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This paper proposes a NH3/CO2 cascade refrigeration system that sets an auxiliary refrigeration loop in the high-temperature cycle to increase the subcooling degree in the low-temperature cycle (CRSS). Based on the basic principles of thermodynamics, a mathematical model is established and theoretical simulation is carried out to obtain the influence of key parameters on the cycle performance. Compared with the conventional cascade refrigeration system (CCRS), the conclusions find that there exists an optimal condensation temperature of low-temperature cycle (TMC.opt) to maximize COP. The maximum COP of CRSS is 4.58% higher than that of CCRS when the subcooling degree is 10 °C. The maximum exergy efficiency is 0.391, increasing by 4.40%. With the increase of the subcooling degree, both the COP and the TMC.optof CRSS increase. When the subcooling degree increases from 5 °C to 15 °C, the performance increment increases from 2.73% to 6.00%. When the evaporation temperature of the system changes, both the COP and exergy efficiency decreases slightly and it is found that the performance is better when the evaporation temperature is lower when condensation temperature is kept constant. Besides, the discharge temperature of the NH3 compressor in CRSS can be reduced by 9.9 °C when the evaporation temperature is −30 °C.

Experimental research on flow characteristics of open natural circulation system was performed, to figure out the mechanism of the open natural circulation behaviors. The influence factors, such as the heating power, the inlet subcooled and the level of cooling tank on the flow characteristics of the system were examined. It was shown that within the scope of the experimental conditions, there are five flow types: single-phase stable flow, flash and geyser coexisting unstable flow, flash stable flow, flash unstable flow, and flash and boiling coexisting unstable flow. The geyser flow in flash and geyser coexisting unstable flow is different from classic geysers flow. The flow oscillation period and amplitude of the former are more regular, is a newly discovered flow pattern. By drawing the flow instability boundary diagram and sorting out the flow types, it is found that the two-phase unstable flow is mainly characterized by boiling and flash, which determine the behavior of open natural circulation respectively or jointly. Moreover, compared with full liquid level system, non-full liquid level system is more prone to boiling phenomenon, and the range of heat flux density and undercooling degree corresponding to unstable flow is larger.

Subcooling systems are positioned in recent years as one of the best solutions to improve the efficiency of transcritical CO2 cycles. Specifically, the integrated mechanical subcooling cycle allows the improvement of these systems only using CO2 as a refrigerant. This integrated cycle can be designed with three different architectures: extracting the CO2 from the gas-cooler outlet, from the subcooler outlet or from the liquid tank. In this work, the three configurations are experimentally analysed and the main differences between them are studied. An experimental plant has been tested at three heat rejection levels (25.0, 30.4 and 35.1°C) and a fixed temperature of the secondary fluid at the evaporator inlet of 3.8°C. The results show that from an energy efficiency point of view, all the configurations have practically the same COP, with certain variations in the cooling capacity and the greatest differences in the cycles are found in the subcooler. Funding for open access charge: CRUE-Universitat Jaume I

The MARS code has been assessed for the prediction of onset of flow instability (OFI) in a vertical channel. For assessment, we built an experiment database that consists of experiments under various geometry and thermal-hydraulic condition. It covers pressure from 0.12 to 1.73 MPa; heat flux from 0.67 to 3.48 MW/m2; inlet sub-cooling from 39 to 166 °C; hydraulic diameters between 2.37 and 6.45 mm of rectangular channels and pipes. It was shown that the MARS code can predict the OFI mass flux for pipes reasonably well. However, it could not predict the OFI in a rectangular channel well with a mean absolute percentage error of 8.77%. In the cases of rectangular channels, the error tends to depend on the hydraulic diameter. Because the OFI is directly related to the subcooled boiling in a flow channel, we suggest a modified subcooled boiling model for better prediction of OFI in a rectangular channel; the net vapor generation (NVG) model and the modified wall evaporation model were modified so that the effect of hydraulic diameter and heat flux can be accurately considered. The assessment of the modified model shows the prediction of OFI mass flux for rectangular channels is greatly improved.

In some thermally driven two-phase natural circulation systems, bubble pumps serve as the key driving powers for the cycles. Recently a type of distribute heating bubble pump generator (BPG) is gradually receiving attention due to its compact structure and great potentials to utilize solar energy and low-grade waste heat recovery. The BPG provides a variety of promising features (e.g., passive heat transfer, enhanced reliability), which can benefit the advancing of heat transfer technology. For the primary study, we performed an experiment in a distributed heating BPG. Through utilizing multiple lift tubes and partial visualization configurations, it provides accesses to observe the flow pattern transition and monitor the flow instability, and thus to explore some of the underlying mechanisms affecting BPG performance. Results showed that heat input and immersion height were crucial parameters to enable the operation of distribute heating BPG. With low heat input or high inlet water subcooling level, the flow within the pump was unstable with intermittent flow interruptions. As the heat input increased, the fluid flow became more stable, the vapor generation increased linearly, while the lifted liquid flow rate initially increased then decreased. Correspondingly, the flow pattern at the outlet section of lift tubes gradually changed from slug flow to churn flow, and then to annular flow. The higher of the immersion was, the higher heat input was needed for the flow pattern transition. It was in the churn flow regime at the outlet of lift tubes for the BPG to lift a maximum liquid. At lower immersion level, liquid reflux in the lift tubes was obvious and affected the flow stability as well as the lifting performance. At higher immersion level, the fluid flow was more stable and faster, which lifted more liquid while generated less vapor depending on the inlet subcooling. In general, the BPG showed better performance (both the lifted liquid and vapor generation increased) at smaller inlet subcooling level or lower system pressure. This study highlights the flow pattern evolution and flow stability, which is helpful to the reliable design and effective operation of the distributed heating BPG.

