The application of thermal diodes, which allow heat to flow more readily in one direction than the other, is an important way to reduce energy consumption in buildings and enhance the battery heat dissipation of electric vehicles. Depending on various factors including the specific design, materials used, and operating conditions, the convective thermal diode can exhibit the best thermal rectification effect in intended applications compared to the other  thermal diodes. In this study, a novel convective thermal diode with a wick was proposed based on the phase change heat transfer mechanism. This design takes advantage of both capillary forces provided by the wick and gravity to achieve enhanced unidirectional heat transfer performance for the designed convective thermal diode. The effect of the filling liquid ratio on the thermal performance of the thermal diode was experimentally investigated, which was in good agreement with the theoretical analysis. The research findings showed that with an optimal liquid filling ratio of 140%, the thermal diode with a wick can achieve a better thermal rectification ratio when subjected to a lower heating power, and the maximum thermal rectification ratio of 21.76 was experimentally achieved when the heating power of the thermal diode was 40 W.

With the advancement of micro machining technology, the high-heat-flux removal from miniature electronic devices and components has become an attractive topic. Flow boiling in micro-channels is an optimal form of heat transfer and has been widely employed in high-heat-flux cooling applications. This comprehensively-reviewed article focused on the available recent literatures of experimental investigation regarding the flow boiling heat transfer and unstable behaviors of the fluid with lower boiling point in micro-channels. The thermal-fluid characteristics and potential heat transfer mechanisms of low-boiling-point fluids flow boiling in different narrow passages were summarized and discussed. The literatures regarding the pressure drop and occurrence of the unstable phenomena existing in two-phase flow boiling process were also discussed. The emphasis was given to the heat transfer enhancement methods as well as instability elimination, and various methods such as modification of surface and channel flow geometries were considered. Some future researches in the field of micro-scale flow boiling were suggested.

Hybrid deflagration/auto-ignition flame structures coexist in the combustion of advanced engines. Decoupling exergy destruction caused by different irreversible processes under varied flame regimes is thus important for understanding engine thermodynamics. In this study, the flame propagation modes for the premixed DME/air mixtures are numerically investigated under engine-relevant conditions. Local entropy generation and exergy destruction characteristics are compared under different flame structures. Results reveal that as the typical premixed flame transition towards auto-ignition front, the exergy destruction from heat conduction and species mass diffusion gradually vanish and are dominated by chemical reaction. The distributions of temperature and species mole fraction in the flame domain are analyzed to clarify the exergy destruction behaviors caused by heat conduction and mass diffusion. Furthermore, by dividing the DME oxidation process into four stages, the main reaction channels that contribute to the increase in exergy destruction from chemical reaction have been identified. It is found that the production and consumption of CH₂O and HȮ₂ radical dominate the exergy destruction behavior during DME oxidation. On this basis, the kinetic mechanism of low-temperature chemistry causing greater exergy destruction is elucidated. Specifically, low-temperature chemistry leads to significant exergy destruction due to (a) the large irreversibility of itself and (b) (mainly) the enhancement of H₂O₂ loop reactions by low-temperature reaction intermediates. Thus the reduction of combustion irreversibility is promising to be achieved by reasonably regulating the fuel oxidation path.

Using thermal models to describe the heat dissipation process of FCBGA is a significant topic in the field of packaging. However, the thermal resistance model considering the structure of each part of the chip is still ambiguous and rare, but it is quite desirable in engineering. In this work, we propose a detailed thermal resistance network model, and describe it by using thermal conduction resistance and thermal spreading resistance. For a striking FCBGA case, we calculated the thermal resistance of each part of the structure according to the temperature field simulated by COMSOL. The thermal resistance network can be used to predict the temperatures in the chip under different conditions. For example, when the power changes by 40%, the relative error of junction temperature prediction is only 0.24%. The function of the detailed thermal resistance network in evaluating the optimization space and determining the optimization direction is clarified. This work illustrates a potential thermal resistance analysis method for electronic devices such as FCBGA.

As the only controllable means of a micro gas turbine (MGT) combustor during unit operation, pilot fuel ratio (PFR) is the key to achieving stable combustion and low pollutant emission. This paper discusses the influence of PFR on the inner flow field structure and pollutant emissions. The steady-state three-dimensional RANS method with a 40-step reduced methane-air kinetics mechanism is used to study the reaction flow field and species field with PFR of 9.0%, 12.7%, 15.2% and 17.6%. Results show that, with the decrease in PFR, the axial velocity and temperature near the central axis of the combustion chamber show a tendency to decrease. A similar separation phenomenon occurred in the core pyrolysis reaction zone (measured by HCO) and oxidation zone (measured by OH), which is more conducive to promoting the oxidation of CO. The quantitative effect of the pilot flame on nitrogen oxides (NO_x) was separated by using inert gas instead of nitrogen in combustion air. It was found that the NO_x produced by the pilot flame under the operation condition with a PFR of 9.0% was 3.2×10^–6, accounting for 17.4% of the total NO_x emission, which was twice that of PFR.

