摘要:To ensure that the concentration of indoor pollutants in a laboratory animal room is maintained at a low level, all-fresh-air systems with large air volumes are typically used in traditional air-conditioning systems, which results in high energy consumption for air conditioning. The fresh air volume used to control the concentration of pollutants is considerably higher than that used to control other parameters; therefore, this study proposes an air-conditioning system that realizes independent control of temperature, humidity, and pollutants. The proposed system reduces the fresh air volume and energy consumption of a heating, ventilation, and air-conditioning system. A cage-type laboratory animal room is used as an example; the purification effect of the proposed system is simulated using CFD software, and the system’s energy-saving performance is investigated. The results show that for a fresh air change rate of 8.53 h-1, the circulating air change rate is 13 h-1, and the supply circulating air temperature is 19.4 ℃,the indoor temperature, humidity, and pollutant concentration requirements can be met to ensure the normal survival of mice. Compared with the traditional all-fresh-air system, the fresh air volume and energy consumption of the proposed system is decreased by 57.4% and 47.9%, respectively.
关键词:laboratory animal room;HVAC system;fresh air volume;independent control;circulating air purification
摘要:Natural refrigerants such as CO2 have become a research hotspot. Under high-pressure conditions, a large throttling loss becomes the primary reason for the low circulation efficiency of the CO2 refrigeration cycle. Therefore, reducing the throttling loss is an effective method to improve efficiency. This study proposes a supersonic two-phase expander with a Laval nozzle as the core component. A CO2 supersonic two-phase expansion refrigeration cycle model is established, and an ideal cycle thermodynamic analysis and simulation investigation are conducted. The results show that the inlet pressure and temperature of the supersonic two-phase expander and the outlet pressure of the cyclone separation section affect the cooling performance of the system. The coefficient of performance (COP) of the CO2 supersonic two-phase expansion refrigeration cycle is 6.69, which is 1.63 times that of the existing CO2 transcritical refrigeration cycle, with relatively optimal refrigeration performance. Additionally, the operating pressure of the system is considerably reduced. The loss of liquid-phase velocity during gas-liquid separation affects the refrigeration performance of the system. The COP of the system decreases from 9.56 to 6.01, and the relative Carnot efficiency decreases from 0.95 to 0.60; however, it still remains at a high level. The preliminary thermodynamic analysis and simulation show that the proposed CO2 supersonic two-phase expansion refrigeration cycle is feasible and exhibits good development prospects.
关键词:refrigeration cycle;supersonic two-phase expander;thermodynamic analysis;natural refrigerant CO2
摘要:Air refrigeration has been extensively applied to the fields of aircraft environmental control and food refrigeration. However, these scenarios are equipped with high-pressure air sources or separate air compressors, which limits the flexible application of air refrigeration technology, and the overall coefficient of performance (COP) is low. A high-speed centrifugal compression-and-expansion integrated machine driven by a high-speed motor can considerably simplify the system and improve the COP by recovering the expansion work. This study uses a high-speed motor-driven oil-free floating bearing compression-expansion integrated machine as the core component to build a fresh-air household air-refrigeration system with an open reverse booster cycle. The temperature, pressure, and flow of the expander and compressor inlet and outlet of the system is measured, and the system is connected with an enthalpy difference chamber to obtain the refrigeration capacity and air supply parameters of the refrigeration system. Performance tests are conducted at multiple speeds under standard air-conditioning conditions. At a rated speed of 38 000 r/min, the cooling capacity is 1.6 kW. The supply-air temperature decreases and the cooling capacity increases with an increase in speed. Based on this feature, the air refrigeration system has the advantages of a direct air supply and adjustable supply-air temperature, which enables air-cooling technology in the field of fresh-air conditioning applications.
