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.

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.

Amorphous hafnium dioxide (a-HfO₂) has attracted increasing interest in the application of semiconductor devices due to its high dielectric constant. However, the thermal transport properties of a-HfO₂ are not well understood, which hinders its potential application in electronics. In this work, we systematically investigate the thermal transport property of a-HfO₂ using the molecular dynamics method. The non-equilibrium molecular dynamics simulations reveal that the thermal conductivity of a-HfO₂ is length-dependent below 100 nm. Spectrally decomposed heat current further proves that the thermal transport of propagons and diffusons is sensitive to the system length. The thermal conductivity is found to increase with temperature using Green-Kubo mode analysis. We also quantify the contribution of each carrier to the thermal conductivity at different temperatures. We find that propagons are more important than diffusons in thermal transport at low temperatures (<100 K). In comparison, diffusons dominate heat transport at high temperatures. Locons have negligible contribution to the total thermal conductivity.

In Iran, the intensity of energy consumption in the building sector is almost 3 times the world average, and due to the consumption of fossil fuels as the main source of energy in this sector, as well as the lack of optimal design of buildings, it has led to excessive release of toxic gases into the environment. This research develops an efficient approach for the simulation-oriented Pareto optimization (SOPO) of building energy efficiency to assist engineers in optimal building design in early design phases. To this end, EnergyPlus, as one of the most powerful and well-known whole-building simulation programs, is combined with the Multi-objective Ant Colony Optimization (MOACO) algorithm through the JAVA programming language. As a result, the capabilities of JAVA programming are added to EnergyPlus without the use of other plugins and third parties. To evaluate the effectiveness of the developed method, it was performed on a residential building located in the hot and semi-arid region of Iran. To obtain the optimum configuration of the building under investigation, the building rotation, window-to-wall ratio, tilt angle of shading device, depth of shading device, color of the external walls, area of solar collector, tilt angle of solar collector, rotation of solar collector, cooling and heating setpoints of heating, ventilation, and air conditioning (HVAC) system are chosen as decision variables. Further, the building energy consumption (BEC), solar collector efficiency (SCE), and predicted percentage of dissatisfied (PPD) index as a measure of the occupants’ thermal comfort level are chosen as the objective functions. The single-objective optimization (SO) and Pareto optimization (PO) are performed. The obtained results are compared to the initial values of the basic model. The optimization results depict that the PO provides optimal solutions more reliable than those obtained by the SOs, owing to the lower value of the deviation index. Moreover, the optimal solutions extracted through the PO are depicted in the form of Pareto fronts. Eventually, the Linear Programming Technique for Multidimensional Analysis of Preference (LINMAP) technique as one of the well-known multi-criteria decision-making (MCDM) methods is utilized to adopt the optimum building configuration from the set of Pareto optimal solutions. Further, the results of PO show that although BEC increases from 136 GJ to 140 GJ, PPD significantly decreases from 26% to 8% and SCE significantly increases from 16% to 25%. The introduced SOPO method suggests an effective and practical approach to obtain optimal solutions during the building design phase and provides an opportunity for building engineers to have a better picture of the range of options for decision-making. In addition, the method presented in this study can be applied to different types of buildings in different climates.

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.

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.

The unsteady cloud cavitation shedding in fuel nozzles greatly influences the flow characteristics and spray break-up of fuel, thereby causing erosion damage. With the application of high-pressure common rail systems in diesel engines, this phenomenon frequently occurs in the nozzle; however, cloud cavitation shedding frequency and its mechanism have yet to be studied in detail. In this study, a visualization experiment and proper orthogonal decomposition (POD) method were used to study the variations in the cavitation shedding frequency and analyze the cavitation flow structure in a 3 mm square nozzle. In addition, large eddy simulation (LES) was performed to explore the causes of cavitation shedding, and the relationship between cavitation and vortices. With the increase of the inlet and outlet pressure differences, and fuel temperatures, the degree of cavitation intensified and the frequency of cavitation cloud shedding gradually decreased. LES demonstrated the relationship between the vortices, and the development, shedding, and collapse of the cavitation clouds. Further, the re-entrant jet mechanism was found to be the main reason for the shedding of cavitation clouds. Through comparative experiments, the fluctuation of the vapor volume fraction in the nozzle hole accurately predicted the regions with stable cavitation, re-entrant jet, cavitation cloud shedding, and collapse. The frequency of cavitation shedding can then be calculated. This study employed an instantaneous POD method based on instantaneous cavitation images, which can distinguish the evolution process and characteristics of cavitation in the nozzle hole of diesel engines.

