Frosting is one main adverse factor hindering the application of finned tube evaporators in refrigeration systems. To accurately predict the growth characteristics of the frost layer, its growth behavior on the surface of a three-dimensional finned tube under forced convection conditions was numerically simulated based on the coupled VOF (volume of fluid) multiphase flow and phase change mass transfer rate model method. The maximum difference between the simulated frost layer thickness and the experimental results was within 15%. The estimated value of the frost layer density was within a confidence interval of up to 90%; this is in good agreement. By building a visualization experimental platform for frosting on the surface of the finned tube of the evaporator, the influences of ambient temperature, relative humidity, and frontal wind speed on frost growth characteristics under single-factor changes were analyzed. The results showed that the thickness of the frost layer gradually decreased from front to back in the direction of the airflow. The lower the air temperature, the greater the wind speed, the higher the relative humidity, the larger the thickness of the frost layer, the more frost, and the larger the thermal resistance of the frost layer. The thickness of the frost layer reached its highest value of 2.344 mm at 85% relative humidity. According to the Morris sensitivity analysis, the relative humidity had the greatest influence on the frost layer thermal resistance, and the sensitivity coefficient reached 2.41. The correlation of the frost layer thermal resistance with different environmental parameters was obtained using the least squares regression method.

The combination of a super-long gravity heat pipe and a heat pump system for harvesting deep geothermal heat has the advantages of low cost, high efficiency, and no groundwater contamination. Direct heat exchange between the evaporator of the heat pump system and the condenser of the gravity heat pipe can simplify the heat exchange process and improve the heating efficiency of the system. Therefore, a U-shaped evaporator-condenser was developed, and its heat transfer performance was studied by building an experimental platform combining a heat pump and a heat pipe. Notably, the heat transfer coefficient of the U-shaped evaporator-condenser reached 2 037.92 W/(m²·℃) when the working fluid on the heat pump side passed through the tube. Based on the homogeneous flow model, a one-dimensional steady-state evaporator-condenser heat transfer model was established by integrating the mass, energy, and momentum conservation equations with empirical formulas for condensation outside the tube and boiling heat transfer inside the tube. Using Python, the simulation results were compared with experimental data. Notably, the average deviation of the heat transfer in the evaporator-condenser was 18.91%, confirming the accuracy of the model and providing a theoretical calculation method for designing an efficient evaporator-condenser.

The high redundancy of the measured data from heating, ventilation, and air conditioning (HVAC) systems significantly reduces the computational efficiency of model calibration. To address this challenge, a model calibration method based on mining feature operating conditions and a priori probability guidance was introduced in this study. Correlation analysis was conducted on the operational data for mining feature operating conditions. Feature variables related to HVAC system operation were selected, and a grid sampling technique based on these characteristic variables was employed to obtain representative operating conditions, enhancing the efficiency of the model calculations. Additionally, a prior probability model was established for the parameters to be calibrated during the model calibration process. A priori interval estimation was then performed, and the objective function was improved based on the prior probability to guide the model towards faster convergence. The proposed method was validated using a one-month operational dataset from a cooling plant in an industrial building located in Wuhan, China. The results indicated that the proposed method achieved significant improvements in performance metrics. Specifically, mean absolute percentage error (MAPE) and cross-validated root mean square error (CV-RMSE) were reduced by 16.0% and 12.0%, respectively, compared to the K-means clustering-based method, and by 20.9% and 15.2%, respectively, compared to the baseline data-based method. Furthermore, the normalized mean bias error (NMBE) was closer to zero, and the coefficient of determination (R²) increased by 4.7% and 8.5%, respectively, compared to the two aforementioned methods. Additionally, our method enhanced the computational efficiency by approximately 39.3%. This method provides technical guidance and data support for achieving an efficient and accurate modeling of HVAC systems.

