Performance evaluation and modeling of a hybrid cooling system combining a screw water chiller with a ground source heat pump in a building

The performance of a hybrid cooling system that combines a screw water chiller with a ground source heat pump (GSHP) was measured and analyzed at various cooling loads. In addition, the hybrid cooling system in a building was modelled sophisticatedly using EnergyPlus and then validated with the measured data. The coefficient of performance of the GSHP was lower than that of a conventional chiller in the monitored building, but the hybrid cooling system helped to stably provide the required cooling capacity at high-load conditions. The mean bias error and the normalized root-mean squared error of the predicted cooling load of the building were −8% and 12.4%, respectively. The hybrid cooling system was simulated by varying four operating parameters: the operating schedule, chilled water temperature (T_CW), dry-bulb temperature (T_DB), and entering water temperature (T_EW). The T_CW is ascertained as being the most effective control parameter in the hybrid cooling system.     

Film condensation heat transfer characteristics of R134a on horizontal stainless steel integral-fin tubes at low heat transfer rate

Condensation heat transfer characteristics of R134a on the integral-fin tubes are experimentally investigated. The test tubes are made of stainless steel, and the root diameter of the tubes is 13.27 mm. The height of fin is 1.19 mm, and the densities of the integral fin are 19 fpi and 26 fpi. The present tests were conducted at the saturation temperatures of 20 °C and 30 °C. The condensation heat transfer coefficients of the tubes having 19 fpi and 26 fpi at the saturation temperature of 20 °C are higher than that of the plain tube by 4.4 and 3.1 times, respectively. When the temperature difference across the condensate film is less than 0.7 °C, the enhancement of the tube of 19 fpi is much larger than that of the tube of 26 fpi. The Honda and Nozu model shows the smallest mean deviation between the estimated values and experimental results among the existing models.     

双通道两视干涉相位差解径向速度模糊方法

针对合成孔径雷达地面动目标显示(SAR-GMTI)系统中动目标径向速度估计存在模糊的问题,提出了基于双通道两视干涉相位差实现无模糊估计径向速度的方法．该方法在距离频率域构造两视数据,将不同通道数据做干涉处理以得到两视数据的干涉相位．两视数据的干涉相位差不存在缠绕问题,可以用来无模糊地估计径向速度．理论推导证明,所提方法有效提高了最大不模糊速度．文中着重分析了两视数据距离频率间隔对径向速度估计性能的影响．通过仿真和实测数据处理验证了所提方法．     

Effects of accumulator heat exchangers on the performance of a refrigeration system

An accumulator heat exchanger (AHX) consists of an accumulator and an inner heat exchanger (IHX) contained in a shell. The AHX has been used in multi-air-conditioners to obtain system reliability and high performance by providing liquid refrigerant into expansion devices and preventing wet-compression. Energy is exchanged between the evaporator exit and the condenser exit in the AHX. In this study, the heat transfer characteristics of the AHX were investigated experimentally, and the effects of the AHX on the performance of a refrigeration system using R22 were measured. The operating characteristics of the refrigeration system with the AHX were considerably different from those without the AHX. The AHX system showed higher refrigerant flow rate than the non-AHX system at a constant EEV (electronic expansion valve) opening because of higher subcooling, resulting in better performance and reliability of the refrigeration system. At 50% EEV opening, the cooling capacity and COP of the AHX system were higher than those of the non-AHX system by 7.5% and 3.2%, respectively.     

Cooling performance of a variable speed CO2 cycle with an electronic expansion valve and internal heat exchanger

The cooling performance of a CO₂ cycle must be improved to develop a competitive air-conditioning system with the conventional air-conditioners using HFCs. In this study, the cooling performance of a variable speed CO₂ cycle was measured and analyzed by varying the refrigerant charge amount, compressor frequency, EEV opening, and length of an internal heat exchanger (IHX). The basic CO₂ system without the IHX showed the maximum cooling COP of 2.1 at the compressor discharge pressure of 9.2 MPa and the optimum normalized charge of 0.282. The cooling COP decreased with the increase of compressor frequency at all normalized charges. The optimum EEV opening increased with compressor frequency. Simultaneous control of EEV opening and compressor frequency allowed optimum control of the compressor discharge pressure. The optimal compressor discharge pressure of the modified CO₂ cycle with the IHX was reduced by 0.5 MPa. The IHX increased the cooling capacity and COP of the CO₂ cycle by 6.2–11.9% and 7.1–9.1%, respectively, at the tested compressor frequencies from 40 to 60 Hz.