Spray experiments of RP-3 jet fuel at non-evaporating and evaporating environments were studied on a constant volume spray chamber, and diffusive back-imaging technique was used to capture the transient spray development processes. Spray tip penetration, projected spray area and cone angle of RP-3 jet fuel were derived from the spray development images, and compared to those of diesel fuel. It is observed that non-evaporating sprays of RP-3 jet fuel and diesel fuel do not exhibit significant differences, as their spray penetration distances, projected spray areas and spray cone angles are consistent at most test conditions. The evaporating sprays of RP-3 jet fuel produce shorter liquid-phase penetration distances and lower projected spray areas than those of diesel fuel, and these differences are particularly pronounced at low ambient temperatures. However, fuel effects on the evaporating spray cone angle are insignificant. Further, increased ambient density or ambient temperature shortens the liquid-phase spray penetration distance and reduces the liquid-phase spray area, and these effects are more pronounced for diesel fuel than RP-3 jet fuel.

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.

Printed circuit heat exchanger (PCHE) has been widely used in supercritical carbon dioxide (S-CO₂) power systems because of its high heat transfer efficiency and good compactness. However, due to the large variety of PCHE configurations, channel selection in practical applications lacks a basis. Therefore, this paper discussed the heat transfer and friction characteristics and the synergy of three fields in the channel under the guidance of the field synergy principle for four typical PCHE channels. Additionally, the comprehensive performance of four channels was compared. Finally, the heat transfer and friction factor correlations for S-CO₂ in four channels were established. The findings demonstrate that the synergy of velocity and pressure fields of the straight channel PCHE is better (β≈180°), so its resistance loss is relatively small. The zigzag and sinusoidal wavy channels and the airfoil fins can reduce the angle α between the temperature gradient and velocity, thus enhancing the heat transfer. The sinusoidal wavy channel can reduce flow resistance compared to the zigzag channel due to the rounded corners. The streamlined airfoil structure can guide the flow and reduce backflow, thus reducing resistance losses. In the range of Re studied in this paper, the maximum error of the proposed heat transfer and friction factor correlations of PCHE is 7.0%, which shows good fitting accuracy. The research in this paper can provide a reference for the selection and design of PCHE with different channel configurations.

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.

Hyperloop has become one of the key reserve technologies for future high-speed rail transit. The gas in the tube is compressed and rubbed, leading to a strong aerodynamic heating effect. The research on the flow field characteristics and aerodynamic heating effect of hyperloop is in its infancy, and that on the flow field structure is lacking. In this study, the nozzle theory was used to make a preliminary judgment on the choked flow phenomenon in the hyperloop. Based on the flow results obtained under different working conditions, the identification basis of the choked flow phenomenon in the hyperloop was obtained. Furthermore, the effect of the choked/unchoked flow on the flow structure, temperature, and pressure distribution of the annular space in the tube was analyzed. Based on traditional high-speed railway aerodynamics, according to relevant theories and calculation in aerospace field, and combined with the model test data, the reliability verification analysis on the characteristics of the flow field are carried out. The structure of the flow filed in the tube can be divided into choked and unchoked. The judgment is dependent on whether the throat reaches the speed of sound. Under the choked flow, a normal shock wave is formed in front of the tube train. The temperature rise of the local flow field exceeds 50 K; the temperature rise of the stagnation region exceeds 88 K, and the pressure is approximately 1.7 times that of the initial pressure in the tube. When the flow is unchoked, differences arise in the distribution of the flow field corresponding to different incoming Mach numbers. When the incoming flow is supersonic, the flow field maintains a supersonic speed, and a bow-shaped shock wave is formed at the front of the tube train. Owing to the shock wave or expansion wave, the local flow field exhibits significant fluctuations in temperature and pressure. Conversely, when the incoming flow is subsonic, the flow field in the tube maintains a subsonic speed, and no shock wave structure is observed.

