最新刊期
卷 44 ， 期 1 ， 2023
- 摘要：Latent heat storage (LHS) technology can solve the time-space non-continuity in the transfer of thermal energy, which can effectively reduce heat loss and accelerate the realization of China's carbon neutrality goal. However, the LHS system in practical applications needs to be equipped with heat transfer enhancement devices, considering the low thermal conductivity of phase change materials (PCMs), which significantly increases the system complexity and capital cost. In this review, heat transfer enhancement methods, namely single heat transfer enhancement technology, combined heat transfer enhancement technology, multi-stage heat transfer enhancement technology, and liquid PCM flow enhancement technology, are briefly classified. The corresponding shortcomings of the last three technologies are described and explained. Based on the obtained conclusions, three relative geometric relations between the heat source and heat sink are summarized, namely the expansion type, parallel type, and contraction type, which can effectively guide the overall design of the latent heat storage units. Finally, the future application prospectives of latent heat storage technology are discussed. It is necessary to design latent heat storage from the system level based on knowledge of thermodynamics and system theory to achieve optimal thermodynamic performance and economic efficiency.关键词：phase change;convection;heat transfer;enhancement;heat storage1448|1708|2发布时间：2024-08-14
- 摘要：The drying process consumes large amounts of energy in industrial and agricultural production. Research and practical applications show that compared with other drying technologies, the heat pump drying technology has better energy-saving potential and economic benefits. As a natural working medium, CO2 is environmentally friendly. The transcritical CO2 cycle has a clear temperature glide, which is more suitable for heat pump drying than the subcritical cycle with a constant condensing temperature of a conventional working fluids. In this paper, the research status of the transcritical CO2 heat pump drying systems is described. The feasibility of the application, system model establishment and key parameter analysis, experimental research, and system optimization of CO2 heat pump drying system are discussed in depth. The advantages and disadvantages of various CO2 heat pump drying system optimization methods are summarized. Existing limitations in the current research of transcritical CO2 heat pump drying systems are proposed, thereby providing a reference for future research applications of CO2 heat-pump-drying technology.关键词：heat pump drying;transcritical cycle;theoretical analysis;experimental research;system optimization1235|1091|0发布时间：2024-08-14
- 摘要：With the increased awareness in the international community, the problems of ozone depletion and greenhouse effect have received extensive attention, more and more scholars are focusing on environmental-friendly refrigerants. CO2 demonstrated its value in terms of simplicity, availability, safety, reliability, and good heat transfer performance. This paper analyzes and summarizes the research progress of small-scale CO2 gas-liquid two-phase expanders. It is found that replacing the throttle valve with a CO2 expander in the cycle can effectively improve refrigeration efficiency. The cooling and heating efficiency of the cycle is 6.9% and 4.6% higher than that of the R134a cycle, and 25.76% and 16.17% higher than that of the cycle with a throttle valve, respectively. The higher the isentropic efficiency of the CO2 expander, the greater the system efficiency. As long as the isentropic efficiency of the expander is greater than 32%, the performance of the system with a CO2 expander is better than that with a regenerator. Different CO2 expanders have different efficiency, leakage, and friction loss due to their different structures. Piston type, rotor type, and other expanders have simple structures, while their leakage loss is relatively large. The efficiency of the sliding vane type, rotary vane type, and scroll type expander has been greatly improved, but their structure is more complicated, leading to more complicated manufacturing and increased production costs. The compression work is supplied to the compressor, increasing the flow rate during the expansion process, which requires designing a CO2 expander with an adjustable expansion ratio. At present, the research of CO2 expanders is basically concentrated in the laboratory, and this technology has not been commercialized yet. Therefore, the future development direction should combine experiments and theory to further study the efficiency improvement of CO2 expanders and the development of key components technology.关键词：transcritical cycle;expander;expansion work;isentropic efficiency;leakage;CO21124|1176|1发布时间：2024-08-14
