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.

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.

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.

The supercritical CO₂ Brayton cycle has potential to be used in electricity generation occasions with its advantages of high efficiency and compact structure. Focusing on a so-called self-condensing CO₂ transcritical power cycle, a model was established and four different layouts of heat recuperation process were analyzed, a without-recuperation cycle, a post-recuperation cycle, a pre-recuperation cycle and a re-recuperation cycle. The results showed that the internal normal cycle’s share of the whole cycle increases with increasing the cooling pressure and decreasing the final cooled temperature. Heat load in the supercritical heater decreases with increasing the cooling pressure. From perspective of performance, the re-recuperation cycle and the pre-recuperation cycle have similar thermal efficiency which is much higher than other two layouts. Both thermal efficiency and net power output have a maximum value with the cooling pressure, except in the condition with the final cooled temperature of 31°C. Considering both the complexity and the economy, the pre-recuperation cycle is more applicable than the other options. Under 35°C of the final cooled temperature, the thermal efficiency of the pre-recuperation cycle reaches the peak 0.34 with the cooling pressure of 8.4 MPa and the maximum net power output is 2355.24 kW at 8.2 MPa of the cooling pressure.

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.

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.

A tandem blade configuration is a significant flow control method that delays the onset of flow separation. This study numerically investigates the effects of diffusion factor and percentage pitch on the flow structure of tandem blades. Diffusion factors vary from 0.328 to 0.484. Percentage pitches change from 0.80 to 0.92. Results show that the loss coefficient increases with diffusion factor and decreases with percentage pitch. There is a hub-corner stall of the forward blade in all cases. Gap flow determines the rear blade corner separation. Varying the percentage pitch and diffusion factor changes the momentum distribution of the gap flow. Corner separation of the rear blade is inhibited as low-momentum gap fluids are involved in the passage vortex along with the hub-corner stall of the forward blade. Increasing diffusion factor causes a change in incidence at the leading edge of the rear blade, resulting in a variation at the corner separation of the rear blade. A tandem blade is compared with the reference outlet vane. The performance of the tandem blade is superior to that of the reference outlet vane in all incidences, with a 26.35% reduction in the loss coefficient and a 7.89% enhancement in the pressurization at the designed incidence. Tandem blades stall at positive incidence because of the hub-corner stall of the forward blade. The intensity of the gap flow increases with incidence, preventing corner separation of the rear blade at positive incidences.

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.

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. 

In this paper, a novel ternary eutectic salt Na₂CO₃-Li₂CO₃-LiF was designed and investigated for concentrated solar power (CSP). The FactSage software was used to predict the composition and eutectic point of Na₂CO₃-Li₂CO₃-LiF. The microstructure, thermophysical properties, and thermal stability of eutectic salts were experimentally measured using various analytical methods. With a mass ratio of 57%:32%:11%, the eutectic salt exhibited excellent thermal storage properties with a fusion enthalpy of 413 J/g and a melting point of 426.8°C. The excellent thermal stability of the eutectic salt was reflected by a weight loss of only 0.8% at 600°C.

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.

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.

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.

In this paper, a novel NH₃/CO₂ ejector-cascade refrigeration system with regenerator is proposed, which can recycle the waste heat at the outlet of the compressor. After establishing the mathematical model of the system, the theoretical energy and exergy analysis are carried out and compared with the conventional cascade refrigeration system. It is concluded that compared with the conventional cascade refrigeration system, the novel ejector-cascade refrigeration system with regenerator has the advantages of less power consumption of the compressor, less component exergy destruction, high system performance, and is more suitable for working at a lower temperature. Under the working conditions studied in this paper, compared with the conventional cascade refrigeration system, the COP of the novel ejector-cascade refrigeration system with regenerator is increased by 9.58%; the exergy efficiency is increased by 9.50%, and the optimal evaporation temperature is about –45°C.

