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Salt hydrates are promising candidates for long-term thermochemical heat storage (TCHS) in the building environment. In such storage systems, the surplus of energy will be exploited in an endothermic reaction to dehydrate the salt hydrates. Once it is demanded, the stored energy will be released through an exothermic reaction by hydrating the salt, which results in an increase in the mass and temperature of salt particles as well as changes in the species of material. In order to construct an improved storage system, it is very important to deeply understand the details of the heat and mass transfer processes in the packed beds of salt hydrates. Poor heat (in the closed systems) and mass transfer (in open systems) can be the bottleneck in this technology. The main objective of this work is to investigate how heat transfer will be affected by applied pressure, particle size, and packing arrangement through calculating/measuring the effective thermal conductivity of the packed beds of salt hydrates. This is achieved by applying and developing a CFD-DEM model and by experimental measurements in a vacuum oven. Comparisons are carried out for the numerical results at low and high ambient pressures with the experimental measurements, which show a very good agreement. The obtained results show the effect of natural convection in the packed bed when the higher vapor pressure is applied.

Accepted: 2021-06-23, PA2210008000

The present study deals with the experimental research of heat transfer and air flow behaviour of pin fin heat sink with and without wings under forced convection. For inline and staggered arrangements of pin fins, the fin pitch ratio (S/D) and the wing size ratio (Lw/D) are varied for the Reynolds number range of 6800–15,100. The heat transfer rate and frictional losses are increased by decreasing the (S/D) and (Lw/D) ratios. The optimum performance corresponds to the the pin fin heat sink with wings having (S/D) ratio of 2 and (Lw/D) ratio of 0.2.

The interfacial heat transfer coefficient at metal/mold plays an important role for explaining the energy extraction during solidification process. Experiments on heat transfer of binary metallic alloys has been constantly made for microstructure analysis, which are mainly based on cooling rates of liquid metals. Solidification process is still a challenge to numerical techniques due to typically presence of multiphase transport phenomena. In this paper, the Levenberg-Marquardt technique is used to obtain the parameters of a representing function for the contact resistance at metal/mold interface, aiming to determine the interfacial heat transfer coefficient. The application of finite volume method for representing the cooling process of an alloy casting in a mold is also proposed. To assess the numerical approach, the removal of heat in only one side of the mold is taken into account, in order to ensure upward, and unidirectional solidification. The objective function used is obtained calculating the difference between numerical and experimental temperatures. The results are compared with data available in the literature, yielding good performance for both direct and inverse problems.

Large eddy simulation was applied for studying the effects of plasma aerodynamic actuation on the coherent structures and cooling efficiency of a cylindrical cooling hole. Two arrangement-typed plasma actuators were placed immediately downstream of the cooling hole exit and a phenomenological plasma model was employed to provide plasma force vectors. Results show that the single plasma actuator installed in streamwise direction exerts downward force and streamwise momentum injection effects on film cooling flow. Therefore, the coolant is deflected to the wall and stretched farther downstream, and hairpin vortices are reduced in number and size, and then happen to breakup. The pair of plasma actuators installed in spanwise direction acts a predominant spanwise momentum injection effect on the film cooling flow, thus inducing the coolant flows along spanwise direction. The beneficial vortex pair created by plasma aerodynamic actuation mitigates the detrimental entrainment and lift-off effect of counter rotating vortex pair. Moreover, hairpin vortices stay much closer to the wall thanks to the reduction of the Kutta-Joukowski lift and mutual vortex induction, and soon break into much smaller-scale vortices. Eventually, both the streamwise and spanwise coverage of the coolant are expanded, and thus improved the cooling efficiency.

This paper aims to investigate the hydrothermal and entropy generation characteristics of a non-Newtonian nanofluid containing CuO nanoparticles in an offset strip-fin microchannel heat sink (MCHS). The base fluid is solution of 0.5 wt% Carboxymethyl Cellulose (CMC) in water. This study investigates the effects of nanoparticles concentration, Reynolds number and geometric size of strip-fin on the performance of MCHS from the viewpoint of both the first and the second thermodynamic law. The results reveal that enhancing the Reynolds number improves the performance of MCHS by boosting the convective heat transfer coefficient of the working fluid which favourably reduces the CPU surface temperature and thermal entropy generation rate and importantly leads to the temperature uniformity of the CPU surface. However, increase in Reynolds number adversely affects both the pumping power and the frictional entropy generation in the system. Therefore, the optimal strip-fin size is investigated to find the optimum performance of the offset strip-fins MCHS from the viewpoint of both the first and the second thermodynamic law. The optimal results show that the highest ratio of heat transfer enhancement to pressure drop increment, using the nanofluid instead of base fluid, is 2.29. In addition in the optimal case, the minimum total entropy generation rate of the nanofluid is 2.7% less than the base fluid.

The present study describes a new two-stage transcritical CO2 refrigeration cycle with two ejectors (NERC). At this studied cycle (NERC), vapor compression is performed in two stages and each vapor compression line includes separate ejector. Thermodynamic and exergetic analysis of the proposed cycle are performed using Engineering Equation Solver (EES) software. The new cycle (NERC) is compared with a conventional cycle. It is found that depended on conditions the new cycle (NERC) improves the COP from 20% to 80% compared to the conventional cycle. The effects of main cycle parameters i.e. the high-side pressure of a compressor (discharge pressure), intermediate pressure, the outlet temperature of gas cooler and evaporator temperature on COP are studied.

In this paper, based on the structural characteristics of the double-P radiation tube, a novel type of flat double-P radiant tube was designed to improve the heating uniformity of the radiator tube and the heating efficiency of the workpiece. A new three-dimensional computational fluid dynamics modeling of the flat double-P radiant tube was conducted to discuss the heat transfer and combustion phenomenon. The influences of air and gas flow rate, air oxygen content, air consumption coefficient and air preheating temperature on flue gas circulation, surface temperature distribution and radiation power of the flat double-P radiant tube were analyzed and discussed, respectively. The amount of circulating flue gas, ratio of circulating flue gas, surface temperature non-uniformity coefficient, surface temperature distribution and radiation power of the flat double-P radiant tube with different sizes and positions of the burner nozzle characteristics were also studied in depth. The results showed that the fuel speed should be control between 40 m/s and 60 m/s. When the distance between the air nozzle and the fuel nozzle was 50 mm, or the position of the nozzles was 0 mm, the temperature non-uniformity coefficient was the lowest, which were both 0.058.

