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.

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.

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.

As an important and effective indicator of contact heat transfer, thermal contact resistance is a widespread phenomenon in engineering. It can directly affect product reliability, full-load performance, power consumption and even life cycle in energy, aerospace, electronic packaging, cryogenic refrigeration, etc. Therefore, enhancing the interface heat transfer and suppressing thermal contact resistance have become increasingly important. Against this background, this paper seeks to elaborate on conceptions of thermal contact resistance and the ways to reduce it. After reviewing the existing methods of measuring thermal contact resistance and characterizing the interface morphology, we highlight the theoretical underpinnings of thermal contact resistance, including the two-dimensional mathematic characteristics of the contact interface and the theoretical and empirical models for quantifying it. Three categories of influencing factors, i.e., thermal, geometrical and mechanical states, are then presented. Based on the macroscopic formation mechanism, the paper summarizes the existing methods for suppressing thermal contact resistance, with close attention paid to polymer composite thermal interfacial materials and metal interfacial materials filled with high thermal conductivity filler. In light of the findings, this review provides five promising directions for future research on thermal contact resistance. It suggests that the failure modes and service life of interface materials are essential to apply such technologies to suppress thermal contact resistance in practice. This review will be a guide for future research in thermal contact resistance and for the widespread use of composite interface materials.

Fast fluid transport on graphene has attracted a growing body of research due to a wide range of potential applications including thermal management, water desalination, energy harvesting, and lab-on-a-chip. Here, we critically review the theoretical, simulational, and experimental progress regarding the fluid slippage on graphene. Based on the summary of the past studies, we give perspectives on future research directions towards complete understanding and practical applications of slip flow on graphene.

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.

Recent research and development on ramjet and supersonic combustion ramjet (scramjet) engines is concerned with producing greater thrust, higher speed, or lower emission. This is most likely driven by the fact that supersonic/hypersonic propulsion systems have a broad range of applications in military sectors. The performances of such supersonic/hypersonic propulsion systems depend on a series of physical and thermodynamic parameters, such as the fuel types, flight conditions, geometries and sizes of the engines, engine inlet pressure/velocity. As a propulsion system, a stable and efficient combustion is desirable. However, self-excited large-amplitude combustion oscillations (also known as combustion instabilities) have been observed in liquid- and solid-propellant ramjet and scramjet engines, which may be due to acoustic resonance between inlet and nozzle, vortex kinematics (large coherent structures), and acoustic-convective wave coupling mechanisms due to combustion. Such intensified pressure oscillations are undesirable, since they can lead to violent structural vibration, and overheating. How to enhance and predict the engines’ stability behaviors is another challenge for engine manufacturers. The present work surveys the research and development in ramjet combustion and combustion instabilities in ramjet engines. Typical active and passive controls of ramjet combustion instabilities are then reviewed. To support this review, a case study of combustion instability in solid-fueled ramjet is provided. The popular mode decomposition algorithms such as DMD (dynamic mode decomposition) and POD (proper orthogonal decomposition) are discussed and applied to shed lights on the ramjet combustion instability in the present case study.

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 complex flow phenomenon of rotating instability (RI) and its induced non-synchronous vibration (NSV) have become a significant challenge as they continuously increase aerodynamic load. This study aims to provide an understanding of the non-synchronous blade vibration phenomenon caused by the rotating instability of a transonic axial compressor rotor. In this case, blade vibrations and non-synchronous excitation are captured by strain gauges and unsteady wall pressure transducer sensors. Unsteady numerical simulations for a full-annulus configuration are used to obtain the non-synchronous flow excitation. The results show that the first-stage rotor blade exhibits an NSV close to the first bending mode; NSV is accompanied by a sharp increase in pressure pulsation; amplitude can reach 20%, and unsteady aerodynamic frequency will lock in a structural mode frequency when the blade vibrates in a large-amplitude motion. The predicted NSV frequency aligns well with the experimental results. The dominant mode of circumferential instability flow structure is approximately 47% of the number blades, and the cell size occupies 2–3 pitches in the circumferential direction. The full-annulus unsteady simulations demonstrate that the streamwise oscillation of the shedding and reattachment vortex structure is the main cause of NSV owing to the strong interaction between the tip leakage and separation vortices near the suction surface.