Subcooled boiling is the most effective form of heat exchange in the water jacket of the cylinder head. Chen's model is the most widely used correlation for predicting boiling heat transfer, but the selection of the correlation for the nucleate boiling is controversial. The work of this paper is to simulate the heat transfer process in the water jacket of the cylinder head with a horizontal rectangular channel that is heated on one side. Using the coolant flow velocity, inlet temperature and system pressure as variables, the heat flux and heat transfer coefficient were obtained. The results show that the increase of the coolant flow velocity can effectively promote the convection heat transfer, and the change of inlet temperature and system pressure will affect the occurrence of nucleate boiling. However, the Chen’s model predictions doesn’t fit well with the experimental data. Four nucleate boiling correlations were selected to replace Chen's model nucleate boiling correlation. The correlation proposed by Pioro coincides best with the experimental data. The mean error after correction is 18.2%.

Situated at the bottom of the vacuum vessel, the divertor extracts heat and ash produced by the fusion reaction, minimizes plasma contamination, and protects the surrounding walls from thermal and neutronic loads. The vertical targets of divertor are designed to be able for up to 20 MW/m2 high heat flux. It is a great ordeal for both the material performance and the cooling ability. Critical heat flux margin is very crucial during the design of divertor. The ANSYS FLUENT is used in this paper to predict the critical heat flux on a monoblock structure with a twisted tape inside the tube. Numerical results are validated with the corresponding sets of experimental results. In this paper, CFD method used to predict critical heat flux of divertor cooling channel was first introduced. On the other hand, influence of inlet subcooling on critical heat flux is studied in detail. The inlet subcooling affect the critical heat flux much complicated for the single-side heated and swirl flow channel. Whether the influencing trend or the locations of critical heat flux occurrence are different under different inlet sub-cooling. The derivations between the simulation and experimental results were no more than 32%. This study proves the CFD tools can provide efficient help on the understanding of the critical heat flux phenomenon of complex construction.

In this study, we experimentally investigate of a two-phase natural circulation loop that functions as a passive containment cooling system (PCCS). The experimental apparatus comprises two loops: a hot loop, for simulating containment under severe accidents, and a natural circulation loop, for simulating the PCCS. The experiment is conducted by controlling the pressure and inlet temperature of the hot loop in the range of 0.59–0.69 MPa (abs) and 119.6–158.8 °C, respectively. The heat balance of the hot loop is established and compared with a natural circulation loop to assess the thermal reliability of the experimental apparatus, and an additional system is installed to measure the vapor mass flow rate. Furthermore, the thermal–hydraulic characteristics are considered in terms of a temperature, mass flow rate, heat transfer coefficient (HTC), etc. The flow rate of the natural circulation loop is induced primarily by flashing, and a distortion is observed in the local HTC because of the fully develop as well as subcooled boiling. As a result, we present the amount of heat capacity that the PCCS can passively remove according to the experimental conditions and compared the heat transfer performance using Chen's and Dittus-Boelter correlation.

In the paper, the main results of thermodynamic analysis and optimization of the hybrid two–stage carbon dioxide (СО2) transcritical mechanical compression–ejector cooling cycle are presented. The proposed hybrid cooling cycle contains a two–stage CO2 transcritical mechanical compression refrigeration machine and an ejector cooling machine using refrigerants R245ca, R601b, and R1233zd(E). The operation of the ejector cooling cycle is ensured by using some part of the heat rejected from the compressed superheated CO2 vapor of the mechanical compression refrigeration cycle. The cold obtained in the ejector cooling machine is used for subcooling the high–pressure CO2 vapor after the gas cooler. In addition, the intermediate cooling of the CO2 vapor after the low–pressure stage of the mechanical compression refrigeration cycle is realized by preheating and boiling the refrigerant of the ejector cooling cycle in the additional generator–precooler before entering the main generator–precooler. This paper presents a method for determination of the optimal design parameters and performance of the proposed hybrid cooling cycle. The results showed an increase in efficiency of the two–stage CO2 transcritical mechanical compression refrigeration machine in the proposed cooling system of up to 32.7% compared to the conventional system at transcritical operating conditions with high an environmental temperature.

Increasing the efficiency of disperse phase crystallization is of great interest for melt emulsion production as the fraction of solidified droplets determines product quality and stability. Nucleation events must appear within every single one of the μm-sized droplets for solidification. Therefore, primary crystallization requires high subcooling and is, thus, time and energy consuming. Contact-mediated nucleation is a mechanism for intensifying the crystallization process. It is defined as the successful nucleation of a subcooled liquid droplet induced by contact with an already crystallized droplet. We investigated contact-mediated nucleation under shear flow conditions up to shear rates of 457 s$^{-1}$ for a quantitative assessment of this mechanism. Rheo-nuclear magnetic resonance was successfully used for the time-resolved determination of the solids fraction of the dispersed phase of melt emulsions upon contact-mediated nucleation events. The measurements were carried out in a dedicated Taylor–Couette cell. The efficiency of contact-mediated nucleation $\lambda$$_{sec}$ decreased with increasing shear rate, whereas the effective second order kinetic constant k$_{coll, eff}$ increased approximately linearly at small shear rates and showed a linear decrease for shear rates higher than about 200 s$^{-1}$. These findings are in accordance with coalescence theory. Thus, the nucleation rate is optimal at specific flow conditions. There are limitations for successful inoculation at a low shear rate because of rare contact events and at a high shear rate due to too short contact time.

