低频率下电子膨胀阀调节对制冷系统性能的影响

以R32变制冷剂流量制冷系统实验装置为研究对象,通过改变变频压缩机频率和电子膨胀阀开度,对低频率下电子膨胀阀调节对系统性能的影响进行了实验分析。研究结果表明:1)系统制冷量和质量流量在各频率下变化规律相同;2)当控制蒸发器出口过热度在2 K附近时,低频率范围(25~35 Hz)内存在一个最佳频率点,与其他各频率相比,其系统COP最高;3)低频率下压缩机在过热度0 K附近极少量的吸气带液就会对系统性能造成严重影响,这在实际运行中需要极力避免;4)低频率下(25~35 Hz)控制过热度从0 K变为10 K,电子膨胀阀调节区间为4%~9%,而高频率下(40~50 Hz)调节区间为15%~23%,提高冷冻水温度可以有效改善低频下电子膨胀阀的调节性能。以上结论可以推广到其他变制冷剂流量系统,如变频热泵空调和汽车空调等。     

变流量制冷系统中频率与电子膨胀阀开度的协同控制研究

针对变流量(VRF)制冷系统控制中的滞后或超调现象,本文以变频滚动转子式制冷系统为研究对象,分别通过改变压缩机频率与电子膨胀阀开度,建立了两者单独控制下的曲线拟合模型,并对不同工况下两者的同步控制方法进行了实验研究。结果表明:过热度随膨胀电子膨胀阀开度的增大而减小,在电子膨胀阀开度为28%～32%与频率为44.5～46.5 Hz时,过热度控制难度上升,通过调节冷冻水温度可改善这一状况;在一定的工况下,通过控制质量流量,可以得到频率与电子膨胀阀开度的关系式,实现电子膨胀阀开度与压缩机频率的同步控制,使系统迅速达到稳定状态。     

不同空气侧风速下微通道蒸发器换热特性实验研究

搭建微通道蒸发器性能实验台,采用控制变量法研究不同空气侧风速下微通道蒸发器表面温度分布、制冷剂进出口压力的变化规律,计算换热量和换热系数,从而分析空气侧风速对微通道蒸发器的流量分配特性和换热效果的影响。结果表明,随着风速增大,微通道蒸发器制冷剂流量分配不均匀性增大,进出口压力波动振幅和周期增加,压降增大,风速2 m/s时微通道蒸发器换热效果最佳。     

蒸发器流程布置的数值模拟研究与分析

基于湿球温度效率法，建立了蒸发器的稳态分布参数模型，分析了三种不同流路布置蒸发器的换热性能，并与前人的实验研究结果进行对比，计算所得的换热量与试验值的最大偏差为9．44％，说明该模型切实可行。运用该模型计算并分析了六种不同流路布置蒸发器的流动和换热特性，结果表明：逆流布置蒸发器换热最好，错流其次，顺流最差。各种流路布置方案管外迎风面风速不变时，随着管内冷媒流量的增大，管内压降、总换热景、显热换热量、潜热换热量均增大，但潜热所占比重增大。管内冷媒流量保持不变，蒸发器迎风面风速增大时，总换热量、显热换热量均增大，潜热换热量、压降、吸湿系数均减小；当风速增大到一定程度时，蒸发器换热量、压降的变化都趋于平缓。同时，在蒸发器流路布置中，重力的影响不可忽略。研究结论为蒸发器流程布置的优化设计提供了理论基础和指导方向。     

涡旋式水源热泵系统性能仿真

为了预测涡旋式水源热泵系统变结构和变工况稳态性能,建立了稳态涡旋式热泵系统仿真模型.其中涡旋式压缩机模型考虑了吸、排气换热对工质流量和排气温度的影响以及流量、排气温度和输入功率三者的耦合关系;通过增加电子膨胀阀开度对蒸发器出口过热度的控制模型,反映了过热度对膨胀阀流量的影响.系统算法综合了顺序模块法和连续迭代法,改善了迭代收敛性,且易于实现部件模型的模块化.与实验结果对比表明:模型预测值与实验值的误差小于4.4％.     

