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[en] A mathematical model of an automotive air conditioning (AAC) system with a variable displacement compressor (VDC) is developed in order to simulate and analyze its steady-state performance. A test system is established to validate the system model, and the simulated results agree well with the experimental data. The simulation results show that there is a performance band for the system parameter relationship due to the frictional forces between the moving components of the VDC within which all the steady-state points fall. Different from the one-one parameter relationship in an AAC system with a fixed displacement compressor, it is a multiple-one parameter relationship in the AAC system with a VDC. The influence of the compressor rotary speed, air temperature at the condenser inlet and air flow rate through the evaporator on the performance band is simulated. The evaporating pressure performance band moves in the direction of increasing evaporator inlet air temperature and increasing evaporating pressure when the compressor rotary speed increases. The evaporating pressure performance band moves in the direction of decreasing evaporating inlet air temperature and decreasing evaporating pressure when air temperature at the condenser inlet or air flow rate through the evaporator increases
[en] Multi-unit air conditioners (MUACs) are widely used in light commercial buildings and residential buildings due to their higher thermal comfort and energy efficiency. To investigate the transient characteristics of MUACs, a dynamic simulation model with the framework of two-phase fluid network is developed. The state-space forms are used to model the system and components, and the component submodels are embedded in the fluid network model, which makes it possible to update the system model and components submodels independently. In the model of state-space form, the differentials are obtained by taking the inverse of coefficient matrix, and then the state parameters are calculated by integrating the differentials with time. The simulation outputs are compared with the experimental data in the step changes of the compressor speed and electronic expansion valve openings. The comparison shows that the proposed model can catch the dynamic characteristics of MUACs with high accuracy. Therefore, it can be used as an effective tool to analyze the transient performance and optimize the control algorithm of MUACs. - Highlights: ► A dynamic model is developed for multi-unit air conditioners (MUAC). ► The model is built in the two-phase fluid network for different MUACs. ► State-space method model is built for evaporators, condensers and MUACs. ► The component submodels are embedded in the fluid network model for easy updating. ► The model can catch the dynamic characteristics of MUACs with high accuracy.
[en] Highlights: • A multi-unit heat pump is proposed for simultaneous temperature and humidity control. • Condensation heat is non, partly or fully recovered for temperature regulation. • Highly integrated heat pump for residential cooling, dehumidification and heating. • High energy saving potential for all-year-round operation in wet and warm regions. - Abstract: A multi-unit heat pump is presented for simultaneous humidity and temperature control to improve the energy efficiency and the thermal comfort. Two parallel connected condensers are employed in the system, locating at the back of the indoor evaporator and the outdoor unit, respectively. The heat pump can operate in four modes, including heating, cooling and dehumidification without and/or with partial or total condensing heat recovery. The experimental investigation shows that the temperature control capacity is from 3.5 kW for cooling to 3.8 kW for heating with the cooling and heating efficiency higher than 3.5 kW kW−1, and the dehumidification rate is about 2.0 kg h−1 with the efficiency about 2.0 kg h−1 kW−1. The supply air temperature and humidity can be simultaneously regulated with high accuracy and high efficiency by adjusting the indoor and/or outdoor air volumes. It provides an integrated and effective solution for simultaneous indoor air temperature and humidity control for all-year-round operation in residential buildings
[en] Some ionic liquids have dehumidification capacity and are non-corrosive to metals. Based on the Finite Difference Method, the heat and moisture transfer model of the counter-flow dehumidifier was established. Using this model, the dehumidification capacity of one kind of ionic liquids 1-Ethyl-3-methylimidazolium Tetrafluoroborate ([EMIM]BF4) and traditional desiccant lithium bromide (LiBr) was compared in equivalent conditions. The results show that the dehumidification performances of the two solutions both improve with the increase of air flow rate, solution flow rate and air humidity. Even though the dehumidification rate of [EMIM]BF4 is a little lower than that of LiBr, such difference could be reduced by increasing the mass concentration of [EMIM]BF4. Meanwhile, compared with traditional desiccants, [EMIM]BF4 solution has significantly lower corrosion to metals and does not crystallize at high mass concentration, which render it a possible substitute of existed desiccants in liquid desiccant air conditioning systems. - Highlights: → The paper researched on using [EMIM]BF4 solution as desiccant in the liquid desiccant air conditioning system by simulation. → By comparing the dehumidification performance of [EMIM]BF4 solution with traditional desiccant LiBr solution, it confirms the feasibility of using [EMIM]BF4 as desiccant. → [EMIM]BF4 solution has significantly lower corrosion of metals and does not crystallize at high mass concentration, which make it a possible substitute of existed desiccants.
[en] There are often abnormal working conditions at evaporator outlet of a refrigeration system, such as two-phase state in transient process, and it is essential to investigate such transient behaviours for system design and control strategy. In this paper, a dynamic lumped parameter model is developed to simulate the transient behaviours of refrigeration system with variable capacity in both normal and abnormal working conditions. The appropriate discriminant method is adopted to switch the normal and abnormal conditions smoothly and to eliminate the simulated data oscillation. In order to verify the dynamic model, we built a test system with variable frequency compressor, water-cooling condenser, evaporator and electronic expansion valve. Calculated values from the mathematical model show reasonable agreement with the experimental data. The simulation results show that the transient behaviours of the variable capacity refrigeration system in the abnormal working conditions can be calculated reliably with the dynamic model when the compressor rotary speed or the opening of electronic expansion valve changes abruptly.