To address the pressing need for intelligent and efficient control of circulating fluidized bed (CFB) units, it is crucial to develop a dynamic model for the key operating parameters of supercritical circulating fluidized bed (SCFB) units. Therefore, data-knowledge-driven dynamic model of bed temperature, load, and main steam pressure of the SCFB unit has been proposed. Firstly, a knowledge-driven method is employed to develop a dynamic model for key operating parameters of SCFB units. The model parameters are determined based on the operating data of the unit and continuously optimized in real time. Then, Bidirectional Long Short-Term Memory combined with Convolutional Neural Network and Attention Mechanism is utilized to build the dynamic model of bed temperature, load, and main steam pressure. Finally, a collaboration and integration method based on the critic weight method and the variation coefficient method is proposed to establish data-knowledge-driven model of key operating parameters for SCFB units. The model displays great accuracy and fitting ability compared with other methods and effectively captures the dynamic characteristics, which can provide a research basis for the design of intelligent flexible control mode of SCFB unit.

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.

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 complementary of biomass and solar energy in combined cooling, heating and power (CCHP) system provides an efficient solution to address the energy crisis and environmental pollutants. This work aims to propose a multi-objective optimization model based on the life cycle assessment (LCA) method for the optimal design of hybrid solar and biomass system. The life-cycle process of the poly-generation system is divided into six phases to analyze energy consumption and greenhouse gas emissions. The comprehensive performances of the hybrid system are optimized by incorporating the evaluation criteria, including environmental impact in the whole life cycle, renewable energy contribution and economic benefit. The non-dominated sorting genetic algorithm II (NSGA-II) with the technique for order preference by similarity to ideal solution (TOPSIS) method is employed to search the Pareto frontier result and thereby achieve optimal performance. The developed optimization methodology is used for a case study in an industrial park. The results indicate that the best performance from the optimized hybrid system is reached with the environmental impact load reduction rate (EILRR) of 46.03%, renewable energy contribution proportion (RECP) of 92.73% and annual total cost saving rate (ATCSR) of 35.75%, respectively. By comparing pollutant-eq emissions of different stages, the operation phase emits the largest pollutant followed by the phase of raw material acquisition. Overall, this study reveals that the proposed multi-objective optimization model integrated with LCA method delivers an alternative path for the design and optimization of more sustainable CCHP system.

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.

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.

Supercritical carbon dioxide (SCO₂) centrifugal compressor is a key component of a closed Brayton cycle system based on SCO₂. A comprehensive understanding of the loss mechanism within the compressor is vital for its optimized design. However, the physical properties of SCO₂ are highly nonlinear near the critical point, and the internal flow of the compressor is closely related to its properties, which inevitably influences the generation of aerodynamic losses within the compressor. This paper presents a comprehensive investigation of the compressor’s loss mechanism with an experimentally validated numerical method. The real gas model of CO₂ embodied in the Reynolds-Averaged Navier-Stokes (RANS) model was used for the study. Firstly, the numerical simulation method was validated against the experimental results of Sandia SCO₂ compressor. Secondly, performance and loss distribution of the compressor were compared among three fluids including SCO₂, ideal CO₂ (ICO₂) and ideal air (IAir). The results showed that the performance of SCO₂ was comparable to IAir under low flow coefficient, however markedly inferior to the other two fluids at near choke condition. Loss distribution among the three fluids was distinctive. In the impeller, SCO₂ was the most inefficient, followed by ICO₂ and IAir. The discrepancies were magnified as the flow coefficient increased. This is due to a stronger Blade-to-Blade pressure gradient that intensifies boundary layer accumulation on walls of the shroud/hub. Furthermore, owing to the reduced sonic speed of SCO₂, a shock wave appears earlier at the throat region and SCO₂ encounters more intense boundary layer separation.

Coal-fired power plant is a major contributor to greenhouse gas emissions. The post-combustion capture is a promising method for CO₂ emission reduction but the high thermal demand is unbearable. To address this issue, solar thermal energy and CO₂ capture are jointly integrated into the coal-fired power plant in this study. The solar thermal energy is employed to meet the heat requirement of the CO₂ capture process, thereby avoiding the electricity loss caused by self-driven CO₂ capture. Furthermore, the heat released from the carbonation reaction of MgO adsorbent is integrated into the steam Rankine cycle. By partially substituting the extracted steam for feedwater heating, the electricity output of the power plant is further increased. According to the results from the developed model, the system could achieve a CO₂ capture rate of 86.5% and an electricity output enhancement of 9.8% compared to the reference system, which consists of a self-driven CO₂ capture coal-fired power plant and PV generation unit. The operational strategy is also optimized and the amount of CO₂ emission reduction on a typical day is increased by 11.06%. This work shows a way to combine fossil fuels and renewable energy for low carbon emissions and efficient power generation.