电动汽车电池冷却系统对空调系统性能的影响

为了更好地匹配电动汽车空调系统和电池冷却系统,减少其对乘员舱舒适性和整车续航里程的影响,搭建电动汽车空调系统和电池冷却系统测试台架,试验分析电池冷却系统参数对空调系统性能的影响,确定匹配的电池冷却器容量、电子膨胀阀容量和冷却液流量。结果表明:电池冷却器容量从3 kW减小为2 kW时,蒸发器换热量增加30%,蒸发器出口空气温度降低6.6℃;电子膨胀阀容量从1.0 Rt减小为0.5 Rt时,蒸发器换热量增加5%,蒸发器出口空气温度降低1.0℃;冷却液流量从7.5 L/min减少为3.5 L/min,蒸发器换热量加8%,蒸发器出口空气温度降低1.8℃。     

阀头线型对蒸发器非线性补偿的理论与试验研究

通过将电子膨胀阀与受控对象（蒸发器）细化分开来，并依据恒定开环增益准则从理论上分析了阀头补偿原理。依据特性试验结果量化分析了蒸发器变增益的非线性特征和电子膨胀阀的流量特性增益，并根据阀头补偿原理设计出了具有补偿功能的阀头线型。通过冷启动、蒸发器变负荷和过热度设定值改变的对比试验检测了补偿性阀头的控制特性，结果表明补偿性阀头的变增益特性使电子膨胀阀控制特性得到了改善，蒸发器过热度被降低。     

并联式汽车空调系统的试验研究

为分析压缩机转速、电子膨胀阀开度及车内设定温度变动对双蒸发器并联式汽车空调系统性能的影响,制作试验样机,并利用标准焓差试验台对系统进行测试。结果表明:压缩机频率改变对制冷量的影响幅度可达26.8%;当车内设定值低于28℃时,平均每降低2℃设定值,制冷能力下降6.0%,能效比降低4.9%;在电子膨胀阀对过热度的调节在1~10 K范围内时,系统制冷量和能效比的变化幅度不明显。     

二氧化碳翅片管式蒸发器模拟与分析

运用分布参数法建立采用CO2的翅片管式蒸发器的数学模型，分析制冷剂侧和空气侧温度、压力和换热的变化情况。同时讨论迎面风速和制冷剂质量流量对蒸发器换热和流动性能的影响，结果表明提高迎面风速可以增加换热效果，但增加的趋势趋于平缓。制冷剂侧压降则成近似线性增大；随着管内工质流量的增大，蒸发器总换热量和制冷荆侧压降都成近似线性增大。这些工作有助于进一步了解CO2在翅片管式蒸发器中的换热和流动特性，并为换热器的优化设计和系统的匹配提供理论依据。     

R22替代工质蒸发器的性能研究

基于湿球温度效率法,建立了蒸发器的稳态分布参数模型。分别以R22及其3种替代产品(R407C,R410A,R134A)为工质,运用该模型详细比较了在流量变化及风量变化两种情况下2排管蒸发器的换热和流动特性。结果表明：当迎风面风速不变时,随着管内冷媒流量的增大,各种工质蒸发器总换热量、显热换热量、潜热换热量均增大,但潜热换热量增大得更快。压降亦随着管内流量的增大而增大。当管内冷媒流量不变时,随着蒸发器迎风面风速的增大,各种工质蒸发器总换热量、显热换热量均增加,潜热换热量下降,同时压降亦下降,但下降的趋势随着风速的增大而最终趋于平缓。四种工质中,R134A性能较好,但其压降也最大,R410其次,压降最小,但其蒸发压力比R22高出60％左右。R407C与R22在换热量及压降方面最为接近,是其理想的替代产品。     