To investigate the combined effects of orientation and environmental pressure on flame characteristics, steady burning experiments of inclined polymethyl methacrylate (PMMA) slabs were conducted at reduced environmental pressures ranging from 0.5 to 1 atm. According to the visual observations of flame appearance and fitting results of burning rate at different inclination angles, diffusion flames can be classified into three regions: pool-fire region, transition region and wall-fire region. Experimental result shows that flame pulsation frequency depends only on sample width in pool-fire region, however, mainly on fuel inclination angle in wall-fire region. A theoretical analysis is performed to predict the flame pulsation frequency as a function of modified gravity and the outlet diameter of gas-phase flame at different fuel inclination angles. In addition, the dimensionless flame pulsation frequency St is found to correlate well with the modified Froude number Fr ∗ , St = 0.46 1 / Fr ∗ 0.53 . Flame length shows a tendency to peak at 0.8 atm except 2.5 cm wide samples. Correlations between flame length and heat release rate are found with a power-law exponent of 1.41 for pressures smaller than 0.8 atm and 1.04 for larger pressures. Both experimental result and the developed correlations of flame length suggest that the roles of molecular diffusion and air entrainment played in diffusion burning are closely related to the environmental pressure and sample dimension.

Reactive species such as asphaltenes play an important role in crude oil fouling. Crude preheat trains in petroleum refineries are the most affected due to crude oil fouling. Fouling characteristics of crude oils are generally determined through experiments in lab-scale or pilot-scale units using very high surface temperatures/heat fluxes. At very high surface temperature values, the heat transfer regime may change from forced convection to boiling. Determination of maximum heat flux or surface temperature beyond which heat transfer regime changes from forced convection to boiling is important for the study of crude preheat train fouling. In the present study, experiments were conducted using two Malaysian crude oils to estimate the maximum heat flux/surface temperature and a model to determine the maximum heat flux has been developed and validated with the experimental data. This will form the basis for selecting the operating conditions (heat flux/surface temperature) prior to the crude oil fouling characterization studies.

The present article proposes an improved spray breakup model considering the effect of carrier phase turbulence on both the critical Weber number and the breakup rate. For this aim, the effective surface tension as a new concept was introduced to combine the effects of turbulent and aerodynamic induced breakup. A simple analytical expression has been developed for calculating the effective surface tension according to the local turbulence characteristics of the carrier phase. Afterwards, the proposed effective surface tension was used for calculating the critical Weber number in turbulent flow fields and an excellent agreement was shown in comparison with the available experimental data. In the following, an improved secondary atomization model was developed by implementation of the effective surface tension in the classic Pilch-Erdman model. To assess the capabilities of the new breakup model, it was implemented into a CFD code and the results compared with experimental data and those obtained by the previous breakup models. The present model showed a better agreement with experimental data in predicting the spray tip penetration length and local Sauter mean diameter (SMD). It is concluded that the improved model is effectively more reliable for simulation of spray droplet breakup in turbulent gaseous environments.

Nowadays, molten salts are widely used in concentrated solar plants as working media to transfer and store solar energy. However, the relatively poor thermal properties of molten salts have restricted their further application. Nevertheless, the addition of nanoparticles to molten salts excellently improves these thermal properties. In present work, solar salt-based nanofluids were prepared using a lyophilizer. The specific heat of solar salt-based SiO2 nanofluids was experimentally measured based on the sapphire method, and its heat transfer performance was numerically investigated using the lattice Boltzmann method. Results indicated that an optimum nanoparticle mass fraction that could enhance heat transfer performance existed. This optimum nanoparticle mass fraction, which was found to be 1.0 wt%, improved the heat transfer coefficient and Nu number by 8.58 and 7.29%, respectively. The parameter analysis indicated that the considerably large change in the specific heat dominated the change in heat transfer performance. Meanwhile, the simulation results of the average Nu numbers exhibited good agreement with the Shah-London correlation prediction to within ±3.0%, indicating the feasibility of using this correlation in designing heat exchangers that utilize solar salt-based SiO2 nanofluids.

The operational flexibility of coal-fired power plants has become a priority objective; however, the lifetime of main devices is reduced due to thermomechanical fatigue during transient processes. This study focuses on the safety of high-pressure heaters at operational flexibility measures. The dynamic characteristics of thermal parameters in the 1# HP heater were obtained and analyzed based on transient models of a coal-fired power plant. The change trends of pressure/temperature on the steam/feedwater side of the 1# HP heater at three throttling the extraction steam measures were different with that at the feedwater bypass measure. Subsequently, finite element analysis method was used, and the thermomechanical stresses under the four measures were calculated and analyzed. The time to reach the maximum mechanical stresses was shorter than that of thermal stresses. Finally, the fatigue lifetimes of the heater were estimated. The maximum fatigue damage ratio at the per load cycle was 0.0069%, and the minimal allowable number of cycle was 14413 in the upper region of the tube sheet when the extraction steams of #1, #2, and 3# HP heaters are regulated simultaneously. The study could offer data guidance to the safety and maintenance of coal-fired power plants during operational flexibility regulations.

An experimental investigation is presented of the effects of wind speed (0–9 m/s), yaw angle (0° and 90°), and tilt angle (15° and −90°) on the mixed convective heat losses from a cylindrical cavity heated with different internal wall temperature distributions. The internal wall comprised 16 individually controlled heating elements to allow the distribution of the surface temperature to be well controlled, while the air flow was controlled with a wind tunnel. It is found that temperature distribution has a strong influence on the convective heat losses, with a joint dependence on the wind speed and its direction. For the no-wind and side-on wind conditions, the measured range of the heat losses varied by up to 50% with a change in the wall temperature distribution. However, for high head-on wind speeds, this variation reduced down to ∼20%. In addition, the heat losses from downward tilted were ∼3 times larger than the upward facing heated cavity for high wind speeds (typical of tower-mounted and beam-down configurations, respectively). Also, the measured heat losses were found to be only slightly dependent on wind speed and direction in contrast with the downward tilted cases.