- 摘要：This study investigates the performances of five low GWP (Global Warming Potential) refrigerant/[P6,6,6,14][Cl] working pairs in the compression-assisted single-effect absorption system. The studied refrigerants include hydrocarbon (HC) R290 and R600a, hydrofluoro-olefin (HFO) R1234yf, R1234ze(E), and R1233zd(E). The NRTL model was utilized to correlate the vapor-liquid equilibrium data of five working pairs, and the performance improvement of the compression-assisted absorption system was verified against the traditional single-effect absorption system. Based on this, the effects of the generation and absorption temperatures on COP, exergy efficiency, and circulation ratio of five working pairs were analyzed. The results show that the implementation of a compressor in front of the absorber may significantly increase the COP of the system. Of the five working pairs, R290/[P6,6,6,14][Cl] exhibited the best performance; while R1233zd(E)/[P6,6,6,14][Cl] had the worst performance, and the maximum differences in the COP and exergic efficiency were 0.277 4 and 0.224 5, respectively. As the generation temperature increased, the COP and exergy efficiency of all working pairs had maximum values, whereas the circulation ratios decreased monotonically. As the absorption temperature increased, the COP and exergy efficiency of all working pairs decreased, whereas the circulation ratios increased.关键词：refrigerant;ionic liquid;absorption refrigeration;performance analysis1201|990|1发布时间：2024-08-14
- 摘要：This study develops a zero-emission heat pump system consists of heat pipes, water coils, and heat pumps for exhaust gas to mitigate the incomplete treatment, disgusting smell, and unrecoverable heat of exhaust gas after the drying process in the pharmaceutical industry. The quality of gas recycling and reuse is guaranteed by closed-loop dehumidification, waste gas treatment, and heating, thereby achieving near-zero emissions and recovery of waste heat. The experimental results of the system performance show that when the average temperature of the supply air is 65.1 °C, the relative humidity of the supply air is 21.1%, the air volumetric flow rate is 12 000 m3/h, the coefficient of performance of the heat pump unit is 4.7, and the specific moisture extraction rate of the system is 8.5 kg/(kW?h). The composition of the circulating gas shows that 73.2% of the volatile organic compounds are absorbed, and no non-condensable, flammable, or explosive gases are accumulated in the system during each production period.关键词：heat pump;heat pipe;drying;waste gas treatment;closed circulation912|1037|0发布时间：2024-08-14
- 摘要：Aviation applications are facing the challenges of cooling high-power and high-heat-flux electronic equipment. Traditional cooling methods cannot cope with thermal requirements greater than a heat flux of 100 W/cm2. In this study, the ground test bench of a pump-driven two-phase cooling loop (MPCL) system is constructed, and the control strategy of the system is designed. The cooling ability and resistance characteristics of the system are tested, and the mathematical model is developed. The results show that the mechanical pump drives the two-phase cooling system with good thermal performance. The designed copper cold plate is able to effectively handle 6 kW concentrated heat sources with a heat flux of 120 W/cm2. A 10 kW heat source can be effectively cooled by the MPCL system using 70% less working fluid than single-phase cooling under designed working conditions. The surface temperature of the heating element can be stabilized at 63–70 °C, which meets the temperature requirements of the chip. Additionally, the temperature is uniform between the evaporator branches, with a temperature difference below 5 ℃. The pressure drop of the phase-change segment is below 400 kPa, and the resistance characteristics can be described by the Kim and Mudawar models.关键词：mechanical pump drive;two-phase cooling loop;cooling properties;R134a1060|1516|1发布时间：2024-08-14