Steam-assisted-gravity-drainage (SAGD) is currently an effective technology for heavy oil recovery. The pre-heating stage is important to affect the overall process of SAGD development. In this paper, flow and heat transfer model in vertical well section and horizontal well section and thermal connectivity judgment model during cyclic pre-heating stage were established. Model calculation results are in good agreement with actual measurement results. The effects of steam injection rate and steam quality on the distribution of steam parameters along the whole wellbore and the distribution of heat transfer were compared. Besides, the optimization of steam injection parameter combination was provided. Ultimately, the effect of horizontal well spacing on pre-heating thermal connectivity was discussed. The results show that: With the increase of steam injection rate and steam injection quality, the pressure and temperature drop of the fluid increase. Wellhead required steam injection pressure and temperature increases and even superheated steam appear with the increase of steam injection rate and steam injection quality. The position where the pre-heating backflow turns from wet steam to subcooled water gradually shifts to the wellhead. With the increase of steam injection rate and steam injection quality, the heat transfer between annulus and reservoir decreases, but the uniformity of pre-heating in the horizontal section increases. The time required for cyclic pre-heating to form thermal communication between double wells increases significantly as the horizontal well spacing increases. To simultaneously integrate the uniformity and economy of pre-heating on the oilfield site, the steam injection quality at the heel end of the horizontal section of 0.7, the steam injection rate of 80 t/d, and the horizontal well spacing of 5 m were recommended.

A visualized experimental test rig of flow boiling in high temperature thermal storage module was presented. The effects of high-temperature sensible heat thermal storage boundary condition on the oscillation characteristics at the onset of dynamic instability were investigated in this work. The flow boiling tests were carried out in a 1.6-meter-long horizontal tube with an inner diameter of 16.0 mm, which was embedded in a sensible thermal storage module using solid graphite. A series of thermal storage temperatures (200–340 °C), inlet subcooling degrees (5–45 °C) and mass flow rates (0–593.5 kg/m2s) were applied. Based on the dimensionless stability map, an early onset of dynamic instability was observed in comparison with the traditional constant heat flux condition. Meanwhile, both the pressure drop oscillations (PDO) and density wave oscillations (DWO) were observed. Based on the development of flow pattern and heat transfer distribution characteristics, the regulation mechanisms of the high-temperature thermal storage boundary on the development of dynamic instability were analyzed. The results show that the characteristics of dynamic instability are highly related to the development of flow pattern. The thermal storage boundary has affected the development of flow pattern by determining the wall superheat. The comprehensive effects of superheat, subcooling degree and mass flux on the development of flow pattern were also interpreted.

Being a novel energy storage medium, ice slurry has the advantage of good fluidity, high latent heat and heat transfer efficiency, which has a broad application prospect. In this work, the ice making characteristics of different types of additives (MgCl2, ethylene glycol and magnetic iron oxide nanoparticles) in a binary ice system by vacuum flash evaporation under the action of solid adsorption were experimentally investigated. Subcooling, flash rate, ice packing factor, latent heat and thermal conductivity were measured to evaluate the performance of ice production. The results showed that 0.075% wt of iron oxide nanoparticles and 1% wt of MgCl2 are more suitable to be used as additives for binary ice preparation than ethylene glycol, with the former outperforming the latter. Under optimal conditions, compared with pure water for ice making, the subcooling degree of iron oxide nanofluid was reduced by 55.80%, and the thermal conductivity and ice packing factor were increased by 98.80% and 54.22%, respectively. The subcooling degree of the MgCl2 solution was reduced by 37.52% and the ice packing factor was improved by 59.51%. Meanwhile, DSC tests showed that the latent heat of the nanofluid was almost independent of the nanoparticle concentration. However, the increase of MgCl2 solution would cause the decrease of latent heat. In addition, high concentrations of additives can inhibit ice crystal nucleation and growth, but a certain concentration of additives facilitates ice crystal refinement in vacuum ice making process, which is conducive to the transportation of ice slurry.

A few researchers recently reported the presence of vapor bubbles of R600a at the inlet of the capillary tube of refrigerators under subcooled conditions. However, the effect of a non-equilibrium two-phase state on the refrigerant mass flow rate has not yet been addressed. The aim of this study was firstly to investigate the effects of ambient temperature and R600a charge amount on the refrigerant state at the inlet of the capillary tubes and, secondly, to examine the ability of empirical correlations for prediction of the refrigerant mass flow rate under a non-equilibrium two-phase state. A transparent tube was installed at the inlet of the capillary tubes of a parallel two-circuit cycle refrigerator-freezer for visualization of the refrigerant flow. The visualization results demonstrated the presence of a non-equilibrium state under subcooled conditions. The experimental mass flow rates were compared with predictions by empirical correlations of Rasti et al. and Wolf and Pate. Both correlations predicted the mass flow rate with a large deviation under a non-equilibrium state and acceptable deviation under a fully liquid state. Results showed that the mass flow rate under a non-equilibrium two-phase state significantly decreased compared to the fully liquid condition. Finally, a new empirical parameter was introduced to consider the non-equilibrium two-phase flow. The new parameter was applied to previous empirical correlations and the modified version of Rasti et al.’s correlation showed good agreement with the experimental data under a non-equilibrium two-phase state.

In this study, the two-phase pressure drop in a one-sided high-heat-loaded screw tube was analyzed under subcooled flow boiling conditions. When the loaded heat flux is gradually increased under one-sided heating conditions, although the pressure drop changes slightly in the single phase, it starts to increase when loading above the onset of significant void (OSV) heat flux. The effect of system parameters on the two-phase pressure drop in relation to the change of heat transfer mechanism or fluid properties was analyzed according to each variable change. In addition, this study evaluated the prediction performance of existing two-phase pressure drop multiplier correlations developed under subcooled flow boiling conditions. Because most existing correlations are based on a smooth channel with a small diameter, they tend to overpredict the experimental values. The correlation proposed by Ramesh showed the lowest average error rate (31.94%); however, the results were underpredicted for high-heat-flux conditions. To obtain more accurate results, we developed a Python code using an artificial intelligence regression method to proceed with the optimization process based on the experimental values. Using this code, we developed a novel two-phase pressure drop multiplier correlation for a one-side-heated screw tube that can be used in fusion divertors under high-heat-load conditions.