电子膨胀阀内啸叫噪声特性及发声规律的实验研究

空调系统中电子膨胀阀节流时出现的尖锐啸叫噪声严重影响用户使用舒适性,了解制冷剂流经电子膨胀阀时的啸叫噪声产生机理及其发声规律是解决上述噪声问题的关键。本文设计了可以对阀前后制冷剂状态进行调节控制并对产生的啸叫噪声进行测量分析的实验系统,得到不同工况及阀开度下的啸叫噪声发声规律。研究表明：啸叫噪声来源于阀内流体高频压力脉动引起的流体周期性振荡,其发声特性是阀内环锥形节流通道与阀腔构成的亥姆霍兹共振腔结构对共振频率附近的噪声源信号选择性放大的结果。啸叫噪声声压级主要与阀内制冷剂流速及阀开度有关,阀开度为700 pls下制冷剂速度由2.5 m/s增至3 m/s时,噪声声压级提高了21%;阀内制冷剂流速决定了流体振荡频率,阀开度决定了阀内声腔共振频率;通过改变阀腔结构以增大共振频率,使常见空调工况下阀内制冷剂冲击引起的振荡频率低于共振频率,可以有效避免啸叫噪声产生。     

Refrigerant R134a vaporisation heat transfer and pressure drop inside a small brazed plate heat exchanger

This paper presents the experimental heat transfer coefficients and pressure drop measured during refrigerant R134a vaporisation inside a small brazed plate heat exchanger (BPHE): the effects of heat flux, refrigerant mass flux, saturation temperature and outlet conditions are investigated. The BPHE tested consists of 10 plates, 72 mm in width and 310 mm in length, which present a macro-scale herringbone corrugation with an inclination angle of 65° and corrugation amplitude of 2 mm.The experimental results are reported in terms of refrigerant side heat transfer coefficients and frictional pressure drop. The heat transfer coefficients show great sensitivity both to heat flux and outlet conditions and weak sensitivity to saturation temperature. The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow.The experimental heat transfer coefficients are also compared with two well-known correlations for nucleate pool boiling and a correlation for frictional pressure drop is proposed.     

Flow boiling of R32 inside a brazed plate heat exchanger

This paper presents measurements of heat transfer coefficient obtained during flow boiling of R32 inside a brazed plate heat exchanger (BPHE). Although R32 is known as a very interesting refrigerant for its thermodynamic and thermophysical properties, very limited flow boiling data are published in the open literature for R32 working in brazed plate heat exchangers.The present experimental data are measured to investigate the effect of refrigerant heat flux, mass velocity, inlet vapor quality and superheating at the outlet. The saturation temperature is kept constant at around 5 °C, which is a usual temperature level for evaporation in liquid coolers. As a significant result, differently from other studies on flow boiling with HFC refrigerants, mass flux is found to be very important, meaning a high contribution of the convective term on the heat transfer coefficient.The present data are also analyzed to assess available correlations for flow boiling inside BPHEs, in order to provide useful information on the accuracy of predicting methods that can be used for evaporators with R32.     

大容量新风处理机制热控制目标的优化与实现

设计出一款基于VRF空调系统的16HP大容量新风处理机,制热采用变目标冷凝温度的优化设计模型控制压缩机的能力输出,通过调节新风机电子膨胀阀开度来达到目标出风温度,模型中的换热修正系数由试验确定,通过优化室外机电子膨胀阀开度的控制解决低温制热出风温度低的问题.实验验证了该控制方法的可行性,系统运行稳定可靠,出风温度控制精度高,而且实现节能,为新风机的应用提供指导.     