Improving indoor air quality in homes requires fresh air; however, this is a strain on air conditioning systems. To address this issue, the use of energy-efficient fresh air units equipped with exhaust air heat recovery is recommended. These units include both passive and active types, with prominent examples being air-to-air enthalpy heat exchangers and heat pump units. Currently, the evaluation of the energy efficiency in fresh air units predominantly revolves around air-to-air enthalpy heat exchangers, rendering the commonly used heat exchange efficiency inapplicable to heat pump units. The concept of exhaust air heat recovery is perplexing and contradictory. Furthermore, the assessment of fresh air units primarily focuses on the units themselves, without considering their impact on air conditioning units and the overall system performance once combined. This study aims to establish a unified definition of exhaust air heat recovery for fresh air units, elucidating its intrinsic meaning. Additionally, it proposes a comprehensive energy efficiency evaluation method for a combined fresh air and air conditioning system. Through a case study of seven existing fresh air unit types, the necessity of exhaust air heat recovery is highlighted, and the energy efficiency levels of different unit types are compared.

Seasonal thermal energy storage (STES) can effectively mitigate the supply and demand imbalance of solar energy between winter and summer. Large-scale water pit thermal storage systems require efficient and accurate computational simulations to avoid investment waste. This study proposes a simplified numerical analysis method and establishes a cylindrical underground pit with a total volume of 11 304 m³ to describe the operation of a STES system. The model establishes a one-dimensional heat transfer model for the water body and a two-dimensional heat transfer model for the soil, separately solving for the water and the soil temperature field. The two models are connected through the temperature boundary at the pool wall to simulate the entire system. To comprehensively verify the accuracy of the numerical simulation model, validation was conducted under standby, charging, and discharging modes. The results indicate that the developed model has good accuracy and reliability. Under the standby mode, the temperature error of the five water layers in the sandbox test is less than 10%, with the highest accuracy in the middle and lower-middle water layers, with an average absolute error of 1.75% and 1.24%, respectively. Under the charging mode, the average relative error is 1.57%, and the average temperature error is 0.44 ℃. Under the discharging mode, the average relative error is 0.46%, and the average temperature error is 0.24 ℃.

Cryogenic liquid fuel launch vehicles encounter longitudinal unstable vibrations during flight, which is a serious threat to the normal operation of rockets. Such vibrations exhibit typical low-frequency characteristics and often occur during the jet condensation of cryogenic liquid oxygen in propellant pipelines. To solve this problem at the source, the characteristics of the jet condensation oscillation and flow pattern transition must be investigated. Based on the height function method, a modified mass transfer model was used to dynamically capture the interfacial curvature. The relationship between the condensation pulsation frequency and two-phase interfacial curvature was established, and the frequency of the pressure oscillation was found to be 9.8-10.6 Hz. The results indicated that three typical types of jet condensation oscillations exist: stable pulsation, gas plume oscillation, and suck-back flow. The pressure amplitude of the suck-back and oscillation flows was up to 130 kPa, whereas that of the stable pulsation was only 1-3 kPa. From the dimensional analysis, the transition threshold of the flow pattern was Jc^*=7.3 when dimensionless structure parameter L^*=2.2. When Jc^*>7.3, a suck-back oscillating flow pattern appeared. The dimensionless criterion could precisely predict the condensation flow pattern. This provides a theoretical basis and technical support for the design of cryogenic liquid fuel rockets.

A high-efficiency condensation dehumidification system utilizing copper foam driven by a Stirling refrigerator was developed to address the demands for high-efficiency heat transfer and a compact lightweight design in space stations. An experimental study was conducted to investigate its heat and mass transfer characteristics under various conditions. The experimental parameters were set as follows: air inlet temperature ranging from 20 ℃ to 30 ℃, relative humidity between 50% and 80%, cold plate temperature from 8 ℃ to 13 ℃, and inlet wind speed from 0.4 m/s to 1.4 m/s. The results indicated a positive correlation between the increase in the air inlet temperature and the enhancement of both the heat and mass transfer coefficients. Specifically, when the air inlet temperature increased from 20 ℃ to 30 ℃, the heat transfer coefficient increased by 10.5%, whereas the mass transfer coefficient exhibited a more substantial increase of 57.1%. Furthermore, variations in the relative humidity of the air inlet distinctly affected the heat and mass transfer coefficients: the heat transfer coefficient decreased by 31.6% with an increase in the relative humidity, whereas the mass transfer coefficient increased by 11.4%. Although reducing the temperature of the cold plate can effectively improve heat transfer, it leads to the accumulation of condensate water and reduces the efficiency of heat and mass transfer. Therefore, an appropriate cold plate temperature must be selected. Additionally, the efficiency of heat and mass transfer was markedly enhanced with increasing inlet wind speed. However, a continuous increase in wind speed resulted in higher system energy consumption. Thus, a balance between efficient heat transfer and high system energy consumption was essential. Based on extensive experimental data, the heat transfer model was refined using regression analysis. The standard deviation between the theoretical and experimental values was 8.21%, and the maximum deviation was 19.76%, demonstrating the strong predictive accuracy of the model.