The costs of conventional fuels are rising on a daily basis as a result of technical limits, a misallocation of resources between demand and supply, and a shortage of conventional fuel. The use of crude oil contributes to increased environmental contamination, and as a result, there is a pressing need to investigate alternate fuel sources for car applications. Biodiesel is a renewable fuel that is derived chemically by reacting with the sources of biodiesel. The present research is based on analyzing the effect of fish oil biodiesel-ethanol blend in variable compression engine for variable compression ratio (VCR). The processed fish oil was procured and subjected to a transesterification process to convert fatty acids into methyl esters. The obtained methyl esters (biodiesel) were blended with ethanol and diesel to obtain a ternary blend. The ternary blend was tested for its stability, and a stable blend was obtained and tested in VCR engines for its performance, combustion, and emission characteristics. In the second phase, experiments are conducted in the diesel engine by fueling the fish oil methyl ester and ethanol blended with diesel fuel in the concentration of 92.5 vol% of Diesel+7.5 vol% of Fish oil+1.25 vol% ethanol, 92.5 vol% of Diesel+7.5 vol% of Fish oil+5 vol% ethanol, 87.5 vol% of Diesel+12.5 vol% of Fish oil+1.25 vol% ethanol, 87.5 vol% of Diesel+12.5 vol% of Fish oil+5 vol% ethanol, 82.5 vol% of Diesel+17.5 vol% of Fish oil+1.25 vol% ethanol, 82.5 vol% of Diesel+17.5 vol% of Fish oil+ 5 vol% ethanol to find out the performance parameters and emissions. Because the alternative fuel performs better in terms of engine performance and pollution management, the percentage chosen is considered the best mix. The results showed that the use of a lower concentration of ethanol in the fish oil biodiesel blend improved the engine thermal efficiency by 5.23% at a higher compression ratio. Similarly, the engine operated with a higher compression ratio reduced the formation of HC and CO emissions, whereas the NO_x and CO₂ emissions increased with an increased proportion of biodiesel in diesel and ethanol blends.

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.

The specific heat capacity of working fluid is an important influence factor on heat transfer characteristic of the pulsating heat pipe (PHP). Due to the relatively large specific heat capacity of micro encapsulated phase change material (MEPCM) suspension, a heat transfer performance experimental facility of the PHP was established. The heat transfer characteristic with MEPCM suspension of different mass concentrations (0.5% and 1.0%) and ultra-pure water were compared experimentally. It was found that when the PHP uses MEPCM suspension as its working fluid, operating stability is impoverished under lower heating power and the operating stability is better under higher heating power. At the inclination angle of 90°, the temperature at heating side decreases compared to ultra-pure water and the temperature at heating side decreases with the raising of MEPCM suspension mass concentration. The heat transfer characteristic of the PHP is positively correlated with the inclination angle and the 90^° is optimum. The unfavorable effect of the inclination angle decreases with heating power increasing. When the inclination angle is 90^°, the PHP with MEPCM suspension at 1.0% of mass concentration has the lowest thermal transfer resistance and followed by ultra-pure water and MEPCM suspension at 0.5% of mass concentration has the highest thermal transfer resistance. When the inclination angles are 60^° and 30^°, the effect of gravity on the flow direction is reduced to 86.6% and 50% of that on the inclination angle of 90°, respectively, and the promoting effect of gravity on the working fluid is further weakened as the inclination angle further decreases. Due to the high viscosity of MEPCM suspension, the PHP with ultra-pure water has the lowest heat transfer resistance. When the inclination angles is 60^°, the thermal resistance with MEPCM suspension at 0.5% of the mass concentration is lower than that at 1.0% at the heating power below 230 W. The thermal resistance of MEPCM suspension tends to be similar for heating power of 230–250 W. At the heating power above 270 W, the thermal resistance with MEPCM suspension at 1.0% of the mass concentration is lower than that at 0.5%.

Targeted regulation of heat transfer in carbon/carbon composite structure is built for cooling electronic device. A three-dimensional data-driven design model coupling genetic algorithm (GA) with self-adaption deep learning for targeted regulation of heat transfer in built structure is proposed. The self-adaption deep learning model predicts the temperature of built structure closer to optimal value in GA model. The distributions of pore and carbon fiber bundles in built structure are optimized by the proposed model. The surface temperature of electronic device in the optimized structures is 19.1%–27.5% lower than that in the initial configurations when the porosity of built structure varies from 3% to 11%. The surface temperature of electronic device increases with an increase in porosity. The built structure with carbon fiber bundles near the surface of electronic device and pore distribution in the middle of structure has a higher heat dissipation capacity compared with that in the initial configuration. Besides, the computation time of the proposed model is less than one tenth compared with that of the traditional genetic algorithm.

