect of operating conditions on Specific Energy Consumption. It can be seen that methane mole% has the most significant impact on the thermal efficiency of the heat exchanger. [Display omitted] [ABSTRACT FROM AUTHOR]

Heat exchanger studies have been conducted for several decades since the beginning of the development of heat transfer theories due to their widespread use in various industries. The ongoing research topics include heat exchanger efficiency, design parameter optimization, material development, and heat transfer technology exploration. Research has been continued to evolve nanofluid compositions, concentrations, and operating conditions that affect heat exchanger efficiency. They have used hybrid nanofluids in heat exchangers, which contain multiple types of nanoparticles or nanoparticles with additional additives, as hybrid nanofluids improve thermal performance. As a result, the development and optimization of hybrid nanofluids as a working medium in heat exchangers are gaining popularity, and the use of hybrid nanofluids in heat exchangers has already drawn a lot of interest as a viable strategy in industrial sectors. Therefore, such a surge of interest in using hybrid nanofluids in heat exchangers has resulted in a significant body of literature on this. In this review paper, the authors aim to incorporate the findings of these studies and offer a thorough evaluation of the state-of-the-art research on the application of hybrid nanofluids in various types of heat exchangers. This review provides a synopsis of the published literature on the efficacy of heat exchangers utilizing hybrid nanofluid. The researchers have also encountered and documented several challenges, including stability and aggregation, improving thermal conductivity, fluid flow characteristics, corrosion and material compatibility, cost and scalability, and determining the ideal size and shape of nanoparticles while considering environmental and health concerns. In addition, this review identifies research gaps in preventing agglomeration, ensuring nanoparticle dispersion, optimizing fluid composition to minimize flow effects, exploring and identifying materials compatible with hybrid nanofluids, investigating cost-effective methods for hybrid nanofluid production, and addressing scalability issues for widespread implementation in industrial heat exchangers. This review paper will be a valuable resource for researchers and practitioners seeking to overcome these challenges and advance the field of hybrid nanofluids for heat exchanger applications. [ABSTRACT FROM AUTHOR]

• The steady-state operation of high-temperature water flow boiling was investigated within 100–250 °C for space heat dissipation. • Four parameters of heat exchanger efficiency (HEE), heat transfer power (HTP), liquid-side outlet quality (LSOQ), and two-phase heat transfer coefficient (HTC) were analyzed under different operating pressures and mass flow rates. • The parameter values were calculated and variation rules were summarized. With the development of space nuclear reactors, the transmission and emission of a large amount of waste heat in space are inevitable. Because of its large heat transfer, clean nature, and safe working medium, flow boiling of high-temperature water is ideal for space applications. This study focuses on high-temperature heat dissipation and experiments on the flow boiling heat transfer of high-temperature water. The steady-state operation characteristics of high-temperature water flow boiling heat transfer in the operating temperature range of 100–250 °C are studied with an emphasis on the variations in heat exchanger efficiency (HEE), heat transfer power (HTP), liquid-side outlet quality (LSOQ) of the high-temperature heat exchanger, and two-phase heat transfer coefficient (HTC). The HTP can be improved to a peak of 16 kW under a moderate air mass flow rate, and the LSOQ can be increased to approximately 0.95. The maximum HTC in this system could reach approximately 2100 W/(m2*K), which offers significant design guidance for the high-temperature water flow boiling system. [ABSTRACT FROM AUTHOR]

• To investigate the accuracy of 3D borehole thermal resistance (R b,3D) and total internal thermal resistance (R a,3D). • To establish the 3D computational model incorporated with a four-resistance model technique to calculate R b,3D and R a,3D for 130 cases. • To Develop two prediction models: Response Surface Methodology (RSM) and the hybrid Deep Feedforward Neural Network-Genetic Algorithm (DFNN-GA). • To rigorously evaluate the performance of the prediction models using statistical metrics, such as R2, RMSE, MAE, and AARE. • The outcomes derived through the implementation of diverse statistical approaches showcase the proficiency of both the RSM and DFNN-GA predictive models in precisely predicting the values of R b,3D and R a,3D. The thermal efficiency of ground heat exchanger (GHEs) relies on optimizing borehole thermal resistance and total internal thermal resistance for cost-effective and efficient design. This study aims to establish a robust correlation among various parameters, including thermal conductivity of pipe, grout, and ground, borehole depth, borehole and pipe diameter, pipe-pipe distance, fluid velocity, and heat transfer rate of the ground, with the 3D borehole thermal resistance (R b,3D) and total internal thermal resistance (R a,3D). This exploration of eight influential factors results in an l-130 design matrix with 130 data points. A 3D computational model, utilizing a four-resistance method, accurately calculates R b,3D and R a,3D , while analyzing heat transfer in both solids and fluids. Two prediction models, Response Surface Methodology (RSM) and a hybrid Deep Feedforward Neural Network-Genetic Algorithm (DFNN-GA), establish the correlation between R b,3D and R a,3D and the eight variables. The results highlight the efficacy of the prediction models, achieving R2 values of 79.87% and 87.90% for RSM and 94.00% and 99.15% for DFNN-GA, corresponding to the predictive accuracy of R b,3D and R a,3D , respectively. Furthermore, lower RSME, MAE, and AARE values are observed. A rigorous comparative analysis assesses the relative effectiveness of these methodologies in achieving the research goals. [ABSTRACT FROM AUTHOR]

High-efficiency microchannel heat exchangers (MHEs) can remarkably meet the thermal requirements in many areas of technology. Several active and passive techniques can improve the efficiency of heat exchangers; the improvement enables engineering processes to decrease operational costs through waste heat reduction. In the present article, two different passive techniques are applied to improve the heat transfer efficiency of a counter-flow heat exchanger. To do this, four various microchannels with airfoil fins, circular fins, trapezoidal fins, and triangular fins are designed. First, the temperature distribution and effectiveness for each microchannel are calculated and compared with each other. Results of the comparison showed that the optimum structure belongs to the microchannel with trapezoidal fins. Second, BeO ceramic is utilized as the microchannel heat exchanger's material. At a given mass flow rate of 30.5 kg/h, the superior thermal features of BeO lead to more uniform temperature distribution, and also it makes an improvement of 219% in the effectiveness of MHE with trapezoidal fins compared to alumina-made MHE with simple channels. [ABSTRACT FROM AUTHOR]