With the development of computer technology and the application of artificial intelligence, electronic chips are becoming increasingly miniaturized and integrated, leading to a rapid increase in their volumetric heating power, thus affecting their normal operation. To address this problem, a heat sink with an array of finned porous microjets was designed, and HFE-7100, which has good thermal stability and electrical insulation, was selected as the cooling medium. Through a combination of numerical simulations and experimental research, the influence of factors such as the longitudinal aspect ratio of the slotted fins, inlet subcooling, inlet volumetric flow rate, and jet Reynolds number on the heat transfer process of microjet boiling was investigated. The results showed that the optimized structure with an aspect ratio of 0.5 met the requirements of chip cooling and had a better cooling effect. In the single-phase convection heat transfer stage, under the same working condition, the inlet subcooling degree had little effect on heat transfer, and increasing the volume flow rate or jet Reynolds number could strengthen the convection heat transfer, and the maximum heat transfer coefficient could reach 15 724.40 W/(m²·K). However, in the jet boiling stage, the heat flux corresponding to the onset of nucleate boiling (ONB), and it decreased with a decrease in the inlet subcooling degree. Increasing the inlet volume flow rate or jet Reynolds number inhibited the occurrence of boiling, thus weakening the heat transfer. However, compared with the single-phase convective heat transfer stage, the heat transfer coefficient increased by 20.6%.

Room-temperature magnetic heat pumps, one of the main applications of the magnetocaloric effect near room temperature, offer high efficiency, environmental protection, low noise, and low vibrations. This study discusses the application potential of a cascade magnetic heat pump cycle with a large temperature span by comparing the theoretical magnetic heat pump and refrigeration cycles. The results show that meeting the kilowatt-level heating capacity and achieving a wide temperature span of approximately 30 K in an actual heating scenario poses challenges to room-temperature magnetic heat pumps. The differences in design and application between the room-temperature magnetic heat pump and the existing room-temperature magnetic refrigeration prototypes are discussed, with a focus on magnetocaloric material selection strategies and performance evaluation indices suited for large temperature spans. The comparative analysis of magnetic heat pumps and magnetic refrigeration in this study also helps researchers to clarify the key to the design and application of large temperature span magnetic heat pumps based on existing research on room-temperature magnetic refrigeration. It promotes the comprehension and application of room-temperature magnetic heat pumps.

The current greenhouse effect and the "dual-carbon" goal have set off a new wave of refrigerant substitution, requiring new refrigerants to achieve a comprehensive balance between environmental protection, safety and thermal properties. However, there are currently no ideal substitutes for these refrigerants. Most environmentally friendly refrigerants, such as R290, R32, and R1234yf, which have low ozone depletion potential (ODP) and Global Warming Potential (GWP), are flammable and pose safety risks. Identifying suitable flame retardants for environmentally friendly flammable refrigerants has emerged as a crucial focus of current research on refrigerant alternatives. This article provides a recent overview of research progress on the compatibility of flame retardants with flammable refrigerants. The main focus was categorizing the various flame retardants and assessing their efficacy across different flammable substances. It also discusses their performance, effects, mechanisms, and environmental impacts. Furthermore, the article analyzes their potential applications and development trends and recommends flame retardants compatible with flammable substances.