This study evaluates the effectiveness of phase change materials (PCMs) inside a storage tank of warm water for solar water heating (SWH) system through the theoretical simulation based on the experimental model of S. Canbazoglu et al. The model is explained by five fundamental equations for the calculation of various parameters like the effectiveness of PCMs, the mass of hot water, total heat content, and duration of charging. This study simulated eleven PCMs to analyze their effectiveness like Sodium hydrogen phosphate dodecahydrate (SHPD), OM 37, N-Eicosane (NE), Lauric acid (LA), Paraffin wax (PW), OM 48, Paraffin wax C_20-33 (PW-C20-33), Sodium acetate trihydrate (SAT), Palmitic acid (PA), Myristic acid (MA), and Stearic acid (SA). Among all PCMs, the SHPD has found the highest value of effectiveness factor of 3.27. So, it is the most recommended PCM for the storage tank of the SWH system. The study also includes the melt fraction analysis of all enumerated PCMs corresponding to container materials of stainless steel, glass, aluminum mixed, tin, aluminum, and copper. This melt fraction analysis is performed by making a coding program in the FORTRAN programming language. Through the analysis, copper container material is found to have high melting rate for all PCMs so it is superior to other container materials. 

This study is to utilize the heat-absorbing and releasing capabilities of phase change materials (PCM) to regulate the surface temperature fluctuations of batteries during charging and discharging. The goal is to keep the battery within the optimal operating temperature range. The impact of PCM thickness and phase change temperature on battery temperature is investigated by encircling a cylindrical battery with a PCM ring. To improve the thermal conductivity of PCM, expanded graphite (EG) is added to make a composite phase change material (CPCM), and the effects of various EG mass ratios on battery surface temperature and CPCM utilization level are investigated. The findings indicate that increasing PCM thickness effectively extends temperature control time, but its impact is limited. The difference in phase change temperature of PCM controls the battery temperature in different temperature ranges. Lower phase change temperatures are unsuitable for controlling battery temperature in high temperature environments. The addition of EG enhances the thermal conductivity of PCM, leading to further control of battery temperature. The results show that the addition of 6% (mass ratio) EG to CPCM extends the effective temperature control time by 11 min and improves by 28% compared to a single PCM. The CPCM utilization is also more satisfactory and achieved a balance between heat storage and thermal conductivity in a battery thermal management system (BTMS) based on PCM.

High energy consumption is a serious issue associated with in situ thermal desorption (TD) remediation of sites contaminated by petroleum hydrocarbons (PHs). The knowledge on the thermophysical properties of contaminated soil can help predict accurately the transient temperature distribution in a remediation site, for the purpose of energy conservation. However, such data are rarely reported for PH-contaminated soil. In this study, by taking diesel as a representative example for PHs, soil samples with constant dry bulk density but different diesel mass concentrations ranging from 0% to 20% were prepared, and the variations of their thermal conductivity, specific heat capacity and thermal diffusivity were measured and analyzed over a wide temperature range between 0°C and 120°C. It was found that the effect of diesel concentration on the thermal conductivity of soil is negligible when it is below 1%. When diesel concentration is below 10%, the thermal conductivity of soil increases with raising the temperature. However, when diesel concentration becomes above 10%, the change of the thermal conductivity of soil with temperature exhibits the opposite trend. This is mainly due to the competition between soil minerals and diesel, because the thermal conductivity of minerals increases with temperature, whereas the thermal conductivity of diesel decreases with temperature. The analysis results showed that, compared with temperature, the diesel concentration has more significant effects on soil thermal conductivity. Regardless of the diesel concentration, with the increase of temperature, the specific heat capacity of soil increases, while the thermal diffusivity of soil decreases. In addition, the results of a control experiment exhibited that the relative differences of the thermal conductivity of the soil samples containing the same concentration of both diesel and a pure alkane are all below 10%, indicating that the results obtained with diesel in this study can be extended to the family of PHs. A theoretical prediction model was proposed based on cubic fractal and thermal resistance analysis, which confirmed that diesel concentration does have a significant effect on soil thermal conductivity. For the sake of practical applications, a regression model with the diesel concentration as a primary parameter was also proposed.

To achieve high-performance compressor cascades at low Reynolds number (Re), it is important to organize the boundary layer transition and separation processes efficiently and reasonably. In this study, the airfoil is focused on at a 5% blade height at the root of the orthogonal blade in the downflow passage of the high-load booster stage. The bionics modeling design is carried out for the leading edge of the original blade cascade; the response characteristics of laminar transition and separation to blades with different leading edge shapes at low Reynolds numbers are studied by using large eddy simulations combined with Omega vortex identification. The findings of this study demonstrate that bionic leading edge modeling can significantly improve the aerodynamic performance of blades at low Reynolds numbers. The blades effectively suppress the formation of separation bubbles at low Reynolds numbers and weaken or even eliminate large-scale flow separation at the trailing edge. In addition, the blades can weaken the vortex intensity on the blade surface, reduce the areas of high-velocity fluctuations, and minimize aerodynamic losses caused by turbulence dissipation. These results should serve as a valuable reference for the aerodynamic design and flow control of the high-load booster stage blade at low Re.

Local heat transfer and flow characteristics, which is crucial to the overall performance of supercritical CO₂ recuperators, is rarely examined in details in reported studies. In this paper, the local heat transfer and flow characteristics of supercritical CO₂ in sinusoidal channel printed circuit heat exchangers are numerically investigated under the working conditions of recuperators. Based on the simplified physical model constituted by 10 pitches, the variations of Re, heat flux, Nu, secondary flow and other relevant parameters along the flow direction are analyzed firstly. Comparison is further made between the high and low temperature recuperators (HTR and LTR). It’s observed that the local heat transfer and flow characteristics vary greatly from the inlet to the outlet, especially in the LTR. Differences of 127.5% and 61.7% can be observed on the cold side of the LTR for Re and heat transfer coefficient h, respectively. Results indicate that the temperature difference and heat transfer coefficient h should not be regarded as constant and the distribution of h should be carefully considered in the design of the LTR. The complicated interactions among the varying thermophysical properties, buoyancy and the periodically changed centrifugal force are believed to be the key that shapes the flow fields. Furthermore, the changes of the wavy angle θ are found to have greater influence than the changes of mass flow rate in reshaping the flow field when θ>25°. Though gravity direction strongly affects the local heat transfer and flow characteristics, it’s also found that the effects on the overall thermal-hydraulic performance are relatively minor. Yet the installation direction that yields a_y=g should better be avoided for the sinusoidal channel printed circuit heat exchanger.