In this paper, a numerical simulation method is used to calculate a 1.5-stage axial transonic compressor to explore its unsteady flow mechanism. The performance curve is compared with the experimental data to verify the calculation method with a high numerical accuracy, which shows that the unsteady calculation has good reliability. According to the analysis of the data from the monitoring points under the near-stall condition, the unsteady disturbances originate from the tip region of blade and perform the strongest at the blade pressure surface with a broadband characteristic. Further analysis is conducted by combining with the characteristics of the transient flow field at the tip of blade. The results show that the unsteady pressure fluctuations are caused by the migration of the new vortex cores. These new vortex cores are generated by the breakdown of leakage vortex in the downstream, which is induced by the leakage vortex and shock wave interference. Moreover, the relationship between the unsteady flow characteristics and the working conditions is also studied. The leakage vortex intensity and the shock wave strength gradually increase with the decrease of flow rate. When the combination of the leakage vortex intensity and shock wave strength reaches the first threshold, a single frequency of unsteady disturbances appears at the blade tip. When the combination of the leakage vortex intensity and shock wave strength reaches the second threshold, the frequency of unsteady disturbances changes to a broadband.

Supercritical carbon dioxide printed circuit board heat exchangers are expected to be applied in third-generation solar thermal power generation. However, the uniformity of supercritical carbon dioxide entering the heat exchanger has a significant impact on the overall performance of the heat exchanger. In order to improve the uniformity of flow distribution in the inlet header, this article studies and optimizes the inlet header of a printed circuit board heat exchanger through numerical simulation. The results indicate that when supercritical carbon dioxide flows through the header cavity, eddy currents will be generated, which will increase the uneven distribution of flow rate, while reducing the generation of eddy currents will improve the uniform distribution of flow rate. When the dimensionless factor of the inlet header is 6, the hyperbolic configuration is the optimal structure. We also reduced the eddy current region by adding transition segments, and the results showed that the structure was the best when the dilation angle was 10°, which reduced the non-uniformity by 21% compared to the hyperbolic configuration, providing guidance for engineering practice.

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.

Supersonic wind tunnel experiment is one of the important measurements for developing advanced gas turbines, and supersonic multi-hole probes are sophisticated tools to measure pneumatic parameters in such experiments. However, shock waves form around the probe head in supersonic flow, which affect the accuracy of results. In this study, a supersonic five-hole probe is selected as the research object. Firstly, a compound five-hole pressure-temperature probe was designed and produced with 3D-printing technology. Then, the shock wave spectrum was numerically calculated by three methods, which were the Mach number, density gradient, and shock function; in contrast to the other two methods, the shock function could accurately identify the types and ranges of shock and expansion waves. The results show that a strong shock wave is formed at the front section of the probe head, and the shock wave generated around the pressure measuring tube affects the total pressure and Mach number of the flow field, which causes the increase of entropy. The intensity of the shock wave at the head of the pressure measuring tube is the largest, causing a decrease in the total pressure around the flow field. Afterwards, to reduce the calculation errors caused by neglecting the compressibility of gases and the entropy increase, a gas compression factor δ_s was introduced. It is proved that the error of the calculated pneumatic parameters is less than 5% and 10% in subsonic and supersonic condition, respectively, with the gas compression factor considered. The research results of this paper provide theoretical reference for the design and use of pneumatic probes during subsonic to supersonic flow.

Experimental analysis was conducted to study the impact of fuel-air mixing and dilution jet on the temperature distribution in a small gas turbine combustor using various optical diagnostic techniques. The strength and velocity of the swirler at the venturi exit were adjusted to modify the fuel-air mixture, which is presumed to dominate the heat release of the main combustion zone. Additionally, the dilution hole configuration, including the number and size of the holes, was varied to investigate the dilution effect on outlet temperature distribution. Various optical diagnostic techniques, such as particle image velocimetry, planar Mie scattering, and OH^* chemiluminescence, were used to measure the flow field, fuel spray distribution, and flame structure, respectively. A reduction in swirling strength led to a decrease in the average flow rate in the throat, which improved the structure and symmetry of the axial vortex system in the sleeve, enhanced the mixing of fuel and gas in the dome swirling air, and ultimately, improved the temperature uniformity of the heat release zone. Compared to larger and sparse dilution jets, smaller and dense dilution jets tended to generate hot spots shifted towards the radial middle area.