The standing-wave thermoacoustic engines (TAE) are applied in practice to convert thermal power into acoustic one to generate electricity or to drive cooling devices. Although there is a number of existing numerical researches that provides a design tool for predicting standing-wave TAE performances, few existing works that compare TAE driven by cryogenic liquids and waste heat, and optimize its performance by varying the stack plate spacing. This present work is primarily concerned with the numerical investigation of the performance of TAEs driven by cryogenic liquids and waste heat. For this, three-dimensional (3-D) standing-wave TAE models are developed. Mesh- and time-independence studies are conducted first. Model validations are then performed by comparing with the numerical results available in the literature. The validated model is then applied to simulate the standing-wave TAEs driven by the cryogenic liquids and the waste heat, as the temperature gradient ΔT is varied. It is found that limit cycle oscillations in both systems are successfully generated and the oscillations amplitude is increased with increased ΔT. Nonlinearity is identified with acoustic streaming and the flow reversal occurring through the stack. Comparison studied are then conducted between the cryogenic liquid-driven TAE and that driven by waste heat in the presence of the same temperature gradient ΔT. It is shown that the limit cycle frequency of the cryogenic liquid system is 4.72% smaller and the critical temperature ΔT_cri =131 K is lower than that of the waste heat system (ΔT_cri=187 K). Furthermore, the acoustic power is increased by 31% and the energy conversion efficiency is found to increase by 0.42%. Finally, optimization studies on the stack plate spacing are conducted in TAE system driven by cryogenic liquids. It is found that the limit cycle oscillation frequency is increased with the decreased ratio between the stack plate spacing and the heat penetration depth. When the ratio is set to between 2 and 3, the overall performance of the cryogenic liquid-driven TAE has been greatly improved. In summary, the present model can be used as a design tool to evaluate standing-wave TAE performances with detailed thermodynamics and acoustics characteristics. The present findings provide useful guidance for the design and optimization of high-efficiency standing-wave TAE for recovering low-temperature fluids or heat sources.

The low power consumption of the near-critical compressor is the key factor for the high efficiency of supercritical CO₂ Brayton cycle. In the numerical simulation of the compressor, the rapid changes in the thermophysical properties of the CO₂ near the critical point make it difficult to capture the condensation phenomenon. This paper investigates the influence of fluid physical properties on the condensation phenomenon. Firstly, the differences in the physical properties of CO₂ in the SRK EOS (equation of state), PR EOS, and SW EOS are compared. Then, the simulation of nozzles and compressors were carried out and discussed. Results show that the condensation positions predicted by the three EOSs are basically the same. Compared with SW EOS, the disparities between the maximum condensation mass fraction predicted by the PR and SRK EOSs is 5.7% and 11.5%, and that of total pressure ratio is 0.3% and 3.8%, respectively. The results show that PR EOS can be considered for numerical simulation in engineering practice. Since its physical property calculation results are closer to the actual physical properties while the physical properties change more gently, it has considerable accuracy and numerical stability.

Gas turbines are increasingly and widely used, whose research and production reflect a country’s industrial capacity and level. Due to the changeable working environment, gas turbines usually work under the condition of simultaneous changes of ambient temperature, load and fuel. However, the current researches mainly focus on the change in single condition, and do not fully consider the simultaneous change in different conditions. On the basis of single condition, this paper further studies the dual off-design performance of gas turbines under three conditions: temperature-load, fuel-load and fuel-temperature. Firstly, the whole machine model of a gas turbine is established, in which the compressor model has the greatest impact on the performance of gas turbines. Therefore, this paper obtains a more accurate compressor model by combining the engineering modeling advantages of gPROMs and the powerful mathematical calculation ability of MATLAB neural network. Then, according to the established gas turbine model, the dual off-design performance is studied, which is mainly based on the parameter of output and efficiency. The result shows that the efficiency and power output of gas turbines will decrease with the increase of ambient temperature. With the decrease of fuel calorific value, power output and efficiency will increase. As the load decreases, the efficiency of the gas turbines will decrease, and these changes are consistent with the single off-design performance. However, when the fuel and temperature change simultaneously, only adjusting the IGV angle cannot avoid the surge when the temperature is above 30°C. At this time, it is necessary to adjust the extraction rate in order to ensure the safe and stable operation of gas turbines. Therefore, the research on dual off-design performance of gas turbines has an important significance for the peak shaving operation of gas turbines.

