On the morning of July 31, the 8th National Symposium on New Refrigeration and Air Conditioning Technologies was held at California Garden Hotel in Chengdu. Co-hosted by Shanghai Jiao Tong University and the Chinese Society of Refrigeration, and co-organized by Southwest Jiaotong University and Sichuan Society of Refrigeration, the conference gathered more than 300 participants nationwide, including experts, professors and students majoring in refrigeration and air conditioning, as well as researchers and enterprise representatives from the industry. The three-day event consisted of a seminar on the disciplinary development and teaching of refrigeration, heating, ventilation and air conditioning on the first day, and a symposium on new refrigeration and air conditioning technologies on the second and third days.

The first half of the session was presided over by Professor Yang Zhao from Tianjin University and Professor Zhang Chunlu from Tongji University, while the second half was chaired by Professor Wang Hongbin from Shandong Normal University. A total of 17 reports were delivered in this sub-forum, including 9 in the first half and 7 in the second half.

Student Xu Mengfei from Southeast University presented a report entitled Thermodynamic Analysis of Absorption Refrigeration Cycle Based on Dual Working Fluid Pairs. A novel absorption refrigeration cycle adopting dual working fluid pairs was proposed in this study. Lithium chloride (LiCl) aqueous solution with high surface vapor pressure was used as the high-pressure stage working pair, while lithium bromide (LiBr) aqueous solution with low surface vapor pressure served as the low-pressure stage working pair. Theoretical calculation and analysis on the thermodynamic performance of the cycle under various operating conditions were conducted. The results show that LiCl aqueous solution delivers better thermodynamic performance than LiBr aqueous solution at low driving heat source temperatures. Furthermore, the dual working fluid pair-based absorption refrigeration cycle broadens the utilization temperature range of low-grade solar energy to 55–75℃, with a maximum thermal coefficient of performance (COP) reaching 0.47. It also extends the utilization duration of low-temperature solar energy in daytime and improves the efficiency of solar heat collection. In addition, intermediate pressure exerts a significant impact on cycle thermodynamic performance, and an optimal intermediate pressure exists to maximize overall performance, with the optimal range determined at 2.5–4.0 kPa.

Student Xing Meibo from Xi’an Jiaotong University delivered a report titled Thermodynamic Analysis of Transcritical CO₂ Two-stage Compression Heat Pump Cycle Enhanced by Dual Ejectors. This study proposed to adopt dual ejectors as expansion devices to improve the performance and expand the operating range of transcritical carbon dioxide two-stage compression heat pump cycles. Gas-liquid two-phase ejectors were respectively applied in the low-pressure and high-pressure stages to fully recover the expansion work during refrigerant throttling and reduce throttling losses. A thermodynamic model was established and compared with the conventional two-stage compression cycle equipped with a flash tank. The results indicate that compared with the traditional system, the dual-ejector two-stage compression cycle effectively increases the heating COP and volumetric heating capacity, with the maximum heating COP improved by 30.62%.

Student Ju Haohong from Xi’an Polytechnic University presented a report named Energy-saving Analysis of Indirect Evaporative Cooling Economizer for Data Centers. Targeting the practical engineering application of an evaporative cooling economizer in a data center in Xi’an, the research tested and analyzed the operating modes of the economizer and obtained key performance parameters including temperature, relative humidity, air volume and cooling capacity of the tubular indirect evaporative cooling unit. Test data from two groups reveal that the cooling capacity of the indirect evaporative cooling economizer accounts for approximately one quarter of the total cooling capacity. The economizer reduces the reliance on mechanical refrigeration, which further verifies the practical energy-saving effect of air-side economizers for industrial air conditioning in areas with moderate humidity.