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.

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.

Thermal energy conversion and also storage system is to advance knowledge and develop practical solutions at the intersection of micro and nano-scale engineering, energy conversion, and sustainability. This research addresses the challenge of enhancing these critical aspects to ensure prolonged system performance and durability in the context of evolving energy technologies. This research analyses the anti-oxidation and filtration behaviours of micro and nano-scale structures in the context of electro- and photo-thermal energy conversion and also storage systems. A micro multiscale hierarchical structure strategy is presented to fabricate multi-scale double-layer porous wick evaporators with the electrospun nanofibers made of gelatin-polyamide 6 (GPA6) and Ti₃C₂T_x MXene/silver nanowire with Cellulose Micro/NanoFibers (CMNF) cryogens by using spark plasma sintering (SPS) based high-pressure hydrothermal treatment model. An excellent anti-oxidation effect was offered by coating the film in thermal conditions and the anti-oxidation properties were further examined from 500°C to 850°C. The results are analysed using Matlab software to improve the efficiency of energy conversion processes by integrating nanostructures into thermal systems, to increase energy output while minimizing losses. The silver nanowire is with a heat transfer coefficient of 78%, a mass remaining rate of 98.7%, and an energy storage efficiency of 23.8%. This study enhances energy density and duration by integrating nanostructures into thermal systems while minimizing energy losses, and it not only exhibits excellent anti-oxidation properties but also possesses superior filtration capabilities for designing and engineering multifunctional nanomaterials.

The seemingly useless reeds are prepared as thermal insulation materials, which not only meet the requirements of environmental sustainability but also enhance the added value of reeds, creating new economic benefits. The hydrophobicity of raw biomass surfaces leads to problems such as weak bonding strength and non-dense structure in the formed materials, as well as issues related to the residual insect infestations on the surface. In this study, reed straw was used as the raw material, and foamed geopolymer was used as the binder to prepare building insulation materials based reed. To improve the interfacial adhesion performance between reed straw and foamed geopolymer, a thermochemical modification method-thermal carbonization, was proposed. In this study, the mechanical properties and hydraulic properties of the studied materials with different degrees of surface thermal modification were tested, especially the fire resistance performance, and weathering resistance performance rarely found in published literature. When the surface thermal modification condition of reed straw was 250°C (30 min), the comprehensive performance of reed-based building insulation materials was the best, when the studied material density was 321.3 kg/m³; the compressive strength was 0.59 MPa; the thermal conductivity was 0.101 W/(m∙K); the pH was 11.27; the moisture absorption rate was 25.1%, and the compressive strength loss rate in wet-dry cycles was 18.5%. In addition, it had excellent fire resistance performance and weathering resistance performance. This new material can be widely used to improve the thermal insulation of traditional buildings and as sandwich filler in prefabricated buildings, such as preparing insulating walls.

Stall in compressors can cause performance degradation and even lead to disasters. These unacceptable consequences can be avoided by timely monitoring stall inception and taking effective measures. This paper focused on the rotating stall warning in a low-speed axial contra-rotating compressor. Firstly, the stall disturbance characteristics under different speed configurations were analyzed. The results showed that as the speed ratio (RR) increased, the stall disturbance propagation speed based on the rear rotor speed gradually decreased. Subsequently, the standard deviation (SD) method, the cross-correlation (CC) method, and the discrete wavelet transform (DWT) method were employed to obtain the stall initiation moments of three different speed configurations. It was found that the SD and CC methods did not achieve significant stall warning results in all three speed configurations. Besides, the stall initiation moment obtained by the DWT method at RR=1.125 was one period after the stall had fully developed, which was unacceptable. Therefore, a stall warning method was developed in the present work based on the long short-term memory (LSTM) regression model. By applying the LSTM model, the predicted stall initiation moments of three speed configurations were at the 557th, 518th, and 333rd revolution, which were 44, 2, and 74 revolutions ahead of stall onset moments, respectively. Furthermore, in scenarios where a minor disturbance preceded the stall, the stall warning effect of the LSTM was greatly improved in comparison with the aforementioned three methods. In contrast, when the pressure fluctuation before the stall was relatively small, the differences between the stall initiation moments predicted by these four methods were not significant.