Lubricant-infused surfaces (LISs) can promote stable dropwise condensation and improve heat transfer rates due to a low nucleation free-energy barrier and high droplet mobility. Topographical differences in the oil surface cause water microdroplets to rigorously self-propel long distances, continuously redistributing the oil film and potentially refreshing the surface for re-nucleation. Using high-speed microscopy, we reveal that during water condensation on LISs, the smallest visible droplets (diameter ~ 1um, qualitatively representing nucleation) predominantly emerge in oil-poor regions due to a smaller thermal activation barrier. Considering the significant heat transfer performance of microdroplets (< 10um) and transient characteristic of microdroplet movement, we compare the apparent nucleation rate density and water collection rate for LISs with oils of different viscosity and a solid hydrophobic surface at a wide range of subcooling temperatures. Generally, the lowest lubricant viscosity leads to the highest nucleation rate density. We characterize the length and frequency of microdroplet movement and attribute the nucleation enhancement primarily to higher droplet mobility and surface refreshing frequency. Interestingly and unexpectedly, hydrophobic surfaces outperform high-viscosity LISs at high subcooling temperatures, but are generally inferior to any of the tested LISs at low temperature differences. To explain the observed non-linearity between LISs and the solid hydrophobic surface, we introduce two dominant regimes that influence the condensation efficiency: mobility-limited and coalescence-limited. Our findings advance the understanding of dynamic water-lubricant interactions and provide new design rationales for choosing surfaces for enhanced dropwise condensation and water collection efficiencies., submitted to Langmuir

Although the wettability of hydrate surfaces and hydrate film growth are key to understanding hydrate agglomeration and pipeline plugging, a quantitative understanding of the coupled behavior between both phenomena is lacking. In situ measurements of wettability coupled with film growth were performed for cyclopentane hydrate surfaces in cyclopentane at atmospheric pressure and temperatures between 1.5-6.8 °C. Results were obtained as a function of annealing (conversion) time and subcooling. Hydrate surface wettability decreased as annealing time increased, while hydrate film growth rate was unaffected by annealing time at any subcooling. The results are interpreted as a manifestation of the hydrate surface porosity, which depends on annealing time and controls water spreading on the hydrate surface. The wettability generally decreased as the subcooling increased because higher subcooling yields rougher hydrate surfaces, making it harder for water to spread. However, this effect is balanced by hydrate growth rates, which increase with subcooling. Also affecting the results, surface heating from heat release (from exothermic crystallization) allows excess surface water to promote spreading. The hydrate film growth rate on water droplets increased with subcooling, as expected from a higher driving force. At any subcooling, the instantaneous hydrate growth rate decreased over time, likely from heat transfer limitations. A new phenomenon was observed, where the angle at the three-phase point increases from the initial contact angle upon hydrate film growth, named the crystallization angle. This is attributed to the water droplet trying to spread while the thin film is weak enough to be redirected. Once the hydrate film grows and forms a "wall" around the droplet, it cannot be moved, and further growth yields a crater on the droplet surface, attributed to water penetrating the hydrate surface pore structures. This fundamental behavior has many flow assurance implications since it affects the interactions between the agglomerating hydrate particles and water droplets.

A CO2 cryogenic separation process is proposed and designed for the new liquefied natural gas (LNG) purification cold box. This process is based on the liquefaction process using brazed plate heat exchanger (BPHE) and two separators are embedded between the liquefaction and subcooling heat exchangers to remove frozen CO2. The separator adopts one-use one-standby mode to ensure uninterrupted operation. The main parameters are first obtained through the phase diagram and solubility curve, and then verified by HYSYS simulation. The simulation results show that the preset values are in good agreement with the simulation. LNG enters the separator at 163 K and then is cooled down to 132.5 K to freeze and separate the CO2, so that the CO2 solids will not block the heat exchanger when they are further subcooled. When the mole fraction of CO2 inside the separator accumulates to 10%, the process will switch to another separator to work. The mixed refrigerants (MRs) heat the frozen separator to above 189 K, and the mixtures (solid CO2 and LNG) become liquid and flow out. The change of the process does not cause the temperature crossover inside the heat exchanger. Furthermore, the exergy losses of the proposed process are slightly increased by 1.3% compared to the original process.

Metal cutting fluids for improved cooling and lubrication are an environmental risk and a health risk for workers. Minimizing water consumption in industry is also a goal for a more sustainable production. Therefore, metal cutting emulsions that contain hazardous additives and consume considerable amounts of water are being replaced with more sustainable metal cutting fluids and delivery systems, like vegetable oils that are delivered in small aerosol droplets, i.e., via minimum quantity lubrication (MQL). Since the volume of the cutting fluid in MQL is small, the cooling capacity of MQL is not optimal. In order to improve the cooling capacity of the MQL, the spray can be subcooled using liquid nitrogen. This paper investigates subcooled MQL with machining simulations and experiments. The simulations provide complementary information to the experiments, which would be otherwise difficult to obtain, e.g., thermal behavior in the tool-chip contact and residual strains on the workpiece surface. The cBN hard turning simulations and experiments are done for powder-based Cr-Mo-V tool steel, Uddeholm Vanadis 8 using MQL subcooled to −10 °C and regular MQL at room temperature. The cutting forces and tool wear are measured from the experiments that are used as the calibration factor for the simulations. After calibration, the simulations are used to evaluate the thermal effects of the subcooled MQL, and the surface residual strains on the workpiece. The simulations are in good agreement with the experiments in terms of chip morphology and cutting forces. The cutting experiments and simulations show that there is only a small difference between the subcooled MQL and regular MQL regarding the wear behavior, cutting forces, or process temperatures. The simulations predict substantial residual plastic strain on the workpiece surface after machining. The surface deformations are shown to have significant effect on the simulated cutting forces after the initial tool pass, an outcome that has major implications for inverse material modeling.