- 摘要：There are significant height differences between outdoor and indoor units in variable refrigerant flow (VRF) systems deployed in high-rise buildings. Moreover, there are many branches in the pipeline structure in such systems, resulting in the performance degradation of indoor units. Simulation software is needed to determine the influence of these factors to improve the capabilities of such VRF indoor units. This study establishes a distributed parameter model for the pipeline and a flow distribution model for the system based on fluid network theory to accurately reflect the flow characteristics of the refrigerant in the pipeline. A multi-zone separation model for the indoor unit heat exchanger and a fast pulse number calculation model for the electronic expansion valve in the indoor unit were also established to quickly reflect the operating status of indoor units. In addition, a hierarchical iterative solution algorithm was developed to improve the calculation speed of the software. These models were validated using existing data. The results showed that the errors for each model were <12%, and the calculation time for the VRF system with a common structure was less than 1 min. Furthermore, the calculated example using the simulation software showed that the indoor unit performance decreased by 2.7%–11.9% when the height difference between the indoor and outdoor units increased by 20 m and 0.4%–9.7% when the number of branches in the pipeline increased by one.关键词：VRF system;pipeline structure;large height difference;multiple branches;performance of indoor unit988|931|0发布时间：2024-08-14
- 摘要：The indoor thermal environment of data centers is based on the supply airflow distribution, which is significantly affected by the geometric configuration of the air-supply system. This study compares four commonly used air supply layouts, namely the cold/hot-aisle airflow open-ended system, hot aisle containment system, cold aisle under racks system, and cold aisle containment system. For each air supply layout, geometric factors such as the plenum height, perforation rate, baffle position, and baffle angles were investigated. Using ANSYS Fluent, up to 288 models of CRAC with different air supply modes and configurations were built. The models were validated using the temperature measurement data at the SmartAisle? cold aisle containment system. The average rack and hot spot temperatures, thermal performance evaluation index, and return temperature index were used as the metrics for comparison. The results show that both increasing the plenum height and decreasing the perforation rate within a certain range contribute to thermal performance improvement. Considering the room temperature profile and evaluation metrics, the overall best performance is achieved in the cold aisle containment case. Finally, the effects of various factors when using \/-shaped and /\-shaped baffles were compared using the models of the cold aisle containment system. The results show that when using \/-shaped and /\-shaped baffles, the optimal perforation rate is 20%, while the optimal static pressure height is 0.5 m and 0.6 m, respectively. Overall, \/-shaped baffles achieve better temperature uniformity than /\-shaped baffles.关键词：data center;geometric configuration;air supply mode;thermal performance evaluation index938|858|0发布时间：2024-08-14
- 摘要：The temperature-controlling characteristics of a thermal mechanical package (TMP) are crucial to the experiments in inertial confinement fusion. This study addresses the semi- and holo-TMP structures. The influences of relative factors, including the pressure of helium inside the hohlraum, width of bulge loops, and power of heat rings, on the temperature-controlling characteristics of TMP are investigated systematically. The results show that a semi-TMP structure has a better temperature-control performance than a holo-structure under the same thermal conditions. Increasing the pressure of helium makes a positive but limited contribution to the TMP temperature-control effect. The presence of bulge loops enhances the heat transfer between the TMP and gold hohlraum, thereby improving the temperature-control performance. In addition, the widths of the bulge loops slightly improve the temperature-controlling effect. The temperature response of the gold hohlraum to variations in the heat ring power exhibits clear differences between the semi- and holo-TMP structures. As the heating power increases, the maximum temperature difference of the gold hohlraum increases linearly at a rate of 0.38 K/W and 0.25 K/W for the semi- and the holo-TMP structures, respectively, which indicates a better temperature-controlling effect on the semi-TMP structure.关键词：inertial confinement fusion;cryotarget;temperature-controlling characteristics;numerical simulation872|922|0发布时间：2024-08-14