An circulating fluidized bed evaporator of Na2SO4 solution is designed and built to investigate heat transfer and antiscaling performance by varying the amount of added particles (0.5%–2.0%), heat flux (6.47–13.65 kW·m−2) and circulation flow velocity (0.38 m·s−1–2.07 m·s−1). SiC particles are used as inert solid particles. The experiments are operated under normal pressure, and the operating temperatures of fluid is about 101–103 °C within the experimental range. The research results show the addition of SiC particles can not only effectively enhance the heat transfer performance in Na2SO4 solution, but also remove and prevent scaling. The effect of heat transfer performance is strongly dependent on the particle size. The maximum enhancing factor of 1 mm SiC particles is 17.1% at e = 2%, u = 0.38 m·s−1 and a heat flux of 8.97 kW·m−2. With an appropriate amount of added particles, 1 mm SiC particles can effectively prevent scaling in the evaporation of Na2SO4 solution. The research findings may be beneficial to the application of fluidized bed heat transfer and antiscaling technology to industry.

Subcooled flow boiling heat transfer is one of the most preferred heat removal methods, especially in areas where high heat fluxes are encountered. The design of heat sinks employing subcooled flow boiling involves prediction of pressure drop, in addition to heat transfer coefficient, as it exerts a huge influence on the local heat transfer, stability of the system and the critical heat flux. This paper provides a systematic review of the performance of the prior correlations and their applicability for different conditions based on a large experimental database compiled from 12 sources containing 1365 data points in total, focussing on water as the working fluid. The results of the assessment are used to identify the right correlation applicable for a specific operating range. New correlations are proposed for the cases and ranges for which the predictability of the prior correlations is unsatisfactory.

Better understanding of the leakage characteristic through thin-walled pipes is of great importance to maintain the integrity of reactor pressure boundary. This experimental study focuses on the characteristics of two-phase critical flow through short slits (L/d = 4–21), and the test pressure covers the range of 0.1–10.0 MPa. Cracks with small passage opening (as low as 46.5 μm) were adopted to better simulate the real crack, where both axial and circumferential crack were considered. Therefore the test database was enlarged. Influences of stagnation conditions and crack morphologies were analyzed. The flow mechanism was explained on how flash decides the flow rate by affecting flow pattern and choking: circumferential crack shows a higher leakage than axial crack, due to a weaker choking phenomenon; the flow pattern transition is discovered at low subcooling degree. A dimensionless number β, indicating the fraction of flashing, was proposed and two different flow phenomenon stages were identified and distinguished by β = 15. Besides, correlations were developed for the flow rate through short slits, showing a reasonable accuracy in predicting the test data both in this study and from literature.

In-vessel retention (IVR) is considered as an effective mitigation strategy in realms of nuclear safety. Extensive experimental investigations have been done on the natural convection characteristics of corium pools according to the IVR concept, using various simulant materials. Based on the known COPRA (Corium Pool Research Apparatus) molten salt experiments, in which the eutectic salt (50 mol% NaNO3-50 mol% KNO3) and the non-eutectic salt (20 mol% NaNO3-80 mol% KNO3) were used as simulant materials, this paper evaluates extensively the simulant effect of simulant material on natural convection behaviors inside the corium pools through numerical simulation. The Wall-Modeled LES (WMLES) method was employed for the turbulence calculation. The distributions of corium pool temperature and heat flux as well as the crust thickness from the numerical simulations were validated with experimental data. Through numerical simulations, this work provides detailed temperature fields and velocity fields inside a large internally heated corium pool, and further discusses in depth the interplay between the heat transfer characteristics and the natural convection behaviors. This work can provide references for the selection of simulant material in the future experimental research.

This study presents the numerical and experimental investigations of air/water direct contact heat and mass transfer in the humidifier. The mathematical model of humidifier is developed based on the Multiphase-Eulerian model. The regression equation for Merkel number utilized in the numerical model was proposed on the basis of experimental results. According to the numerical simulation results, the error between the numerical and experimental results is within 12%. The distributions of air temperature, the mass fraction of vapour in the air and total pressure have been studied to reveal the heat and mass transfer characteristics in the humidifier. And, the distributions of the relative humidity at different water inlet temperature and air-and-water mass flow ratio are investigated. It is found that the evaporation rate of the humidifier increases with the increase in the air mass flow rate, water inlet temperature and water mass flow rate. Comparing with the previous theoretical calculation results based on the Poppe model, the numerical results are closer to the experimental results. This study has important significance on the development of humidification-dehumidification desalination technology.

Owing to their high energy storage density, thermochemical energy storage (TCES) systems are promising alternatives for seasonal storage of heat, which can be charged with solar thermal energy during the summers, and employed for space heating application in the winter season. In the present work, an open TCES system with a single reactive bed of solid strontium bromide salt reacted with moist air flow is considered. A two-dimensional model considering diffusion effect of water vapour inside the reactive bed is developed and validated. Parametric studies are carried out for both the charging as well as the discharging phases using the two-dimensional model to evaluate the effect of inlet air temperature, inlet partial pressure of water vapour and inlet air flow rate on the performance of the open TCES system. A case study is carried out for the discharging phase for a single night during the winter season for Pune, India. The use of a suitable humidity control at the inlet of the reactive bed is found to increase the energy saving by 18.9% and is found to be an effective way in maintaining the temperature of the air at the outlet of the reactive bed within comfortable limits for space heating application.

An industrial condensate stabilizer unit is simulated with HYSYS 3.1 software when the excessive water is present on the trays. Effects of reboiler temperature, excess water flow rate, and incoming stream temperature on the Reid vapor pressure (RVP), temperature distribution along the column and impurity concentrations along the stabilization column, as well as, energy usage are studied. In addition, the optimal location for the water draw tray for the condensate stabilizer column is found. It is observed that the presence of water draw pan in the column leads to about 15% and 28% reduction, respectively, in the reboiler and the compressor duties compared to the industrial case without water draw pan. For the optimal case, 73% of excess water is recovered from the water draw pan. The presence of the water draw pan in the column also results in lower amount of impurities and higher heating value in the off-gas stream. Finally, the column with the water draw pan shows higher exergy efficiency of about 81% in comparison with the column without water draw pan (with exergy efficiency of about 72%).

