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

The high-frequency pulse tube cryocooler (HPTC) represents a promising miniature cryocooling technology due to its compact structure and the absence of low-temperature moving components. However, limited to the non-ideal gas effect of ⁴He, the HPTC is hard to obtain high cooling performance in the liquid helium temperature range. ³He as the working fluid can effectively improve the cooling performance of the HPTC, but the high cost hinders its wide application. In consideration of both cooling performance and cost-effectiveness, this paper explores the feasibility of utilizing ³He-⁴He mixtures as the working fluid for HPTCs. Firstly, the experimental results of a developed HPTC based ⁴He are reported. With a total power consumption of 575 W, the lowest temperature of 3.26 K was observed. And the measured cooling power at 4.2 K was 20.8 mW. Then the theoretical utmost efficiency of the cryocooler was calculated in terms of the thermophysical properties of the working fluids, using ³He-⁴He mixtures with different compositions as the working fluids. The whole machine modeling of the HPTC was further carried out, and the influence of the working fluids with different components on the structural parameters such as double-inlet and inertance tube, and operating parameters such as pressure and frequency were analyzed. The calculated results show that the cooling power is expected to be increased to 36 mW and 53 mW if the equimolar 3He-4He mixture and pure 3He are used, respectively.

Achieving efficient thermal management urges to exploit high-thermal-conductivity materials to satisfy the boosted demand of heat dissipation. It is critical to adopt standardized characterization protocols to evaluate the intrinsic thermal conductivity of thermal management materials. However, for the most representative laser flash method, the lack of standard measurement methodology and systematic description on the thermal diffusivity and influencing factors has led to significant deviations and confusion of the thermal conduction performance in the emerging thermal management application. Here, the measurement error factors of thermal diffusivity by the common laser flash analyzer (LFA) are discussed. Taking high-thermal-conductivity graphitic film (GF) as a typical case, the key factors are analyzed to guide the measurement protocol of related carbon-based thermal management materials. The basic principle of the LFA measurement, actual pre-processing conditions, instrument parameters setting, and data analysis are elaborated for accurate measurements. Furthermore, the graphene thick films and common isotropic materials are also extended to meet various thermal measurement requirements. Based on the existing practical problems, we propose a feasible test flow to achieve a unified and standardized thermal conductivity measurement, which is beneficial to the rapid development of carbon-based thermal management materials.

Existing photovoltaic cells with high infrared emissivity generate huge radiative heat loss in photovoltaic/thermal applications and degrade the photothermal performance. The purpose of this work is to evaluate the full spectral absorptivity of CdTe cells to find a spectrally selective photovoltaic cell for photovoltaic/thermal applications. To this end, the solar absorptivity and mid-infrared thermal emissivity of CdTe cells were tested by ellipsometry, UV-Vis-NIR spectrophotometer, and Fourier transform infrared spectrometer. The experimental results show that the AM 1.5 solar spectrum weighted absorptivity of the substrate configuration CdTe cell reaches 0.91, and the mid-infrared emissivity is only 0.29, while the superstrate configuration cell emissivity is as high as 0.9. Further research shows that substrate configuration with a transparent conductive layer on top can be flexibly grown on metal foils and has spectral selectivity with high solar absorptivity and low mid-infrared emissivity should be considered in the future for photovoltaic/thermal applications.

Phase change materials (PCMs) are a kind of highly efficient thermal storage materials which have a bright application prospect in many fields such as energy conservation in buildings, waste heat recovery, battery thermal management and so on. Especially inorganic hydrated salt PCMs have received increasing attention from researchers due to their advantages of being inexpensive and non-flammable. However, inorganic hydrated salt PCMs are still limited by the aspects of inappropriate phase change temperature, liquid phase leakage, large supercooling and severe phase separation in the application process. In this work, sodium acetate trihydrate was selected as the basic inorganic PCM, and a novel shape-stabilized composite phase change material (CPCM) with good thermal properties was prepared by adding various functional additives. At first, the sodium acetate trihydrate-acetamide binary mixture was prepared and the melting point was adjusted using acetamide. Then the binary mixture was incorporated into expanded graphite to synthesize a novel shape-stabilized CPCM. The thermophysical properties of the resultant shape-stabilized CPCM were systematically investigated. The microscopic morphology and chemical structure of the obtained shape-stabilized CPCM were characterized and analyzed. The experiment results pointed out that acetamide could effectively lower the melting point of sodium acetate trihydrate. The obtained shape-stabilized CPCM modified with additional 18% (mass fraction) acetamide and 12% (mass fraction) expanded graphite exhibited good shape stability and thermophysical characteristics: a low supercooling degree of 1.75°C and an appropriate melting temperature of 40.77°C were obtained; the latent heat of 151.64 kJ/kg and thermal conductivity of 1.411 W/(m·K) were also satisfactory. Moreover, after 50 accelerated melting-freezing cycles, the obtained shape-stabilized CPCM represented good thermal reliability.