The design of thermal conductivity enhancers (TCE) is quite critical to overcoming the disadvantage of the poor thermal conductivity of phase change materials (PCM). The main contribution of this study is firstly to discuss how to actively enhance natural convection of the melted PCM in cellular structure by the fin formed in the structure under the condition of the same metal mass, apart from simultaneously improving heat conduction, which can boost the heat transfer performance. Also, a tailored hybrid fin-lattice structure (HFS) as TCE is designed and fabricated by additive manufacturing (AM). A two-equation numerical method is applied to study the heat transfer of the PCM, and its feasibility is validated with the experimental data. The numerical results indicate that enhanced natural convection and improved heat conduction can be obtained simultaneously when a well-designed fin is embedded into a lattice structure. The enhanced natural convection results in the improved melting rate and the decreased wall temperature; e.g., the complete melting time and the wall temperature are reduced by 11.6% and 19.7%, respectively, because of the fin for metal aluminum. Moreover, the parameters of HFS including the porosity, pore density, and fin dimension have a great impact on the heat transfer. The enhancement effect of the fin for HFS on the melting rate of the PCM increases as the thermal conductivity of the base material decreases. For example, when the fin is introduced into the lattice structure, the complete melting time is reduced by 24.1% for metal titanium. In summary, this study enables us to obtain a good understanding of the mechanism of the heat transfer and provides necessary experimental data for the structural design of HFS fabricated by AM.

Plastic crystal neopentyl glycol (NPG) exhibits colossal barocaloric effect with high entropy changes. However, their application is restricted in several aspects, such as low thermal conductivity, substantial supercooling effect, and poor springback properties. In this work, multi-walled carbon nanotubes (MWCNTs) with ultra-high thermal conductivity and high mechanical strength were selected for performance enhancement of NPG. The optimal mixing ratio was determined to be NPG with 3 wt% MWCNTs composites, which showed a 6 K reduction in supercooling without affecting the phase change enthalpy. Subsequently, comprehensive performance of the composites with optimal mixing ratio was compared with pure NPG. At 40 MPa, 390 J·K^–1·kg^–1 change in entropy and 9.9 K change in temperature were observed. Furthermore, the minimum driving pressure required to achieve reversible barocaloric effect was reduced by 19.2%. In addition, the thermal conductivity of the composite was increased by approximately 28%, significantly reducing the heat exchange time during a barocaloric refrigeration cycle. More importantly, ultra-high pressure release rate resulted in a 73.7% reduction in the springback time of the composites, offering new opportunities for the recovery of expansion work.

Energy efficiency issues are being focused on the growing concern of global warming and environmental pollution. The high-temperature heat pipe (HTHP) is an effective and environmental-friendly heat transfer device employed in many industries, including solar power generation, high-temperature flue gas waste heat recovery, industrial furnaces, nuclear industries, and aviation. As a critical factor in HTHPs, thermal performance is mainly introduced in the entire paper. To date, most reviews have been published concerning one or several application scenarios. However, to the best of authors’ knowledge, it is hard to find a review discussing how to improve the thermal performance of HTHPs comprehensively. First, the impact on the performance of three main components of HTHPs over the past 30 years is introduced: the working fluid, the HTHP structure, and the wick structure. Herein, it is a considerable review of the optimal operating conditions for each direction, and we expect this paper contribute to improving the thermal performance of HTHPs. Then, current numerical simulations and theoretical research on the heat transfer limit of HTHPs are recommended. The significant hypotheses used in numerical simulations and the present theoretical studies are compiled here. Finally, some potential future directions and tentative suggestions for HTHP research are put forward.