- 摘要：This study investigates the atomizing law of the nucleator nozzles, which are an important atomizing device for domestic outdoor snow-making units. Air and water are used as the medium, and the Sauter mean diameter, atomization flux, and spray angle of the domestic KBJD-1 nucleator nozzle are studied by a high-speed camera and laser particle size analyzer. The results show that the Sauter mean diameter decreases exponentially with an increase in air supply pressure but increases exponentially with an increase in water supply pressure. Moreover, the rate of change of the particle size gradually decreases with an increase in the gas-liquid pressure ratio. At the same experimental condition of water supply pressure at 1.0 MPa, when varying the air supply pressure from 0.3 MPa to 0.8 MPa, the atomization flow rate approximately decreases linearly with the air supply pressure. The spray angle first increases and then decreases with an increase in the air supply pressure. When the air supply pressure reaches 0.55 MPa, the spray angle reaches its maximum value.关键词：outdoor snow maker;atomization performance;Sauter mean diameter;spray angle856|941|1发布时间：2024-08-14
- 摘要：While small household refrigeration equipment such as air conditioners and refrigerators provide a comfortable home environment, the associated noise has become one of the key reasons behind consumer complaints. This study uses the theory of air-entrained cavitation to optimize the structure of the electronic expansion valve. The refrigerant at the inlet of the evaporator is introduced to the outlet of the electronic expansion valve through the installation of a bypass pipe to increase the local pressure of the refrigerant, such that the bursting of bubbles in the two-phase flow can be effectively suppressed. The improved double-bypass electronic expansion valve and four-bypass electronic expansion valve may significantly reduce the noise generated by the refrigerant bubble burst at the outlet of expansion valves. Numerical simulation and experimental verification studies show that the double-bypass electronic expansion valve and four-bypass electronic expansion valve reduced noise by 2.729 dB(A) and 3.778 dB(A) than those of the original electronic expansion valve.关键词：electronic expansion valve;flow noise reduction;aerated cavitation;structure optimization;cavitation bubbles1067|887|0发布时间：2024-08-14
- 摘要：In-situ CaCl2 impregnation/chimerization of carbon precursors and graphene blending were introduced into the preparation process of agglomerated activated carbon to simultaneously enhance the heat and mass transfer efficiency of the activated carbon-methanol adsorption refrigeration system. Two types of innovative carbons—CA-GC and SC-GC—were prepared in this study, with synchronously developed meso-pores and micro-pores. This study includes a comparative study of the adsorption/desorption characteristics, thermal conductivity, and refrigeration characteristics of the new carbon-methanol working pair. In-situ CaCl2 impregnation/chimerization resulted in significant improvement in terms of the abundance of calcium oxide adsorption sites on the carbon surface and inside the carbon framework (ash content reached 19.38%). The Sokoda & Suzuki equilibrium adsorption capacity (533.38±6.97 mg/g) and thermal conductivity (1.058±0.77 W/(m?K)) of CA-GC were significantly improved. The blending process of graphene further enhanced the thermal conductivity (2.61±0.15 W/(m?K)) and equilibrium adsorption capacity (712.84±30.66 mg/g) of the composite carbon material SC-GC. When the desorption temperature was 100 °C and the cycle time was 60 min, the desorption capacity, refrigeration capacity, and refrigeration power of the continuous refrigeration cycle of the adsorption refrigeration bed constructed by SC-GC reached 533.10±14.17 mg/g, 486.95±9.79 kJ/kg, and 973.86±15.28 kJ/(kg?h), respectively.关键词：adsorption refrigeration;activated carbon;methanol;thermal conductivity;refrigeration power816|962|0发布时间：2024-08-14
- 摘要：In this study, a simulation model of a water-cooled direct contact cold thermal energy storage system using compressed CO2 hydrate was established, and the charging characteristics and total cold thermal energy storage capacity of the cold thermal energy storage system under charging pressures of 3.5 MPa, 3.6 MPa, 3.7 MPa, and 3.8 MPa were simulated. The results show that the relative errors between the simulation and experimental results of the cooling curve are 4.02%, 4.43%, 3.38%, and 1.89%, and those of the total cold thermal energy storage capacity are 0.82%, 3.41%, 1.45%, and 1.81% for the four charging pressures, respectively. All the relative errors are less than 5%; thus, the model has a good prediction ability. The model is used