摘要:A two-phase circulation cooling system with a multi-channel direct cooling plate was developed. The R1233zd(E) cooling medium was used. The thermodynamic cycle and cooling performance were investigated under the following conditions: the condensing temperature was set to 10, 15, and 20 °C, the mass flux varied from 147–882 kg/(m2?s), and the heat flux varied from 7.73–39.75 kW/m2. The results show that the enthalpy of the refrigerant at the cooling plate outlet decreases with an increase in the mass flux. Additionally, the system pressure and enthalpy of the refrigerant increase with an increase in heat flux. Under different heat fluxes, the wall temperature of the cold plate varies differently. When the heat flux is 7.73 kW/m2, the maximum temperature difference decreases from 2.9 to 1.6 K, whereas the mass flux rises from 147 to 735 kg/(m2?s). For a heat flux of 39.75 kW/m2, the maximum temperature of local wall difference increases from 3.6 to 5.2 K. For different mass flux, the heat transfer coefficients rise to different degrees with an increase in heat flux. When the mass flux is 147 kg/(m2?s), the heat transfer coefficient increases from 1 843 to 4 528 W/(m2?K). However, for a mass flux of 588 kg/(m2?s), the heat transfer coefficient under the same condition rises from 1 536 to 3 569 W/(m2?K).
关键词:two-phase cooling;surface heat transfer coefficient;cooling performance;direct cooling plate
摘要:This study proposes an absorption-compression cascade refrigeration cycle. The cycle consists of a high-temperature stage combined absorption-compression refrigeration cycle, which is driven by the waste heat of the internal combustion engine, and a low-temperature stage CO2 subcritical compression refrigeration cycle, which is driven by power. The performances of different refrigerant pairs in the cycle are compared, and the influence of key parameters on the performance of the cycle is further discussed. Finally, an economic analysis is performed. The results show that under the same condensation temperature of 40 °C and evaporation temperature of ?35 °C, R124-DMAC/R744 exhibits excellent performance with a coefficient of performance of 2.864. It is an ideal working pair, and the annual total cost of the system is 15 150.14 USD.
摘要:A dynamic simulation model of a transcritical CO2 parallel compression system was established using GT-SUITE simulation software to explore an efficient control method for the transcritical CO2 parallel compression system. Based on the system performance dataset obtained by the simulation, the second-order polynomial model and the neural network model were established and compared as the system performance prediction models. Based on the neural network model, a model predictive controller for the transcritical CO2 parallel compression system was developed, and the performance of the controller in terms of the stability, high efficiency, and real-time control of the system was studied. The results show that under the action of the model predictive controller, the system can reach a stable operating state within 150 s for different cooling conditions. The performance of the system using model predictive control is 13.3% higher than that using fixed value control. The simulation verifies that the proposed model predictive control strategy is feasible and optimizes the real-time control performance of the CO2 parallel compression system; the overall performance is improved by 7.3% compared with the fixed value control under the given working conditions. The control strategy proposed in this study is significant for the use of machine learning methods in designing system controllers to improve the performance of heat pump air-conditioning systems.
关键词:transcritical CO2 system;machine learning;model prediction control;dynamic simulation
摘要:There are many natural cold sources in the northern China in winter, and thermodynamic analysis shows that the heat released by the working medium subcooling can be compensated in the isothermal heat absorption process of the evaporator. To determine the influence of a natural cold source on the heating performance of a heat pump, an outdoor sub-cooler was added, and an air source heat pump experimental device using a natural cold source was built. The influence of subcooling on the heating performance of the air source heat pump was experimentally investigated. The experimental results show that subcooling has minimal influence on the heating capacity and heating coefficient of performance (COP) of the heat pump when the outdoor temperature is above 0 °C and the condensing temperature is less than 45 °C. The heating capacity of the system is maintained between 6.22and 6.70kW, the heating COP is maintained at approximately 3.03, and the discharge temperature of the compressor does not exceed 103 °C. However, when the outdoor and condensing temperatures are ?10 °C and 50 °C, respectively, as the subcooling increases, the compressor power and its discharge temperature increase significantly, the heating capacity of the system increases slightly at first and then decreases, and the heating COP decreases to 2.3. The results show the feasibility of the proposed air source heat pump system, which uses subcooling for defrosting and can achieve defrosting without stopping heating.