Present work investigates the heat transfer and melting behaviour of phase change material (PCM) in six enclosures (enclosure-1 to 6) filled with paraffin wax. Proposed enclosures are equipped with distinct arrangements of the fins while keeping the fin’s surface area equal in each case. Comparative analysis has been presented to recognize the suitable fin arrangements that facilitate improved heat transfer and melting rate of the PCM. Left wall of the enclosure is maintained isothermal for the temperature values 335 K, 350 K and 365 K. Dimensionless length of the enclosure including fins is ranging between 0 and 1. Results have been illustrated through the estimation of important performance parameters such as energy absorbing capacity, melting rate, enhancement ratio, and Nusselt number. It has been found that melting time (to melt 100% of the PCM) is 60.5% less in enclosure-2 (with two fins of equal length) as compared to the enclosure-1, having no fins. Keeping the fin surface area equal, if the longer fin is placed below the shorter fin (enclosure-3), melting time is further decreased by 14.1% as compared to enclosure-2. However, among all the configurations, enclosure-6 with wire-mesh fin structure exhibits minimum melting time which is 68.4% less as compared to the enclosure-1. Based on the findings, it may be concluded that fins are the main driving agent in the enclosure to transfer the heat from heated wall to the PCM. Proper design and positioning of the fins improve the heat transfer rate followed by melting of the PCM in the entire area of the enclosure. Evolution of the favourable vortices and natural convection current in the enclosure accelerate the melting phenomenon and help to reduce charging time.

As an important component of the stirling-type pulse tube cryocooler (SPTC), an efficient phase shifter can significantly improve the cooling capacity. Compared to the common phase shifter, the active warm displacer (AWD) has a wider phase adjustment range and therefore can obtain a better phase relationship easily. Based on a two-stage thermal-coupled SPTC operating in the 20 K range, this paper studied the influence of the swept volume ratio between the compressor and displacer. The research found that the swept volume ratio changes the cooling capacity and efficiency of the cryocooler mainly by changing the phase difference between the pressure wave and the volume flow at the cold end. It was found from the results of the simulation and experiments that there is an optimal displacement of the displacer (Xd) of 2.5 mm and an optimal phase angle of 15° to obtain the highest cooling efficiency while the displacement of the compressor is constant. The cooling capacity at 20 K is 1.3 W while the input electrical power of the second stage compressor is 202 W, which indicates an overall relative Carnot efficiency (rCOP) of 0.055 in terms of input electrical power. In addition, due to the reasonable setting of precooling temperature and capacity, the swept volume ratio and phase at the maximum cooling capacity and maximum efficiency are consistent in this study. The research improves the understanding of phase shifters and has guiding significance for the optimization of the SPTC working below 20 K.

Proton exchange membrane electrolysis cell (PEMEC) is one of the most promising methods to produce hydrogen at high purity and low power consumption. In this study, a three-dimensional non-isothermal model is used to simulate the cell performance of a typical PEMEC based on computational fluid dynamics (CFD) with the finite element method. Then, the model is used to investigate the distributions of current density, species concentration, and temperature at the membrane/catalyst (MEM/CL) interface. Also, the effects of operating conditions and design parameters on the polarization curve, specific electrical energy demand, and electrical cell efficiency are studied. The results show that the maximum distribution of current density, hydrogen concentration, oxygen concentration, and temperature occur beneath the core ribs and increase towards the channel outlet, while the maximum water concentration distribution happens under the channel and decreases towards the channel exit direction. The increase in gas diffusion layer (GDL) thickness reduces the uneven distribution of the contour at the MEM/CL interface. It is also found that increasing the operating temperature from 323 K to 363 K reduces the cell voltage and specific energy demand. The hydrogen ion diffusion degrades with increasing the cathode pressure, which increases the specific energy demand and reduces the electrical cell efficiency. Furthermore, increasing the thickness of the GDL and membrane rises the specific energy demand and lowers the electrical efficiency, but increasing GDL porosity reduces the specific electrical energy demand and improves the electrical cell efficiency; thus using a thin membrane and GDL is recommended.

The heat storage performance of latent heat storage systems is not good due to the poor thermal conductivity of phase change materials. In this paper, a new type of pointer-shaped fins combining rectangular and triangular fins has been employed to numerically simulate the melting process in the heat storage tank, and the fin geometry parameter effects on heat storage performance have been studied. The results indicate that compared with the bare tube and the rectangular finned tank, the melting time of the phase change material in the pointer-shaped finned tank is reduced by 64.2% and 15.1%, respectively. The closer the tip of the triangular fin is to the hot wall, the better the heat transfer efficiency. The optimal height of the triangular fin is about 8 mm. Increasing the number of fins from 4 to 6 and from 6 to 8 reduces the melting time by 16.0% and 16.7% respectively. However, increasing the number of fins from 8 to 10 only reduces the melting time by 8.4%. When the fin dimensionless length is increased from 0.3 to 0.5 and from 0.5 to 0.7, the melting time is shortened by 17.5% and 13.0%. But the melting time is only reduced by 2.9% when the dimensionless fin length is increased from 0.7 to 0.9. For optimising the design of the thermal storage system, the results can provide a reference value.