• Constant gas cooler pressure control is adequate to low temperature heating systems. • An adaptive control is recommended for high temperature heating systems. • Performance is similar for flooded or dry evaporator if superheating is below 4 K. • The use of an internal heat exchanger is always positive for hot water generation. • An increase in the surface always increases COP when using plate heat exchangers. This paper reports the results of an experimental and numerical study of different configurations of a CO 2 water-to-water heat pump for hot water generation. The experimental installation developed to test the heat pump unit is briefly described and then the experimental results are presented. Numerical results of a model previously developed are compared with experimental results, allowing the validation of the model. Once the model is validated, it is used to predict untested conditions. The results show, for several hot water generation scenarios, the influence that different parameters have on the system performance, including the gas cooler pressure, the internal heat exchanger (IHX) effectiveness, and the heat exchangers area on the efficiency of the system. For low and intermediate heating temperatures, the optimal gas cooler pressure is nearly independent of the range of evaporation water temperature (5 to 30 °C), therefore a constant pressure control able to maintain the pressure slightly above the optimal value, can be the best solution. On the other hand, for medium, and high temperatures, an adaptive control system is strongly recommended. Although the influence of a variation in the heat exchangers area depends on the operating conditions, in general terms, the area of the gas cooler has the higher influence on the efficiency of the system and the area of the IHX has the smaller influence. As the gas cooler temperature increases or the evaporation temperature decreases, the improvement obtained by increasing the HXs area decreases. [ABSTRACT FROM AUTHOR]

With the increasing demand for environmental protection, CO 2 , as a natural refrigerant, has always been environmentally friendly and safe, which makes transcritical CO 2 heat pump system attract more attention. The internal heat exchanger is employed to improve the performance of transcritical CO 2 heat pump system to match the traditional heat pump circulation system. In this work, two evaluation methods are proposed, one is the actual operation thermal effectiveness of internal heat exchanger, the other is the actual increase rate of COP. Compared with the experimental results, the conclusions can be extracted that the actual increase rate of COP can effectively evaluate the impact of the internal heat exchanger on system performance. In addition, this research deeply analyzes the influence of the internal heat exchanger on exergy efficiency. The results show that the internal heat exchanger greatly reduces the exergy efficiency under the discharge pressure of 7310 kPa and the ambient temperature of 15 °C, and the difference value between the system with and without internal heat exchanger reaches 39%. However, the influence of the internal heat exchanger on exergy efficiency decreases with the increase of discharge pressure and ambient temperature, the minimum difference value is only 0.1% at 11000 kPa and 30 °C. [ABSTRACT FROM AUTHOR]

• Electric vehicle air conditioning operating points and cooling loads provided. • Ultrahigh-speed centrifugal compression system simulated using low-GWP refrigerants. • Greater cooling loads generated at lower input power requirements. • Requires improved heat exchangers to handle condenser and evaporator temperatures. • Azeotropic refrigerants offer higher cooling capacity at higher pressures. Electric vehicles (EVs) are an increasingly popular option to decrease global greenhouse emissions in the transportation sector. The largest limiting factor is their range, significantly decreased by the cooling load required. In conjunction with the shift to incorporating low-Global Warming Potential (GWP) refrigerants in cooling systems, it is critical to reexamine the design of traditional air conditioning (AC) systems. To this end, a model of a cooling system for an EV was created, while incorporating an ultrahigh-speed miniature centrifugal compressor, allowing for the performance comparison of multiple refrigerants. From these simulations, a centrifugal compression system using HFC-134a provided sufficient cooling capacity for the modeled vehicle (13.07 kW) with lower input power requirements (2.05 kW) than a commercial EV AC system. However, this requires improved heat exchanger efficiencies. A straightforward system design may be attainable with lower operating pressures utilizing the low-GWP HFO-1336mzz(Z) while achieving a nearly equivalent cooling capacity (12.39 kW) at reduced power requirements (1.86 kW). Performance and safety tradeoffs were examined for a variety of refrigerants and azeotropic mixtures. The use of azeotropic refrigerants increased the cooling capacity and energy efficiency compared with the individual components, but with the tradeoff of higher pressure and compressor exit temperatures. Overall, this effort provides specified conditions for EVs while being among the first to demonstrate the potential of low-GWP and low-Ozone Depletion Potential refrigerants in AC systems utilizing ultrahigh-speed centrifugal compressors. [ABSTRACT FROM AUTHOR]

For enhancing the heat transfer efficiency of heat exchanger, silica-water (SiO 2 -H 2 O) nanofluids were used as a working medium in this study. Further, the effects of tube structure, twisted tape hole spacing, and hole shape on the flow and heat exchange characteristics of nanofluids in the tube were systematically researched by numerical simulation. The results displayed that a triangular tube exhibited larger flow resistance, better heat transfer effect and more comprehensive performance compared to the round tube. The conclusion also showed that the highest comprehensive evaluation index (PEC) was obtained by producing open circular holes on the twisted tape. The usage of porous twisted tape, nanofluids, and triangular tube could significantly improve the system economy and enhance the heat exchange ability. At the same time, the flow in the tube was disordered and accompanied with the increase in entropy. The maximum improvement of heat exchange efficiency of nanofluids in round tube and triangular tube with porous twisted tape was 74.80 and 55.97%, respectively. [Display omitted] • Effects of different hole shapes in twisted tape on forced convection are studied. • Effects of hole spacing in twisted tape on forced convection are studied. • Tube with round hole twisted tape shows the best heat transfer performance. • Round tube and porous twisted tape can improve heat transfer by 74.80% at most. [ABSTRACT FROM AUTHOR]

