最新刊期
卷 38 ， 期 6 ， 2017
- 摘要：The packing is a crucial part of a liquid-desiccant dehumidifier, and its mass-transfer characteristics have a great impact on the overall performance of liquid-desiccant-based air-conditioning systems. An experimental test rig was built to study the performance of a counter-flow packed-type liquid-desiccant dehumidifier that employs cross-corrugated cellulose-structured packing with a high specific surface area (650 m2/m3). A LiCl aqueous solution was used as the desiccant solution. The water-vapor pressure difference between the gas and liquid interface was considered as the mass-transfer-driven force. New experimental correlations for the mass-transfer coefficient and the dehumidification effectiveness were developed. The performance of this type of packing was compared with other types of packing in the literature. The results show that this type of packing has the highest volumetric mass-transfer coefficient. In addition, the volumetric mass-transfer coefficient is found to increase with the specific surface area of the packing. The deviations between the experimental data and the predicted data using the experimental correlation for the mass-transfer coefficient are within ±20%, and within ±15% for the dehumidification effectiveness. This indicates that the two new correlations can be used to accurately predict the mass-transfer performance of a dehumidifier.关键词：liquid desiccant;mass transfer coefficient;effectiveness;experimental correlation;vapor pressure difference2514|2571|4发布时间：2024-07-18
- 摘要：Recently, a CaCl2-BaCl2-NH3 chemisorption freezing cycle driven by a low-temperature heat source has drawn an increasing amount of attention. To develop a working pair with a more desirable performance than CaCl2-BaCl2-NH3, working pairs of CaCl2-NaBr-NH3, SrCl2-BaCl2-NH3, and SrCl2-NH4Cl-NH3 were investigated and compared. Most composite adsorbents were developed with a novel matrix of expanded natural graphite treated with sulfuric acid (ENG-TSA), leaving one serial with expanded natural graphite (ENG) for comparison. For SrCl2-NH4Cl-NH3 and CaCl2-NaBr-NH3, experimental results show that the maximum adsorption quantities are 95.4% and 88.6% of the theoretical values, respectively. Simulation results indicate that the best results are obtained from CaCl2-NaBr-NH3, and its specific cooling power (SCP) and coefficient of performance (COP) are as high as 260.74 W/kg and 0.285, respectively, which are 15.1% and 5.6% better than the values for CaCl2-BaCl2-NH3. It is also noted that, for different working pairs, the COP, cooling capacity, and SCP range from 0.215 to 0.285, 2 to 3.65 kW, and 161.4 to 260.74 W/kg, respectively. Comparisons also show that the ENG-TSA matrix effectively improves the SCP. For example, the SCP of CaCl2-BaCl2-NH3 is improved by 40.2%, when compared with a working pair with the ENG matrix.关键词：sorption;refrigeration cycle;working pairs;ENG-TSA2242|2506|1发布时间：2024-07-18
- 摘要：In this study, an ammonia refrigeration room was selected as a typical scenario. Computational fluid dynamics (CFD) was applied to study the rules of the ammonia-leakage diffusion. The ammonia-leakage diffusion in a refrigeration room with up-down ventilation and without ventilation was numerically simulated using the Fluent CFD software package. The ammonia concentrations in the longitudinal profile and the cross-section of the refrigeration room were analyzed at different times after the ammonia leakage. The numerical simulation results show that the ammonia concentration increases with height in the refrigeration room, and a lower fresh-air inlet can quickly decrease the ammonia concentration. Based on the above results, it is proposed that an ammonia-gas alarm should be installed near the ceiling to detect the leakage, and an emergency exhaust inlet should be installed near the floor to decrease the ammonia concentration.关键词：ammonia;leakage;diffusion;simulation;refrigeration room2148|2062|4发布时间：2024-07-18
- 摘要：LiBr/H2O falling-film absorption is a common heat-and mass-transfer process in an absorption air-conditioning system. By analyzing the coupled heat and mass transfer of the falling-film absorption process, two-dimensional physical and mathematical models of the falling-film absorption along a vertical wall were established. The model was solved using the Fluent computational-fluid-dynamics software. The interfacial temperature, the concentration profile across the falling film, and the average heat-and mass-transfer fluxes and coefficients were obtained. The effect of the Reynolds number on the heat and mass transfer of the falling film was investigated. The results show that, when the film Reynolds number is less than 150, the average interfacial temperature increases with the increasing Reynolds number; however, the rising trend of the average interfacial temperature is more and more gentle. The average mass-transfer coefficient increases with the increasing Reynolds number, while the average heat-transfer coefficient decreases. Simultaneously, with the increasing Reynolds number, the average heat-and mass-transfer fluxes first increase and then decrease. An optimal Reynolds number exists that leads to the maximum heat- and mass-transfer fluxes. The average heat-and mass-transfer fluxes reach their maximums, 7.2 kW/m2 and 2.9×10-3 kg/(m2?s), respectively, when the Reynolds number is 50.关键词：heat and mass transfer;lithium bromide solution;falling film absorption;Reynolds number1978|2505|1发布时间：2024-07-18