Based on the COMSOL Multiphysics simulation software, this study carried out modeling and numerical simulation for the evaporation process of liquid metal lithium in the vacuum free molecular flow state. The motion of lithium atoms in the evaporation process was analyzed through a succession of studies. Based on the available experimental values of the saturated vapor pressure of liquid metal lithium, the relationship between saturated vapor pressure and temperature of liquid lithium in the range of 600 K–900 K was obtained. A two-dimensional symmetric model (3.5 mm×20 mm) was established to simulate the transient evaporation process of liquid lithium at wall temperatures of 750 K, 780 K, 800 K, 810 K, 825 K, and 850 K, respectively. The effects of temperature, the evaporation coefficient, back pressure, and length-to-diameter ratio on the evaporation process were studied; the variation trends and reasons of the molecular flux and the pressure during the evaporation process were analyzed. At the same time, the evaporation process under variable wall temperature conditions was simulated. This research made the evaporation process of liquid lithium in vacuum molecular flow clearer, and provided theoretical support for the space reactor and nuclear fusion related fields.

Using the efficient, space-saving, and flexible supercritical carbon dioxide (sCO₂) Brayton cycle is a promising approach for improving the performance of nuclear-powered ships. The purpose of this paper is to design and compare sCO₂ cycle power systems suitable for nuclear-powered ships. Considering the characteristics of nuclear-powered ships, this paper uses different indicators to comprehensively evaluate the efficiency, cost, volume, and partial load performance of several nuclear-powered sCO₂ cycles. Four load-following strategies are also designed and compared. The results show that the partial cooling cycle is most suitable for nuclear-powered ships because it offers both high thermal efficiency and low volume and cost, and can maintain relatively high thermal efficiency at partial loads. Additionally, the new load-following strategy that adjusts the turbine speed can keep the compressor away from the surge line, making the cycle more flexible and efficient compared to traditional inventory and turbine bypass strategies.

As a new type of energy transport medium with high efficiency and high heat transfer performance, nanofluids have shown broad application prospects in the fields of thermodynamics, solar heat collection, microelectronics, thermal energy, and material science. The wide liquid range and environmental properties of ionic liquids have drawn ample attention to their application when used as a working fluid, especially as a base solvent of nanofluids. The ionic liquid-based nanofluids were prepared by a two-step method using 1-ethyl-3-methylimidazole trifluoroacetate ionic liquid as a base solvent and graphene oxide (GO) as a nanofiller. Thermophysical properties study reveals that the thermal conductivity could be enhanced by 3.0% with the addition of 0.05 wt% GO, and the viscosity and the specific heat capacity were also subject to study as a function of testing temperature and concentration of nanofiller. Additionally, the photothermal conversion efficiency of these nanofluids was studied comprehensively under different conditions. The results show that the photothermal conversion efficiency can reach 83% within an irradiation time of 6000 s and the highest temperature of the nanofluids is up to 105.89°C with a maximum photothermal conversion efficiency increase by 29%.

The oscillatory response of multiple shock waves to downstream perturbations in a supersonic flow is studied numerically in a rectangular duct. Multiple shock waves are formed inside the duct at a shock Mach number of 1.75. The duct has an exit height of H, and the effect of duct resonance on multiple shock oscillations is investigated by attaching exit ducts of lengths 0H, 50H, and 150H. The downstream disturbance frequency varied from 10 Hz to 200 Hz to explore the oscillation characteristics of the multiple shock waves. The oscillatory response of shock waves under self-excited and forced oscillation conditions are analyzed in terms of wall static pressure, shock train leading-edge location, shock train length, and the size of the separation bubble. The extent of the initial shock location increases with an increase in exit duct length for the self-excited oscillation condition. The analysis of the shock train leading edge and the spectral analysis of wall static pressure variations are conducted. The variation in the shock train length is analyzed using the pressure ratio method for self-excited as well as forced oscillations. The RMS amplitude of the normalized shock train length (ζ_ST) increases with an increase in the exit duct length for the self-excited oscillation condition. When the downstream perturbation frequency is increased, ζ_ST is decreased for exit duct configurations. For all exit duct designs and downstream forcing frequencies, the size of the separation bubble grows and shrinks during the shock oscillations, demonstrating the dependence on duct resonance and forced oscillations.

Understanding the residence time distribution (RTD) of a continuous hydrothermal reactor is of great significance to improve product quality and reaction efficiency. In this work, an on-line measurement system is attached to a continuous reactor to investigate the characteristics of RTD. An approach that can accurately fit and describe the experimental measured RTD curve by finding characteristic values is proposed for analysis and comparison. The RTD curves of three experiment groups are measured and the characteristic values are calculated. Results show that increasing total flow rate and extending effective reactor length have inverse effect on average residence time, but they both cause the reactor to approach a plug flow reactor and improve the materials leading. The branch flow rate fraction has no significant effect on RTD characteristics in the scope of the present work except the weak negative correlation with the average residence time. Besides, the natural convection stirring effect can also increase the average residence time, especially when the forced flow is weak. The analysis reveals that it is necessary to consider the matching of natural convection, forced flow and reactor size to control RTD when designing the hydrothermal reactor and working conditions.