In the process of biomass gasification syngas waste heat recovery, water and organic compounds in the syngas will condense at the same time, resulting in immiscible condensates adhering to the wall, which can reduce the heat exchanger heat transfer efficiency and working life. Changing the wettability of the wall surface can affect the condensate flow state on the wall surface, thus affecting the condensation heat transfer performance. Water and organic cyclohexane were used as immiscible working fluids in this paper, the condensation flow and heat transfer characteristics of the mixed vapors on hydrophilic, super-hydrophilic, and super-hydrophobic wall surfaces were experimentally investigated. The results showed that cyclohexane in immiscible condensates existed as a liquid film on hydrophilic, super-hydrophilic, and super-hydrophobic wall surfaces, while the water existed in droplets on the hydrophilic and super-hydrophobic wall surfaces, and there were two forms of channels and droplets on the super-hydrophilic wall surface. The condensation heat transfer coefficient on the super-hydrophobic wall surface was higher than that on the hydrophilic wall surface and the super-hydrophilic wall surface, with a maximum of 34% higher than hydrophilic and 45% higher than super-hydrophilic, which was related to the smaller droplet departure diameter on the super-hydrophobic surface. In addition, the super-hydrophilic wall surface had better heat transfer performance than the hydrophilic wall surface only when the immiscible condensate was a "film-channel" flow pattern. The results can provide guidance for enhancing the condensation heat transfer of the water and organic compounds binary mixed vapors. • Wall wettability can change the immiscible condensates flow behavior and thus affect condensation heat transfer. • The smaller the condensate droplets departure diameter on the wall, the greater the condensation heat transfer performance. • The condensation heat transfer coefficient on the super-hydrophobic wall surface was the highest. [ABSTRACT FROM AUTHOR]

A liquid piston system (LP) is proposed to recover energy during the discharge of a liquid air energy storage (LAES) plant. The traditionally used air turbine is replaced with an LP system which will expand the evaporated air to generate power. Moreover, an NH 3 and transcritical CO 2 cycle are integrated to enhance heat and cold utilisation. The integrated LAES system was modelled using gPROMS. The numerical results have a maximum discrepancy of less than 6.4% from experimental data previously reported in the literature. The round-trip efficiency (RTE) was studied for various scenarios, including the sensitivity to thermal oil flow rate and heat exchanger efficiencies. Additionally, the study examined the influence of several LP configurations and concluded with a comparison to a conventional LAES system. With a liquefaction pressure of 40 bar, the highest RTE reached was 37%, surpassing the conventional LAES system's 25% RTE at a discharge pressure of 25 bar. The heat utilisation was 96%, which doubles that of the conventional LAES system, while the cold utilisation was enhanced by 10%. Furthermore, the LP, NH 3 , and CO 2 cycles thermal efficiencies were 88%, 19%, and 30%, respectively. A detailed energy assessment for heat and cold utilisation is presented. • In the novel system, power was produced using Liquid Piston and Rankine cycles. • The idealised case round-trip efficiency of the integrated LAES system was 37%. • The conservative case round-trip efficiency of the integrated LAES system was 29%. • The Liquid Piston, NH 3 and CO 2 efficiencies were 88%, 19%, and 30%, respectively. [ABSTRACT FROM AUTHOR]

Changing the inner tube shape proves to be an effective method to enhance the efficiency of horizontal latent heat thermal energy storage exchangers (H-LHTES), which can improve the heat transfer efficiency of exchangers. This paper proposed an I-beam inner tube exchanger, which contained I-1 and I-2 types, then under the constant mass of phase change material, the charging and discharging performance of the H-LHTES with I-beam, circular, rectangular and rhombic inner tube shapes were comparatively investigated using FLUENT numerical simulation software. The results demonstrated that compared with the circular inner tube, the charging and discharging time reduces remarkably for the H-LHTES with I-beam, rectangular and rhombic inner tubes, and the I-beam inner tube shows the best enhancement effect, additionally, the I-1 has the shortest total time at the inner tube height of 74.8 mm, the charging and discharging time reduces by 82.41% and 43.56%, the total time decreases by 50.46%, the rectangular and rhombic inner tubes have the shortest total time at the inner tube height of 82.8 mm and 90.8 mm, the total time decreases by 34.7% and 36.99%, respectively. The results can supply a reference for the optimal design of inner tube shape of the H-LHTES. [ABSTRACT FROM AUTHOR]

This article presents experimental analysis of heat removal capabilities of a wavy finned tube heat exchanger operating under natural convection conditions Two types of heat exchangers are considered in this work: standard construction exchangers, most often found in waste heat rejection devices and exchangers based on a novel geometry, which supports intensification of heat transfer processes under natural convection. The aim of the research was to experimentally verify the novel conception and to determine the thermal efficiency of heat exchangers under the influence of selected design parameters. The factors analyzed in the research include: temperature difference between the heat transfer surface and the surrounding air, different angles of the units transferring heat and the height of the exchanger casing. Four variants of the casing height were considered in this work. The conducted research also allowed to determine the influence of the fin spacing on the performance of the heat exchangers under natural convection conditions. The test results obtained show that the novel geometry allows for significance improvement of heat transfer efficiency. The proposed solution allowed to increase the density of the heat flux from 12% in (fin spacing of 2.8 mm) up to 24% (fin spacing of 5.6 mm). It was also observed that the height of the casing has significant impact on the heat transfer efficiency. For each of the analyzed cases, the heat exchanger without a casing was characterized by the lowest thermal efficiency, while the highest heat flux was removed from the exchanger with the highest casing (up to four times higher heat transfer rate). It was determined that the largest amount of heat was removed from exchangers located horizontally, while the exchanger with an angle of 60°, was characterized by the lowest rate of heat removal. Another factor which was determined to have significant impact on the performance of the heat exchangers is the spacing between the fins. The research results obtained shows that increasing the spacing of the finned surface from 2.8 mm to 5.6 mm resulted in an average twofold increase in thermal efficiency in the case of four-row exchangers and an almost threefold increase in the case of two-row exchangers. The obtained results can be used as basic guide for the development of finned tube heat exchangers operating under natural convection conditions. [ABSTRACT FROM AUTHOR]