In this study, the mathematical modeling of microchannel heat exchangers (MCHEs) has been experimentally examined for applications where there is a single-phase refrigerant flow such as pre-heating/cooling, superheating/subcooling. A mathematical simulation model has been developed for louvered fin MCHEs where the working fluid is R600a, which estimates the outlet temperature, total heat transfer capacity and entropy generation of the R600a. To imrpove the accuracy of the model, different mass velocities in the passes and uniform air velocities at the face of MCHE have been taken into consideration in the proposed model. Different from other models in the literature, a calculation system created by two discretization level has been applied to take into account these effects. Non-uniform air velocities have been taken into consideration via dividing the face of the MCHE into airflow regions in the model. Experimental study has been performed to validate the model results. It concluded that the model predicts the outlet temperature with an average absolute deviation within ±10% for all investigated test conditions. It is found that the taking into consideration non-uniform air velocity improves accuracy of the model. The entropy generation mechanisms in the MCHE have been investigated and it has been determined that the contribution of the fluid flow irreversibility to entropy generation is quite low compared to heat transfer irreversibility., Bu çalışmada, özellikle iklimlendirme sistemlerinde ön ısıtma/soğutma, aşırı kızdırma/soğutma gibi tek fazlı soğutkan akışının olduğu uygulamalar için mikrokanal eşanjörlerin matematiksel modellemesi deneysel doğrulamalı olarak ele alınmıştır. Çalışma akışkanının R600a olduğu panjurlu kanatlı mikrokanal eşanjörler için R600a’nın çıkış sıcaklığını, toplam ısı transfer kapasitesini ve entropi üretimini tahmin eden bir matematiksel benzetim modeli geliştirilmiştir. Modelin doğruluk hassasiyetini arttırmak için geçişlerdeki farklı kütle hızları ve uniform olmayan hava hızı modelde dikkate alınmıştır. Bu etkileri dikkate almak için literatürde yer alan diğer modellerden farklı olarak iki seviye ayrıklaştırma ile oluşturulan bir hesaplama sistemi modelde uygulanmıştır. Modelde mikrokanal ısı eşanjörünün ön yüzü hava akış bölgelerine bölünerek uniform olmayan hava hızları dikkate alınmıştır. Model sonuçlarını doğrulamak için deneysel çalışma yapılmıştır. Deneysel doğrulama sonucunda modelin, incelenen tüm test koşulları için çıkış sıcaklığını ±%10 aralığında bir ortalama mutlak sapma ile öngördüğü sonucuna varılmıştır. Mikrokanal ısı eşanjörünün, ısı transfer performansını tahmin etme kabiliyeti, deneylerle değerlendirilmiş, uniform olmayan hava hızının modele dahil etmenin, modelin doğruluk hassasiyetini arttırdığı görülmüştür. Mikrokanal ısı eşanjöründeki entropi üretim mekanizmaları incelenmiş ve akışkan akımı tersinmezliklerinin entropi oluşumuna katkısının, ısı transfer tersinmezliklerine kıyasla oldukça düşük olduğu tespit edilmiştir.

Superior anti-icing and active deicing multifunctional surfaces are highly desired in outdoor infrastructure and work of aircraft, automobiles, wind turbines, power transmission, etc. Herein, inspired by microstructures and functions of natural opal and Nepenthes pitcher plants, we present a novel multifunctional inverse opal slippery anti-icing surface (IOSAS) by employing a combined method of colloidal nanoparticles self-assembly, microstructure mold replication, and hydrophobic lubricating fluid infiltration. Because of the 3D long range ordered nanoscale porous structure of artificial inverse opal, the lubricating liquid could wick into, wet, and stably adhere to IOSAS, forming thousands of solid-liquid interfaces and defect-free and inert slippery surface. Thus, the IOSAS was imparted with inhibiting ice nucleation property and hindering heat transfer and ice crystals’ growth characteristics. Owing to the existence of a thin liquid layer on the surface, the mechanical interlock between porous substrate and ice is greatly weakened, which imparts the slippery surface with low ice adhesion strength, and the frozen droplet and ice bulk could be easily removed. In addition, we have demonstrated that the supercooled IOSAS has no adhesion to moving water droplets, while various subcooled metal surfaces are adhesive, and this exhibits liquid-infused inverse opal surfaces dynamic low-temperature water-repellent performance. These characteristics endow IOSAS with multifunctional anti-icing performances, making it a promising option for a wide range of ice-phobic applications.

High-heat-flux heat removal is critical for the long-term safety of the high-power electronic device. However, the precise and uniform temperature control of the device is challenging due to various nonlinearities, couplings and disturbances. To this end, active disturbance rejection controller (ADRC), hybrid with an auxiliary feed-forward flow rate controller and an auxiliary subcooling degree controller, is introduced in this paper for the wall temperature control of a microchannel mechanically pumped two-phase cooling loop. First, the optimal flow rate in terms of the heating power is identified to minimize the temperature level and variance. The relative curves between the flow rate and heating power are like character ‘v’ which are used as a feed-forward law for the flow rate regulation. Second, the reference of the sub-cooling degree is determined based on the closed-loop control of the preheater to get a uniform temperature field with least variance. Finally, ADRC is used in the primary loop to control the average wall temperature. The experimental results indicate the efficiency of thermal control for two-phase cooling loop via combination of primary and auxiliary loop. In addition, ADRC performs better in both temperature tracking and disturbance rejection when compared with the conventional PI controller, with superiority in terms of faster tracking and stronger robustness to disturbances and couplings.