- 摘要：In a lithium bromide absorption-type accumulator system, allowing the solution to crystallize greatly improves the system storage density. However, the crystals seriously affect the discharge process of the energy-storage system. To rapidly dissolve the crystals, a flat-membrane framed accumulator was designed to address this difficulty. The heat-transfer and mass-transfer performance in the accumulator will directly affect the energy-discharge rate of the accumulator. Therefore, a test rig was built to test the heat- and mass-transfer performance in the membrane accumulator. Using experimental data, the heat- and mass-transfer characteristics of the membrane accumulator under different working conditions were analyzed, as well as the influence of the water-vapor cross-sectional area on the heat-and mass-transfer performance. The experimental results show that the crystals in the lithium-bromide solution with an initial mass fraction of 65% can be completely dissolved in 60 min. The feasibility of the flat-membrane framed accumulator is confirmed by this experiment.关键词：dissolving crystal;energy storage;flat membrane;accumulator;lithium bromide1942|2170|0发布时间：2024-07-18
- 摘要：Phase-change heat-storage material (PCM) is widely used in many fields, and its thermophysical properties are the key factor for its application. In this study, an improvement of the T-history method is presented, based on the basic principle of the T-history curve method, the theoretical step curve of the PCM and the actual step-curve measurement process. Firstly, according to crystalline-melting thermodynamic conditions, a certain degree of supercooling is necessary for crystal crystallization; therefore, there is sensible heat between the phase-change-start phase and the phase-change-end phase. Then, the energy-balance equation was improved by considering the sensible heat effect on the energy-balance equation in the phase-change stage, and the temperature-difference effect on the energy-balance equation in the cooling process of the phase-transition phase. Secondly, the phase-change material was placed in a constant-temperature water source at 70 ℃ until it melted. Then, it was placed in a constant-temperature box at 10 ℃. The step curve was obtained with a two-second interval measurement. Finally, the experimental results for three phase-change materials, sodium dodecahydrate disodium phosphate, lauric acid, and stearic acid, were calculated according to the improved equations to verify the improvement of the T-history method. The results show that the calculated value of the latent heat is closer to that of the differential-scanning calorimetry process, and the error is less than 10%. Therefore, the improvement of the T-history method has a certain practical significance for the development and testing of phase-change thermal energy storage material.关键词：T-history method;phase change;latent heat2234|2885|1发布时间：2024-07-18
- 摘要：Coal-fired boilers and thermal power plants are the primary heating resource in the district heating field. However, they have great energy-saving potential because of the significant heat loss from the high-temperature discharging flue gas. In view of the existing problems, a waste-heat recovery system for coal-fired boilers based on a spray tower is presented. It could efficiently recover the latent heat and improve the system efficiency and economic performance by using a spray-tower heat exchanger and absorption heat pump. In this study, a model was built for this system and validated with measured data in a practical project. The system was then evaluated from the points of view of energy saving, emission reduction, and economic feasibility. The results show that the simulation results of the model are roughly in line with the trend of the observed results, and the absolute and relative errors are in a reasonable range, which proves that the model is reasonable. Meanwhile, the boiler thermal efficiency can rise to 104.5%, the pollutant emissions can obviously decrease, and the quantity of collected condensate water can reach 18,000 tons a year. In addition, the payback period is 3.96 years. In summary, the proposed system has higher energy efficiency and better economic benefits than existing systems.关键词：waste heat recovery;EES;coal-fired boiler;direct-contact heat exchange;energy conservation and emission reduction1994|2543|4发布时间：2024-07-18