Taking full advantage of LHP’s efficient long distance heat transport capability, a LHP-based power-free cooling system has been developed in this work. The heat source was simulated by a film heater, which was connected to the cylindrical evaporator of the LHP via an aluminum saddle. The condenser line was embedded into the bottom surface of a fin radiator where heat dissipation to the ambient was completely by air natural convection. Extensive experiments on this cooling system were conducted, mainly focusing on its startup and system thermal resistance. Experimental results show that this cooling system can successfully start up with a very small heat load. With the film heater temperature not exceeding 80 °C, this cooling system can manage a heat load as large as 150 W over a distance of 1.05 m, corresponding to a system thermal resistance about 0.30 °C/W. Moreover, this cooling system exhibited very strong anti-gravity capability. With an adverse elevation of 0.5 m, it can still maintain normal operation, and no obvious performance decay was observed except an increased operating temperature at small heat loads. This work provides good design guidance and reference for future applications of this advanced cooling system.

This paper presents the experimental investigation results of ignition and combustion characteristics of fuels based on typical coal flotation waste which is formed and accumulated around the world in large volumes (about 200–300 million tons per year). We focus on the comparison of these characteristics for waste-derived fuels in different states: slurry, gel, and solid. We analyze the ignition delay times, minimum (threshold) ignition temperatures, combustion heat, concentrations of anthropogenic emissions, and the relative efficiency of fuels. We have determined that minimum ignition temperatures and shorter ignition delay times are achieved in the solid state (50–80% lower than in the slurry and gel state). The lowest concentrations of the most hazardous emissions are present in the liquid (slurry) state (NOx and SOx are 18–75% lower than from the combustion of coal in the solid state). The conditions were identified for the efficient use of coal and oil processing waste as part of composite fuels. The relative fuel effective utilization factor of waste-derived fuel mixtures in different states ranged from 0.11 to 45.5. It is maximal for fuels in the slurry state. The obtained data has been compared with the results of other studies.

One-dimensional modeling is considered as one of the most approved methods of studying the characteristics and performance of aircraft reciprocating engines, operating under different flight conditions. Owing to the recent progresses, the one-dimensional modeling of piston engine components including air filter, carburetor, air manifold, inlet and outlet valves, cylinder, piston, crankshaft, exhaust manifold, and turbocharger have become possible using some of the engineering software such as the GT-Power. The present paper is devoted to extensive performance analysis of one of the most widely used aircraft engines, the Rotax-914 which is a spark-ignition (SI) turbocharged aircraft engine, during the climb phase. The survey is conducted through the preparation of a one-dimensional model of the Rotax-914 using GT-power software. Then, to be more consistence with the real flight condition, its turbocharger is accurately modeled by considering the performance curves provided by the manufacturer. Next, to ensure its accuracy, the developed model was validated against the manufacturer data. The important engine performance parameters and turbocharger performance curves were analyzed for different flight aspects up to 30,000 feet. The obtained results indicated that the turbocharger prevents a dramatic decline in engine performance parameters up to 18,000 feet. Thereafter, the compressor reaches the choking region, fails to supply air demand, leading to extreme performance loss. The effect of ambient temperature difference from the international-standard-atmosphere (ISA) on engine performance was also investigated. A 40 °C increase in the ambient temperature causes a 10% drop in the brake horsepower and consequently, a performance loss was observed during the climb. Ambient temperature and altitude differently affect engine performance. The effect of ambient temperature on the engine performance was greater as the compressor reaches the choking region. Beyond the choking region, altitude variations had a greater impact on engine performance than the temperature variations.

The thermo-hydraulic and entropy generation characteristics of a square substrate imposed to a uniform heat flux (q″ = 200 W/m2) have been numerically investigated in the laminar regime (99.6 ≤ Re ≤ 1998) using finite volume method of Ansys Fluent R16. Six new constructal designs triangular (Case-1), dumbbell (Case-2), diamond (Case-3), H-shaped (Case-4), two-lobbed (Case-5) and three-lobbed (Case-6)) are obtained when the embedded pipe is freely morphed to have a greater and greater access to the heated regions of the square plate. First, the convection effect is neglected and the temperature of the plate as a function of the length of the loop is quantified. Second, the convection is taken into consideration and the temperature of the plate for all such configurations is quantified. Also, the pressure drop and entropy generation for all the designs have been compared with each other to get the best design among them. It has been observed that the diamond shaped configuration provides better cooling to the plate when their lengths are in range of 2 ⩽ L c / L ⩽ 2.74 . For longer loops, H-shaped and three-lobbed designs are found to be the better design than the other designs. The pressure drop and total entropy generation for case-3 is found to be the lowest among all designs.

An accurate condensation model plays an important role in the development of high-efficiency turbine. Many efforts have been devoted into the research on steam non-equilibrium condensation. However, aiming at the cryogenic gas at high-speed flow the studies on the condensation model are still inadequate and incomprehensive. In present paper, different condensation models, established via the alternations of nucleation model and droplet growth model are compiled into commercial CFD software to simulate the non-equilibrium condensation flow within a cryogenic radial-inflow turbo-expander. Firstly, the rotation effect on droplets diameter and distribution within the impeller passage are analyzed. The results point out the region where large droplets are prone to form and gather. Furthermore, the influence mechanism of heat and mass transfer in varying condensation models on the flow fields, including rapid nucleation, supercooled degree, liquid mass fraction, droplet diameter, are revealed based on the analysis of Nusselt number and Knudsen number. The results imply that the predicted location of condensation onset is little sensitive to the droplet growth model. At last, the effect of modifications for nucleation rate and droplet growth rate on the thermodynamic losses in turbo-expander are quantificationally discussed.

Micro-porous ceramic membrane tube is a new type of condensation material which can synchronously recover both water vapor and its accompanied latent heat from exhausted flue gas. The recovery outcome is helpful to reduce amount of make-up water and enhance boiler efficiency of thermal power plant. In this study, an experiment has been carried out to investigate condensation heat-transfer, water-recovery, and flow-resistance performance of micro-porous ceramic membrane tube bundle (CMTB), respectively. In addition, real exhausted flue gas with inlet flow rate ranging from 390 to 750 m3/h, temperature ranging from 323 K to 373 K was used as experimental feed gas. Results showed that the condensation heat-transfer process got more apparently with increasing of flue gas inlet temperature (from 335.3 K to 364.8 K). In addition, both cooling water flow rate and heat-transfer temperature difference had a remarkable influence on the water-recovery performance of the CMTB, and maximal water-recovery mass flow rate could reach 0.0336 × 103 kg/h. Most of all, velocity drop of flue gas was controlled within an appropriate range (from 28.94 m3/h to 178.78 m3/h). The application analysis on the CMTB gives a good perspective for designing high efficiency membrane-type condenser to recover water vapor from exhausted flue gas and make good use of its latent heat in the thermodynamic system of power plant.