摘要:In this study, a mechanical subcooling transcritical CO2 heat pump heating system using a zeotropic refrigerant is proposed. A thermodynamic model is developed for this system and compared with that using a pure refrigerant. The results indicate that the coefficient of performance employing R1234ze(E)/R601(60/40) with a large temperature glide is up to 2.45, which is 13.82% higher than that of the pure component, at an ambient temperature of -12 ℃ and supply/return water temperatures of 65°C/40°C. The discharge pressure of the CO2 system can be efficiently reduced, and a higher subcooling degree is achieved using a zeotropic refrigerant; hence, the throttling irreversible loss decreases. The optimal discharge pressure is reduced by 27.85% using R290/R601(70/30). The temperature match in the subcooling process can be efficiently improved, and the exergy efficiency is up to 14.09% higher than that of the pure refrigerant component when using R1234ze(E)/R601(60/40). The relatively large temperature glide and suitable concave-convex characteristic of the temperature-enthalpy curve are the two key principles for zeotropic refrigerant selection used for CO2 heat pump systems with mechanical subcooling. Finally, the utilization of R1234ze(E)/R601(60/40) is recommended.
摘要:The gravity recirculation refrigeration system has two significant advantages compared with the ordinary direct-expansion refrigeration system. The lower the evaporation temperature, the clearer the efficiency advantage, and the lower temperature obtained in the chamber. In this study, the pressure enthalpy diagram of a gravity recirculation system is redesigned based on the principle of a gravity recirculation system, and the thermodynamic cycle of a gravity recirculation system is divided into two cycles for analysis. By analyzing the flow resistance in the refrigerant flow process, the variation trend of the pressure in each part of the system is obtained. The experimental results show that the pressure at each state point is identical to the theoretical analysis results. The coefficient of performance (COP) of the gravity recirculation refrigeration system can be improved by up to 19% at different evaporation temperatures compared with the experimental results of the ordinary direct-expansion refrigeration system, and the COP reaches 1.07 at ?30 °C.
摘要:A chilled water system with large temperature difference is essential for the economical operation of air conditioning. This design also enables air handling units (AHU) to achieve an energy-saving effect. Based on the mechanism of the heat exchange in AHU, this study analyzes how chilled water with large temperature difference affects the heat transfer of AHU. Experiments are conducted to test the influence of large temperature difference of chilled water in the cooling coils with differential enthalpy method. The pipe shapes and flow routes are optimized. The performances of the cooling coils are compared for different coil pipe shapes, flow routes, and fin spacings. Using the same heat exchange section, the AHU of this design reduces the energy consumption by approximately 20% compared with a conventional unit. Furthermore, a preferable scheme is proposed to reduce energy consumption by 25%, in which cooling coils are composed of four-row high-efficiency inner threaded pipes that have the same ratio (cooling capacity to airflow) as those of six-row non-threaded pipes. The results provide applicable and practical cases for the design of AHU with large temperature difference.
关键词:large temperature difference;air handling unit;coefficient of heat transfer;flow path
摘要:The limitations of non-uniform frosting along the direction of air flow and large frost mass in the first row of the finned tube heat exchanger of an air-source heat pump under low ambient temperature may cause a short defrost interval, heating capacity decline, and other negative effects. To overcome these challenges, an experimental study was conducted on finned tube heat exchangers with different fin pitches operating at low ambient temperatures. The results show that finned tube heat exchangers with different fin pitches can prolong the frosting blocking time of the windward tube row. This inhibits the frosting rate and frost quality and reduces the heat loss caused by frequent defrosting, which is also beneficial for increasing the total heat transfer in one defrosting cycle. The heat transfer capacity of the finned coil heat exchanger with variable pitch is higher in the middle and later stages of frosting. Under a reasonable combination of the fin pitch, the finned coil heat exchanger with variable pitch can prolong the frosting blocking time of the windward side of the heat exchanger without losing substantial heat transfer capacity.
关键词:air-source heat pump;different fin pitch;frosting condition;finned tube heat exchanger
摘要:In this study, the start-up time, thermal resistance, and evaporation temperature fluctuation of a pulsating heat pipe were experimentally studied based on the influence of alcohol polyoxyethylene ether nonionic surfactant (AEO-9). The experimental conditions included heating powers of 40 W, 60 W, 80 W, 100 W, and 120 W and AEO-9 mass concentrations of 10 mg/kg, 20 mg/kg, 30 mg/kg, and 40 mg/kg. The results indicate that AEO-9 can reduce the start-up time compared with deionized water; the start-up time first decreases and then increases as the AEO-9 mass concentration increases. When the heating power is 40 W and the AEO-9 mass concentration is 20 mg/kg, the start-up time is reduced by a maximum of 29.2%. AEO-9 can also reduce the thermal resistance. At a low heating power (≤60 W), the thermal resistance increases as the AEO-9 mass concentration increases. At a high heating power (≥100 W), the thermal resistance decreases as the AEO-9 mass concentration increases. When the heating power is 40 W and the AEO-9 mass concentration is 10 mg/kg, the thermal resistance is reduced by a maximum of 35.8%. AEO-9 can reduce the evaporation temperature fluctuation, and the fluctuation amplitude decreases as the AEO-9 concentration increases. When the heating power is 120 W and the AEO-9 mass concentration is 40 mg/kg, the temperature fluctuation amplitude is reduced by a maximum of 68.4%.
