Applied Thermal Engineering最新文献

To achieve optimal energy efficiency in buildings, accurately forecasting the energy consumption of air conditioning systems is crucial. This study develops an energy consumption prediction model based on a deep belief network, which is constructed according to the principles of a restricted Boltzmann machine. Actual experimental data from a water source heat pump system are collected, and feature variables are selected. The study discusses the impact of model parameters and training set sizes on the performance of energy consumption prediction model. Additionally, the trend in model prediction performance is analyzed through parameter adjustments. The results show that the coefficient of determination (R²) for the optimized model has increased to 0.585. The mean square error (MSE), root mean square error (RMSE), and mean absolute error (MAE) have been reduced to 6.311, 2.512, and 1.625, respectively. The deep belief network energy consumption prediction model outperforms other common machine learning models for water source heat pump systems.

The thermal sensitivity of lithium-ion batteries (LIBs), crucial for electric vehicles, poses a significant challenge, especially under harsh ambient conditions. This study introduces an innovative cooling strategy that combines phase change materials (PCMs) with active cooling to achieve uniform temperature distribution across LIBs and optimize recovery time for PCM solidification. Using the Newman, Tiedemann, Gu, and Kim (NTGK) model for numerical analysis, this study investigates the heat transfer behaviour of a single Li-ion cell equipped with PCM for passive cooling under different battery C-rates, ambient temperatures, PCM thickness, internal and external fins, and convective heat transfer coefficients during 3C–0C and 3C–1C discharging–charging cycles. The addition of a 2 mm layer of PCM to the cell results in a reduction of the maximum temperature by 28.2 °C at a discharging rate of 3C at 20 W/m²·K when compared to an uncooled, bare cell configuration at the ambient temperature of 30 °C. Adding six internal fins decreases the cell temperature by 0.63 °C and the PCM temperature by 0.73 °C at the ambient temperature of 30 °C. Furthermore, increasing the convective heat transfer coefficient to 100 W/m²·K and extending with 6 fins of 4 mm each reduces the maximum battery temperature by 40.63 °C, optimizing the solidification time of PCM to 800 s at an ambient temperature of 40 °C. The findings reveal that optimally configured extended fins integrated with PCM reduce peak temperatures during high C-rate operations and shorten the PCM recovery time during the discharging-standalone and discharging-charging phases, facilitating uninterrupted functionality across repeated cycles, even in extreme ambient environments.

Flexible thermal insulation membrane plays a key role in outdoor wear of human body and thermal management of electronic products. This study used electrospinning to prepare thermal insulation hollow silica/polytetrafluoroethylene (HSi/PTFE) fiber membranes. HSi were prepared using tetraethylorthosilicate as the silicon source and hydrothermal carbon spheres as templates. A spinning solution of PTFE containing the HSi was used to prepare fiber membranes. The heat transfer resistance of the fiber is improved by embedding HSi into the PTFE fiber, resulting to improved heat insulation capability of the fiber membrane. The influence of HSi content on the thermal insulation performance of PTFE fiber membrane was studied. When the HSi content was 5 %, the fiber membrane showed the lowest thermal conductivity (0.0197 W/(m·K)), which was not only lower than most fiber thermal insulation materials, but also had excellent tensile properties (tensile deformation capacity of 168 %), which was convenient for practical application. In addition, this kind of fiber membrane also has high hydrophobicity (water contact angle of 147°), effectively reducing the influence of moisture on thermal insulation performance. This work presents innovative prospects for the future advancement of thermal insulation materials.