Restrictive environmental regulations are driving the use of CO 2 as working fluid in commercial vapour compression plants due to its ultra-low global warming potential (GWP 100 = 1) and its natural condition. However, at high ambient temperatures transcritical operating conditions are commonly achieved causing low energy efficiencies in refrigeration facilities. To solve this issue, several improvements have been implemented, especially in large centralized plants where ejectors, parallel compressors or subcooler systems, among others, are frequently used. Despite their good results, these measures are not suitable for small-capacity systems due mainly to the cost and the complexity of the system. Accordingly, this work presents a new subcooling system equipped with thermoelectric modules (TESC), which thanks to its simplicity, low cost and easy control, results very suitable for medium and small capacity plants. The developed methodology finds the gas-cooler pressure and the electric voltage supplied to the TESC system that maximizes the overall COP of the plant taking into account the ambient temperature, the number of thermoelectric modules used and the thermal resistance of the heat exchangers included in the TESC. The obtained results reveal that, with 20 thermoelectric modules, an improvement of 20% in terms of COP and of 25.6% regarding the cooling capacity can be obtained compared to the base cycle of CO 2 of a small cooling plant refrigerated by air. Compared to a cycle that uses an internal heat exchanger IHX, the improvements reach 12.2% and 19.5% respectively. The authors would like to acknowledge the support of the Spanish Ministry of Science, Innovation and Universities for the funding under the FPU Program (FPU16/05203).

The mal-distribution of gas–liquid mixtures has serious influences on the heat-transfer performance of plate–fin heat exchangers (PFHEs), which would result in an adverse effect on the stable and efficient operation of a natural gas liquefaction plant. Therefore, it is necessary to investigate the phase distribution performance of PFHEs and its influence on the natural gas liquefaction performance. In this study, a two-phase flow distribution experimental system was built to investigate the flow distribution characteristics of a PFHE under different working conditions. The effects of gas–liquid ratio, tilt angle, and sloshing on the gas–liquid distribution were studied. In addition, the influences of mal-distribution of the feed gas, nitrogen refrigerant, and mixed refrigerant (MR) on the heat transfer and natural gas liquefaction performance were analyzed through the Aspen Muse software. The results showed that the greater the gas–liquid ratio and tilt angle, the more uneven the liquid flow distribution. The efficiency of the MR process in the tilt condition was reduced by 5.2% to 18.5% compared to that in the horizontal condition. Compared with the tilt working condition, the sloshing could reduce the unevenness of the flow distribution. To satisfy the natural gas liquefaction rate of more than 90%, the critical standard deviation values of flow distribution unevenness of the feed gas, nitrogen refrigerant, MR, and MR under the tilt condition were 8.33, 2.95, 4.42 (liquid phase), and 1.42 (liquid phase), respectively. In the process design, to ensure the output and liquefaction rate, the amount of the refrigerant circulation and power consumption should be kept at the margin of approximately 6% and 4% for the nitrogen expander and MR liquefaction processes, respectively.

Direct-expansion solar assisted heat pump (DX-SAHP) is a promising technology for saving energy. In this paper, vapor injection (VI) cycle system to DX-SAHP system is studied. Thermodynamic comparative analyses of ideal VI cycle and ideal conventional vapor compression cycle was carried out for verifying the advantages by applying VI cycle into DX-SAHP. Additionally, photovoltaic/thermal (PVT) modules were used in the system to harvest thermal energy and electric power. A PVT based solar heat pump system employing VI cycle (PVT-VISHP) was proposed, fabricated and investigated experimentally. Performance of the experimental prototype was subsequently evaluated for seven days in winter. The system’s performance was evaluated based on the average ambient temperature of −1.13 °C and solar irradiation of 164.03 W/m2. Results showed that the total generated electricity, heat, and consumed power of the system were 0.51 kWh, 23.68 kWh, and 7.24 kWh, respectively. Moreover, average heating coefficient of performance (COPth) and advanced coefficient of performance (COPPVT) of the PVT-VISHP system were 3.27 and 3.45, respectively. In the study, the ambient-evaporating temperature difference ranged from 9.2 °C to 17.1 °C. Furthermore, variations of injected vapor’s pressure, mass flow rate, mass flow rate ratio, and subcooling effects were analyzed in detail. Performance of the PVT modules during a sunny day was tested. It was found that the PVT modules’ average thermal collecting and PV efficiencies were 49.9% and 7.51%, respectively.

In this paper, an oscillating heat pipe (OHP) with external expansion structure and compact layout for thermal energy recovery and storage was designed and fabricated. The thermal performance of the OHP with different filling ratios (FRs) of self-rewetting fluid under continuous heating mode was tested at first. Then pulsing heating mode and alternate heating mode were explored to enhance the heat transport performance. The temperature fluctuation characteristic and the thermal resistance under different heating modes and heating periods were compared and analyzed. The result showed that the OHP can function well and sustain large heating power under appropriate range of FR. The pulsing mode and alternate mode are both helpful to increase the fluctuation frequency and amplitude of evaporator temperature. For the condition of relative low heating power and short heating period, the enhancement effect is more obvious. The thermal resistance at 100 W can be reduced by more than 10% when the heating period varies from 2 s to 10 s. Under short heating period of 0.4 s, the thermal resistance of the OHP under pulsing heating mode at 200 W can be even reduced by over 30%.

The self-healing of cracks in asphalt mixture is mainly due to the drain of the bitumen contained in the space between the aggregates, into the cracks. Until now, it was believed that the physical principles affecting the flow of bitumen are influenced by gravity and surface energy of bitumen and the aggregates. In this study, we show that the thermal expansion of bitumen plays an important role in the self-healing of asphalt mixture. To demonstrate this, asphalt mortar beams were manufactured and broken in two pieces by means of three-point bending tests. Self-healing was induced in asphalt mixture by increasing its temperature using a convection oven, at temperatures that ranged from 40 °C to 120 °C. The self-healing ratio was calculated by comparing the force required to break the test specimens before and after heating. Furthermore, a test was designed that consisted of bitumen raising through a capillary tube from a bitumen container. To account for the effect of thermal expansion, the bitumen container was fully enclosed except for the capillary tube. To account for the effect of surface energy on the bitumen’s capillary flow, the capillary tube was placed in a container that was open to the atmosphere. The rise of bitumen was monitored at temperatures that ranged from 40 °C to 120 °C. Finally, activation energies were derived from the rise of bitumen in the capillaries, viscosity changes and the self-healing progression. It was found that the activation energy of asphalt self-healing is similar to that of bitumen rising due to thermal expansion, which confirms the contribution of thermal expansion on asphalt self-healing by the effect of increasing temperature.