In this research paper, a solar air heater with triangular fins has been experimentally analysed and optimized. Initially, an experimental set-up of a solar air heater having triangular fins has been developed at the location of 28.10°N, 78.23°E. The heat transfer rate through fins and fins efficiency has been determined by the Finite Difference Method model equations. The experimental data and modeled data of response parameters have been optimized in MINITAB-17 software by the Response Surface Methodology tool. For creating the response surface design, three input parameters have been selected namely solar intensity, Reynolds number, and fin base-to-height ratio. The range of solar intensity, Reynolds number, and fin base-to-height ratio is 600 to 1000 W/m², 4000 to 6000, and 0.4 to 0.8 respectively. The response surface design has been analyzed by calculating the outlet temperature, friction factor, Nusselt number, fin efficiency, thermal performance factor, and exergy efficiency. The optimum settings of input parameters: solar intensity is 1000 W/m²; Reynolds number is 4969.7, and the fin base to height ratio is 0.6060, on which these response: namely outlet temperature of 92.531°C, friction factor of 0.2350, Nusselt number of 127.761, thermal efficiency of 50.836%, thermal performance factor of 1.4947, and exergy efficiency of 8.762%.

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.

Thin-film catalysts are recently recognized as promising catalysts due to their reduced amount of materials and good catalytic activity, leading to low-cost and high-efficiency catalysts. A series of CuFeOx thin-film catalysts were prepared with different Fe contents using a one-step method as well as tested for the catalytic reduction of nitrous oxide (N₂O) in the presence of CH₄ at a high GHSV of 185 000 mL/(g∙h). The increase of iron strongly affects the dispersion and leads to the creation of a less-active segregated Fe₂O₃ phase, which was confirmed by XRD, EDX, and XPS outcomes. The results show that the synergistic properties between Cu and Fe, which affect the CuFeOx film catalysts in many aspects, such as the hollow-like texture, specific surface area, nano-crystallite size, the surface contents of Cu+, Fe³⁺, and oxygen species, the reductive strength and the strong active sites on the surface. Using DFT calculations, the adsorption and decomposition energy profiles of N₂O on the CuFeO₂ (012) surface model were explored. The surface Fe-site and hollow-site are active for N₂O decomposition, and the decomposition energy barriers on the Fe-site and the hollow-site are 1.02 eV and 1.25 eV respectively at 0 K. The strategy adopted here to tailor the activity through low-doping Fe-oxide catalysts could establish a promising way to improve the catalytic reduction of N₂O with CH₄.

Lithium-ion batteries are used in a wide range of applications. However, their cycle life suffers from the problem of capacity fade, which includes calendar and cycle aging. The effects of storage time, temperature and partial charge-discharge cycling on the capacity fade of Li-ion batteries are investigated in this study. The calendar aging and cycle aging are presented based on the storage and cycling experiment on LiCoO₂/graphite cells under different storage temperature and different ranges of state of charge (SOC). Based on the measurement data, a one-component and a double-component aging model are presented to respectively describe the capacity fade caused by calendar and cycle aging. The calendar aging of LiCoO₂/graphite batteries is mainly affected by temperature and SOC during the storage. Mean SOC and change in SOC (∆SOC) are the main factors affecting battery degradation during cycling operation.

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.

With countries proposing the goal of carbon neutrality, the clean transformation of energy structure has become a hot and trendy issue internationally. Renewable energy generation will account for the main proportion, but it also leads to the problem of unstable electricity supply. At present, large-scale energy storage technology is not yet mature. Improving the flexibility of coal-fired power plants to suppress the instability of renewable energy generation is a feasible path. Thermal energy storage is a feasible technology to improve the flexibility of coal-fired power plants. This article provides a review of the research on the flexibility transformation of coal-fired power plants based on heat storage technology, mainly including medium to low-temperature heat storage based on hot water tanks and high-temperature heat storage based on molten salt. The current technical difficulties are summarized, and future development prospects are presented. The combination of the thermal energy storage system and coal-fired power generation system is the foundation, and the control of the inclined temperature layer and the selection and development of molten salt are key issues. The authors hope that the research in this article can provide a reference for the flexibility transformation research of coal-fired power plants, and promote the application of heat storage foundation in specific coal-fired power plant transformation projects.