Thermochemical recuperation heat recovery is an advanced waste heat utilization technology that can effectively recover exhaust waste heat from oxy-fuel Stirling engines. The novel combustor of a Stirling engine with thermochemical recuperation heat recovery system is expected to utilize both reformed gas and diesel fuels as sources of combustion. In this research, the effects of various factors, including the H₂O addition, fuel distribution ratio (FDR), excess oxygen coefficient, and cyclone structure on the temperature distribution in the combustor, combustion emissions, and external combustion system efficiency of the Stirling engine were experimentally investigated. With the increase of steam-to-carbon ratio (S/C), the temperature difference between the upper and lower heating tubes reduces and the circumferential temperature fluctuation decreases, and the combustion of diesel and reformed gas remains close to complete combustion. At S/C=2, the external combustion efficiency is 80.6%, indicating a 1.6% decrease compared to conventional combustion. With the increase of FDR, the temperature uniformity of the heater tube is improved, and the CO and HC emissions decrease. However, the impact of the FDR on the maximum temperature difference and temperature fluctuation across the heater is insignificant. When the FDR rises from 21% to 38%, the external combustion efficiency increases from 87.4% to 92.3%. The excess oxygen coefficient plays a secondary role in influencing temperature uniformity and temperature difference, and the reformed gas and diesel fuel can be burned efficiently at a low excess oxygen coefficient of 1.04. With an increase in the cyclone angle, the heater tube temperature increases, while the maximum temperature difference at the lower part decreases, and the temperature fluctuation increases. Simultaneously, the CO and HC emissions increase, and the external combustion efficiency experiences a decrease. A cyclone angle of 30° is found to be an appropriate value for achieving optimal mixing between reformed gas and diesel fuel. The research findings present valuable new insights that can be utilized to enhance the performance optimization of Stirling engines.

In order to reduce the environmental impact of conventional sludge treatment methods and to utilize the energy in sludge more effectively, a coupled system based on sewage sludge gasifier (SSG), solid oxide fuel cells (SOFC), supercritical CO₂ cycle (S-CO₂), and organic Rankine cycle (ORC) is proposed. The clean syngas obtained from sludge gasification is mixed with CH₄ and then first utilized by the fuel cell. The exhaust gas from the fuel cell is fully combusted in the afterburning chamber and then enters the bottom cycle system consisting of S-CO₂ & ORC to generate electricity. To understand the performance of the system, thermodynamic and economic analyses were conducted to examine the project’s performance. The thermodynamics as well as the economics of the coupled system were analyzed to arrive at the following conclusions, the power production of the system is 37.34 MW; the exergy efficiency is 55.62%, and the net electrical efficiency is 61.48%. The main exergy destruction is the gasifier and SOFC, accounting for 62.45% of the total exergy destruction. It takes only 6.13 years to repay the construction investment in the novel system, and the project obtains a NPV of 17 723 820 USD during 20 years lifetime. The above findings indicate that the new coupled system has a better performance in terms of energy utilization and economy.

Humid air turbine cycle (HAT) has potential of electrical efficiencies comparable to combined cycle, with lower investment cost and NOx emission. The typical heat exchanger network of HAT consists of intercooler (if there is), aftercooler, recuperator, economizer and humidifier, which brings higher efficiency but makes the system more complex. To simplify HAT layout, a novel humidifier concept is proposed by integrating the aftercooler into traditional counter-current humidifier. Based on this concept, a one-dimensional model including pressure drop and exergy calculation is established to distinguish the thermodynamic and hydrodynamic characteristics, and then the structural parameters, such as the number of rows and columns, tube diameter, pitch and type for a micro HAT are identified. The results show that the aftercool-humidifier plays the same role as original aftercooler and humidifier, and can match the in-tube air, out-tube air and water stream well with lower volume. In the case of micro HAT cycle, the volume of heat and mass transfer area can be reduced by 47% compared with traditional design. The major thermal resistance occurred in the convection heat transfer process inside the tube; however, using enhanced tube cannot effectively improve the compactness of device.