Sintered porous coating tubes are high performance heat transfer components which are used to enhance boiling heat transfer. Sintering metal powder particles on the surfaces of plain tubes form porous coatings with numerous cavities which can promote nucleation of bubble generation in boiling processes and thus enhance boiling heat transfer enhancement. In the present study, experiments of the subcooled and saturated flow boiling heat transfer characteristics on the sintered porous coating tubes were conducted. The test tubes with porous coatings have an outer diameter of 25 mm, a length of 1 m and the coating thicknesses are 0.06 mm, 0.12 mm, 0.18 mm, and 0.25 mm, respectively. The heat transfer performance of high flux tubes is evaluated with a mass flow rate ranging from 128.3 to 252.03 kg/m²·s and the saturation temperature of the experimental section is controlled between 45 and 50℃. The influence of flow conditions, heat flux, and properties of the sintered layer on boiling heat transfer was discussed. The results indicate that sintered porous media can effectively reduce the degree of superheating required for boiling heat transfer, but they also inevitably increase in flow resistance. Remarkably, the heat transfer enhancement due to the porous media increases up to a certain point and then decreases, while the flow resistance increases as the sintered layers thicken. The maximum heat transfer coefficient of the sintered tube with a sintered thickness of 0.06 mm is 1.6 times greater than that of a smooth tube. However, increasing the thickness of the porous layer does not always enhance heat transfer. The effects of different particle sizes of the sintered grains and the thickness of the sintered layers under the conditions of subcooled boiling and the onset of nucleate boiling have been analyzed to understand the physical mechanisms. An empirical heat transfer correlation has been proposed according to the experimental results for the sake of design calculation in industry.

In this study, we have established an experimental platform featuring a shell and tube heat exchanger (STHE) combined with phase change material (PCM) to investigate its energy storage and release performance. Paraffin 25 and water have been selected as the energy storage material (ESM) and the heat transfer fluid (HTF), respectively. Besides, numerical simulations of different energy storage units by changing the phase change unit structures are carried out with FLUENT software. The effect of different specific surface area (surface area per bulk volume, m⁻¹) and length-to-diameter (L/D) ratios on the energy storage and release process is numerically studied. The findings show that as the specific surface area rises, the heat conduction effect gets stronger. When the specific surface area rises by 223.8 %, the melting time and solidification time can be cut by about 75.9 % and 87.4 %, respectively. Furthermore, the L/D ratio also has a great influence on the average energy storage rate since the average energy storage rate decreases by 9.6 % when the L/D ratio is increased from 7.9 to 10.5. In contrast, the average energy release rate decreases by only 1.6 %. Additionally, the cooling capacity of the STHE has been extensively explored in this research. For instance, under specific conditions (e.g., with a L/D of 7.9 and a specific surface area of 111.1), the cooling performance is evaluated. The study reveals that when the power of data center servers is set at 100 W and 200 W, the emergency cooling periods are observed to be 1680 s and 330 s, respectively.

Loop heat pipe (LHP), as passive heat transfer system, is one of the methods for thermal management of electronic components. To improve the heat transfer performance of LHPs, there is a pressing need for high-performance wicks. In this study, the hydrothermal carbonization method was used to fabricate a carbon spheres modified nickel wick (CSs-Ni-Wick) based on a biporous wick. The physical characteristics of the CSs-Ni-Wick were then analyzed experimentally. This unique CSs-Ni-Wick combined the advantages of large pores for reducing flow resistance and small pores for enhancing capillarity. Furthermore, the CSs-Ni-Wick surface exhibited a higher concentration of hydrophilic functional groups, effectively facilitating the replenishment of subcooled liquid to the vapor–liquid interface and preventing wick drying. Based on these advantages, a flat plate LHP was constructed and subjected to multiple tests in horizontal condition to evaluate the heat transfer performance of the CSs-Ni-Wick. Experimental results revealed that the LHP achieved a maximum heat load of 140 W (20 W/cm²) and a minimum thermal resistance of 0.357 °C/W, while maintaining the heat source temperature below 85℃. Additionally, the implementation of a micro-carbonized surface increased the density of vaporization cores, facilitating faster vapor nucleation, particularly at low heat loads. This enables vapor to be transferred more quickly from the evaporator to the condenser, leading to a smooth startup in the brass LHP using methanol as the working fluid, characterized by the absence of temperature overshoot or oscillation.