The investigation of the melting behaviors of the molten salt at micron scale during the melting process is critical for explaining the solid-liquid phase transition mechanism. In this paper, a novel experimental system and analysis method were proposed to study the melting process with three heating rates in the range of 1–10°C/min of the solar salt at micron scale. The solid-liquid boundary morphology and phase transition kinetics of molten salt particles were focused on. Meanwhile, the correlations between liquid fraction, temperature and time under different heating rates were studied. The solid-liquid boundary morphology was depicted by the visualized experimental set-up, and the instantaneous liquid volume fraction during the non-isothermal phase transition was obtained. Then, the correlation between temperature and liquid volume fraction was proposed to reveal the evolution of the solid-liquid boundary with temperature at different heating rates. Furthermore, the non-isothermal phase transition kinetic equation was established by introducing a constant parameter (V_a,b), and more kinetic parameters such as lg V_a,b and –lg V_a,b/b were studied. The results showed that the exponent b is not sensitive to the heating rate with a range of 3–5 for solar salt particles. However, the heating rate influences the value of V_a,b and presents a positive relationship. Besides, the non-isothermal phase transition kinetic equations at different heating rates in the range of 1–10°C/min can be quickly predicted by the proposed improved experimental test method. This study could fill the research insufficiency and provide significant guidance for future research on the solid-liquid transition mechanism of molten salts at micron scale.

Oxidation of acetylene (C2H2) has been investigated in a high-pressure jet-stirred reactor (HP-JSR) with equivalence ratios Φ=0.5, 1.0, 2.0 and 3.0 in the temperature range of 650 K–900 K at 1.2 MPa. 18 products and intermediates were analyzed qualitatively and quantitatively by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). Generally, with Φ increasing, the production of intermediates increases significantly. CH₄, C₂H₄, C₂H₆, C₃H₆ and C3H8 were important intermediates, which were formed abundantly at Φ=3.0. Sufficient light hydrocarbon intermediates could be an important reason for significant formation of cyclopentadiene, benzene, toluene and styrene at Φ=3.0. A detailed kinetic mechanism consisting of 299 species and 2041 reactions has been developed with reasonable predictions against the present data and previous results obtained at 0.1 MPa. According to flux and sensitivity analysis, H and OH radicals play important roles in the consumption of C₂H₂. The combinations among light hydrocarbons and their free radicals are the main generation pathways of aromatics. C₃H₃, IC₄H₅ and AC3H5 are important precursors for the formation of aromatics. By comparing the results of atmospheric pressure and high pressure, it can be found that increasing the pressure is conducive to fuel consumption and aromatics generation.

The chloride salts have great potential used as high-temperature thermal energy storage (TES) medium for the concentrated solar power system. In this study, LiCl, KCl and CaCl₂ were selected as energy storage materials in order to further broaden the working temperature of ternary chloride salt and improve its energy storage density. The new high-temperature energy storage ternary chloride composed of LiCl, KCl, and CaCl₂ was developed based on the phase diagram generated by FactSage. Three components of LiCl-KCl-CaCl₂ with the mass ratio of 37.85%-53.38%-8.77%, 30.90%-13.82%-55.28% and 1.78%-18.61%-79.61% were developed, of which the corresponding melting temperature (T_m) 340.93, 433.57 and 626.85°C, respectively, was obtained either. Considering that T_m of the third group of salts was too high, only the first two groups of salts were tested. DSC test showed that the actual melting point was only 0.46% and 1.64% different from the melting point predicted by Factsage. The thermal properties of the two ternary chloride salts were also compared. The solid and liquid-specific heat of ternary salts was determined by DSC using sapphire as the standard reference. The vapor pressure and decomposition temperature of ternary chloride salts were investigated. The results showed that the vapor pressure of salt 1 was almost constant below 650^°C by FactSage. Meanwhile, the TG results showed that the upper working temperature of salt 1 was 650^°C under the air atmosphere. In addition, the ternary chloride salts after short-term cycling still exhibited excellent thermal properties, which revealed that these good thermal properties make them have broad application prospects in high-temperature thermal energy storage systems.

Compressed Air Energy Storage (CAES) is one of the methods that can solve the problems with intermittency and unpredictability of renewable energy sources. A side effect of air compression is a fact that a large amount of heat is generated which is usually wasted. In the development of CAES systems, the main challenge, apart from finding suitable places for storing compressed air, is to store this heat of compression process so that it can be used for heating the air directed to the expander at the discharging stage. The paper presents the concept of a hybrid compressed air and thermal energy storage (HCATES) system, which may be a beneficial solution in the context of the two mentioned challenges. Our novel concept assumes placing the thermal energy storage (TES) system based on the use of solid storage material in the volume of the post-mining shaft forms a sealed air pressure reservoir. Implementation of proposed systems within heavily industrialized agglomerations is a potential pathway for the revitalization of post-mine areas. The potential of energy capacity of such systems for the Upper Silesian region could exceed the value of 10 GWh. In the paper, the main construction challenges related to this concept are shown. The issues related to the possibility of storing air under high pressure in the shaft from the view of the rock mass strength are discussed. The overall concept of the TES system installation solution in the shaft barrel is presented. The basic problems related to heat storage in the cylindrical TES system with a non-standard structure of high slenderness are also discussed. The numerical calculations were based on the results of experiments performed on a laboratory stand, the geometry of which is to reflect the construction of a TES tank in a post-mining shaft. The article presents the results of numerical analysis showing the basic aspects related to difficulties that may occur at the construction stage and during the operation of the proposed HCATES system. The paper focuses on the methodology for determining the energy and exergy efficiency of a section of a Thermal Energy Storage tank, and presents the differences in the performance of this tank depending on its geometric dimensions, which are determined by the different sizes of mine shafts.