• The model of external fin backfill coupled heat exchanger was established. • The thermal performance of FBCHE were simulated. • The materials and structures of different fins are compared. • The heat transfer factors affecting FBCHE were analyzed. There are abundant geothermal resources in mine goaf. The effective exploitation of geothermal energy is of great significance to realize the long-term sustainable development of mines. In this paper, a three-dimensional unsteady heat transfer model of a typical serpentine backfill coupled heat exchanger is established using COMSOL Multiphysics simulation software, and the accuracy of the model is verified. A serpentine external finned backfill coupled heat exchanger is proposed. Comparative analysis of the heat transfer performance of serpentine external finned backfill coupled heat exchanger and smooth backfill coupled heat exchanger is conducted. The effects of different fin materials, fin structures and system design parameters on the thermal performance of serpentine external finned backfill coupled heat exchanger are studied. The results show that the temperature difference between the inlet and outlet, total heat storage and total heat release increases with the increase of fin thickness, fin height, and surrounding rock temperature, decreases with the increase of inlet velocities and inlet temperature. The heat exchanger energy efficiency coefficient increases with increasing fin thickness and height, decreases with increasing surrounding rock temperature, inlet temperature and inlet velocities. The temperature difference between the inlet and outlet, total heat storage and total heat release of copper fins are higher than those of other materials. The energy efficiency coefficient of using round fin is higher than that of using square fin, square H-fin and round H-fin. When copper round fins are used, the heat storage of the system is 1457.03 kJ, the heat release is 2831.95 kJ, and the energy efficiency coefficient is 51.45%. [ABSTRACT FROM AUTHOR]

• A novel air-flue gas total heat exchanger is proposed, which can solve the problem caused by flue gas nonlinearity. • The limit efficiency of the novel total heat exchanger is up to 100 %. • Heat transfer model of the proposed heat exchanger is established by employing CFD simulation. • Efficiency of the novel total heat exchanger is 87 %–92 %. • Thermal performance of the novel heat exchanger has been compared with a double tower system and an enthalpy wheel. Because of high humidity and nonlinearity of flue gas, waste heat from flue gas is hard to recovery. Boilers equipped with vapor-pump system is developed to solve the problem caused by high humidity. In this system, double spray towers subsystem is equipped to realize total heat waste heat recovery. However, caused by nonlinearity, limited waste heat recovery efficiency is just 83 % (1 segment) and 93 % (2 segment). Further, based on boilers equipped with vapor-pump (BEVP) system, enthalpy wheel system is developed to solve the problem caused by nonlinearity. However, enthalpy wheel system cannot solve the problem completely. In this article, a novel air-flue gas total heat exchanger is put forward to achieve full waste heat recovery. In this system, waste heat recovery efficiency limit is up to 100 %. Then, the limit condition of total heat transfer process is discussed. Performance of the total heat exchanger is discussed and compared to double spray towers system and enthalpy wheel system. As the result, considering heat transfer temperature difference, the total heat exchanger total heat transfer efficiency of the total heat exchanger is 7 % higher than 2-segment BEVP system and 10 % higher than enthalpy wheel system. [ABSTRACT FROM AUTHOR]

• A predictive Multi-Layer Perceptron network was implemented for vertical tubes in presence of non-condensable and light gases. • Extensive databases with varying inputs for passive containment cooling systems for nuclear power plants (NPPs) were utilised. • A new correlation for condensation heat transfer coefficient was developed and evaluated. • The developed correlation exhibited outstanding effectiveness in forecasting the condensation HTC with high accuracy. This research focuses on developing a predictive model that can effectively estimate the condensation heat transfer coefficient (HTC) on vertical tube surfaces during free-fall. The model takes into consideration the presence of non-condensable gases. The main objective is to assemble an extensive and varied database that encompasses different geometric parameters and operational conditions. The motivation here stems from the urgent need to develop practical models and/or correlations that can accurately predict the condensation heat transfer efficiency for industrial heat exchangers affected by non-condensable gases, with a particular focus on passive containment cooling systems (PCCS) employed in nuclear power plants (NPPs). This innovative cooling system utilizes the force of gravity to condense water steam, effectively removing heat from the containment vessel in the event of an accident. For the development of the model, we employed the neural network toolbox in MATLAB to construct an Artificial Neural Network (ANN) known as a Multi-Layer Perceptron (MLP) network. This model was designed to predict the HTC during the condensation process involving two different types of Non-Condensable Gases (NCG): Air and Nitrogen, along with Helium as a representative light gas. To build the model, we utilized a dataset comprising 1,888 data points sourced from 21 experimental references. The input layer of our model receives a range of parameters, encompassing total pressure (P tot) , subcooling temperature (Δ T sub ) , mass fraction of non-condensable gases (w nc ) , wall length (L) , hydraulic diameter (D h) , the ratio of the Helium to non-condensable gases (R He / n c) , liquid density (ρ l), gas density (ρ g) , liquid viscosity (μ l) , gas viscosity (μ g) , saturation temperature (T sat) , latent heat (h fg) , liquid thermal conductivity (k l) , and gas specific heat (c p , g). The output data of our model corresponds to the condensation HTC (h). To ensure consistent scaling, the input data was normalized by scaling each feature within the range of 0 to 1. On the other hand, the output data underwent a transformation using the natural logarithm. By utilizing Pearson correlation coefficient analysis and importance analysis for parameter selection, a new correlation for heat transfer was developed and evaluated. The developed machine learning model exhibited outstanding effectiveness in precisely forecasting the condensation HTC with an accuracy range of ± 10 % and ± 20 %. The outcome of this investigation represents the notable progress in analyzing extensive datasets gathered through experiments. Specifically, it addresses the previously unexplored influence of Helium gas by harnessing the capabilities of machine learning. [ABSTRACT FROM AUTHOR]