关键词:pulsating heat pipe;surfactant;start-up time;heat transfer characteristics
摘要:The ice storage air conditioner plays a vital role in the energy consumption of buildings, economic cost, and peak-load regulation of the electricity grid. However, the effects of different flow in the tube of ice storage tank, which is the key component of ice storage air conditioning system, on the ice layer have not been clearly determined, and there is a lack of studies on the influence of nanofluid on ice storage. In this study, the inlet conditions of cold storage materials and the influence of the volume fraction of nanocomposite phase-change materials on the solid-liquid phase change and temperature in the cold storage process are investigated, and the optimal cold storage conditions are obtained via numerical simulation. The results show that the effect of the inlet velocity is smaller than that of the temperature, and an increase in the inlet velocity decreases the temperature difference between the inlet and outlet, which results in a more uniform temperature distribution. The addition of nanoparticles improves the efficiency of the ice accumulation, and the volume fraction of the nanoparticles is the most important factor because it may decrease the efficiency.
关键词:phase change ice storage tank;nanofluid;ice storage;liquid phase rate;velocity of flow
摘要:The computational fluid dynamics (CFD) method is used to simulate the removal of fine particulate matter (PM2.5) by three displacement-like ventilation systems (horizontal supply air from the middle of the workshop and exhaust air from the upper part, horizontal supply air from the lower part of the workshop and exhaust air from the upper part, downward supply air from the lower part of the workshop and exhaust air from the upper part). Field measurements verify the accuracy of the simulation. The results show that the three displacement-like ventilation systems form pollutant stratification in the workshop, and the concentration of pollutants in the lower part is lower than that in the upper part of the workshop. Pollutants in the personnel activity area of the workshop can be reduced by 26.3%–70% by decreasing the height of the exhaust outlet. Compared with the other two ventilation systems, system 3 does not cause drafts, and it exhibits a high contaminant-removal effectiveness, which is more suitable for ventilation in industrial plants.
摘要:The dual purpose of ensuring sleep thermal comfort and saving energy can be achieved by developing a non-uniform sleeping environment. A thermoelectric-based cooling mattress system for sleep thermal comfort is proposed and constructed. Using the coupling heat-transfer model of thermoelectric-based cooling and the sleep thermal load, the thermal characteristics of the thermoelectric-based cooling mattress system are parametrically analyzed. The system design is optimized, and the energy-saving potential is evaluated. The results show that the range of neutral temperatures can be expanded while reducing the risk of condensation by increasing the number of semiconductors in the mattress and decreasing the current input. The energy-saving ratio of a thermoelectric-based cooling mattress is directly related to the indoor temperature, working current, and room area. The energy-saving ratio under Condition1 (setting temperature of 27 °C) and Condition2 (setting temperature of 28 °C) can reach up to 10.26% and 23.37%, respectively, compared with the Reference Condition (setting temperature of 26 °C).
关键词:thermoelectric cooling;heat transfer analysis;local cooling;sleep thermal environment;building energy saving
摘要:A new composite phase change material composed of sodium formate (HCOONa), ammonium chloride (NH4Cl), and water is prepared for low-temperature cold chain logistics below ?23 °C. The material (mass ratio of HCOONa-NH4Cl-H2O is 2:1:7) is named SF70. The supercooling, thermal conductivity, and phase separation of SF70 are optimized by adding nanomaterials and thickeners. Then, experiments are conducted on the circulation and application of the optimized material in an insulated container. The experimental results show that the 0.4% TiO2 nanomaterial exhibits the best effect on improving the supercooling and thermal conductivity of the composite; 1% PAAS can effectively eliminate phase separation. The optimized material is composed of SF70 + 0.4% TiO2 + 1% PAAS, for which the phase change temperature, latent heat, supercooling, and thermal conductivity are ?29.9 °C, 255 kJ/kg, 2.8 °C, and 0.652 2 W/(m?K), respectively. After 200 cycles, the thermal properties of the material remain stable. When the phase change material is used in an insulated container, meatballs can be kept below ?23 °C for more than 20 h, which meets the requirements of short-distance cold chain transportation.