to optimize the system to improve its cold storage performance: when the cooling water flow rate increased from 65 mL/s to 100 mL/s, the pre-cooling time of the system decreased from 22.5 min to 20.8 min, and the COP of the system first increased and subsequently decreased. When the cooling water flow was 95 mL/s, the system COP reached maximum. When the cooling water temperature increased from 22 °C to 30 °C, the pre-cooling time of the system increased from 16.1 min to 21.9 min, and the COP of the system decreased from 1.77 to 1.53. When the charging pressure increased from 3.5 MPa to 5.0 MPa, the discharge pressure increased from 5.4 MPa to 12.1 MPa. When the charging pressure was 5.0 MPa, the discharge pressure was 12.1 MPa, which exceeded the maximum operating pressure of the compressor. Therefore, the charging pressure should be lower than 5.0 MPa for the sake of safety.关键词：CO2 hydrate;compression refrigeration cycle;cold thermal energy storage system;simulation;optimization841|987|0发布时间：2024-08-14
- 摘要：Mixer diameter significantly influences the performance of two-phase ejectors. This study proposes a design method for the mixer diameter of two-phase ejectors based on the turbulent jet mixing mechanism. When the mass flow rate of the mixed jet increases to the target total mass flow rate of the ejector, the diameter of the jet's external boundary should be the optimal mixer diameter. The mixer diameter of a CO2 ejector is optimized based on this method. The optimal diameter is 2.4 mm when the entrainment ratio is 0.5. The performance of the two-phase CO2 ejector is investigated for varying mixer diameters via numerical simulation. The results show that the optimal diameter is 2.2 mm under the same condition. When the mixer diameter is 2.4 mm, the pressure lift is 0.9 MPa, which is 57.9% higher than the baseline performance. Based on the turbulent jet theory, this study also proposes a correlation for the dimensionless optimal mixer diameter. The dimensionless optimal mixer diameter ranges from 1.7 to 2.1 when the entrainment ratio is 0.4–0.8. The correlation exhibits a -6%–10% deviation from the CFD simulation results and can be used to predict the optimal diameter.关键词：ejector;mixer;two-phase flow;jet;optimization860|1092|0发布时间：2024-08-14
- 摘要：This study conducts visualization research on the flow pattern of R290 flow boiling in horizontal micro-tubes. The main flow patterns and influence factors of flow boiling heat transfer under different pipe diameters were analyzed, in addition to the summary of the flow pattern transformation law by comparing to the theoretical flow pattern diagrams. The experimental conditions were as follows: heat flux (1–70 kW/m2), mass flow rate (50–1 020 kg/(m2?s), saturation temperature (-10–25 °C), pipe diameter (1–3 mm), and vapor quality (0–1). Eight types of flow patterns of R290 flow boiling heat transfer in micro-channels were observed, among which intermittent flow and wave flow were the main flow patterns in 3 mm-tubes, while slug flow and annular flow were the main heat transfer patterns in 1 mm-tubes. Bubble flow and mixed flow in 3 mm-tubes, bubble flow in 2 mm-tubes, and slug flow in 1 mm-tubes were consistent with the Damianides and Westwater flow pattern transition criterion. However, the discrete flow regions in 2 mm-tubes and 1 mm-tubes were poorly matched; the change in pipe diameter had an important influence on the flow pattern. A decrease in pipe diameter caused changes in the bubble shape, flow pattern, flow pattern distribution, and flow pattern transition curve, and the micro-scale effect of the pipe diameter can be observed.关键词：horizontal micro-tube;flow boiling;R290;flow pattern;D&W flow pattern transition criterion1058|1000|0发布时间：2024-08-14
- 摘要：This study conducts a visual experimental investigation on low-pressure refrigerant R1233zd(E) in a parallel mini-channel heat sink. The evolution and distribution of flow patterns in parallel channels and the influence of flow patterns on heat-transfer characteristics were analyzed. Four flow patterns (bubbly flow, slug flow, churn flow, and wavy-annular flow) were observed in the channels when increasing the heat flux. At high heat flux levels, reverse flow occurs in some channels. In addition, it is observed that the distribution of flow patterns in different channels is slightly different. The variation trend of the local surface coefficient of the heat transfer can be divided into different stages based on the evolution of the flow pattern. Moreover, the middle and outlet local surface coefficients of the heat transfer exhibit different variation trends. When the vapor quality is less than 0.1, the surface coefficient of the heat transfer is almost independent of the mass flux. With an increase in vapor quality, the surface coefficient of the heat transfer is positively correlated with the mass flux.关键词：parallel channel;flow boiling;visualization;flow pattern distribution;heat transfer characteristics1134|985|0发布时间：2024-08-14