Student Tan Yingying from Henan University of Science and Technology gave a report entitled Research on a Novel Hybrid Working Fluid Ejection Refrigeration Cycle. Driven by low-grade heat, an innovative hybrid working fluid ejection refrigeration cycle was proposed by introducing the principle of auto-cascade ejection refrigeration. Two-stage fractional condensation and separation were adopted to reduce the pressure ratio of the new cycle, achieving lower refrigeration temperature and higher cooling efficiency. Thermodynamic mathematical models for each component of the system were established. Under stable operating conditions, the effects of ejector pressure ratio and condensation temperature on system performance were analyzed. The research demonstrates that the ejection refrigeration cycle using R134a/R23 mixed working fluid can achieve a refrigeration temperature below -40℃.

Zou Huiming from Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, delivered a report on Control System Research of Linear Compressors for Refrigerators. Based on the control objectives of linear compressors for household refrigerators, combined with compressed air experiments and theoretical analysis, the study investigated key control technologies for stroke detection, top dead center detection and performance optimization of linear compressor control systems. Corresponding control strategies suitable for refrigerator refrigeration operation were formulated, and refrigeration performance tests were carried out. Joint operation experiments show that the proposed control system enables efficient and reliable operation of linear compressors, with the refrigeration COP reaching 1.74 under rated working conditions of household refrigerators.

Student Yuan Han from Tongji University presented a report titled Performance Optimization Analysis of Air Refrigeration Cycle. Environmentally friendly and pollution-free, the air refrigeration cycle is a promising alternative to traditional vapor compression refrigeration cycles. A dimensionless thermodynamic model of the single-stage air refrigeration cycle was established, and formulas for the optimal pressure ratio corresponding to maximum COP as well as analytical expressions of cycle performance parameters under optimal pressure ratios were derived. In-depth numerical analysis was conducted under different operating conditions and rotating component efficiencies. The results show that an increased temperature difference between high and low-temperature heat sources raises the optimal pressure ratio, lowers the optimal COP and increases the unit cooling capacity. Improving the efficiency of rotating components slightly reduces the optimal pressure ratio, while significantly increasing the optimal COP and decreasing the unit cooling capacity. Among all components, expander efficiency has the most prominent impact on the overall optimal performance. The findings provide references for the optimal design of practical air refrigeration systems.

Student Luo Jiangyu from Zhejiang University delivered a report named Simulation Study on Valve Switching of Pump-free Ejection Refrigeration Systems. To reduce system vibration, improve the utilization efficiency of low-grade heat and cut down power consumption, a pump-free ejection refrigeration system with valve switching was proposed and theoretically simulated. Water was adopted as the refrigerant, with a design condensation temperature of 304 K and evaporation temperature of 288 K. Due to the intermittent water return mode of the switching valve system, this study compared the average COP under two generating temperatures (423 K and 419 K) with constant condensation and evaporation temperatures. Theoretical calculation results show that the average system COP is 0.499 at the generating temperature of 423 K and 0.555 at 419 K.

Zhang Luyao from Xi’an Polytechnic University presented a report entitled Influence of Sub-wet-bulb Temperature Cold Water on the Cooling Performance of Wet Curtains in Agricultural Greenhouses. Mathematical modeling was established to theoretically analyze the influence of sub-wet-bulb temperature cold water on wet curtain cooling performance. Field tests were conducted on a greenhouse equipped with a wet curtain-fan cooling system in Fuping, Shaanxi Province. The results indicate that when the average temperature of circulating water is 3.4℃ lower than the outdoor average wet-bulb temperature, the evaporative cooling efficiency of the wet curtain reaches 92.37%, and the outlet air temperature is 0.6℃ lower than the outdoor wet-bulb temperature. The research proves that adopting sub-wet-bulb temperature cold water as circulating water for direct evaporative cooling can further reduce the dry-bulb temperature of outlet air and improve the cooling efficiency of wet curtains.