Growing environmental restrictions in energy production industry calls for greater efficiency and cleaner fuel burning processes. Biomass (wood chips) as a fuel is in great demand for boiler and power plants as it is considered widely available and relatively clean. While combining woodfuel flue gas and condensing economizers significantly raises the efficiency and makes it even more viable solution for energy production although the biomass fuel usage still has reservations in waste heat, which could be utilized. The calculation algorithm is presented for evaluation of subcooled biomass flue gas components concentration values which determine the leftover heat energy value carried by flue gas flow. Several cases of biomass quality (regarding moisture w=45%, 50%, 55% and 60%) and combustion process quality (regarding air excess value λ=1,2; 1,5; 1,8) in the flue gas temperature range of 50 to 20°C and effects for flue gas internal dynamic were examined. It was determined that water vapour amount depends only on temperature, while every other component concentration change with different air excess and temperature values. It was observed that further usage of biomass flue gas could result in up to 13% additional heat energy recovery for 1MW of fuel input, system combination together with condensing economizers could result in up to 31% of heat energy recovery.

Visualization experiments are carried out to investigate the atomization characteristics of R1336mzz flash spray cooling. The influences of superheat, spray distance, and nozzle orifice diameter on spray cooling performance are analyzed experimentally. As the superheat increases, finer droplets and thinner liquid film are observed; this is helpful to improve the two-phase heat transfer efficiency. Enlarging atomization angle under high superheat is also observed for flash spray cooling, and it benefits for reducing the spray distance. It can be found that when the inlet superheat is 19.8°C and the spray distance is 6 mm, the critical heat flux (CHF) reaches 251 W/cm2 and the maximum heat transfer coefficient (HTC) reaches 37.4 kW/(m2 °C), which are 55% and 11.6% higher than those when the inlet subcooling is 6.9°C and the spray distance is 12 mm, respectively. Using flash spray reduces the spray distance, which benefits for designing compact spray cooling device. In addition, the nozzle orifice diameter has great influence on the cooling performance of flash spray, and the choice of the nozzle depends on the superheat. This study provides a physical insight into the heat transfer enhancement in flash spray cooling.

With the development of industrial technology, heat transfer at the microscale has attracted more and more attention. In this work, 200 µm platinum wire and 150 µm nickel-chromium wire were used as experimental objects, which the power was provided by DC power with the range of 15.6 W to 56.2 W. Distilled water was used as the experimental liquid. Various bubbles on the micro wire were observed and the heat mechanism was analyzed. A variety of bubble attachment phenomena were captured on the 200 µm platinum wire, including the adhesion during bubble detachment, the rotation of small attached bubbles on the surface of large bubble, multiple bubbles circling at the top of the same bubble, and different attached bubble departure phenomena. Marangoni force in the vertical direction triggered the formation of bubble attachment. In addition, the effects of surface tension, adhesion force and buoyancy force on the circling of the bubble were also considered. The analysis of the bubble sweeping on the 150 µm nickel-chromium wire was analyzed. The results showed that the static bubble would interact with the sweeping bubbles on the other side, thereby changing the heat transfer mechanism, which was not discussed in detail before. The bubble jet flow generated by thermocapillary convection on the vertical direction of the bubble surface was the main influencing factor, which would change the microlayer encountered in front of the bubble. The effects of bubble diameter and liquid subcooling on the sweeping velocity were also studied. The results showed that the larger the bubble diameter was, the lower the sweeping velocity would be achieved while the liquid subcooling temperature had less impact on the velocity of sweeping bubble.

The heat transfer process in the condenser is often divided into superheated, two-phase and subcooled zone. But the three-zone model has discontinuities of the heat transfer coefficient between single phase and two-phase regions. This study applies a computational fluid dynamics (CFD) study on the two phase flow and the heat transfer characteristics in the condensing-superheated zone. The condensation process of R410A (Tsat=44.59 °C) in a horizontal pipe (Di=6.1 mm) is simulated by using VOF (volume of fluid) model. The formation and the development of the condensation film are detailed analyzed. The heat transfer mechanism in the condensing-superheated zone includes both the single-phase convection and the phase transition. The heat transfer coefficient in the superheated condensation area increases from the single-phase convection to the saturated condensation. Combined with the development of liquid film morphology, this study is helpful for understanding the flow and heat transfer mechanism in the condensing-superheated zone.

In this paper, thermo-economic optimization and comparative analysis of a cascade refrigeration system configured with flash gas removal in its high-temperature cycle (HTC) and flash intercooling with indirect subcooling in lower temperature cycle (LTC) using different natural refrigerant pairs is performed. Thermo-economic optimization is carried out to maximize the exergetic efficiency and minimize the overall cost rate. The optimization model involves six design variables which include subcooling and de-superheating parameters, LTC evaporation and condensation temperatures, HTC condenser temperature and cascade temperature difference. The comparative analysis of twenty-two natural refrigerant pairs based on the results of thermodynamic and economic optimizations reveals that R717-R290 is most efficient pair and R290-R1150 is least efficient refrigerant pair thermodynamically whereas R717-R1270 is the best and R600a-R290 is the worst pair economically. Seven potential refrigerant pairs are chosen via the thermodynamic and economic optimization results and they are further compared based on their performances obtained through multi-objective optimization (maximization of exergetic efficiency and minimization of total cost rate). Multi-objective genetic algorithm is used for optimization which results in seventy non-dominated Pareto optimal solutions where the TOPSIS method is used to select a unique solution for each refrigerant pair. A comparison of refrigerant pairs using these unique solutions shows that R717-R1270 is the best refrigerant pair for the cascade system under consideration. It is also found that R717-R1270 results in 7.77% rise in COP and 5.32% reduction in overall cost when compared with NH3–CO2 refrigerant pair working under identical operating conditions.

In-vessel retention (IVR) is a viable method of preserving reactor pressure vessel integrity in severe reactor accidents. Accurate knowledge of the bubble lift-off diameter for a downward-facing in...