This study investigates the concept of using heat to enhance reverse osmosis (RO) desalination. An analytical model is used to evaluate the effect of temperature on water permeate flux, specific energy, permeate quality, and applied operating pressures. When feed is heated from 20 to 50 °C, specific energy savings of up to 24 % are observed for high flux seawater desalination and up to 33 % for brackish water. Such improvements are consistent with the literature, but until now the thermal energy input required to heat feed has been mostly neglected from analysis. For the first time, the overall energy balance of thermally-enhanced RO is considered to evaluate the tradeoff between savings in mechanical pump work and thermal energy input. Results suggest that this tradeoff is favorable under the right conditions. In particular, there is a need for high thermal efficiencies including both a very high heat pump coefficient of performance and very high heat exchanger efficiency for recycling thermal energy. Under these conditions, overall energy savings of up to 12 % are observed for seawater desalination when feed is heated from 20 to 41 °C, and up to 18 % for brackish water with feed heated to 46 °C. [Display omitted] • Heating RO feed increases water flux, enabling operation at lower applied pressure. • Mechanical pumping work can be reduced by as much as 24 % in seawater desalination. • Considering thermal energy input in the overall energy balance, savings reach 12 %. • Favorable energy balance requires high thermal efficiencies or low-cost heat source. • Savings are greatest in applications with high water flux and recovery ratio targets. [ABSTRACT FROM AUTHOR]

Increasing efficiency in heat exchangers is always a major concern for researchers and industrial owners. Then, mechanical engineers have made a significant contribution to increasing the efficiency of heat exchangers and ultimately reducing energy consumption by making fundamental changes in the geometry of heat exchangers as well as adding vanes and barriers. In this study, in addition to using the wavy channel, the effect of the presence of plate barriers in 15 different locations in a wavy channel have been investigated. The governing equations solved by SIMPLE method. Although, the presence of barriers caused pressure drop, the increase of Nusselt number in several cases was a convincing justification for using barrier. The flow was laminar and the working fluids were air and water at three different Reynolds numbers of 75, 150 and 225. The governing equations were solved by placing the barriers in positions in three transverse rows with the same relative distances. Finally, the performance coefficient, which is a relationship resulting from the role of pressure drop coefficient and Nusselt number, was defined. The results showed that for one of the barriers and the air as working fluid, the performance coefficient reaches 1.28 and for the water as working fluid and one of the barriers position, it reaches 1.50. Regardless of the discussion of pressure drop, in several cases, the Nusselt number experiences an increase about 30% compared to the no barrier state. [ABSTRACT FROM AUTHOR]

U-tubes are nowadays widely used in large industries, especially in heat exchangers, as well as solar absorbers. Optimizing and increasing the thermal efficiency of heat exchangers reduces fossil fuel consumption and costs savings. A comparative experimental study was conducted on friction factor, and heat transfer of oil-based nanofluids flow (CuO and Fe 3 O 4 with 0.5%, 1%, and 2% mass concentrations) inside inclined U-tube at 0°, 30°, 60°, and 90° inclination angles under constant heat flux. The nanofluids prepared in two steps method by dispersing nanoparticles in SN-500 base oil. The tests were performed in five different flow rates. The results showed that compared to the base fluid, using nanoparticles increased the friction factor and convective heat transfer coefficient. According to the results, it was found that the highest heat transfer rate belonged to 0.5% CuO and 1% Fe 3 O 4 , and the maximum increase in Nusselt number for nanofluids happened at 30° inclination angles. Finally, it was found that the Fe 3 O 4 nanofluid with 1% concentration had the highest value among all the test nanofluids samples in horizontal and inclined modes. Unlabelled Image • Thermo-hydraulic properties of oil-based nanofluids have been studied in U-tubes. • 0.5% CuO and 1.0% Fe3O4 nanofluids have the highest Nusselt number. • CuO nanofluids show anti-wear properties and reduce the friction factor. • Fe3O4 nanofluid with a 1% concentration has the highest thermal performance index. • Maximum increase in Nusselt number happens at 30° inclination angle. [ABSTRACT FROM AUTHOR]

In the present study, the effects of rotary tubes and magnetic-induced nanofluid on heat transfer characteristics of a compact heat exchanger are individually investigated. Two-phase Eulerian model is employed to predict the hydrothermal and entropic characteristics of Fe 3 O 4 /water ferrofluid in the heat exchanger. Results indicate that utilizing each rotary tubes and magnetic field method can improve the energy and exergy efficiencies of the compact heat exchanger under specific circumstances by forming different types of secondary flow. It is found that employing each method individually can increase the maximum heat transfer rate by more than 60%. In comparison with methods like passive vortex and swirl generators, subtle pressure drop and entropy generation is observed in the new proposed methods since no additional obstacles were employed. Results also reveal that 12.5% of the possible maximum energy can be regenerated along with a 3.5% increase in the exergy efficiency of the heat exchanger at low Reynolds numbers by employing each method. Unlabelled Image • Two-phase model was employed in analysis of a FTCHE with Fe 3 O 4 /water ferrofluid. • Each of the employed method has a similar effect on the FTCHE. • Energy and exergy efficiencies were improved by each utilized technique. • A minimization in pressure drop and entropy generation is resulted. • Individual employment of magnetic field and rotary tubes was recommended. [ABSTRACT FROM AUTHOR]

Phosphorus effect on ash fouling deposition produced during combustion process of sewage sludge solid fuel is a very important factor. Previous studies have only focused on decrease of the ash melting temperature and increase of slagging and sintering by phosphorus content. Therefore, research regarding combustion fouling formation and its effect on temperature reduction of deposit surface by phosphorus content is insufficient. Ash fouling is an important factor, because ash in the combustion boiler process deposits on the surface of heat exchanger and interferes with heat exchange efficiency. In particular, temperature reduction of heat exchanger surface via fouling should be considered together with fouling deposition, because this is related to the heat exchanger efficiency. Synthetic ash, phosphorus vaporization, and drop tube furnace experiments were performed to investigate effect of phosphorus on ash fouling formation and temperature reduction of deposit surface under combustion condition. Phosphorus was highly reactive and reacted with ash minerals to produce mineral phosphate, which promoted ash fouling deposition during the combustion experiments. In contrast, the occurrence of sintering on deposited fouling resulted in formation of a large hollow structure, which alleviated the temperature reduction on the deposit surface. Phosphorus content had a substantial correlation with fouling deposition behavior and influenced reduction in the surface temperature of the heat exchanger, because it led to generating low temperature mineral phases. • Ash fouling of sewage sludges treated by HTC was evaluated under combustion condition. • Phosphorus content had a substantial correlation with ash fouling formation. • Sintering increased the porosity of ash fouling structure. • Porosity of ash fouling structure had a correlation with temperature reduction of deposit surface. [ABSTRACT FROM AUTHOR]