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.

Phase change materials (PCMs) designate materials able to store latent heat. PCMs change state from solid to liquid over a defined temperature range. This process is reversible and can be used for thermo-technical purposes. The present paper aims to study the thermal performance of an inorganic eutectic PCM integrated into the rooftop slab of a test room and analyze its potential for building thermal management. The experiment is conducted in two test rooms in Antofagasta (Chile) during summer, fall, and winter. The PCM is integrated into the rooftop of the first test room, while the roof panel of the second room is a sealed air cavity. The work introduces a numerical model, which is built using the finite difference method and used to simulate the rooms’ thermal behavior. Several thermal simulations of the PCM room are performed for other Chilean locations to evaluate and compare the capability of the PCM panel to store latent heat thermal energy in different climates. Results show that the indoor temperature of the PCM room in Antofagasta varies only 21.1°C±10.6°C, while the one of the air-panel room varies 28.3°C±18.5°C. Under the experiment’s conditions, the PCM room’s indoor temperature observes smoother diurnal fluctuations, with lower maximum and higher minimum indoor temperatures than that of the air-panel room. Thermal simulations in other cities show that the PCM panel has a better thermal performance during winter, as it helps to maintain or increase the room temperature by some degrees to reach comfort temperatures. This demonstrates that the implementation of such PCM in the building envelope can effectively reduce space heating and cooling needs, and improve indoor thermal comfort in different climates of Chile.

Due to the wide application of floor heating systems, the radiant floor cooling systems has developed rapidly in recent years. In this paper, TRNSYS numerical simulation methods are used to study the influence of chilled water supply temperature and flow rate on the cold storage characteristics of a standard floor structure for office buildings in northern China. The results are verified by experimental measurements. The functional relationship between the saturated cold storage time and the chilled water flow rate is quadratic polynomial, while the changes of supply-water temperature have no effect on the saturation time; the supply-water temperature has a linear relationship with the saturated cold storage volume, while the chilled water flow rate has almost no effect on the saturation cold storage volume. The accumulated cold volume of floor changes with time in an exponential distribution with four coefficients, and the floor has the characteristics of rapid cold storage. This paper is instructive for the design, application and promotion of radiant floor cooling systems.

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.

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.

Since the transition from rotating stall to surge in a transonic compressor at high speed is very quick, quite often there is no time to take measures to prevent the surge. Therefore, it is desired to find any rotating stall precursors, of which the occurrence can offer sufficient time for stall or surge prevention. In this study, a series of unsteady flow analyses were performed on a transonic compressor under operating conditions before rotating stall with unsteady results scrutinized to find rotating stall precursors. Particular attention is paid to the spatial modes and time modes of static pressure near the casing and around the blade leading and trailing edges. The results show that the characteristics of the precursor in both spatial and time domains can be used as rotating stall warnings.

The two-stage transcritical CO₂ refrigeration cycle with dedicated dual-subcooling and mechanical recooling is proposed. The inter-stage pressure is critical for such cycle performances; however, it has not been studied exactly. Therefore, the research aim is to disclose the effect of inter-stage pressure on performances of the proposed cycle. The main work consists of four aspects. Firstly, the comparative study is performed to display advantages of the proposed cycle. Secondly, the key temperatures, heat and power consumptions as well as performance indicators for different inter-stage pressures are analyzed in detail, based on the parametric model. Thirdly, the optimal inter-stage pressure for different conditions is obtained by the nonlinear direct search method. Finally, the economic performance is assessed. It is found that the compressor power of the proposed cycle drops by 12%, and the working temperature lower limit is reduced by 11°C. Furthermore, it is considered that the optimal inter-stage pressure is insensitive to the heat source temperature. The novelty lies in illustrating the effect of inter-stage pressure, obtaining trends of the optimal value, and pointing out the system feasibility. The paper is favorable for design and operation optimization of the proposed system.

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.

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.