This paper deals with evaluation of wall heat loads that occur in a region of detonation propagation in a detonative propulsion combustion chamber. The heat loads were assessed for two types of detonation engine: pulse and rotating detonation. The results were compared to experimental data published in the literature. The procedure undertaken is based on an experimental investigation of heat transfer behind a single detonation wave that was conducted using a detonation tube. Fast response thermocouples were used to measure hot gas side wall temperature and this enabled the researchers to obtain values of heat flux behind a detonation wave propagating through gaseous, homogeneous, stationary mixtures of various compositions and initial pressures. Two mixtures were studied: H2-O2 and CH4-O2. In the case of the hydrogen-oxygen mixture a theoretical investigation was also conducted, where heat flux was obtained on the basis of inviscid CFD calculations supplied with an analytical model of Sichel and David. Numerical results include the expansion wave as well as the influence of wall temperature on the detonation chamber wall heat loads.

The problem of phase-changing with heat and mass transfer has aroused widespread interest and imposed new challenges, especially in understanding the non-eutectic phase-change dynamics. In this paper, experimental study and model development were performed to analyze the crust solidification/melting dynamics with heat transfer and component diffusion, which played an important role in estimating the interface temperature and corresponding heat transfer. The experiments were performed with non-eutectic binary mixture of NaNO3-KNO3 as corium simulant during three heating power stages. Then the transient model was developed to describe crust formation and melting with both heat balance of corium pool and crust, as well as the mass balance at crust interface. Besides, the radiation and natural convection in top space, as well as heat conduction in the crust, gap and wall were taken into consideration. The developed transient model was validated against with experimental data and were proved to be in good agreement with each other. Furthermore, the parameters of crust-wall gap size and diffusive boundary layer thickness were performed for sensitivity study.

The solar energy usage ability is mainly considered in the construction of solar thermal power plant (STPP) which is affected by factors of design direct normal irradiance (DNI), solar multiple (SM), thermal energy storage (TES) size and dispatch fraction (DF). In this paper, a general model is developed in order to obtain the relationship among these factors, the electricity output and the solar energy usage efficiency of STPP. Based on the modeling method, a critical design of DNI is derived and some suggestions for improvement in the design of DNI are presented in order to improve the efficiency, which can result in the reduction of waste solar energy. The effects of SM, TES size and DF on the critical design of DNI are also investigated. It is found that the solar energy usage ability will be only dependent on the TES size when TES size is below 6 h and the solar energy usage ability will be dependent on both TES size and DF when TES size is above 6 h. The increase of TES size and the decrease of DF beyond their corresponding critical design DNI will reduce the solar energy usage ability of STPP. The results obtained can provide an important basis for studying the solar energy utilization capacity of STPP and determine the design parameters of STPP.

The area-to-point heat conduction problem is solved using a discrete binary particle swarm optimization (BPSO) algorithm. The distribution of conductive materials is optimized to form a tree-shaped conducting path, which can significantly reduce the maximum temperature of the heat-generating domain. The effects of the conductivity ratio and the quantity of conductive materials on the geometric structure of the conducting path are investigated. As conductivity ratio and filling ratio increase, the maximum temperature decreases. In addition to the conductivity ratio and filling ratio, the heat sink location is found to have an impact on cooling capacity. That is, the maximum temperature decreases with decrease in distance between the hot spot and heat sink. In addition, the structural features of the conducting path produced according to different objectives are characterized.

In this study, an experimental device is designed and built to investigate the heat transfer characteristics in a downflow circulating fluidized bed evaporator. The tested downer has a length of 1200 mm and a dimension of Ф38 mm × 3 mm. Polyformaldehyde (POM), SiC and glass bead particles are used as the inert solid particles. A range of experimental investigations are performed by varying the amount of added particles (0.5–2.0%), heat flux (8–16 kW·m−2) and circulation flow velocity (0.56–1.78 m/s). Results show that as the addition of the particles destroys the laminar sublayer and increases nucleate sites, the boiling heat transfer coefficient of the three-phase flow is higher than that of the two-phase flow. All three kinds of particles added can enhance the heat transfer. The maximum enhancing factors are 78.5%, 68.8%, and 72.1% within the range of experiments for POM, glass bead and SiC particles, respectively. The flow boiling heat transfer coefficient initially increases and then decreases or fluctuates with the increase in the amount of added particles. The circulation flow velocity has two aspects of influence on the flow boiling heat transfer. The enhancing factor increases with the increase in circulation flow velocity at low heat flux, but initially increases and then decreases at high heat flux. The enhancing factor decreases with the increase in heat flux because of the generation of vapor film on the heating wall. Three dimensional diagrams are established to determine the optimum particles. The results can provide some references for the industrial application of the circulating fluidized bed heat transfer technology.

European standards have set stringent particulate number (PN) regulations for gasoline direct injection (GDI) engine, proposing a great challenge for the particulate emission control. The dual-injection mode, which incorporates direct-injection with port-fuel-injection, is considered to be an effective approach to reduce PN emissions. In this study the combustion, gaseous and particulate emission characteristics under different direct-injection ratios in a dual-injection gasoline engine were investigated. The results show that dual-injection increases the peak of in-cylinder pressure curves and makes the heat release curves shift towards an earlier region. The dual-injection mode yields a notable CO reduction while there is a slight increase in NOx emission. Dual-injection can significantly reduce the PN emission compared with GDI engines. As the direct-injection ratio decreases, the PN concentration continuously declines. The peak of size distributions curves for dual-injection tends to shift towards the smaller diameter region, showing the ability to generate particles in smaller size. The soot particles from dual-injection have lower oxidation activation energy than those from direct-injection. According to the results above, it can be inferred that dual-injection can promote the combustion process and has remarkably positive effects on particle emission control owing to its reduced PN emissions and improved particle oxidation reactivity.