Tree-shaped fin exhibits an excellent performance in balancing the strong and weak heat flow in latent heat exchangers, which renders higher solidification efficiency and more uniform temperature distribution. Based on the enthalpy-porosity method, a transient model of the solidification heat transfer in the heat exchangers with five tree-shaped fins are established and numerically analysed. The solidification behaviours of the phase change material (PCM), including liquid fraction and temperature evolution, in tree-shaped heat exchanger are investigated and compared with corresponding ones. The results indicate that the tree-shaped fin introduces more uniform solidification front and yield a higher solid fraction. Compared with the traditional longitudinal fins, the efficiency of heat exchanger with four-level tree-shaped fins increases by 53%. In terms of solidification efficiency, the tree-shaped fin configuration with four-level is considered as the optimal fin configuration. The initial temperature difference and the thermal conductivity of fins largely determine the solidification trend of the PCM in the heat exchanger. Compared with the width ratio, the heat exchanger with larger length ratio stores more energy during the phase change process. • Tree-shaped fin is introduced to optimize heat transfer performance of heat exchanger. • The solidification behaviour of PCM in the heat exchanger with five fin configurations are analysed. • The efficiency of heat exchanger with tree-shaped fin increases by 53%. • The tree-shaped fins render the solidification front more uniform and yield a higher solid fraction. [ABSTRACT FROM AUTHOR]

The work presents the design and analysis of a novel cryo-compressed hydrogen (CCH 2) supply system. It aims to storage CCH 2 (around 20 MPa and 20 K), but supply hydrogen under suitable conditions (0.16 MPa and 338 K) for fuel cell stacks. Generally, thermal waste of fuel cell occupies nearly half of the entire outcome and cannot be applied for truck driving. But in this system, original wasted energy can be reused to heat cool hydrogen, which relieves the heat burden of cooling device. This process is carefully designed and demonstrated for a 25-ton heavy duty truck. Mass flow rate in the specified CCH 2 system is verified by theoretical calculating. Also, negative throttling effect of hydrogen is carefully considered for comprehensive utilization. At last, different efficiency of cryogenic heat exchangers are compared to explore the characteristics of energy consumption. • An on-board cryo-compressed hydrogen supply system with multistage heating has been designed. • Numerical simulation of this CCH 2 system has been developed and validated well. • Residual heat generated by PEMFC is designed for reusing, which improves total energy utilization. • The promotion effect of negative throttling effect in heating process has been studied. • Performance of 25T truck is discussed to guide equipment selection of hydrogen supply system. [ABSTRACT FROM AUTHOR]

• Raw data of cross-flow heat exchanger is collected and processed. • Relationships between the process variables are determined and outliers are removed by PCA. • Heat exchanger thermal efficiency is modeled by PLS according to all variables collected from the concentration loop. • Heat exchanger thermal efficiency is modeled by PLS according to a reduced number of the system variables. • Thermal efficiency is modeled using ANN. The abundant quantities of impurities resulting from the production of phosphoric acid by dihydrate process promotes fouling in the heat exchanger which is manifested by quick clogging of the tubes, outlet acid temperatures rise to values close to safety measures and operating cycles less than five days. During the acid concentration operation, fouling leads to a significant drop in the overall heat transfer coefficient, which is highly dependent on the thermal efficiency of the heat exchanger. This requires an instantaneous follow-up. The present work aimed at modeling thermal efficiency in a cross-flow heat exchanger by the collection and treatment of the operating data of the concentration loop. To this end, Principal Component Analysis (PCA) was selected for dimensionality reduction. A first fouling factor modeling by Projection to Latent Structures (PLS) showed a decent estimation of the deposition phenomenon with a correlation coefficient R 2 and prediction ability equal to 0.925 and 76%, respectively. In order to improve the results obtained by PLS, Artificial Neural Network (ANN) with backpropagation method was used thereafter. Among the 75 trained topologies, the best performance was obtained with a network consisting of one hidden layer with 7 neurons using tangent sigmoid transfer function for the hidden and output layers. The optimized ANN model for thermal efficiency prediction resulted in reliable quality indices which reflect that the model accurately tracks the variability within measurements with AARD = 0.0639%, MSE = 0.00003, RMSE = 0.00573 and r 2 All = 0.9998. [ABSTRACT FROM AUTHOR]

The environment inside laying hen houses has an important effect on hen productivity, health, and well-being. Heat exchangers (HE s) can recover waste heat in ventilation exhaust to reduce supplemental heating needs while maintaining a greater fresh air exchange rate. For laying hen houses, there is limited information on the effect of heat recovery ventilation (HRV). Thus, the objective was to evaluate an air-to-air HE for manure belt drying ventilation in aviary laying hen housing. Temperature (T), relative humidity, ammonia, and manure dry matter (DM) content were characterized during a 4-wk period in October 2018. In weeks 2 and 4, the HRV was shut down and compared to weeks 1 and 3 when the HRV was operational. Average (±SD) ambient T was 10.6°C ± 4.0°C, similar for the 4-wk period. Heat exchanger efficiency was 75.07% ± 9.4% with the average supply temperature increased by 10.0°C ± 3.4°C and an average of 93.94 ± 31 kW heat recovered. Average indoor T (23.1°C ± 0.5°C) was warmer as a function ambient T and daily average T range was lower with HRV (1.8°C ± 0.7°C) compared to without HRV (22.2°C ± 1.2°C; 3.1°C ± 1.1°C). Seven days after manure removal, final average manure DM was 40.6% ± 3.1% (without HRV) and 60.0% ± 3.3% (with HRV). Implementation of HRV positively influenced indoor thermal environment by maintaining less dynamic diurnal fluctuations and greater temporal T uniformity. [ABSTRACT FROM AUTHOR]