The effective cathode flow field design can realize the internal water balance and higher current density output of proton exchange membrane fuel cells (PEMFC). Therefore, a segmented water management flow field is proposed in this study, i.e. a half separated-half coupled cathode (HSHC) flow field which has two inlets but just one outlet. A 3D numerical PEMFC model is applied to study the effect of the HSHC flow field on PEMFC performance and its operating strategy in terms of operating conditions. The study results are shown as follows: Compared with the two conventional cathode flow fields, the HSHC flow field improves the water balance along the channel and increases the current density by 17.1% at a cathode stoichiometry of 3.25. It is because the HSHC flow field can overcome the water loss at channels upstream and the water accumulation at channels downstream. The draw water phenomenon (DWP) in the HSHC flow field is observed, which is mainly affected by the water vapor pressure of channel. Based on the DWP, cooling channel inlet flow rates can be used to adjust water balance, but severe water loss should be avoided. In addition, the inlet temperature control in HSHC flow field should be that cell temperature>cathode channel inlet temperature>cooling channel inlet temperature> ambient temperatures for better water balance.

700°C double reheat advanced ultra-supercritical power generation technology is one of the most important development directions for the efficient and clean utilization of coal. To solve the great exergy loss problem caused by the high superheat degrees of regenerative steam extractions in 700°C double reheat advanced ultra-supercritical power generation system, two optimization systems are proposed in this paper. System 1 is integrated with the back pressure extraction steam turbine, and system 2 is simultaneously integrated with both the outside steam cooler and back pressure extraction steam turbine. The system performance models are built by the Ebsilon Professional software. The performances of optimized systems are analyzed by the unit consumption method. The off-design performances of optimization systems are analyzed. The results show that: the standard power generation coal consumption rates of optimization systems 1 and 2 are decreased by 1.88 g·(kW·h)^–1, 2.97 g·(kW·h)^–1 compared with that of the 700°C reference system; the average superheat degrees of regenerative steam extractions of optimized systems 1 and 2 are decreased by 122.2°C, 140.7°C (100% turbine heat acceptance condition), respectively. The comparison results also show that the performance of the optimized system 2 is better than those of the optimized system 1 and the 700°C reference system. The power generation standard coal consumption rate and the power generation efficiency of the optimized system 2 are about 232.08 g·(kW·h)^–1 and 52.96% (100% turbine heat acceptance condition), respectively.

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.

Optimized fin arrangement and dimension of heat exchanger can improve the maximum output power of thermoelectric generator (TEG) system which converts the wasted heat into electricity with thermoelectric modules (TEMs). Considering that the geometric symmetry contributes to the temperature uniformity improvement and convenient TEMs arrangement, a low-backpressure TEG system based on a polyhedral-shape heat exchanger was developed. To assess the effect of inner topology and fin parameters on the heat transfer and output power of the TEG system, a realizable k-ɛ turbulence based numerical model was established and validated to perform numerical simulations. The results demonstrate that increasing fin length, fin width and fin intersection angle are beneficial to the average surface temperature, temperature distribution uniformity and maximum output power of the TEG system. Moreover, decreasing fin spacing distance contributes to the enhanced average surface temperature and maximum power of TEG system, and has insignificant effect on its temperature uniformity. The inserted fins with optimal length, width, intersection angle and spacing distance enhance higher output power, whereas result in increasing backpressure. The maximum difference between the experimental and simulation results is 3.2%, which validates the feasibility of the established numerical model. It also provides a theoretical reference to the optimal design and performance analysis of low-backpressure TEG systems used in automobile exhaust heat recovery.

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.

A composite biomass insulation material, which uses geopolymers as adhesives and forestry waste as fillers, was proposed and experimentally tested. The orthogonal experimental method was adopted to analyze the optimum theoretical oxide molar ratios and the mass ratio of mixing water to binder (m_w2/m_B) for preparing geopolymers. The influences of curing regimes (including one-stage and two-stage curing methods) and m_w2/m_B ratios of the insulation materials on mechanical, thermal, and hydraulic performances were also studied by experiment. The results indicated that the optimum combination scheme of preparing geopolymers was molar
ratio  ,  and m_w2/m_B =0.5 with the highest mechanical strength of 34.21 MPa. Besides, the best curing conditions of the composite material were the curing temperatures of 85°C and 70°C under the two-stage curing regime, which could achieve the low heat conductivity of 0.123 and 0.125 W/(m·K), and the high mechanical strength of 1.70 MPa and 1.71 MPa, respectively. The optimum m_w2/m_B ratios of the biomass material were 0.5 to 0.55 with heat conductivity of 0.114 to 0.125 W/(m·K). This novel composite insulation material has satisfying physical performances, which is helpful for achieving building energy conservation.

This paper presents a detailed and comprehensive multiphysics design process of an 80 kW, 60 000 r/min high-speed permanent magnet machine (HSPMM) for a micro gas turbine application. First, the preliminary design of the HSPMM is carried out according to the mechanical and electromagnetic theory. Afterwards, the influence of carbon fiber sleeve (CFS) thickness, rotor diameter and core length on rotor stress and rotor dynamics is carefully analyzed to obtain the optimal range of rotor diameter and core length. On this basis, the electromagnetic and power loss characteristics are analyzed in detail to obtain the final design scheme. Fluid-solid coupling model is used to calculate the temperature field of the HSPMM to verify the rationality of the scheme. The rotor thermal stress analysis considering the multi-layer and multi-angle winding of CFS is carried out to obtain the rotor models suitable for prototype and mass production, respectively. Finally, the prototypes are manufactured and tested to verify the reliability of the multiphysics design process.

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.