The performance of a low-cost extruded heat sink prototype for a Low Concentration Photovoltaics system is studied using various analytical correlations and CFD simulations. An experimental test, with temperature measurements at different monitoring points of heat sink surface, was carried out in order to select the most appropriate approach. Large deviations (of up to 20 °C) in the estimation of base plate temperature from correlations were found when compared to experimental results, probably due to the specific geometry characteristics of the heat sink, with variable fin thickness and high fin length. Furthermore, discrepancies of up to 48% have been found among the different correlations. The numerical CFD results of temperature at monitoring points and at the base plate showed relative errors of about 1%, which were at least 15 times smaller than the results given by analytical correlations. Also, numerical simulations allowed the identification of stagnation zones due to the great length of the heat sink and characteristic air flow patterns, which help to explain its performance under different operating conditions. Thus, results showed that multiple chimney flow pattern and air stagnation zones seem to disappear for inclination angles greater than 30°.

Uncertainty on operating and manufacturing conditions for gas turbines leads to a reduced knowledge of their behavior along with high risk of unexpected failures. This study presents quantification analysis of the impacts of geometric uncertainty on film cooling performance at the blowing ratio of 0.5 and 1.5, and a classical flat-plate model with trench hole is used for test case. Four geometric parameters including fillet radiuses and deviation angles of trench are selected as uncertain inputs, and it is assumed that these inputs follow truncated Gaussian distributions. Radial basis function neural network is used for surrogate model based on the CFD (Computational fluid dynamics) calculation results, and Monte Carlo simulation is applied to investigate statistical characteristics of the outputs. The uncertain analysis results show that the increase of the blowing ratio results in the increase of uncertain degree of adiabatic film cooling effectiveness, especially in the near-filed region. Furthermore, by sensitivity analysis, the impact of fillet radius of trench-trailing corner on film cooling performance is the greatest, while the impact of deviation angle of trench-leading edge is the weakest. It illustrates that uncertainty of fillet radius of trench-trailing corner should be paid extra attention in design and manufacturing processes.

In the context of renewables penetration, coal-fired power plants are required to operate at the minimum load in low-demand periods of the grid. However, combustion in the boiler will become unstable in low load conditions, which constrains the load reduction of coal-fired power plants, especially the plants fueled by low-rank coal (LRC). This work proposed a thermal energy storage (TES) concept based on LRC-drying (LD-TES) to reduce the minimum load of LRC-fired power plants (LCPPs). A simple experiment was employed to verify the feasibility of energy storage through LRC drying. The advantage of LD-TES compared with traditional low-temperature TES was revealed based on the thermodynamic analysis. Moreover, an LCPP system integrated with LD-TES was proposed, and the minimum load reduction attributed by LD-TES and the energy/environmental performance of the integrated system is determined quantitatively. To achieve this goal, a thermodynamic model is employed for system simulation and evaluation, and the TES scheme based on hot water tank (HWT-TES) using the same heat source as LD-TES is chosen for comparison. The results show that, pre-drying and storage of LRC is feasible for hourly thermal energy storage, and LD-TES can equivalently store electric power in megawatt magnitude (11.1–25.0 MW) with high round-trip efficiency (92.8%). Besides, LD-TES can also significantly reduce the CO2 emission of LCPP systems.

Upgrading bioreactor under solar radiant heating can maintain the anaerobic fermentation process by utilizing absorption and scattering of solar energy in media to improve slurry temperature. Finding the suitable correction factor of temperature fluctuation for calculating kinetic rate of microbial fermentation process can improve simulation accuracy of microbial fermentation process. In present work, a coupled model for simulating the processes of photothermal transmission and anaerobic fermentation was built. Experimental and numerical researches about the anaerobic fermentation process of upgrading bioreactor were made to investigate the effect of temperature fluctuation on the kinetic rate of biochemical reactions. The results show that the temperature fluctuation has big effects on the biochemical reaction rate of each substance in the fermentation process. The n values for calculating heat exchange number BT are discrepant for both different reactants and temperature differences. For instance, when the temperature difference is bigger than 4 K, n = 2 is suitable to the one of cow dung in reactor A, and n = 3 is suitable to the one of pig dung and straw pitch in reactor B. The effect of n values on daily biogas production of straw is bigger than the one of cow dung, since hydrolytic bacteria are more sensitive to temperature fluctuation.

Arc characteristics are found greatly changed by the negative pressure from the hollow cathode. These changes affect the distribution of arc pressure, arc plasma flow and temperature along the arc column. The characteristics of HCCNPA (hollow cathode centered negative pressure arc) along the arc column were investigated in this study to understand the changes and influences. It was found that the negative pressure applied to the cathode reduced the arc pressure and the arc radius, and complicated the plasma flow distribution with a clear zero-speed dividing line in the arc column. Above the boundary line, the plasma in the middle of the arc column flowed to the cathode, and the peripheral plasma flowed to the anode, presenting two peak values in the arc pressure distribution. The inner negative pressure also could adjust the total arc force acting on the work-piece. Under the same inner negative pressure conditions, the difference in arc force between GTA (gas tungsten arc) and HCCNPA did not change at varying arc lengths.

Experimental results of spacer grid effect on the heat transfer under supercritical pressure conditions are investigated. The experiments are carried out in an electrically heated 19-rod bundle cooled with supercritical Freon R134a. Standard spacer grids with blockage ratio of 0.29 are equipped into the bundle. The local wall temperatures and heat transfer coefficients along the flow direction of the bundle are measured. An interpolation method to determine the fully developed Nusselt number at supercritical conditions is reported. The results indicate that the enhanced heat transfer is limited within 30 bundle diameters downstream of the spacer grids. A peak heat transfer enhancement is observed near the pseudo critical region, which cannot be captured by the subcritical correlations. Considering the maximum heat transfer enhancement as a function of dimensionless temperature ratio, an original approach for deriving the grid-enhanced heat transfer correlation for supercritical applications is presented and the corresponding correlation has been established.