This paper studied the thermal physical properties of foundation materials in the molten salt tank of thermal energy storage system after molten salt leakage by Transient plane source experiment and X-ray computed microtomography simulation methods. The microstructure, thermal properties and pressure resistance with different particle diameters were addressed. The measured heat conductivities from Transient plane source experiment for three cases are 0.49 W/(m∙K), 0.48 W/(m∙K), and 0.51 W/(m∙K), and the porosity is 30.1%, 30.7%, and 31.2% respectively. The heat conductivity simulating results of three cases are 0.471 W/(m∙K), 0.482 W/(m∙K), and 0.513 W/(m∙K). The ratio of difference between the results of simulation and Transient plane source measurement is as low as 1.2%, verifying the reliability of experimental and simulation results to a certain degree. Compared with the heat conductivity of 0.097–0.129 W/(m∙K) and porosity of 71.6%–78.9% without leaking salt, the porosity is reduced by more than 50% while the heat conductivity increased by 4 to 5 times after molten salt leakage. This significant increase in heat conductivity has a great impact on security operation, structure design, and modeling of the tank foundation for solar power plants.

The steam ejector is a crucial component in the waste heat recovery system. Its performance determines the amount of recovered heat and system efficiency. However, poor ejector performance has always been the main bottleneck for system applications. Therefore, this study proposes an optimization methodology to improve the steam ejector’s performance by utilizing computational fluid dynamics (CFD) techniques, response surface methodology (RSM), and genetic algorithm (GA). Firstly, a homogeneous equilibrium model (HEM) was established to simulate the two-phase flow in the steam ejector. Then, the orthogonal test was presented to the screening of the key decision variables that have a significant impact on the entrainment ratio (ER). Next, the RSM was used to fit a response surface regression model (RSRM). Meanwhile, the effect of the interaction of geometric parameters on the performance of the steam ejector was revealed. Finally, GA was employed to solve the RSRM’s global optimal ER value. The results show that the RSRM exhibits a good fit for ER (R²=0.997). After RSM and GA optimization, the maximum ejector efficiency is 27.94%, which is 48.38% higher than the initial ejector of 18.83%. Furthermore, the optimized ejector efficiency is increased by 46.4% on average under off-design conditions. Overall, the results reveal that the combination of CFD, RSM, and GA presents excellent reliability and feasibility in the optimization design of a two-phase steam ejector.

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.

Thermal energy storage (TES) is of great importance in solving the mismatch between energy production and consumption. In this regard, choosing type of Phase Change Materials (PCMs) which are widely used to control heat in latent thermal energy storage systems, plays a vital role as a means of TES efficiency. However, this field suffers from lack of a comprehensive investigation on the impact of various PCMs in terms of exergy. To address this issue, in this study, in addition to indicating the melting temperature and latent heat of various PCMs, the exergy destruction and exergy efficiency of each material are estimated and compared with each other. Moreover, in the present work the impact of PCMs mass and ambient temperature on the exergy efficiency is evaluated. The results proved that higher latent heat does not necessarily lead to higher exergy efficiency. Furthermore, to obtain a suitable exergy efficiency, the specific heat capacity and melting temperature of the PCMs must also be considered. According to the results, LiF-CaF₂ (80.5%:19.5%, mass ratio) mixture led to better performance with satisfactory exergy efficiency (98.84%) and notably lower required mass compared to other PCMs. Additionally, the highest and lowest exergy destruction are belonged to GR25 and LiF-CaF₂ (80.5:19.5) mixture, respectively.