In modern aircraft, the icing detector is incorporated into the ice protection system (IPS) to determine if the aircraft is in an icing environment, thereby enabling the IPS to activate promptly when ice forms on the wings, engines, etc. Regardless of the type of icing detector used, certain unique flight conditions can result in ice accumulation on wings or engines before detection by the device, thereby delaying the identification of icing events and compromising aircraft safety. To mitigate the risk of icing detection failure, aircraft must operate in environments above the critical icing detection temperature. However, within the icing envelope, capturing the critical temperature has traditionally been a challenging task. This paper addresses this issue through a combination of ice wind tunnel experiments, numerical simulations, and theoretical analysis, a novel and rapid method for calculating the critical temperature is presented. Current research involves the experimental measurement of critical wing temperatures in an icing wind tunnel, revealing that icing detection failures occur under flight conditions with a large angle of attack. Additionally, a numerical model has been developed to calculate the critical temperature, and its results align well with experimental data. However, due to the high computational cost of numerical simulations across a wide range of icing conditions, this paper proposes an analytical method based on the principle of thermal equilibrium. This method rapidly predicts the critical temperatures of the probe and wing, achieving a deviation of less than 10% between the analytical and experimental values.

To address the issue of instability in power systems caused by the large-scale integration of renewable energy into the grid, the importance of large-capacity energy storage technologies has been increasingly recognized. To cope with the large storage tanks required for compressed carbon dioxide energy storage systems, two carbon dioxide pumped-thermal energy storage systems are proposed and modeled. Thermodynamic analyses of systems are conducted and sensitivity analyses of key parameters are performed. Parameter improvements are conducted based on the results of sensitivity analyses. The results show that the Rankine cycle-based carbon dioxide pumped-thermal energy storage system achieves a higher round-trip efficiency. For the Rankine cycle-based carbon dioxide pumped-thermal energy storage system, most exergy destruction occurs within the heat exchange units, with the highest exergy destruction in the first regenerator, accounting for 18.16% of the total. Through parameter improvement, the round-trip efficiency of the Brayton cycle-based carbon dioxide pumped-thermal energy storage system can be improved from 49.83% to 62.83%, while the round-trip efficiency of the Rankine cycle-based carbon dioxide pumped-thermal energy storage system can be improved from 60.16% to 69.28%.

This study achieves combustion applications of methanol-diesel dual-direct injection in a retrofitted diesel engine by investigating methanol sprays and engine performance/emissions. Methanol draws high attention due to its green production potential and ease of adaptation to existing combustors and supply infrastructure. One of the most promising methods for utilising methanol in diesel engines is the dual direct injection, which provides a wide operating range and flexible injection control. This study provides an effective solution for dual direct injection by implementing a nozzle cap idea, which can mount a conventional direct injector for methanol delivery in an existing diesel engine. The custom-made nozzle cap can provide hole orientation variations, which effectively controls the methanol-air mixture distributions within the piston bowl. To this end, methanol sprays formed through the three-hole nozzle cap are analysed for varied injector pressure of 15 ∼ 35 MPa. High-speed schlieren imaging confirmed working of the new nozzle for methanol injection with expected results of increased liquid penetration length and cone angle for higher injection pressure. The spray images also helped understand how the mixtures would be distributed within the piston bowl due to direct injection. Experiments performed on a 1-litre single-cylinder common-rail diesel engine operating at 1400 rpm, up to 70 % methanol energy fraction and a broad range of methanol injection timings of BDC to TDC, showed that the methanol-diesel dual direct injection combustion produces overall lower power output than the diesel baseline due to lower calorific value and flame temperature of methanol but significantly reduced CO₂ emissions by up to 16 % and very low smoke emissions. The results showed high sensitivity to methanol injection timings and energy fraction in terms of the measured pressure, derived heat release rate and produced power due to an increase in mixture homogeneity for earlier injection timings and stratified charge conditions for later injection timings. However, the nozzle orientation change and expected methanol-air mixture distributions showed no measurable impact on pressure and heat release rate as well as engine power output and combustion stability. The most significant impact of the methanol three-hole nozzle orientation and resulting mixture distributions was found from uHC and NO_x emissions because a nozzle orientation directing methanol more towards the corner of the piston bowl opposite side of the injector led to increased liquid wall wetting and methanol in crevice volumes and thereby causing less complete combustion for higher uHC and lower NO_x. The low sensitivity of methanol mixture distributions to the in-cylinder pressure and engine power output but the measurable impact found on uHC and NO_x emissions empathies the required optimisation of methanol direct injector nozzle depending on the