The solar absorption-subcooled compression hybrid cooling system (SASCHCS), in which the cooling capacity of absorption subsystem serves as the subcooling power of compression subsystem, is energy-...

Transcritical CO 2 refrigeration system is coupled with the single effect vapour absorption with LiBr-water as a working pair having an objective to enhance the performance of low temperature transcritical refrigeration system while using natural working pair and to reduce the electricity consumption to produce low temperature refrigeration. The high grade waste heat rejected in the gas cooler of tc-CO 2 compression refrigeration system (TCRS) is utilized to run the single effect vapour absorption system (SEVAR) to enhance the energy efficiency of the system. The gas cooler in the transcritical CO 2 system is having heat energy at high temperature and pressure, which is utilized to run the vapour absorption system, while the other refrigerant heat exchanger provides subcooling to further enhance the performance. The combined cycle can provide refrigeration temperature at different levels, to use it for different applications. Energetic and exergetic analysis have been done to analyze the combined system to compute the performance parameters and the irreversibilities occurring in different components to further increase the performance. The combined system is optimized for various heat rejection and refrigeration temperatures. The COP of the combined system has been enhanced by to 24.88% while the enhancement in exergetic efficiency (ηex) is observed as 10.14% respectively over tradition transcritical CO 2 compression refrigeration system, with -10°C as an evaporation (TCRS cooling) temperature and exit temperature of gas cooler T 4 being 40°C. Doi: 10.28991/HIJ-2021-02-02-02 Full Text: PDF

Temperature and humidity independent control (THIC) air-conditioning system is a promising technology. In this work, a novel temperature and humidity independent control (THIC) system is proposed, namely VMD-ACERS, which integrates vacuum membrane-based dehumidification and air carrying energy radiant air-conditioning system. This work establishes a novel coefficient of performance (COP) model of VMD-ACERS. The main parameters affecting the COP of conventional fan coil unit cooling system (FCUCS) and VMD-ACERS are investigated. The performance of FCUCS and VMD-ACERS are compared, and the energy-saving potential of VMD-ACERS is proved. Results indicate that, for FCUCS, the importance ranking of parameters is basically stable. However, for VMD-ACERS, the importance ranking will be affected by FCU and refrigerant. The most important parameters of VMD-ACERS are condensation temperature and permeate side pressure. On the contrary, superheating, subcooling are relatively less important parameters. For VMD-ACERS, it is not necessary to pursue the membrane with very high selectivity, because the selectivity of membrane would also be a less important parameter when it reaches 500. The COP of VMD-ACERS is higher than that of FCUCS when the permeate side pressure is higher than 8 kPa. The VMD-ACERS solves two technical problems about power-saving and thermal comfort of conventional THIC, and can extend the application of THIC air-conditioning system.

In this study, energy and exergy analysis of an organic Rankine cycle with internal heat exchanger using R1234yf, R1234ze and R134a is carried out. The energy and exergy performances of an ORC system for different operating temperature have been analyzed. It is seen from the analysis that, boiler, condenser and subcooling temperature has significant effect on both energy and exergy efficiencies. It was observed that energy and exergy efficiency increased with the increase of boiler and supercooling temperature, and energy and exergy efficiency decreased with the increase of condenser temperature. In addition, exergy destructions of each component were determined. Since the turbine and pump are considered isentropic, exergy destructions in these elements have been neglected. It was determined that the highest exergy destruction was in the condenser with the rate of 72% and the least exergy destruction was in the heat exchanger with the rate of 1%. As a result, it has been observed that the R1234yf fluid chosen as an alternative to the R134a working fluid is the most suitable fluid for the ORC system.

Pool boiling is cost-effective, simple and prevalent amongst all available cooling schemes. The lack of quantitative data, qualitative theories and explanations in the area of critical heat flux (C...

The paper analyzes the methods of temperature measurements of cooling the high-temperature bodies in liquids. Experimental studies on cylindrical samples with different embedding of thermocouples have been carried out in three liquids. It is shown that even in the case of cooling in saturated liquids, external thermocouples, whose electrodes play the role of cooling fins, distort the measured temperatures significantly. Using external thermocouples, it is impossible to record correctly the rate of hot body cooling in liquid and the temperature of transition of cooling to an intensive regime. Using only a central thermocouple is permissible at low Biot numbers. However, even in this case, with intensive cooling, the difference between the temperatures of the center and the surface can reach several tens of degrees. With a decrease in the thermal conductivity of a sample and an increase in its linear dimensions, the temperature gradient increases (especially at intense cooling), and the correct physical interpretation of the process becomes impossible.

Liquid hydrogen flow boiling heat transfer in tubes is of great importance in the hydrogen applications such as superconductor cooling, hydrogen fueling. In the present study, a numerical model for hydrogen nucleate flow boiling based on the wall partition heat flux model is established. The key parameters in the model such as active nucleation site density, bubble departure diameter and frequency are carefully discussed and determined to facilitate the modeling and simulation of hydrogen flow boiling. Simulation results of the numerical model show reasonably well agreement with experimental data from different research groups in a wide operation condition range with the means absolute error (MAE) of 10.6% for saturated and 5.3% for subcooled flow boiling. Based on the model, wall heat flux components and void fraction distribution of hydrogen flow boiling are studied. Effects of mass flow rate and wall heat flux on the flow boiling heat transfer performance are investigated. It is found that in the hydrogen nucleate flow boiling, the predominated factor is the Boiling number, rather than the vapor quality. A new simple correlation is proposed for predicting hydrogen saturated nucleate flow boiling Nusselt number. The MAE between the correlation predicted and experimentally measured Nusselt number is 13.6% for circular tubes and 12.5% for rectangular tubes. The new correlation is applicable in the range of channel diameter 4–6.35 mm, Reynolds number 64000–660,000, saturation temperature 22–29 K, Boiling number 8.37 × 10−5–2.33 × 10−3.