As a new generation fuel cell, solid oxide fuel cell (SOFC) has become a hot spot in the industry due to its unique advantages. In order to improve energy utilization and prevent carbon deposition in the reformer, the ejector is usually used to recover the cell anode exhaust. In this paper, the applications related to ejector in SOFC are reviewed, including the ejector design and optimization methods, the ejector performance verification experiment and the performance of ejector in SOFC systems. Besides, in order to adapt to the wide power output characteristics of the stack, a study on extending the working range of the ejector is also introduced. On the one hand, the theory of optimal design of ejector used in SOFC system is obtained, including the influence of main structure parameters of ejector on the performance of the whole system and the theoretical model of performance monitoring of ejector used in SOFC. On the other hand, it is proved that the ejector used in SOFC power system can prevent the occurrence of carbon deposition problems, while the recovery of exhaust heat can improve the energy utilization of the system. Finally, suggestions for future related research work are given, aiming to promote the ejector-based SOFC system to provide higher and more stable performance in the future.

Based on a small perturbation stability model for periodic flow, the effects of inlet total temperature ramp distortion on the axial compressor are investigated and the compressor stability is quantitatively evaluated. In the beginning, a small perturbation stability model for the periodic flow in compressors is proposed, referring to the governing equations of the Harmonic Balance Method. This stability model is validated on a single-stage low-speed compressor TA36 with uniform inlet flow. Then, the unsteady flow of TA36 with different inlet total temperature ramps and constant back pressure is simulated based on the Harmonic Balance Method. Based on these simulations, the compressor stability is analyzed using the proposed small perturbation model.Further, the Dynamic Mode Decomposition method is employed to accurately extract pressure oscillations. The two parameters of the temperature ramp, ramp rate and Strouhal number, are discussed in this paper. The results indicate the occurrence and extension of hysteresis loops in the rows, and a decrease in compressor stability with increasing ramp rate. Compressor performance is divided into two phases, stable and limit, based on the ramp rate. Furthermore, the model predictions suggest that a decrease in period length and an increase in Strouhal number lead to improved compressor stability. The DMD results imply that for compressors with inlet temperature ramp distortion, the increase of high-order modes and oscillations at the rotor tip is always the signal of decreasing stability.

CO₂ Plume Geothermal (CPG) systems are a promising concept for utilising petrothermal resources in the context of a future carbon capture utilisation and sequestration economy. Petrothermal geothermal energy has a tremendous worldwide potential for decarbonising both the power and heating sectors. This paper investigates three potential CPG configurations for combined heating and power generation (CHP). The present work examines scenarios with reservoir depths of 4 km and 5 km, as well as required district heating system (DHS) supply temperatures of 70°C and 90°C. The results reveal that a two-staged serial CHP concept eventuates in the highest achievable net power output. For a thermosiphon system, the relative net power reduction by the CHP option compared with a sole power generation system is significantly lower than for a pumped system. The net power reduction for pumped systems lies between 62.6% and 22.9%. For a thermosiphon system with a depth of 5 km and a required DHS supply temperature of 70°C, the achievable net power by the most beneficial CHP option is even 9.2% higher than for sole power generation systems. The second law efficiency for the sole power generation concepts are in a range between 33.0% and 43.0%. The second law efficiency can increase up to 63.0% in the case of a CHP application. Thus, the combined heat and power generation can significantly increase the overall second law efficiency of a CPG system. The evaluation of the achievable revenues demonstrates that a CHP application might improve the economic performance of both thermosiphon and pumped CPG systems. However, the minimum heat revenue required for compensating the power reduction increases with higher electricity revenues. In summary, the results of this work provide valuable insights for the potential development of CPG systems for CHP applications and their economic feasibility.

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.

Syngas fuel generated by solar energy integrating with fuel cell technology is one of the promising methods for future green energy solutions to carbon neutrality. This paper designs a novel solar-driven solid oxide electrolyzer system integrated with waste heat for syngas production. Solar photovoltaic and parabolic trough collecter together drive the solid oxide electrolysis cell to improve system efficiency. The thermodynamic models of components are established, and the energy, exergy, and exergoeconomic analysis are conducted to evaluate the system’s performance. Under the design work conditions, the solar photovoltaic accounts for 88.46% of total exergy destruction caused by its less conversion efficiency. The exergoeconomic analysis indicates that the fuel cell component has a high exergoeconomic factor of 89.56% due to the large capital investment cost. The impacts of key parameters such as current density, operating temperature, pressure and mole fraction on system performances are discussed. The results demonstrate that the optimal energy and exergy efficiencies are achieved at 19.04% and 19.90% when the temperature, pressure, and molar fraction of H2O are 1223.15 K, 0.1 MPa, and 50%, respectively.