Phase change materials (PCMs) have attracted considerable interest recently due to their remarkable thermal energy storage capabilities for different thermal applications. This would effectively assist in the reduction of CO 2 emissions and encourage the transition to sustainable energy on a global scale. The present study investigates the increase in the thermal efficiency of a shell-and-tube heat exchanger using PCM and a novel arrangement of fins in the form of a tree. The enthalpy-porosity method is used to mimic phase changes under the studied conditions. Moreover, copper nanomaterials are incorporated with the PCM to increase thermal conductivity and speed up the charging process. The influences of various parameters, such as the concentration of nano-additive and the distribution of sub-fins, are investigated. The findings derived from the present model demonstrate that the incorporation of elongated sub-fins in the outer part (far from the inner heating tube) of the PCM unit leads to a noteworthy enhancement in the heat transfer distribution, resulting in a 75.7% reduction in melting time. Adding copper nanomaterials to PCM improves thermal performance. For example, a 6% concentration of nanoparticles leads to a reduction in the melting process by >15% compared to pure PCM. [ABSTRACT FROM AUTHOR]

• Recuperative ORC (RORC) presents a higher efficiency (+20 %) and power (+18 %) than ORC. • RORC works with a lower mass flow rate than ORC for a given evaporating temperature. • RORC needs a lower heat input thus increasing the operating time in absence of solar radiation. • The higher operating time involves a higher yearly electricity energy production (+37 %). • Hot water and coolant temperature play the most important effect on RORC performance. Organic Rankine Cycle -based microcogeneration systems that exploit solar sources to generate electricity and domestic hot water simultaneously are promising solutions for reducing CO 2 emissions in the residential energy-intensive sector. Such systems can be assisted by thermal energy storage reservoirs in which water is heated by solar energy and cooled by the direct use of thermal energy for domestic hot water production and acting as heat source of the power plant. An interesting plant layout optimisation involves the adoption of a recuperative heat exchanger to preheat the working fluid before it enters the heat recovery vapour generator, which is fed with the same working fluid leaving the expander. Despite the positive effect of recuperative heat exchanger adoption on the efficiency of the entire plant, the impact on electrical energy production at this microscale is not straightforward to assess as the heat source is represented by the water inside the reservoir and it is not a continuous high- or medium-temperature stream. The lack of experimental analyses in the literature on these applications makes this a debatable question. Thus, to fill this gap and quantitatively assess the benefits introduced by the adoption of a recuperative stage, a wide experimental comparison between recuperative and not-recuperative Organic Rankine Cycle-based power unit layout was performed. To support the experimental analysis, a comprehensive theoretical model of the Organic Rankine Cycle-based plant was developed and validated against experimental data. The experimental campaign demonstrated that the recuperative Organic Rankine Cycle-based unit required a lower working fluid flow rate for a given expander inlet temperature and pressure ratio. Consequently, a higher electrical power can be produced, requiring lower thermal power from a heat source. The reduction in the thermal power absorbed from the heat source, produced by a higher temperature incoming working fluid entering the Heat Recovery Vapor Generator, had a positive effect, allowing a longer-in-time working condition with a higher conversion efficiency intrinsically produced by the recuperative heat exchanger. Considering the annual electricity consumption of 2700 kWh of a family living in a city in central Italy, the contribution given by the recuperative Organic Rankine Cycle plant operating with a reservoir fed by 15 m2 of flat solar panels was 330 kWh, whereas without the recuperative stage, the production was 240 kWh. The cost of electricity saved justified the adoption of a recuperative heat exchanger. [ABSTRACT FROM AUTHOR]

• Higher fluid flow rate or flow velocity mitigates fouling resistance and particulate deposition. • Impurities accumulate in the region of low velocities. • Higher foulant material concentration induce higher fouling rate. • Physical water treatment methods impart prominent role in fouling retardation. • Fouling rate and particles deposition depicts significant dependency on material surfaces. Mitigation of fouling and particulate deposition in heat exchangers is the foremost concern of this critical review study. Formation of vindictive scale, sludge and fouling layers on heat transfer surfaces have severe influence on thermo-hydraulic performance and overall efficiency of heat exchangers. Fouling on gas or liquid side prevent heat transfer from bulk fluid to the solid surfaces resulting lower thermal performance of heat exchangers. Furthermore, fouling layers on heat transfer surfaces induce blockage to the fluid flow and hence increase pumping power by amplifying pressure drop. The use of various system operating variables in controlling fouling and particulate deposition is comprehensively discussed. The influence of flow rate or velocity, temperature, concentration, material surfaces and other miscellaneous factors on reduction of fouling and particle deposition is extensively investigated. It was concluded that higher flow velocity induces strong shear force across heat transfer surface which in results eradicate deposits and reduces the fouling resistance at the expense of higher pressure drop. The enhancement in fouling concentration leads to the higher fouling resistance and materials with lower thermal conductivity yield inferior fouling resistance. To conclude, this review study will be exceedingly useful for designers to design heat exchangers with higher overall efficiency under the influence of fouling. [ABSTRACT FROM AUTHOR]

• Pure fine silver powder sinter heat exchanger materials were studied. • Bond strength between particles in a sinter was assessed using thermal diffusivity. • A 0.13 μm sinter had surface area 1.4 m2/g and pore volume 5.99 × 10−3 cm3/g. • A 0.13 μm sinter showed bond strength equal to that of a common 0.07 μm material. • The 0.13 μm sinter is suitable for use in ultra-low-temperature cooling. This work investigated sinters made of pure fine silver powders having nominal particle sizes of 0.07, 0.13 and 10.6 μm, using nitrogen gas adsorption, laser flash analysis and field emission-scanning electron microscopy. The goal was to improve the efficiency of heat exchangers used in ultra-low-temperature cooling. This study also examined the bond strength between particles in the sinters, which is associated with the thermal conductivity of the materials, based on thermal diffusivity data. The 0.13 μm sinter had a specific surface area greater than 1.4 m2/g, while the 0.07 μm material, which is most widely used for ultra-low-temperature cooling, had a value of approximately 0.4 m2/g. The latter surface area was less than 25% of previously reported values. The 0.13 μm sinter with a 55% packing fraction exhibited a large total pore volume of 5.99 × 10−3 cm3/g, a high specific surface area of 1.6 m2/g, and the largest room temperature intrinsic thermal diffusivity of 9.12 × 10−5 m2/s in air. Therefore, the 0.13 μm material represents the optimal heat exchange sinter for ultra-low-temperature applications. [ABSTRACT FROM AUTHOR]