Effect of tube diameter on boiling heat transfer and flow characteristic of refrigerant R32 in horizontal small-diameter tubes

This study investigated the effect of tube diameter on flow boiling characteristics of refrigerant R32 in horizontal small-diameter tubes with 1.0, 2.2, and 3.5 mm inner diameters. The boiling heat transfer coefficient and pressure drop were measured at 15 °C saturation temperature. The effects of mass velocity, heat flux, quality, and tube diameter were clarified. The flow pattern of R32 for adiabatic two-phase flow in a horizontal glass tube with an inner diameter of 3.5 mm at saturation temperature of 15 °C was investigated. Flow patterns such as plug, wavy, churn, and annular flows were observed. The heat transfer mechanisms of forced convection and nucleate boiling were similar to those in conventional-diameter tubes. In addition, evaporation heat transfer through a thin liquid film in the plug flow region for low quality, mass velocity, and heat flux was observed. The heat transfer coefficient increased with decreasing tube diameter under the same experimental condition. The fictional pressure drop increased with increasing mass velocity and quality and decreasing tube diameter. The experimental values of the heat transfer coefficient and frictional pressure drop were compared with the values calculated by the empirical correlations in the open literature.     

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.     

热力膨胀阀对空调器性能影响的试验研究

为了提高空调器对变工况的响应速度,通过试验分析热力膨胀阀对空调器(制冷量为3 500 W,R134a制冷剂)的调节特性。试验结果表明,热力膨胀阀的非线性特性使制冷剂质量流量在一定范围内呈现波形变化;热力膨胀阀开度一定时,对于变工况,空调器在40 s内可以达到稳定运行状态;热力膨胀阀开度为40%时,空调器的EER值最大。     

Flow boiling characteristics and flow pattern visualization of refrigerant/lubricant oil mixtures

A comprehensive review of flow boiling characteristics and flow pattern visualization of refrigerant/lubricant oil mixtures is presented in this paper. First, various parameters influenced by the lubricant oil in convective boiling of refrigerants, such as mass velocity, vapor quality, oil concentration and geometric characteristics of the heat transfer tube are discussed. The effects of the unavoidable introduction of the lubricant oil on the thermodynamics properties of a refrigerant are described. Then, a review of the main experimental studies of flow boiling of refrigerant/lubricant oil mixtures is presented and also describes research with halocarbons, carbon dioxide, hydrocarbons and ammonia. There is no agreement among these studies regarding the effect of the oil in the evaporator, with studies showing an increase or decrease in the heat transfer coefficient. However, in relation to pressure drop, all the results presented the same trend, increasing the pressure drop with increasing oil concentration. Next, the flow patterns of refrigerant/oil mixtures are illustrated together with a selection of video images. It is possible to notice the difference in frothing formation with respect to the particular refrigerant and tube geometry. Some predictions of oil effects on the heat transfer coefficient and pressure drops based on the mixture physical properties are then presented and the trends compared to data. Finally, some suggestions for future work are given.     

Flow boiling heat transfer coefficients at cryogenic temperatures for multi-component refrigerant mixtures of nitrogen–hydrocarbons

The recuperative heat exchanger governs the overall performance of the mixed refrigerant Joule–Thomson cryocooler. In these heat exchangers, the non-azeotropic refrigerant mixture of nitrogen–hydrocarbons undergoes boiling and condensation simultaneously at cryogenic temperature. Hence, the design of such heat exchanger is crucial. However, due to lack of empirical correlations to predict two-phase heat transfer coefficients of multi-component mixtures at low temperature, the design of such heat exchanger is difficult.The present study aims to assess the existing methods for prediction of flow boiling heat transfer coefficients. Many correlations are evaluated against available experimental data of flow boiling of refrigerant mixtures. Silver-Bell-Ghaly correlation and Granryd correlation are found to be more suitable to estimate local heat transfer coefficients. A modified Granryd correlation is recommended for further use.     

R134a表面增强型蒸发强化传热管的实验研究

介绍了国外空调冷冻设备中采用的一种新型商用表面增强型蒸发强化传热管,并对其进行了实验研究.发现对使用R134a环保制冷工质,在低热流密度的条件下,表面增强型蒸发强化传热管具有十分优异的强化效果.在实验的换热工况下,保持进口水温不变而改变管内水流速,这种强化管的总换热性能是普通低翅蒸发强化管的 2.2到2.6倍,管外换热系数是普通低翅蒸发强化管的1.3～1.9倍.