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.

The temperature-dependent absorption coefficient and thermal conductivity of a quartz window are obtained through experimental tests at a wide range of temperatures. A Fourier transform infrared spectrometer with a heated cavity is used for performing the transmittance measurements. The spectral absorption coefficient of the quartz window is inverted by the transmittance information at different temperatures using a genetic algorithm. Then, a quartz window-graphite plate-quartz window multilayer structure is designed, and the transient response of the structure subjected to high-temperature heating is recorded by a self-designed setup. Cooperating with the above absorption coefficient, a non-gray radiative-conductive heat transfer model is built for the multilayer structure, and the intrinsic thermal conductivity of the quartz window is identified. Finally, the effects of the temperature-dependent absorption coefficient and spectral selective features of the medium on the heat transfer characteristics are discussed. The results show that the absorption coefficient gradually increases with temperature. The intrinsic thermal conductivity of the quartz window varies from 1.35 to 2.52 W/(m·K) as the temperature rises, while the effective thermal conductivity is higher than the intrinsic thermal conductivity due to thermal radiation, specifically 26.4% higher at 1100 K. In addition, it is found that the influence of the temperature-dependent absorption coefficient on temperature of unheated side shows a trend of first increasing and then decreasing. When the absorption coefficient varies greatly with wavelength, a non-gray radiative-conductive heat transfer model should be built for the semitransparent materials.

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.

The selection of loss models has a significant effect on the one-dimensional mean streamline analysis for obtaining the performance of centrifugal compressors. In this study, a set of optimized loss models is proposed based on the classical loss models suggested by Aungier, Coppage, and Jansen. The proportions and variation laws of losses predicted by the three sets of models are discussed on the NASA Low-Speed-Centrifugal-Compressor (LSCC) under the mass flow of 22 kg/s to 36 kg/s. The results indicate that the weights of Skin friction loss, Diffusion loss, Disk friction loss, Clearance loss, Blade loading loss, Recirculation loss, and Vaneless diffuser loss are greater than 10%, which is dominant for performance prediction. Therefore, these losses are considered in the composition of new loss models. In addition, the multi-objective optimization method with the Genetic Algorithm (GA) is applied to the correction of loss coefficients to obtain the final optimization loss models. Compared with the experimental data, the maximum relative error of adiabatic the three classical models is 7.22%, while the maximum relative error calculated by optimized loss models is 1.22%, which is reduced by 6%. Similarly, compared with the original model, the maximum relative error of the total pressure ratio is also reduced. As a result, the present optimized models provide more reliable performance prediction in both tendency and accuracy than the classical loss models.

In this work, the interactions between the environmentally friendly refrigerant propane (R290) and Polyol Ester (POE) including solubility parameters, diffusion coefficients, binding energies, and radial distribution functions were investigated using molecular dynamics (MD). Specifically, the effect of chain length of Pentaerythritol esters (PEC) as the representative component of POE on the interaction of PEC/R290 was discussed. The solubility parameters difference exhibits the PEC and R290 are more easily miscible as increasing chain length of PEC, and there is plateau as the chain lengths is above 8 units. In addition, it was also found that solubility parameters are various for the isomers due to the different spatial structure. Moreover, the presence of PEC would reduce the diffusion coefficient of R290 in the mixed system of R290/lubricant with the reduction of 20% on average. It is also found that van der Waals forces are dominant in the R290/PEC system. The PEC molecules start to be bound to the H atoms of R290 at the first neighbor shell layer with a radius of 0.219 nm. Finally, the molecular simulation model of POE22 considering various actual components was innovatively developed. The results showed that the solubility of R290 with typical POE lubricant is affected by the composition and proportions of based oil and additives.

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 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.

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.

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.