- 摘要：This paper presents the experimental and numerical results of the heat-transfer characteristics of a water-cooled heat sink with different outlet layouts. An experimental apparatus was set up to validate the numerical results. The coolant flowed vertically into the heat sinks from the center of their top wall. Heat sinks A and B each had only one outlet, located at one corner or at the center of the edge, respectively. For heat sinks C and D, four outlets were set at each corner or at the center of each edge, respectively. The numerical simulation results were validated by comparing them with experimental data. The results show that the relative error of the numerical simulation is less than 7%. The heat-transfer characteristic of the heat sink depends on four parameters: the velocity distribution, pressure loss, comprehensive effect, and the Nusselt (Nu) number, which is the ratio of convective to conductive heat transfer. The numerical results indicate that the Nu numbers of the heat sinks with four outlets are lower than those with one outlet. However, the flow resistance is lower and the comprehensive coefficient is higher. The comprehensive coefficient of four outlets is 50% higher than that of the heat sink with one outlet. The four-outlet heat sink has better heat transfer and flow effects.关键词：water-cooled heat sink;chip cooling;heat transfer characteristic;numerical simulation2000|2948|3发布时间：2024-07-18
- 摘要：The application range of R32, a new alternative refrigerant, is continually increasing. The development and use of a special refrigeration oil has become one of the key issues in R32 applications. This study investigated a new type of polyvinylether (PVE) oil pairing with the R32 refrigerant. Through an experimental comparison of the refrigeration performance of a split air conditioner, which is heat-pump type and the input power is 2 HP, with R410A/polyolester (POE), R32/POE, and R32/PVE, the thermal-resistance change of the evaporator and condenser under different PVE oil contents and how the system coefficient of performance (COP) was affected by the PVE oil content were studied. The results show that when R32 is paired with a more mutually soluble PVE oil, the refrigeration performance is improved effectively. With the increase in PVE oil content, the thermal resistance of the evaporator first increases and then decreases, and the thermal resistance of the condenser gradually increases; thus, the evaporation temperature first increases and then decreases, and the condensing temperature increases continuously. The COP increases and then decreases with the increase of the PVE oil content, reaching a maximum in the vicinity of 270 mL.关键词：R32;PVE oil;air conditioning performance;oil content;heat resistance2473|5135|3发布时间：2024-07-18
- 摘要：To solve the problem of air flowrate measurement in an active chilled-beam system, a test rig of active chilled-beam was designed, and the influences of different primary air flowrate on the induced performance of an active chilled beam were investigated. The results show that the induction ratio of the chilled beam increases with the increasing primary air volume when the primary air volume is less than 80 m3/h. The induction ratio tended to 3.17 when the primary air volume later reached 80 m3/h. There is a quadratic relationship between the pressure of the cold plenum and the primary air flow. Under the 4 mm nozzle of the chilled beam, the fitted curve of the static pressure with the primary air is y = 0.0225x2 + 0.0101x. The active chilled-beam model was then set up, the effect of the primary air volume on the induced performance was simulated numerically, and the simulation results were verified based on the test results. The relative errors between the experimental results and the numerical results reached 10.5% and 8.8% for the induction ratio and the pressure of cold plenum, respectively. The overall results and the trend of the simulation are in good agreement with the measured values, which indicates that the simulation has high reliability.关键词：active chilled beam;air flow measurement;induction ratio;plenum chamber;numerical simulation2040|2420|0发布时间：2024-07-18