The subcooled flow boiling is related to the operation of electronic devices, Hybrid electric vehicle (HEV) Battery module and small catalytic reactors. It is well known that the operational temperature must be maintained to avoid any malfunction of these heat dissipative devices. In this paper the forced convective and subcooled flow boiling heat transfer coefficients of water-ethanol mixture is determined numerically by Volume of fluid analysis (VOF). The interaction between liquid and local vapour is analysed by solving the bubble volume of fraction in the numerical study. Crank Nicolson implicit scheme is used for discretizing the scalar convection equation for bubble void fraction and transforming into algebraic equation. Thomas Algorithm is used to solve the algebraic equations of bubble void fraction. The corrector predictor equation method is used to solve for bubble void fraction when the value obtained is less than 0 or exceeds 1. The thermodynamic and Thermophysical properties are substituted in the x-momentum and energy equation to determine the values of pressure drop, velocity and temperature of the fluid. From the temperature values, the subcooled flow boiling heat transfer coefficient is obtained. It is found that the addition of ethanol to water decreases the forced convective and subcooled flow boiling heat transfer coefficient of the water-ethanol mixture. The numerically determined heat transfer coefficient of water ethanol mixture is compared with that of the experimental results. The average deviation between the experimentally determined and numerically determined subcooled flow boiling heat transfer coefficient of water ethanol-mixture is found to be 24.13%.

The paper proposes a model of heat transfer in the evaporator of the spacecraft thermal control system. The model allows to calculate the average temperature of the evaporator wall and to build a "boiling curve" in a wide range of thermal loads. Adequacy of the model is confirmed by experimental studies on an aluminum thermal sink with high longitudinal thermal conductivity in the range of parameters typical for the thermal control systems of spacecrafts. Ammonia is used as a working fluid. The model might be recommended for use in zero gravity and normal ground conditions.

Cool thermal energy storage systems (CTES) using phase change materials (PCM) are paramount in the energy management of buildings. This work aims to investigate the role of graphitized mesoporous c...

The paper presents results of experimental research on the influence of the mass flux on the heat transfer during flow boiling of subcooled de-ionized water in conditions of three-side heating in a system of two microchannels with an aspect ratio of 13.3. The horizontal experimental section was a copper block with two microchannels 2 mm wide, 0.15 mm deep, and 16 mm long; the inner surface of the microchannels was covered with a thin layer of nickel with hydrophilic nanoparticles of Aerosil 200. The mass flux varied from 500 to 3500 kg/m extsuperscript2s; the initial water subcooling relative to the saturation temperature at the microchannel outlet was 80°C. The dependence of the superheat of the wall on the heat flux density was determined; the critical heat fluxes were measured; comparison of the obtained data with calculation based on the binomial equation for subcooled boiling in a microchannel was carried out.

The present article experimentally explored heat transfer through a horizontal metal foam tube with a constant wall heat flux during the subcooled flow boiling of water, Al2O3/H2O, and CuO/H2O nanofluids. To investigate the effect of using metallic foam on the subcooled flow boiling heat transfer, the results have been compared with a simple tube. Besides, The impacts of significant parameters, including subcooled inlet temperature (25–60 °C), mass flux (150–310 kg m−2 s−1), wall heat flux (100–230 kW m−2), particle type (CuO and Al2O3), and nanofluids concentration (0.5% wt., 1% wt.) were investigated on the transfer of boiling heat while measuring pressure loss and rate of heat transfer. The results show that the use of metal foam increases the heat transfer rate by 3.5–5.8 times compared to the simple tube. Comparing the subcooled flow boiling heat transfer of water and nanofluids in both metal foam and simple tubes, it was found that water has superiority over the nanofluids. Besides, the value of the thermal performance index defined as a ratio of the heat transfer enhancement to the pressure loss degradation is more than one for water and the nanofluids.

This paper describes the computational fluid dynamics (CFD) methodology to simulate the boiling flow in a typical Pressurized Water Reactor (PWR) 5 ⨯ 5 rod bundle. The method includes the Eulerian-Eulerian two-fluid model coupled with the improved wall heat partitioning model. The NUPEC PWR Subchannel and Bundle Test (PSBT) International Benchmark are used for validation. The simulated surface averaged void fraction agree well with the experimental data, which indicate the promising application of the present method for modeling the boiling flow in the fuel rod bundle. The main emphasis of current research has been given to the analysis of the phase distribution around and downstream the spacer grid, the effect of the spacer grid structure, including the mixing vanes, the springs and the dimples on the void fraction distribution is investigated. The findings can contribute to a better understanding of three dimensional flow boiling characteristics and can be used to assist in optimizing the spacer grid.

This study investigates the solidification characteristics of deionized water dispersed with natural graphite flakes (NGF), called micro-particle enhanced phase change material (MePCM), in two sphe...

Systematic regulation of hydrophilic regions plays a key role in optimizing the heterogeneous hydrophilic-hydrophobic surface for promoting condensate transfer ability (CTA) under subcooling or high-humidity conditions. In this work, we develop an operable method to fabricate wettability-controllable coatings by regulating the mass ratio of superamphiphobic and superamphiphilic powder (MRP). By investigating the synergic relationship between CTA and MRP, we display an interesting competition between condensation and detachment of condensates. The initial dewing rate associated with reflecting phase change heat transfer capacity could be continuously strengthened by promoting MRP, while the detachment capacity with respect to improving the long-term condensing rate can be limited by the excessive superamphiphilic microregions. Based on this, we have optimized the threshold of MRP for promoting the condensation heat transfer ability and the water harvesting efficiency with the values of 10:0-8:2 and 10:0-4:6, respectively. This work provides important guidance in designing and optimizing heterogeneous hydrophilic-hydrophobic surfaces for multiple industrial applications including heat management, water harvesting, and desalination.