Student Chen Shaojie from Zhejiang University gave a report on A Pump-free Ejection Refrigerator with Three-way Valves. Ejection refrigeration is a heat-driven cooling technology, in which the circulating pump is the only moving component consuming high-grade electric energy. Compared with other system components, circulating pumps incur high investment and maintenance costs and reduce the reliability and practicality of ejection refrigeration systems. This paper proposed a novel pump-free ejection refrigerator adopting three-way valves. By means of three-way valve switching and height difference between system components, the traditional circulating pump was eliminated. Theoretical research was carried out on the control strategy of the three-way valve and the performance comparison between the new system and conventional systems. The results show that the switching time of the three-way valve is related to liquid storage tank volume, refrigerant flow area and vertical height difference between components. The new system requires a small amount of extra heat for liquid delivery, which is only correlated with the generating temperature.

Professor Wang Hongbin from Lishan College, Shandong Normal University, delivered a report entitled New Options for Solar Absorption Refrigeration. Based on the analysis of operating environments of various solar refrigeration systems, six types of advanced solar absorption refrigeration units were proposed, including two-stage lithium bromide absorption units and evaporative condensation absorption units suitable for low-temperature solar heat sources, as well as absorption air cooler units. Single/double-effect absorption units can achieve efficient cooling with a COP of approximately 1.2 under high-temperature heat sources and stable operation with a COP of 0.72 under low-temperature conditions. Hot water/direct-fired single/double-effect absorption units integrate solar cooling for sunny days and gas-fired refrigeration for cloudy days or insufficient cooling demand. A preliminary performance evaluation verifies that these solar refrigeration units can achieve ideal cooling efficiency under their corresponding operating conditions with optimized design and control strategies.

Professor Yu Hang from Tongji University presented a report on Experimental Study on the Reliability of Supercooling Ice-making Technology. Ice formation inside supercoolers easily leads to the failure of ice-making systems. Based on reliability mathematics theory, the concept of ice-making reliability was defined, and an experimental platform for supercooling ice-making was built for reliability tests. The influences of secondary refrigerant temperature and Reynolds number of supercooled water on ice-making reliability were experimentally investigated.

Wang Dong from University of Shanghai for Science and Technology delivered a report titled Influence of Humidification Capacity on Defrosting of Air Coolers in Low-temperature Cold Storage. Targeting an existing low-temperature cold storage, a partition device for air coolers was designed. Comparative experiments on two defrosting methods (electric heating and hot gas defrosting) were conducted to explore the effects of humidification capacity on defrosting duration, defrosting load and ambient temperature distribution with or without partition devices. The results show that energy consumption increases with the rise of humidification capacity, with an over 18% energy increase at the humidification capacity of 1000 g. Electric heating defrosting causes severe disturbance to the cold storage temperature field, resulting in a 5–10℃ temperature rise after defrosting. The installation of partition devices reduces energy consumption by about 20% and greatly alleviates temperature disturbance. The maximum temperature fluctuation during electric heating defrosting is reduced by 5℃, while the impact on hot gas defrosting is relatively minor.

Chen Shuangtao from Xi’an Jiaotong University presented a report named Design and Construction of Transcritical CO₂ Refrigeration System with Turbo Expanders. Due to the environmental hazards of synthetic refrigerants, carbon dioxide (CO₂) has been widely recognized as an eco-friendly alternative to Freon. Featuring non-flammability, non-toxicity, easy accessibility, high volumetric cooling capacity and excellent heat transfer performance, CO₂ often operates in transcritical vapor compression cycles for heat pumps and air conditioning systems owing to its high heat rejection temperature exceeding the critical point. However, massive throttling energy loss leads to a low COP in traditional transcritical CO₂ cycles. Common efficiency improvement measures include two-stage compression, heat regeneration and the replacement of throttling valves with expanders, ejectors or vortex tubes. Thermodynamic analysis based on the first and second laws of thermodynamics was performed on CO₂ refrigeration cycles with different expansion devices. A 15 kW refrigeration system equipped with micro turbo expanders was designed and constructed. Two micro turbomachines with wheel diameters of 10.6 mm and 10.1 mm were developed, both operating at a shaft speed exceeding 100 krpm. The CO₂ working fluid at the expander outlet presents two-phase flow and subcooled liquid states respectively. This research lays a foundation for the application of turbo expanders replacing throttling valves and the study of high liquid-content two-phase expanders.