A two temperature CO₂ refrigeration system with economizer is proposed and compared with the traditional dual-temperature CO₂ refrigeration system based on energy consumption, exergy and economic analysis. Using genetic algorithm multi-objective optimization method, taking the COP, exergy loss and total economic cost as the objective functions to find the best design conditions of the two systems. The Pareto fronts are obtained at different ambient temperatures. Technique for order preference by similarity to an ideal solution decision-making method is adopted to determine the optimum state points. The simulation results show that when operating at different ambient temperatures, the introduction of economizer can improve COP, reduce exergy loss and the overall economic cost rate of the two-temperature CO₂ refrigeration system. In addition, economic analyses take the impact of carbon dioxide emission cost and electricity price into consideration. The results indicate that with the increase of CO₂ emission cost and electricity price, the hourly economic cost of both systems increases, but the hourly economic cost of the two-temperature CO₂ refrigeration system with economizer system is always lower than that of conventional two-temperature CO₂ refrigeration system.

The coal gasification fly ash (CGFA) is an industrial solid waste from coal gasification process and needs to be effectively disposed for environmental protection and resource utilization. To further clarify the feasibility of CGFA to prepare porous carbon materials, the physicochemical properties of ten kinds of CGFA from circulating fluidized bed (CFB) gasifiers were analyzed in detail. The results of proximate and ultimate analysis show that the CGFA is characterized with the features of near zero moisture content, low volatile content as low as 0.90%–9.76%, high carbon content in the range of 37.89%–81.62%, and ultrafine particle size (d₅₀=15.8–46.2 μm). The automatic specific surface area (SSA) and pore size analyzer were used to detect the pore structure, it is found that the pore structure of CGFA is relatively developed, and part of the CGFA has the basic conditions to be used directly as porous carbon materials. From SEM images, the microscopic morphology of the CGFA is significantly different, and they basically have the characteristics of loose and porous structure. XRD and Roman spectroscopy were used to characterize the carbon structure. The result shows that the CGFA contains abundant amorphous carbon structure, and thus the CGFA has a good reactivity and a potential to improve pore structure through further activation. Through thermal gravimetric analysis, it can be concluded that the order of reactivity of the CGFA under CO₂ atmosphere has a good correlation with the degree of metamorphism of the raw coal. The gasification reactivity of the CGFA is generally consistent with the change trend of micropores combined with the pore structure. According to the physicochemical properties, the CGFA has a good application prospect in the preparation of porous carbon materials.

In this paper, a novel polygeneration system involving plasma gasifier, pyrolysis reactor, gas turbine (GT), supercritical CO₂ (S-CO₂) cycle, and organic Rankine cycle (ORC) has been developed. In the proposed scheme, the syngas is obtained by the gasification and the pyrolysis is first burned and drives the gas turbine for power generation, and then the resulting hot exhaust gas is applied to heat the working fluid for the supercritical CO₂ cycle and the working fluid for the bottom organic Rankine cycle. In addition to the electrical output, the pyrolysis subsystem also produces pyrolysis oil and char. Accordingly, energy recovery is achieved while treating waste in a non-hazardous manner. The performance of the new scheme was examined by numerous methods, containing energy analysis, exergy analysis, and economic analysis. It is found that the net total energy output of the polygeneration system could attain 19.89 MW with a net total energy efficiency of 52.77%, and the total exergy efficiency of 50.14%. Besides, the dynamic payback period for the restoration of the proposed project is only 3.31 years, and the relative net present value of 77 552 640 USD can be achieved during its 20-year lifetime.