Dodecyl alcohol (DDA) is a promising solid-liquid phase change material (PCM) due to its favorable latent heat storage (LHS) characteristics. However, the leakage issue of PCM in a melted state during the heating period and low thermal conductivity restricts its utilization potential in thermal energy storage (TES) practices. Within the same context, the present work aims to overcome the leakage issue and improve the thermal conductivity of the DDA. With this in mind, a novel leak-proof layered double hydroxide (LDH)/DDA composite PCM is proposed through a solution-based impregnation method. The leak-proof impregnation ratio of the DDA impregnated within the cavities of the synthesized Al/Fe-LDH was determined to be 60%. Detailed morphological, physicochemical, and thermal properties of the fabricated composite were studied by scanning electron microscopy (SEM), Fourier transforms infrared (FTIR) spectroscopy, X-ray diffraction (XRD) spectroscopy, differential scanning calorimetry (DSC), thermalgravimetric analysis (TGA), and thermal cycling study. The results show that the LDH/DDA composite has a suitable phase change temperature (about 20°C) for passive solar thermal management of building envelopes. This composite PCM showed high LHS enthalpy (about 136 J/g), good thermal stability, and cycling LHS reliability. It also showed nearly 152% higher thermal conductivity compared to that of pure DDA, ultimately reducing the melting and solidification time of the pure DDA by 44.9% and 45.5%, respectively.

Although extensive efforts have been made to dampen the thermoacoustic instability, successfully controlling the pressure oscillations in modern gas turbines or aeroengines remains challenging. The influence of the acoustic liner on the longitudinal thermoacoustic mode in a model annular combustor is investigated by Large Eddy Simulation (LES) in this work. The result of the self-excited longitudinal thermoacoustic instability without the liner agrees well with the frequency and acoustic analysis of the pressure mode based on experimental data. Three different bias flow velocities of the liner located downstream of the combustor are then simulated. The results reveal that the existence of the liner influences not only the acoustic field but also the flow field. When the bias velocity is large, it leads to intense turbulence-induced fluctuations, and the pressure oscillation is modulated intermittently. It shows that the weak coupling between flow and pressure oscillations plays a significant role in the onset of the intermittency of a thermoacoustic system. Based on the dynamic analysis of the thermoacoustic system with the acoustic liner, this intermittency is caused by the influence of the flow field on the flame-acoustic coupling. Finally, a low-order modeling method based on Van der Pol (VdP) oscillator with additive stochastic forcing is conducted to reproduce the evolving dynamics of the thermoacoustic system. Although the numerical cases demonstrated in this work are relatively simpler than those in a practical combustion system, the results are helpful for us to understand the effect of the acoustic liner and show the attractive potential to apply this device to suppress thermoacoustic instability.

A pair of unidirectional turbines (UT) can operate in oscillatory airflow without additional units. However, this arrangement suffers from poor flow rectification. A fluidic diode (FD) offers variable hydrodynamic resistance based on the flow direction, and this can be coupled with UT to improve flow rectification. In this work, a numerical investigation on the effect of FD with UT is presented using the commercial fluid dynamics software ANSYS Fluent 16.1 with k-ω SST turbulence closure model. Periodic domains of UT and FD are numerically validated individually with experimental results. Later, both are coupled to obtain the combined effect, and these results are compared with the analytical approach. It was observed that coupling FD with UT improved the unit’s performance at the lower flow coefficient (<1), but its performance decreased as the flow coefficient increased. Due to the diode’s presence, fluid leaving the turbine experiences higher resistance at a higher flow coefficient, which decreases the overall performance of the combined unit.

Flow around a pair of flat plates is a basic hydrodynamics problem. In this paper, the flow and heat transfer characteristics of two parallel plates with different edge shapes are numerically calculated. Under different inclined angles, the influence of chamfered and rounded structures with different sizes at the end-edge on unsteady flow and heat transfer characteristics of two parallel plates are analyzed. It is found that the instability and unsteadiness of flow decrease with the increase of end-edge size, and the non-uniformity of wake velocity of both rounded and chamfered plates decreases gradually. The non-uniformity of wake temperature increases firstly and then decreases at a small inclined angle, and the amplitude becomes the largest when S_rou(S_cha)=3, while it basically keeps monotonically increasing at a large inclined angle. Moreover, the global heat transfer performance of the flat plate is obviously affected by the end-edge modification, especially the chamfered structure. With the increase of chamfered size, the global Nusselt number basically shows the decreasing trend. This study provides a theoretical basis for the application of plate-shape structure in engineering fields.