The introduction of fresh air into the indoor space leads to a significant increase in cooling or heating loads. Solid desiccant heat pump fresh air unit which can handle the latent and sensible load of fresh air efficiently have been proposed recently. To improve the performance of the solid desiccant heat pump fresh air unit in the fresh air handling process, in this paper, the application of composite silica gel in a heat pump fresh air unit was investigated. The comparison between silica gel coating (SGC) and composite silica gel coating (CSGC) shows that the adsorption rate and water uptake capacity of CSGC are more than two times higher than those of SGC. An experimental setup for the solid desiccant heat pump fresh air unit was established. The performance of SGC and CSGC was tested in the setup successively. Results show that under summer conditions, compared with the solid desiccant heat pump fresh air unit using silica gel (SGFU), the average moisture removal and COP of the one using composite silica gel (CSGFU) increased by 15% and 30%, respectively. Under winter conditions, compared with SGFU, the average humidification and COP of CSGFU increased by 42% and 17%. With optimal operation conditions of 3 min switchover time and 40 r/s compressor frequency, the COP of CSGFU under summer conditions can reach 7.6. Results also show that the CSGFU and SGFU have higher COP and dehumidification rate under higher outdoor temperature and humidity ratio.

Direct pore-scale and volume-averaging numerical simulations are two methods for investigating the performance of porous volumetric solar receivers. To clarify the difference in the prediction of heat transfer processes, a direct comparison between these two methods was conducted at both steady state and transient state. The numerical models were established based on X-ray computed tomography scans and a local thermal non-equilibrium model, respectively. The empirical parameters, which are indispensable to the volume-averaging simulation, were determined by Monte Carlo ray tracing and direct pore-scale numerical simulations. The predicted outlet air temperature of the receiver by the volume-averaging simulation method corresponded satisfactorily to that in the direct pore-scale simulation. The largest discrepancies were observed when the receiver’s working temperature was elevated, with differences of 5.5% and 3.68% for the steady state and transient state simulations, respectively. However, the volume-averaging method is incapable of capturing the local temperature information of the air and porous skeleton. It underestimates the inlet temperature of the receiver, leading to an overestimation of the receiver’s thermal efficiency, with the largest difference being 6.51%. The comparison results show that the volume-averaging model is a good approximation to the pore-scale model when the empirical parameters are carefully selected.

By combining the advantages of desiccant dehumidification and vapor compression refrigeration, the desiccant-coated heat exchanger-based heat pump (DCHE HP) is regarded as a promising alternative to the traditional vapor-compression air conditioning system (VCAC). Selecting proper desiccants from a large number of candidates is of great importance to improve the performance of DCHE HP. However, this task is challenging using current experimental or modelling strategies. In this work, we developed an equilibrium model to evaluate the power consumption of 39 DCHE HPs coated with different desiccants under various operating conditions. Eventually, five desiccants with low power consumption were selected. It was also demonstrated that under given operating conditions, the DCHE HP based on the five selected desiccants can save 21.3%–32.9% power compared with the VCAC. The power consumption of the DCHE HP is largely dependent on the heat of adsorption, the cyclical water uptakes and the remained moisture contents of the coating desiccants. It was further revealed that the moderate heat of adsorption, the larger cyclical water uptake and the lower remained moisture content are preferable for reducing the system energy demand. This work reported a quick evaluation of 39 desiccants for DCHE HP by an equilibrium model, which may also offer insights into the choosing and designing of desiccants for DCHE HP.

The mixed-flow trapped vortex combustor (TVC) is a new type of combustor that applies trapped vortex flame stabilization technology to mixed-flow combustor. Compared with the traditional mixed-flow combustor, the mixed-flow TVC has many advantages, such as complicated structure of the vortex flow field inside liner, large cooling area, significant local hot spots on the liner, and large wall temperature gradient. In this paper, for a mixed-flow TVC with inclined multi-hole cooling, the liner wall temperature of an annular test rig was examined in experiments. The effects of inlet temperature (T3), inlet Mach number (Ma) and fuel to air ratio (FAR) on the temperature of liner wall were obtained, which provided a valuable reference for understanding the distribution characteristics of liner wall temperature. The experiment results show that the highest temperature is found to be on the fore-wall of the cavity. When T3 and FAR are low, the highest wall temperature was obtained in injector plane. However, the wall temperature in the center plane between two adjacent injectors was higher than that in injector plane under the condition of high FAR and T3. With the increase of FAR and T3, the average wall temperature increases. Ma has a slight impact on the average wall temperature. In addition, this paper provides an effective reference for the design and improvement of the liner cooling structure of the combustor with many discontinuous small-area walls in the flow direction. It is difficult to form a continuous film, and cooling requirements can’t be achieved only by using inclined multi-hole cooling structure. Consideration needs to be given to other efficient cooling structures, or to the combination of multiple cooling structures.

Large eddy simulations have been conducted to study upward transitional flows and heat transfer characteristics of supercritical CO₂ in a vertical mini-channel. The numerical simulation was carried out on a modified buoyantPimpleFOAM solver in OpenFOAM 7, and was verified using experiment data. Numerical results indicate that increasing the Grashof numbers can reduce the flow stability and make the flow transition earlier. There are four stages of heat transfer in the transition process, i.e., weakened, improved, recovered and normal heat transfer. These heat transfer phenomena in the transition process were explained from three perspectives: thermal boundary layer theory, turbulent transport and pseudo-boiling theory. Heat transfer enhancement during transition is related to the transport of supercritical molecular clusters, and these molecular clusters are regarded as pseudo-bubbles in pseudo-boiling theory. The flow pattern of the pseudo-phases in the dia-Widom process contains single-phase flow, steady pseudo-film flow, unsteady pseudo-film flow, partial pseudo-bubbles flow and flocculent pseudo-film flow. Pseudo-bubbles have similar behaviors to subcritical bubbles, i.e., break-up, deformation, condensation and coalescence. Relevant researches in this work are favorable for understanding the heat transfer mechanism of supercritical fluids during flow transition.