- 摘要：A sewage-water heat-pump system (SWHP) for building heating and cooling in Dalian, Liaoning Province, northeastern China, is measured on-site, which is a renewable energy demonstration project. The measurement results show that the average heating coefficient of performance (COP) of a single heat-pump unit and the whole system are 5.02 and 3.93, respectively, when the average sewage-water temperature is 10.45 ℃ during the entire heating season. The average cooling COPs are 6.26 and 4.54, respectively, when the average sewage-water temperature is 18.43 ℃ during the entire cooling season. To improve the system performance, an energy-saving analysis was conducted according to survey data. The test results show that the unit and system COPs increase by 16.08% and 11.36%, respectively, as the heat-transfer coefficient of the sewage heat exchanger increases by 50.08%. Moreover, a SWHP model was established, and the system energy consumption under fixed and variable-frequency water-pump conditions was simulated using the Transient System Simulation Tool (TRNSYS) software. The simulated result shows that the energy consumption of the variable-frequency operation is less than the fixed frequency by 18.52%.关键词：sewage water heat pump;district heating and cooling;performance test;energy saving;variable frequency system2016|2080|0发布时间：2024-07-18
- 摘要：When the air source heat pump water heater is heated to higher water temperature, the system efficiency will decrease obviously. In order to find out the reason of the decrease in heat efficiency and the operation rule of the system, this paper changes the initial water temperature, and studies the effect of temperature change on the parameters of the system. The results show that at the same initial water temperature, when the system operates, the COP has a slowly declining trend, which becomes violent later. With the different initial water temperature, because of the “preheating” stage and the affect by suction liquid of the compressor, the average COP decreases first and then increases with the uprise of the initial water temperature. In addition, a small amount of suction liquid can reduce exhaust temperature and improve system performance. But a large amount of suction liquid can increase the system power consumption, the decreased rate of COP will increase. This paper finally suggests that when the exhaust temperature drops again after the rise, the compressor begins to suction a large amount of liquid.关键词：heat pump water heater;initial water temperature;gas-liquid flow;exhaust temperature2086|2140|0发布时间：2024-07-18
- 摘要：Falling-film evaporation is an efficient heat-transfer technology. The average film thickness is an important factor that affects the heat-transfer performance. Based on the volume of fluid (VOF) algorithm, a computational fluid dynamics (CFD) model was established to describe the flow of a two-phase (air-water) falling film on a two-dimensional vertical channel. The effects of the film velocity, working medium, and co-current and counter-current gas on the liquid film thickness were investigated. The results show that the film thickness grows with the increasing film velocity. The gas-phase medium has little effect on the liquid-film thickness; however, the liquid medium has a great influence, and the film thickness increases with the liquid-film viscosity. The co-current gas flow has little effect on the film thickness in the entry and development regions. However, in the stable regions, the film thickness decreases with the increasing co-current gas velocity. The average film thickness decreases with the increase of the counter-current gas velocity. When the counter-current gas velocity reaches 2.5 m/s, the gas-velocity’s influence on the liquid-film thickness decreases.关键词：two-phase flow;falling film;fluctuation;VOF algorithm;numerical simulation1863|2102|0发布时间：2024-07-18
- 摘要：The influence of different cold end orifice diameter on the performance of vortex tube was studied using the test bench. In the 0.3-0.5 MPa range of inlet pressure, the effect of cold end pore size on the temperature drop, the characteristics of refrigeration capacity, the efficiency of the temperature and the COP characteristics were analyzed. The experimental results showed that the cold end orifice diameter on the vortex tube has a great impact on performance, and there exists a cold end orifice diameter of a vortex tube which makes the cold end temperature drop and refrigeration capacity reach the maximum. Under the current conditions of the vortex tube, the optimal diameter of the cold end orifice is 5 mm, and the ratio between the best cold end orifice diameter and the best hot end diameter is 0.5.关键词：vortex tube;inlet pressure;diameter of cold end orifice2126|3387|8发布时间：2024-07-18
- 摘要：Changing the working conditions of an industrial heat pump can cause a mismatch of the inner and outer pressure ratios of the twin-screw compressor. Over-compression and under-compression increase the compressor power consumption and decrease the heat-pump’s coefficient of performance (COP). Injecting refrigerant vapor into the compression chamber would increase the inner-pressure ratio and avoid under-compression. An experimental study was conducted with different injection pressures when the condenser outlet’s water temperature reached 90 ℃. The results show that the optimal injection pressure causes the COP and adiabatic efficiency to peak at the same time. This result shows that the vapor injection changes the compression process to a quasi-polytropic compression process. Adjusting the compressor’s inner-pressure ratio by changing the refrigerant-injection pressure is feasible.关键词：high temperature heat pump;twin screw compressor;inner pressure ratio adjustment;vapor injection1952|2703|4发布时间：2024-07-18