摘要:To strengthen the heat transfer of a phase change cold storage panel to match the variable cooling demand of refrigerated transportation, this study proposes a type of phase change cold storage panel with embedded heat pipes to quickly balance the heat load fluctuation in the logistics process. The discharging performance of the heat pipe evaporation section under a high heat load is experimentally studied. A dynamic analytical model is developed for the cooling process based on the thermal resistance analysis method. The results show that under the condition of a high heat load, the dynamic characteristics of the heat transfer process for the heat pipe side are observed. Under a 50 ℃ working condition, the highest average heat transfer rate reaches 42.50 W. The temperature difference and heat transfer rate at the airside calculated by the model are consistent with the measured data. The calculation error of the overall cooling capacity is -3.21%–6.16%. The model is used to simulate and analyze the cold storage panel. In the simulation cases, the average heat transfer rate reaches 88.72 W with four heat pipe rows and a 16 mm tube diameter. The average heat transfer rate reaches 112.54 W with an evaporation section length of 80 mm.
摘要:Working under high-temperature conditions is becoming increasingly common in summer; therefore, the working conditions of delivery workers, cleaners, security guards, and other personnel should be ensured, which is in line with China's people-oriented development strategy. To overcome the shortcomings of traditional cooling clothing, such as a poor cooling effect, low battery life, high energy consumption, and high noise, thermal protective clothing with detachable arms for human operation in a high-temperature summer environment is designed using the characteristics of the phase change material Na2SO4?10H2O, which absorbs heat at high temperatures. On this basis, a simulation experiment with water as the heating source and a real human test are conducted. The test results show that in the simulation test with hot water, under the working conditions of 30 °C, 41 °C, and 45 °C, the temperatures of the four measuring points (the front chest, side abdomen, back, and abdomen) increase to the highest level in 60 min, and the average temperature of the four measuring points is finally reduced to approximately 26 °C. The protective clothing exhibits good cooling performance at 30 °C, and poor cooling performance at 45 °C. In the real human test, the average temperature of the four measuring points in the protective clothing can finally decrease to approximately 26 °C within 3 h under the working conditions of 37.1 °C, 39 °C, and 41 °C. Under the high-temperature condition of 60 °C, the average temperature of the four measuring points can finally decrease to approximately 31 °C within 3 h, and the average temperature of the inside and outside of the protective arm finally decrease to approximately 32 °C, which are both lower than the critical temperature of the human body burning at 45 °C. Therefore, the protective clothing exhibits a good cooling effect and can meet the thermal comfort requirements of outdoor workers in summer.
关键词:detachable protective clothing;thermal environment;phase change;latent heat;velocity of temperature falling
摘要:At present, air cooling technology is unable to meet the needs of high-heat-flux electronic devices. Liquid cooling technology has received increasing attention. As a type of liquid cooling technology, jet impingement liquid cooling primarily use nozzles to jet the working fluid directly to the solid surface, thereby improving the heat dissipation performance. The ratio of the jet area, the number of nozzles, and the influence of micro-channels are systematically discussed in this study, which can provide guidance for the optimization of such heat sinks. Simulation results show that the optimal ratio of the jet area is 0.14, in which the average temperature, pressure loss, Nusselt number, and heat transfer coefficient of the heat sink are 55.8 °C, 5.35 kPa, 28.1, and 3.45 kW/ (m2·K), respectively. The optimal number of nozzles is 4, in which the average temperature, pressure loss, Nusselt number, and heat transfer coefficient of the heat sink are 51.4 °C, 5.52 kPa, 35.2, and 4.33 kW/ (m2·K), respectively. In addition, hot spots can be eliminated in this condition. Moreover, the arrangement of the micro-channel decreases from 57.4 °C to 47.1 °C, and the Nusselt number and heat transfer coefficient increase from 35.2 and 4.33 kW/ (m2·K) to 43.4 and 5.32 kW/(m2·K), respectively. The pressure loss increases slightly (approximately 50–70 Pa). With an increase in micro-channels, the overall heat transfer performance of the heat sink is significantly enhanced.