With the advantages of high efficiency and compact structure, supercritical carbon dioxide (sCO₂) Brayton cycles have bright prospects for development in energy conversion field. As one of the core components of the power cycle, the centrifugal compressor tends to operate near the critical point (304.13 K, 7.3773 MPa). Normally, the compressor efficiency increases as the inlet temperature decreases. When the inlet temperature is close to the critical point, the density increases sharply as the temperature decreases, which results in quickly decreasing of volume flow rate and efficiency reducing. The flow loss mechanism of the sCO₂ compressor operating at low flow rate is studied in this paper. Computational fluid dynamics (CFD) simulations for sCO₂ compressor were carried out at various inlet temperatures and various mass flow rates. When the sCO₂ compressor operates at low volume flow rate, the flow loss is generated mainly on the suction side near the trailing edge of the blade. The flow loss is related to the counterclockwise vortexes generated on the suction side of the main blade. The vortexes are caused by the flow separation in the downstream region of the impeller passage, which is different from air compressors operating at low flow rates. The reason for this flow separation is that the effect of Coriolis force is especially severe for the sCO₂ fluid, compared to the viscous force and inertial force. At lower flow rates, with the stronger effect of Coriolis force, the direction of relative flow velocity deviates from the direction of radius, resulting in its lower radial component. The lower radial relative flow velocity leads to severe flow separation on the suction side near the trailing edge of the main blade.

This study investigated the effects of zigzag-flow channel bending angle in printed circuit heat exchangers (PCHEs) using a computational fluid dynamics method with ANSYS-FLUENT simulation. The three-dimensional model of PCHE with a 15° curved, zigzag channel was conducted for preliminary validation. The comparisons between the CFD simulation results and the experimental data showed good agreement with some discrepancies in the heat transfer and pressure drop results. In addition, different bending angle configurations (0°, 3° to 30°) of zigzag channels were analyzed to obtain better thermal-hydraulic performance of the zigzag channel PCHE under different inlet mass flow rates. The criteria of heat transfer and frictional factor were applied to evaluate the thermal-hydraulic performance of the PCHE. The results showed that the 6° and 9° bending channel provided good thermal-hydraulic performance. New correlations were developed using the 6° and 9° bending channel angles in PCHE designs to predict the Nusselt number and friction factor.

This paper experimentally studied the effect of CO₂ opposing multiple jets on the thermoacoustic instability and NO_x emissions in a lean-premixed model combustor. The feasibility was verified from three variables: the CO₂ jet flow rate, hole numbers, and hole diameters of the nozzles. Results indicate that the control effect of thermoacoustic instability and NO_x emissions show a reverse trend with the increase of open area ratio on the whole, and the optimal jet flow rate range is 1–4 L/min with CO₂ opposing multiple jets. In this flow rate range, the amplitude and frequency of the dynamic pressure and heat release signals CH^* basically decrease as the CO₂ flow rate increases, which avoids high-frequency and high-amplitude thermoacoustic instability. The amplitude-damped ratio of dynamic pressure and CH^* can reach as high as 98.75% and 93.64% with an optimal open area ratio of 3.72%. NO_x emissions also decrease as the jet flow rate increases, and the maximum suppression ratio can reach 68.14%. Besides, the flame shape changes from a steep inverted “V” to a more flat “M”, and the flame length will become shorter with CO₂ opposing multiple jets. This research achieved the synchronous control of thermoacoustic instability and NO_x emissions, which could be a design reference for constructing a safer and cleaner combustor.

As the interaction between the combustor and the turbine in the aero-engine continues to increase, the film cooling design considering the combustor swirling outflow has become the research focus. The swirling inflow and high-temperature gas first affect the vane leading edge (LE). However, no practical improved solution for the LE cooling design has been proposed considering the combustor swirling outflow. In this paper, the improved scheme of showerhead cooling is carried out around the two ways of adopting the laid-back-fan-shaped hole and reducing the coolant outflow angle. The film cooling effectiveness (η) and the coolant flow state are obtained by PSP (pressure-sensitive-paint) and numerical simulation methods, respectively. The research results show that the swirling inflow increases the film distribution inhomogeneity by imposing the radial pressure gradient on the vane to make the film excessively gather in some positions. The showerhead film cooling adopts the laid-back-fan-shaped hole to reduce the momentum when the coolant flows out. Although this cooling scheme improves the film attachment and increases the surface-averaged film cooling effectiveness (η_sur) by as much as 15.4%, the film distribution inhomogeneity increases. After reducing the coolant outlet angle, the wall-tangential velocity of the coolant increases, and the wall-normal velocity decreases. Under the swirl intake condition, both η and the film distribution uniformity are significantly increased, and the growth of η_sur is up to 16.5%. This paper investigates two improved schemes to improve the showerhead cooling under the swirl intake condition to provide a reference for the vane cooling design.