Cai Shanshan from Huazhong University of Science and Technology delivered a report on Hygroscopicity and Thermal Conductivity of Mechanical Pipe Cold Insulation Systems under Low-temperature and Humid Conditions. Widely applied in low-temperature transmission systems such as HVAC and refrigeration, mechanical pipe cold insulation materials reduce efficiency loss caused by heat exchange with the external environment. When the pipe surface temperature is lower than the ambient dew point, water vapor condenses and accumulates on cold surfaces, penetrating into insulation materials, deteriorating thermal insulation performance and increasing cooling loss, which directly raises the overall energy consumption of air conditioning systems. At present, there is no unified standard for thermal conductivity measurement of pipe insulation systems under low-temperature and humid conditions, and relevant research data on the correlation between water content and thermal conductivity remain insufficient. To fill the technical gap, a novel experimental scheme was proposed to test the hygroscopicity and thermal conductivity of pipe cold insulation systems. The thermal conductivity and water content of three commonly used insulation materials were measured. Based on experimental data, the diffusion law of water vapor in insulation systems and the influences of adhesives and waterproof membranes on hygroscopic and thermal performance were analyzed.

Student Jia Rong from Shanghai Jiao Tong University presented a report entitled Influence of Heavy Hydrocarbon Components in Mixed Refrigerants on PRICO Liquefaction Process. The composition of mixed refrigerants is a core factor affecting the performance of mixed refrigerant (MR) liquefaction processes. In practical LNG facilities, heavy hydrocarbon components may hinder normal equipment operation. This study analyzed the impacts of C4 and C5 components on the PRICO liquefaction process and proposed targeted control solutions for heavy hydrocarbons in liquefaction production.

Student Chen Jianye from Zhejiang University delivered a report named Flooding Characteristics of LN₂/VN₂ Two-phase Countercurrent Flow in Inclined Tubes. To explore the flooding mechanism of low-temperature fluids, liquid nitrogen/saturated nitrogen vapor (LN₂/VN₂) was adopted as the working medium for flooding experiments in inclined circular tubes. The two-phase flow pattern evolution during flooding was analyzed, and a comparative study on flooding characteristics between LN₂/VN₂ and water/air working pairs was conducted. The results show that the interface wave remains complete with slug flow during water-air flooding, while the interface wave is broken into mist flow by gas phase in the LN₂/VN₂ system. Fast Fourier Transform (FFT) was applied to the pressure difference data at both ends of inclined tubes to analyze the interface wave frequency variation and key influencing factors of flooding intensity. The effect of inclination angle on critical gas velocity was also investigated. No significant difference was observed in the maximum interface wave frequency between low-temperature and normal-temperature fluids. Inclination angle has a minor impact on flooding intensity but a remarkable influence on critical flooding velocity.

Zou Caifu from Qinzhou Coastal Construction Drawing Review Center, Guangxi, presented a report on Renovation Design of Produce Distribution Cold Storage for the South-to-North Vegetable Transportation Project. Cold storage facilities at producing areas are the core infrastructure of the South-to-North Vegetable Transportation project. Different from traditional storage-oriented cold storages designed for maximum cargo storage capacity, produce distribution cold storages in southern regions should prioritize rapid cooling and cold processing capacity. This paper discussed the technical renovation of existing traditional vegetable cold storages to achieve rapid cooling optimization. Combined with the climatic characteristics of southern China, the renovated cold storages were designed to operate under both high and low temperatures to adapt to the storage and processing demands of various vegetables in different seasons. The technical feasibility and economic benefits of the renovation scheme were comprehensively demonstrated.