- 摘要：To study the heat-flux change and cooling rate in the freezing and heat-transfer process of liquid nitrogen, a low-temperature liquid-nitrogen experimental device was made. It was based on the idea of the direct contact heat exchange with the food after the liquid nitrogen vaporized. The experiment was conducted at seven temperature regions ranging from ﹣50 ℃ to ﹣170 ℃ with ﹣20 ℃ interval. A potato was frozen from the initial temperature 18 ℃ to the freezing point ﹣18 ℃. The average heat flux and the temperature distribution of the heat-transfer process were calculated using a curve-fitting formula. The heat-flux change and temperature change rate were analyzed to obtain the most appropriate nitrogen temperature. The results show that when the nitrogen temperature is ﹣122.87 ℃, the heat-flux growth rate reaches the maximum. As the temperature continues to reduce, the heat-flux growth rate decreases; at this point, part of the heat gathered internally from the food results in wasted energy. The temperature change rate for different interfaces was calculated when the food-center temperature was ﹣3 ℃, and the optimum nitrogen temperature was found to be ﹣133.11 ℃. This was only 6.71% different from the previous result. Thus, the optimum nitrogen-freezing temperature is about ﹣128 ℃, which can ensure the rapid freezing of the food and improve the effective utilization of the nitrogen.关键词：frozen food;cooling rate;freezing point;heat flux;fitting formula method1896|2534|1发布时间：2024-07-18
- 摘要：Cold-storage incubators that are easy to disassemble and assemble may become the development trend of the future. An experiment on the effect of cold-chain transportation was performed on an assemble-able cold-storage incubator with pleurotus ostreatus (oyster mushrooms). During the experiment, the temperatures of the air, the center of the pleurotus ostreatus, and the cold-storage agent in the incubator were tested; then, the heat-leakage rate from the gaps in the cold-storage incubator assembly was determined. Meanwhile, the pleurotus ostreatus were sampled every 30 hours to make a sensory evaluation and the nutritional indicators were tested to judge the effect of the cold-chain transportation. The results show that, in an ambient temperature of 25 ℃, it takes 56 hours for the coolant to completely liquefy, which is eight hours less than that estimated. This means that the required coolant amount, which can be adjusted by a factor-correction method, was actually larger than the estimation. Moreover, the heat-leakage rate from the gap increased to 12.97%, which was thought to be a bit larger proportion. Thus, it is necessary to improve the assembly process of the assemble-able cold-storage incubator. In addition, the commodity and nutritional value of pleurotus ostreatus are rarely reduced during cold storage. When the values after 60 h were compared with those at the beginning, the sensory evaluation scores, the soluble-solids content, the soluble-protein content and the vitamin C content had dropped by 3.53%, 5.9%, 4.2% and 9.96% respectively.关键词：assembly;cold storage incubator;heat leakage;cold chain transportation2200|2367|6发布时间：2024-07-18
- 摘要：Freeze-drying is a long-term and effective method for cell preservation. The lyoprotectant can have a significant effect on cell freeze-drying. In this study, different volume concentrations of Me2SO, glycerol, and different mass concentrations polyvinylpyrrolidone (PVP), compound lyoprotectant for lyophilizing human Hepatoma Hep-G2 cells were considered. The effect of intracellular trehalose on the lyophilized cells was investigated by determining the cell-recovery rate, survival rate, and 24 h attachment rate. The results show that, compared with the control group, the cells added in a 40% PVP (w/v) + 10% (v/v) glycerol + 15% fetal bovine serum (FBS) (v/v) + 20% trehalose (w/v) solution received better protection. The cell recovery rate, survival rate, and 24 h adherent rate are 29.58%, 42.18%, and 18.71%, respectively. When the extracellular trehalose molar concentration is 800 mmol/L, the Hepatoma Hep-G2 cell-recovery rate, survival rate, and 24 h adherent rate are 27.81%, 66.65%, and 33.68%, respectively. It is significantly better than the control group and has the best protection for cell freeze-drying.关键词：freeze-drying;lyoprotectant;cell;survival rate2320|2748|0发布时间：2024-07-18
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