A facile chemical etching method for fabricating copper surface with hierarchical nanostructures and low surface energy coating was adopted in this study. Experimental investigation on the condensation heat transfer of refrigerant R141b in a rectangular microchannel with hydraulic diameter of 0.67 mm was conducted. The mass flux was of the range from 70 kg/(m2 s) to 850 kg/(m2 s). The effects of surface wettability and wall sub-cooled on the flow and heat transfer characteristics were analyzed. For channels with same nanostructures, the increase of contact angle resulted in the increase of heat transfer coefficient, but it had little effect on the pressure drop during condensation. The experimental results showed that the channel with the contact angle of 21.6° had a 12.81% enhancement in the heat transfer coefficient compared to the original copper channel with the contact angle of 12.8°. Moreover, a composite channel with gradient contact angle had a 16.67% enhancement in the heat transfer coefficient compared to the original one. The hierarchical nanostructures had significant influences on the heat transfer characteristics and pressure drop during convective condensation in microchannel. A higher mass flux resulted in higher heat transfer coefficient and pressure drop, while a higher wall sub-cooled temperature caused a lower heat transfer coefficient during the R141b condensation in a microchannel.

Conjugate heat transfer in an electronic module containing an array of printed circuit board assemblies (PCBAs) was experimentally and numerically investigated. Each PCBA was assembled with a matrix of rectangular heat sources (4 × 4 with 0.5 W each). It was inserted in parallel in an aluminum case by leaving two opposite opened ends to allow air to flow through. The experiment and simulation were carried out on three different cases based on module orientations and PCBA arrangement; horizontal orientation with PCBA facing upwards (CASE I) and downwards (CASE II), and vertical orientation (CASE III) under both natural and forced convection at an ambient temperature of 30 °C. The simulation was done by varying the number of PCBAs (N) from 5 to 7 sets, the distance (l) between PCBAs array and module wall from an original design of 10 to 20 mm by an increment step of 5 mm, and air velocity from 1 m/s to 4 m/s. The results showed that under natural convection the global maximum temperature increased slightly (from 64.9 °C to 65.9 °C) as the number of PCBAs increased from 5 to 7 sets for the vertical orientation module (CASE III), while it increased significantly (from 97.9 °C to 127.4 °C) for the horizontal orientation module (CASE I). When the number of boards in the module increased, the board that possessed the global maximum temperature would vary in board position in the module according to the module’s orientation. However, the location of global maximum temperature on that board was still in the same region in the heat source matrix. By increasing the distance (l) between PCBA and the module wall, the heat transfer was enhanced. Under forced convection, it can be observed that the drawback of heat removal from the horizontal orientation module diminished under forced convection when air speed was sufficiently high. The global maximum temperature decreased with an increase in the air speed from 63.5 °C to 48.7 °C and from 64.7 °C to 48.8 °C for the six PCBAs of CASE III and CASE I, respectively. A comparison of the experimental and numerical results shows good agreement for both natural and forced convection.

The quest to achieve higher muzzle velocity is one of the key goals for researchers in the fields of propulsion and ballistic. Based on the conventional chemical propulsion, the two-stage combustion system as a new launch principle can provide higher muzzle velocity without exceeding the maximum pressure in the chamber. In this paper, the modified two-fluid model with two moving boundaries is proposed to simulate the detailed multi-dimensional flow and energy conversion behaviors of the combustion gas and propellant grains in the two-stage chamber. The dynamic self-adapting mesh update method is developed to expand and merge the computational domain for both projectile and piston motions. The application to a standard virtual gun as a standard benchmark for interior ballistic codes is used to validate the accuracy and reliability. The calculation results are in good agreement with those of codes in different countries. After that, a two-stage chamber system is set up based on the standard virtual gun. The numerical results present deep understanding of the two-phase flow behaviors in the two-stage chamber. Then the effects of design parameters on the performances in the additional chamber are numerically investigated. Finally, a suggested scheme of the additional chamber is proposed. The findings of the study put a further prediction tool for the understanding and design of the two-stage combustion system with moving boundaries.

This study investigated the thermal performance of quicklime (CaO) energy pile aimed to thaw the warm permafrost ground. Three laboratory experimental models were made by three typical soils in the Tibetan Plateau permafrost regions. The variations of soil temperature, heat flux, and soil volumetric water content were monitored and analyzed during the warm permafrost thawing process. The results show that the same quicklime pile has different thawing effects according to the frozen soil types. The largest thawing range and maximum soil temperature value were observed in the sandy soil, followed by the silty soil, and then the clay. Furthermore, the thawing radii are around 20 cm in all three frozen soils. The highest soil temperature is 38.5 °C in the sandy soil. Soil water content kept almost at the same value before and after the experiments and exerts a limited impact on the physical property of soil. In addition, distinct energy pile expansions were observed at the end of experiments. This pile expansion indicates that the soil density increased around the quicklime piles, and hence improved the strength of the thawed ground. Overall, the quicklime (CaO) pile is an efficient and economic approach to improve the thermal and mechanical stability of the warm permafrost ground.

A key challenge in Exploring oil and gas resources in permafrost regions is to predict the thawing subsidence of frozen soil around the wellbore. At present, the traditional thermo-hydro-mechanic (THM) model calculates thawing subsidence without actually considering the spatial-temporal evolution of the thawed-frozen boundary. The Darcy-Stefan model is used to describe the internal mobile interface problems in the heat transfer of saturated porous media due to ice/water phase transitions. In this paper, we established the coupling model of the Darcy-Stefan model and the elastic theory of porous media, including boundary conditions of heat transfer and Darcy flow on the moving boundary. The new model was validated through a series of small-scale examples and a test well in the Arctic region, which demonstrated the predicted subsidence amount of ground surface around the wellhead was in good agreement with the field test data. Further research indicated that with the increase of the thawing zone, the accuracy of calculation for the thawing subsidence could be significantly improved by using the THM model and considering the internal moving boundary. The models and conclusions in this paper can help design more reliable wellbore in permafrost regions throughout the expected life.

The article presents experimental investigations on a prototype heat exchanger. Presented research is focused on combined active and passive enhancement techniques of surface modification and microjet impingement. The results were compared to reference plate heat exchanger without microjet impingement. The Wilson plot method was applied to determine the heat transfer coefficients in the single phase operation. The heat exchanger was capable of exchanging 300 W of thermal energy at LMTD of 40 K. The obtained overall heat transfer rates reach 600 W/m2. Introducing knurled surface resulted in ∼10% increase heat transfer area, and similar enhancement in transferred heat in reference to plate heat exchanger geometry. Knurled surface decreased heat transfer in combination with a multi-jet impingement in comparison to a smooth surface. A possible explanation of that effect is that the flow in wall jet region is disrupted by the knurled surface and directed in the bulk of the fluid. Initiating mixing with colder fluid, thicker thermal boundary layer, and affecting local temperature distribution.