- 摘要：Superhydrophobic surfaces can reduce the attachment of droplets, reduce the increase of thermal resistance caused by the existence of droplets, and thereby improve the efficiency of air conditioning, power generation, and seawater desalination systems. This study experimentally investigates the growth characteristics of condensate droplets on a superhydrophobic surface under different cold surface temperatures (2–8 ℃), relative humidity values (40%–80%), and inclination angles (0°–90°) and analyzes the effects of different working conditions on superhydrophobic-surface condensation. The results show that with a decrease in the cold surface temperature, the average droplet growth radius and surface droplet coverage gradually increase. The lower the cold surface temperature, the faster the droplet growth rate. The droplets on the superhydrophobic surface grow faster under high humidity, while the droplet growth radius under low humidity will exceed that under medium and high humidity after sufficient time. The droplet coverage on the cold surface under low and medium humidity conditions is considerably less than that under high humidity conditions. The critical sweep radius of droplets decreases gradually with an increase in inclination angle, and the droplet coverage on the vertical surface decreases by 42% compared with that on the horizontal surface.关键词：superhydrophobic;condensation;droplet;droplet radius;surface coverage926|1045|0发布时间：2024-08-14
- 摘要：This study designs and establishes a solar-air dual heat source heat pump system based on a micro-heat pipe array to ensure the stable operation of the multiple heat source heat pump system under complex operating conditions and further optimize the reasonable and efficient energy matching among the system components. A mathematical model of the system is established using the centralized parameter method. The model validation and analysis are based on experimental data from four aspects: system power-generation efficiency, heat-collection efficiency, heating capacity, and heat pump COP. The influence of parameters such as solar irradiance and ambient temperature on the performance of heat pump systems in the photovoltaic/thermal-water & air-source heat pump (PV/T-W&ASHP) operation mode is investigated using the control variable method, and the operational characteristics are comprehensively evaluated. The results show that the proposed mathematical model has considerable accuracy, and the relative errors between the simulated and experimental values are all between ±15%. The average values of the power-generation efficiency and heat-collection efficiency under the PV/T-W&ASHP operation mode are 13.91% and 41.14%, respectively, and the average COP is 2.29. Moreover, when irradiance and ambient temperature are considered a single variable of 500 W/m2 and 15 °C, the system COP is improved by 21% and 29.8%, respectively.关键词：heat pump;coefficient of performance;photovoltaic/thermal;heat transfer performance919|1016|1发布时间：2024-08-14
- 摘要：This study proposes a novel method to improve the cold water supply capacity and reduce energy consumption using a multi-layer water separator to replace the single-layer water separator. This method can delay the mixing intensity between ambient temperature water and cold water and increase the average temperature under the standby state inside the cold tank. The relationship between the structural parameters of the multi-layer water separator, cold water temperature, and cold water volume in each layer is established, and an optimization method for the structural parameters is proposed. An optimized design example is given using a multi-level separator to replace the single-layer water separator in the storage hot and cold water dispenser, and an experiment is conducted. The experimental results prove that the standby energy consumption of the water dispenser fabricated using the newly proposed three-layer water separator is reduced by 15% with the same release capacity when compared with the baseline structure.关键词：storage hot and cold water dispenser;standby power consumption;cold water supply capacity;structure design964|1142|0发布时间：2024-08-14
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