[en] It is essential to ensure the stability for the normal operation of refrigeration systems. This paper reviews the researches on the theory and solutions of the instability of refrigeration systems. The instability of refrigeration systems includes two aspects: the two-phase flow instability in refrigeration system, the instability on refrigeration system control characteristics. As an inherent characteristic of two-phase evaporating flow, several separate explanations for the formation of oscillation of mixture-vapor transition point in the evaporation process by different scholars had been given but there is no general explanation till now. The investigation of instability on refrigeration system control characteristics focused on both static and dynamic researches. The minimum stable signal line theory, as a very important finding for the static instability of the evaporator and thermal expansion valve control loop, presented the different result to other researches. Dynamic researches on simulation and frequency-domain analysis provided various means for forecast and validation with considerable precision while their application range was still confined. With the development of variable capacity compressor and electronic expansion valve, further researches should be carried out to analyze the instability of the variable capacity refrigeration system with considering the influence of parameter coupling and control algorithm.
[en] A novel spray cooling system integrated in the refrigeration circuit is proposed and its performance is investigated experimentally. In this system, the inverter compressor is used to replace the pump in common spray systems, the nozzle plays the role of atomization and throttling, and the spray chamber has function of the evaporator. The nozzle inlet pressure, the evaporation pressure and the degree of subcooling at nozzle inlet are all adjusted to testify the performance of the novel system in experiments. With 60 W/cm2 heat flux, the heat transfer coefficient observed is higher than 30 000 W/m2 K. The critical heat flux up to 110 W/cm2 is obtained, and heater surface temperature is only 31.5 °C under the heat load. Keeping the nozzle inlet pressure (Pin = 390 kPa), the evaporation pressure (Pe = 180 kPa) and the heat flux (q = 72 W/cm2) constant, the experimental results show that the optimal subcooling degree is 5.8 °C. The novel spray cooling system developed in this paper has simple structure and convenient regulation, and its performance can meet the heat removal requirements of most electronic devices in actual applications.
[en] Highlights: • Three parameters are used to evaluate the startup performance. • The startup is faster and overshoot is larger when the distribution is more uneven. • Heating on one evaporator with same Q with the other makes transition time longer. • Heating on one evaporator with same Q with the other makes the overshoot smaller. • Transition time is about twice as much as peak time when peak time exists. - Abstract: Loop thermosyphon with multiple evaporators is a promising device in multi-source heat transfer. The startup performance is very important for its thermal control ability. In this paper, the effect of heating power distribution on the startup of a loop thermosyphon with dual evaporators is investigated experimentally. The startup time and stationarity under different power distributions are analyzed utilizing three parameters: peak time, transition time and temperature (pressure) overshoot. The results show that the startup process is faster and the overshoot of pressure and temperature is larger when the distribution is more uneven; Heating on one evaporator with the same heating power with the other evaporator makes the startup process longer while it makes the overshoot smaller or even disappear; The transition time is approximately twice as much as the peak time when the peak time exists.
[en] Highlights: • Heat performances of RIASHP system for EVs is designed and tested at −20 °C out-car. • Two scroll compressors are used to analyze the effect of injection portholes shapes. • Heating capacity of RIASHP system is raised up to 28.6% than traditional system. • The refrigerant cycle and injection process and the influence factors are discussed. • Summary the analysis: bigger area of injection porthole make better heat capacity. - Abstract: The traditional Air Source Heat Pump (ASHP) for Electric Vehicles (EVs) has many limits in cold region because of poor heating performance and operating safety in low ambient temperature, which can be solved by the ASHP with refrigerant injection. A test bench that can be switched between a traditional ASHP system and a refrigerant injection ASHP system for EVs in the cold region is developed in this study. Compressors applied in the test bench are refitted with different injection portholes from the regular electric scroll compressor to analyze the influence of porthole shape on the system performance. The experimental results show that the heating capacity of refrigerant injection ASHP system is raised up by 28.6% compared with the traditional system. The larger injection porthole helps to increase the heating capacity when in-car inlet air temperature is higher, and the effect of injection porthole shapes to injection process and refrigerant cycle is also analyzed. The research will contribute to the application of refrigerant injection technology in EVs.
[en] Highlights: • An integrated thermal management system is proposed for electric vehicle. • The parallel branch of battery chiller can supply additional cooling capacity. • Heat pipe performance on preheating mode is better than that on cooling mode. • Heat pipe heat exchanger is a feasible choice for battery thermal management. - Abstract: An integrated thermal management system combining a heat pipe battery cooling/preheating system with the heat pump air conditioning system is presented to fulfill the comprehensive energy utilization for electric vehicles. A test bench with battery heat pipe heat exchanger and heat pump air conditioning for a regular five-chair electric car is set up to research the performance of this integrated system under different working conditions. The investigation results show that as the system is designed to meet the basic cabinet cooling demand, the additional parallel branch of battery chiller is a good way to solve the battery group cooling problem, which can supply about 20% additional cooling capacity without input power increase. Its coefficient of performance for cabinet heating is around 1.34 at −20 °C out-car temperature and 20 °C in-car temperature. The specific heat of the battery group is tested about 1.24 kJ/kg °C. There exists a necessary temperature condition for the heat pipe heat exchanger to start action. The heat pipe heat transfer performance is around 0.87 W/°C on cooling mode and 1.11 W/°C on preheating mode. The gravity role makes the heat transfer performance of the heat pipe on preheating mode better than that on cooling mode.