Staged combustion of biomass is the most suitable thermo-chemical conversion for achieving lower gaseous emissions and higher fuel conversion rates. In a staged fixed bed combustion of biomass, combustion air is supplied in two stages. In the first stage, primary air is provided below the fuel, whereas in the later stage, secondary air is supplied in the freeboard region. The available literature on the effects of air staging (secondary air location) at a constant primary air flow rate on combustion characteristics in a batch-type fixed bed combustor is limited and hence warrants further investigations. This study resolves the effect of air staging, by varying the location of secondary air in the freeboard at five secondary to total air ratios in a batch-type fixed bed combustor. Results are reported for the effects of these controlled parameters on fuel conversion rate, overall gaseous emissions (CO₂, CO and NO_x) and temperature distributions. The fuel used throughout was densified hardwood pellets.
Results show that a primary freeboard length (distance between fuel bed top and secondary air injection) of 200 mm has higher fuel conversion rates and temperatures as well as lower CO emissions, at a secondary to total air ratio of 0.75 as compared to primary freeboard length of 300 mm. However, NO_x emissions were found to be lower for a primary freeboard length of 300 mm as compared to 200 mm. An increase in secondary to total air ratio from 0.33 to 0.75 resulted in higher freeboard temperatures and lower CO as well as NO_x emissions. The outcomes of this study will be helpful in the effective design of commercial scale biomass combustors for more efficient and environmentally friendly combustion.

Thermal energy storage (TES) systems use solar energy despite its irregular availability and day-night temperature difference. Current work reports the thermal characterizations of solar salt-based phase change composites in the presence of graphene nanoplatelets (GNP). Solar salt (60:40 of NaNO₃:KNO₃) possessing phase transition temperature and melting enthalpy of 221.01°C and 134.58 kJ/kg is proposed as a phase change material (PCM) for high-temperature solar-based energy storage applications. Thermal conductivity must be improved to make them suitable for widespread applications and to close the gap between the system needs where they are employed. GNP is added at weight concentrations of 0.1%, 0.3%, and 0.5% with solar salt using the ball milling method to boost its thermal conductivity. Morphological studies indicated the formation of a uniform surface of GNP on solar salt. FTIR spectrum peaks identified the physical interaction between salt and GNP.  Thermal characterization of the composites, such as thermal conductivity, DSC and TGA was carried out for the samples earlier and later 300 thermal cycles. 0.5% of GNP has improved the thermal conductivity of salt by 129.67% and after thermal cycling, the enhancement reduced to 125.21% indicating that thermal cycling has a minor impact on thermal conductivity. Phase change temperature decreased by around 2.32% in the presence of 0.5% GNP and the latent heat reduced by 4.34% after thermal cycling. TGA thermograms depicted the composites initiated the weight loss at around 550°C after which it was rapid. After thermal cycling, the weight loss initiated at ~40°C lower compared to pure salt, which was found to be a minor change. Thermal characterization of solar salt and GNP-based solar salt composites revealed that the composites can be used for enhanced heat transfer in high-temperature solar-based heat transfer and energy storage applications.

Condensation temperature is one of the crucial parameters determining the performance of an organic Rankine cycle. It is necessary to consider the differences in the working fluids themselves when setting the condensation temperature of organic Rankine cycle. In current study, temperature-entropy (T-s) diagram is employed to describe the difference in working fluids. Three areas of dry and isentropic fluids in a temperature-entropy (T-s) diagram, which are the area denoting net output work of cycle (A₁), the area denoting net output work of the Carnot cycle (A), and the curved triangle in superheated region denoting condensation characteristics (A₂), are defined. On this basis, the ratio of A₂ to A1 and the ratio of A₁ to A are calculated. Logarithmic Mean Difference of the above two ratios is obtained to determine the operable ideal condensation temperature of 66 dry and isentropic fluids employed in Organic Rankine Cycle. The findings indicate that the operable ideal condensation temperatures for the majority of dry and isentropic fluids are in the range of 305 K to 310 K. The work presented in this study may be useful for designing and establishing an Organic Rankine Cycle system.

Nitrogen oxides (NO_x) from cement industry have drawn more and more attention and the existing denitrification technologies can hardly meet the increasingly stringent emission requirements in China. In our previous work, fuel in-situ reduction (FISR) method was proposed to cut cement NO_x emission. With the pilot-scale precalciner in the previous experiment as objection, optimization of FISR method was conducted using CFD method. The results demonstrated that NO_x emission decreased by 69.86% after adopting FISR method. The effects of initial concentrations of NO and O₂ in kiln gas, feeding location of the first-stage tertiary (tertiary air-I) and cement raw meal (CRM) were further investigated. With increasing initial NO concentration, NO_x emission increased linearly, while the reduction rate of NO in kiln gas maintained above 80%. When O₂ content in kiln gas is more than 4%, oxygen would more significantly promote the formation of NO_x and inhibit the reduction of NO. The dimensionless locations of tertiary air-I and CRM were introduced. The simulation results showed that the optimal dimensionless locations are 0.6 and 1.6 for tertiary air-I and CRM, respectively. The outputs achieved in this study will provide a strong support for the practical application of FISR method in cement industry.

Thermal energy storage (TES) using phase change materials (PCMs) is a powerful solution to the improvement of energy efficiency. The application of Ammonium alum (A-alum, NH₄Al(SO₄)₂·12H₂O) in the latent thermal energy storage (LTES) systems is hampered due to its high supercooling and low thermal conductivity. In this work, modified A-alum (M-PCM) containing different nucleating agents was prepared and further adsorbed in expanded graphite (EG) to obtain composite phase change material (CPCM) to overcome the disadvantages of A-alum. Thermal properties, thermal cycle stability, microstructure and chemical compatibility of CPCM were characterized by differential scanning calorimetry, thermal constant analysis, scanning electron microscopy, X-ray diffraction and Fourier transform infrared spectroscopy. The cold rewarming phenomenon of CPCM was established and explained. Results showed that the latent heat and melting point of CPCM were 187.22 J/g and 91.54°C, respectively. The supercooling of CPCM decreased by 9.61°C, and thermal conductivity increased by 27 times compared with pure A-alum. Heat storage and release tests indicated that 2 wt% calcium chloride dihydrate (CCD, CaCl₂·2H₂O) was the optimum nucleating agent for A-alum. The result of TG and 30 thermal cycles revealed that CPCM exhibited favorable thermal stability and reliability during the operating temperature. The prepared modified A-alum/EG CPCM has a promising application prospect for LTES.