There are various factors having an impact on the energetic and exergetic performance (i.e., COP and eCOP) of an absorption refrigeration systems (ARS) such as the temperatures of the generator, condenser, evaporator and absorber, effectiveness of solution, refrigerant and solution-refrigerant heat exchangers and isentropic efficiency of the solution pump. Many studies have focused on these process parameters, but the importance order and contribution ratios of the parameters due to thermodynamic performance have not been determined by using statistical methods. Firstly, in this study, cycles' thermodynamic model is established and the variation of the COP and eCOP are calculated for different working conditions with different parameters ranges. The effects of these parameters on the COP and eCOP are examined separately on a statistical basis. The importance order of the parameters are determined by using Taguchi and ANOVA methods and the results are compared. Optimum operating conditions are determined by means of statistical analysis for the COP and eCOP. Under these operating conditions, the COP and eCOP of the system are calculated as 0.697 and 0.2829, respectively. Furthermore, for the simultaneous maximization of these two performance indicators, Taguchi-Grey Relational Analysis (GRA) is used. By using this analysis, importance order of the examined parameters on multiple performance characteristics are determined. The absorber and evaporator temperatures are the most efficient parameters on multiple performance characteristics with a contribution ratio of 29.66% and 26.34% of the total effect while the least efficient parameters are the pump efficiency and effectiveness of solution-refrigerant heat exchanger with a contribution ratio of 0.48% and 2.41%, respectively. For the best condition considering the multiple performance characteristics, COP and eCOP of the system are found as 0.6255 and 0.2829, respectively. • The process parameters of ARS and their effects are examined statistically. • The effects of the process parameters on COP and eCOP are investigated, separately. • Contribution ratios of the parameters on the multi-objective function are obtained. • The best/worst conditions of different parameters are determined for COP and eCOP. • Absorber and evaporator temperatures are found to be the most effective parameters. [ABSTRACT FROM AUTHOR]

Abstract In this study, technical and economical evaluation of a multi effect distillation (MED) system is presented which utilizes the vapor produced from waste heat of exhaust flue gases. In order to recover the waste heat of flue gases, a shell and tube heat exchanger was designed with the Polytetrafluoroethylene (PTFE) material in order to resist corrosion as a result of sulfuric acid formation. Recoverable energy, which is obtained from experimental data, serves as the source of energy required to produce water vapor. Due to high fluctuations in the amount of vapor produced, an auxiliary boiler is employed for its compensation. The vapor which has been produced by the heat exchanger and/or auxiliary boiler, is used as the motive steam in the MED distillation. Economic analysis of MED distillation system driven by low-temperature waste heat is carried out and system payback period based on natural gas saving is obtained. Finally, waste-heat-driven MED distillation system is compared to a conventional MED on the basis of the levelized cost of water (LCOW) at different electrical and natural gas tariffs as well as the real interest rates. Economic evaluation shows that the LCOW is about 1.13–2.9 $/m3. Highlights • Techno-economic evaluation of a waste heat driven MED system is performed. • The PTFE heat exchanger and auxiliary boiler are employed to produce vapor for MED. • Comparison is made between the proposed system and conventional MED based on LCOW. • Effect of electrical and natural gas costs and interest rate on LCOW are investigated. • Economic evaluation shows that the LCOW is about 1.13–2.9 $/m3. [ABSTRACT FROM AUTHOR]

Exergy analysis of reliquefaction system based on Reverse Brayton cycle (RBC), which is used to condense boil-off gas (BOG) generated in LNG carrier ships, was presented with its basic configuration in Part 1. In this paper, three modifications of the basic system are evaluated. One modification is based on intercooling and precooling of BOG before compressor suction. Others involve ambient compression of BOG. Effects of pressure ratio of RBC, mass flow rate of nitrogen, thermal sizes of heat exchangers and entry temperature of BOG to reliquefaction system on each configuration have been evaluated. Sensitivity analysis shows that 7.8% improvement of rational exergy efficiency is obtained for systems with ambient compression. When BOG enters as superheated gas, modified configurations could keep up their performance while basic system deteriorated. Higher adiabatic efficiencies of compressors and turbines improve the performance of the reliquefaction systems across the board without any additional weight or space. Design and operating parameters have been optimized with the aid of in-built optimiser tool of Aspen HYSYS® V8.6. It resulted in an improvement of rational exergy efficiency of configurations by 1%–3% and decrease of total UA by 5–7% compared to best values predicted by sensitivity analysis. • Evaluates and compares improved configurations for BOG reliquefaction. • Detrimental effects of higher BOG entry temperature effectively circumvented. • Nondimensionlization of parameters helps to apply results to any size of LNG ship. • Evaluated effects of higher adiabatic efficiency of compressor and turbine. • Optimises rational exergy efficiency with reasonable heat exchanger sizes. [ABSTRACT FROM AUTHOR]

Fully-coupled multiphysics simulations are applied to investigate a number of candidate heat exchanger materials in the Super-High Temperature Additively-Manufactured Resistojet (STAR) thruster. Two mission applications are considered: a low earth orbit (LEO) primary propulsion application and a secondary reaction control system (RCS) application of an all-electric geostationary (GEO) telecommunications platform. High-temperature operation provides a significant increase in specific impulse over the state-of-the-art Xenon-resistojets. Inconel 718 is investigated for moderate-performance for LEO applications, while pure tantalum and pure rhenium are examined for the extreme temperature high-performance GEO application. Simulations determine the attainable performance including heat transfer, Navier-Stokes continuum flow and Joule heating physics in both transient and steady state. Nozzle efficiency, heat exchanger efficiency, electrical characteristics and other key performance indicators are explored. [ABSTRACT FROM AUTHOR]