关键词:jet impingement liquid cooling;ratio of jet area;number of nozzles;micro-channel;performance optimization
摘要:To investigate the effect of the oil circulation rate (OCR) on the performance of a variable-speed scroll compressor (R134a) for electric vehicle air-conditioning, elaborate experiments were conducted to obtain the performance and OCR in a test bench using a secondary fluid calorimeter and OCR measurement by a separator. The results show that the OCR significantly influences the scroll compressor performance when speeds of 3 000 r/min, 4 500 r/min, and 6 000 r/min are compared under two heavy-load conditions. More specifically, when the OCR is between 1% and 10% under the same condition, the volume efficiency and electrical efficiency decline by 0.02–0.05, whereas both the discharge temperature and shell temperature rise by 5–15 K as the OCR decreases from 10% to 1%. This occurs at every speed, and it is especially evident at low speed. Generally, a moderate OCR should be near 5% because an OCR that is less than 5% has a higher impact than that greater than 5%.
摘要:In compression refrigeration systems, an excellent distributor can provide an evaporator with the best heat transfer performance. This study develops a Venturi-type patent CX-distributor, which is composed of a Venturi injection inlet and distributor body. A comparative analysis is conducted between the CX-distributor and the Venturi distributor which is widely used at present. The contrast experiment is conducted in the R410A bus air-conditioning system, and the other parts except the distributor of the system are kept unchanged, including the same R410A charge quantity. Then, a comparative performance experiment of the system is performed under standard working conditions. The experimental results show that, compared with the original distributor, the refrigerant flow resistance of the CX-distributor is reduced by more than 50%, and the uniformity of the temperature distribution at the refrigerant outlet of the evaporator can be improved. Moreover, the refrigeration system performance using the CX-distributor is better than that of the original system. When the system is rated at a constant compressor air volume and frequency, the degree of subcooling of the CX-distributor system is 1–2 °C higher than the original system, the average refrigerating capacity and coefficient of performance increase by an average of 4% and 2.6%, respectively, and the electronic expansion valve opening is only half of that of the original distributor.
摘要:More and more attention is paid to vacuum spray cooling (VSC) because it can effectively reduce the water loss rate in the process of food vacuum cooling. This study investigates the effects of spray height, spray flow, spray time, and spray inclination angle on the cooling time and water loss rate of tofu during VSC. The results show that the spray height has a significant effect on reducing mass loss. With an increase in the spray height, the cooling time increases, and the water loss rate first declines and then remains unchanged. When the spray height is the same, a lower spray flow rate (5–14 mL/min) does not significantly affect the water loss rate; however, the cooling time increases with an increase in the spray flow rate. The cooling time increases as the spray time increases; however, the water loss rate significantly decreases. When the spray time increases from 0 min to 14.50 min, the cooling time increases by 91.83%, from 8.08 min to 15.50 min, and the water loss rate decreases by 72.07%, from 10.49% to 2.93%. When the spray inclination angle is reduced from 90° to 45°, the water loss rate increases from 2.97% to 6.44%, and the cooling time decreases first and then remains unchanged.
摘要:In the cryopreservation of monolayer adherent cells, the detachment of cells from the substrate is one of the main factors that influence the effect of cryopreserved cells. The main reason for this type of shedding is the difference in the coefficient of thermal expansion (CTE) of the matrix and cells. The cooling rate and final temperature are both factors that cause a change in the CTE. In this study, the cryopreservation plan was compared and screened when the difference in thermal expansion was the smallest. The effect of thermal expansion damage on the cryopreservation of hepatocyte complexes was explored by measuring the CTE of hepatocytes and microcarriers during the slow freezing process (cooling rate was 1 ℃/min, 2 ℃/min, and 5 ℃/min, and the final temperature of was -30 ℃. The final experiment concluded that when the cooling rate was 1 ℃/min, the thermal expansion difference between hepatocytes and microcarriers was the smallest, and the cryopreservation effect of the corresponding composite was the best, with survival and adhesion rates of 62.66%±0.67% and 37.2%±1.25%, respectively. Thermal expansion damage between cells and microcarriers exists throughout the cooling process and is one of the main factors affecting the cryopreservation effect of liver cell complexes.