Barocaloric refrigeration technology, one of the caloric-effect refrigeration technologies, is drawing more and more attention. Neopentyl glycol (NPG) was reported to have a giant barocaloric effect, making it a potential barocaloric material. However, the high solid-solid (S-S) phase transition temperature and low thermal conductivity hinder the application of NPG in barocaloric refrigeration. This work lowers the S-S phase transition temperature and improves the thermal conductivity of the NPG-based barocaloric material. An NPG/TMP (TMP: Trimethylolpropane) binary system with an S-S phase transition temperature of 283.15 K is prepared, in which the mass ratio of TMP is 20%. Graphene is then added to the binary system to enhance thermal conductivity, and the optimal mass ratio of graphene was determined to be 5%. The thermal conductivity of this composite is 0.4 W/(m·K), increased by 110% compared to the binary system. To predict the effect of enhanced thermal conductivity on the cold-extraction process of the barocaloric refrigeration cycle, a numerical model is developed. The results show that the cold-extraction time of the barocaloric refrigeration cycle utilizing the composite with 5% graphene as the refrigerant is shortened by 50% compared with that using the binary system.

Multi-fidelity simulations incorporate computational fluid dynamics (CFD) models into a thermodynamic model, enabling the simulation of the overall performance of an entire gas turbine with high-fidelity components. Traditional iterative coupled methods rely on characteristic maps, while fully coupled methods directly incorporate high-fidelity simulations. However, fully coupled methods face challenges in simulating rotating components, including weak convergence and complex implementation. To address these challenges, a fully coupled method with logarithmic transformations was developed to directly integrate high-fidelity CFD models of multiple rotating components. The developed fully coupled method was then applied to evaluate the overall performance of a KJ66 micro gas turbine across various off-design simulations. The developed fully coupled method was also compared with the traditional iterative coupled method. Furthermore, experimental data from ground tests were conducted to verify its effectiveness. The convergence history indicated that the proposed fully coupled method exhibited stable convergence, even under far-off-design simulations. The experimental verification demonstrated that the multi-fidelity simulation with the fully coupled method achieved high accuracy in off-design conditions. Further analysis revealed inherent differences in the coupling methods of CFD models between the developed fully coupled and traditional iterative coupled methods. These inherent differences provide valuable insights for reducing errors between the component-level model and CFD models in different coupling methods. The developed fully coupled method, introducing logarithmic transformations, offers more realistic support for the detailed and optimal design of high-fidelity rotating components within the overall performance platform of gas turbines.

With the development of detection and identification technology, infrared stealth is of great value to realize anti-reconnaissance detection of military targets. At present, infrared stealth materials generally have deficiency, such as complex structure, inconvenient radiation regulation and cumbersome preparation steps, which greatly limit the practical application of infrared stealth materials. In view of the above deficiency of infrared stealth materials, we proposed a kind of multilayer film for infrared stealth using VO₂ thermochromism based on the temperature response mechanism of tuna to adjust its color, and through the intelligent reversible radiation regulation mechanism to meet the infrared stealth requirements of tanks in different actual scenes. The results show that when the temperature increases from 300 K to 373 K, the peak emissivity of the film decreases from 94% to 20% in the 8–14 μm band after structural optimization, which can realize the infrared stealth of the high temperature target in the 8–14 μm band. The average emissivity of the multilayer film for infrared stealth at 3–5 μm and 8–14 μm band can be reduced to 34% and 27% at 373 K, and the peak emissivity at 5–8 μm band can reach 65%, realizing dual-band infrared stealth in the 3–5 μm and 8–14 μm bands and heat dissipation in the 5–8 μm band. The multilayer film for infrared stealth based on VO₂ thermochromism designed by the authors can meet the characteristics of simple film structure, convenient radiation regulation and simple preparation.

The external surface heat transfer coefficient of building envelope is one of the important parameters necessary for building energy saving design, but the basic data in high-altitude area are scarce. Therefore, the authors propose a modified measurement method based on the heat balance of a model building, and use the same model building to measure its external surface heat transfer coefficient under outdoor conditions in Chengdu city, China at an altitude of 520 m and Daocheng city at an altitude of 3750 m respectively. The results show that the total heat transfer coefficient (h_t) of building surface in high-altitude area is reduced by 34.48%. The influence of outdoor wind speed on the convective heat transfer coefficient (h_c) in high-altitude area is not as significant as that in low-altitude area. The fitting relation between convection heat transfer coefficient and outdoor wind speed is also obtained. Under the same heating power, the average temperature rise of indoor and outdoor air at high- altitude is 41.9% higher than that at low altitude, and the average temperature rise of inner wall is 25.8% higher than that at low altitude. It shows that high-altitude area can create a more comfortable indoor thermal environment than low-altitude area under the same energy consumption condition. It is not appropriate to use the heat transfer characteristics of the exterior surface of buildings in low-altitude area for building energy saving design and related heating equipment selection and system terminal matching design in high-altitude area.