变制冷剂流量制冷系统过热度振荡机理实验研究

本文以变制冷剂流量制冷系统实验装置为研究对象,通过改变电子膨胀阀开度、冷冻水侧加热量和冷却水出水温度,对系统过热度振荡机理进行了实验分析。结果表明:1)电子膨胀阀开度较小时(24.7%~25.3%),蒸发器出口过热度振幅在1 K内,随着开度增大(25.6%~26.2%),振幅变大,约为3 K,当开度为26.5%~26.8%时,振幅恢复到1 K以内;2)传热机理的变化是导致过热度振荡的根本原因,影响蒸发器管内沸腾特性的主要参数是蒸发器换热量和质量流量,研究过热度振荡时需将两者综合考虑;3)压比对质量流量的影响较大。在压比增大初期,质量流量逐渐增加,表面传热系数大幅增加,过热度降低;当压比继续增加时,换热机理一直在液膜对流沸腾换热和过热蒸气换热间交替,维持不变。过热度振荡特性在膨胀阀-蒸发器闭环控制时更为复杂,在今后的研究中需要重点关注。     

电子膨胀阀的优势和发展趋势

从制冷系统节流机构的流量调节稳定性、过热度控制原理、系统节能、瞬态反应特性、部分负荷调节特性方面,比较分析了几种节流机构应用于制冷装置中的特点。提出了电子膨胀阀在容量调节和过热度稳定控制方面的优点,并对电子膨胀阀的发展趋势和在各方面的应用进行了分析。     

模糊控制理论在舰船制冷装置中的应用研究

以蒸发器和冷库作为被控对象，利用计算机进行了制冷系统模糊控制的研究．针对蒸发器过热度和库温控制问题设计了模糊控制器．在理论研究的同时，建立了制冷装置计算机控制和数据采集系统．在Visual Basic环境下开发了蒸发器过热度模糊控制程序．对比实验运行表明：利用模糊控制器和电子膨胀阀对蒸发器过热度和库温进行控制较之用热力膨胀阀控制，制冷剂流量调节范围大、过热度小、蒸发器面积利用率高、装置节能效果好．     

基于STC89S52单片机的电子膨胀阀的控制系统研制

本文针对家用变频空调蒸发器过热度控制问题,提出了一种基于带死区的积分分离的PID 控制算法和STC89S52单片机为控制核心的电子膨胀阀的开度控制系统,并阐述了控制系统的工作原理、硬件电路及控制软件设计。通过对变频空调系统的电子膨胀阀的控制实验验证：该控制器具有静态精度高,自适应能力强,可靠性高,抗干扰性强的特点。     

蒸发器出口过热度对汽车空调性能影响的实验研究

蒸发器出口过热度是制冷系统运行状态的一种特殊表征,为了研究在汽车空调中,蒸发器出口过热度对于蒸发器能力和系统影响的大小,本文在标准汽车空调实验台下实验,通过一种新型电子膨胀阀对蒸发器出口过热度进行控制,研究蒸发器出口过热度对于汽车空调性能、COP等参数的影响,研究结果表明,蒸发器能力随蒸发器出口过热度的减小而增大,在本文研究的系统中蒸发器的能力最多能被提升19.4%,系统COP随蒸发器出口过热度的减小先增大后减小,存在最大值,在本文研究的系统中系统COP最多能被提升17.8%。     

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％.     

蒸发器过热度控制中盘管温度测点位置的实验研究

在电子膨胀阀开度缓变条件下,对多流路的蒸发器传热面盘管的温度变化进行了分析,发现各位置出现干涸点的时间不同,并在干涸点形成了温度谷值,讨论了利用过热度控制方案时测点布置的原则,指出在流路中部偏入口位置布点是较适宜的选择。     

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。     

制冷装置中膨胀阀的对比研究

为验证自行开发的电子膨胀阀和采用单片机开发的实时控制器的控制效果,针对热力膨胀阀、步进电机型膨胀阀、电磁阀型膨胀阀,进行了多个制冷系统冷启动和稳定工况下变负荷控制的实验研究。结果表明相对于传统的热力膨胀阀,电子膨胀阀更适用于系统综合控制器;连续调节型的执行机构比离散调节型的执行机构更容易使过热度稳定;积分环节大大改善了过热度的控制品质。自行开发的电子膨胀阀和控制器,性能价格比高,与国外同类产品相比,实际的控制效果也有较明显的提高。     

充注量对空气源热泵热水器性能的影响

本文通过实验比较了热泵热水器在不同制冷剂充注量下的制热性能和电子膨胀阀的调节特性,定量研究了电子膨胀阀调节方式对制热性能的影响。结果表明:阀开度16%和定过热度10℃工况下的最佳充注量均为1.1 kg,相应的最佳整体制热性能COP_a分别为3.05和3.68;过大的充注量降低了制热性能和电子膨胀阀的调节范围;与阀开度调节相比,过热度调节可避免吸气带液的发生;在最佳充注量下,对比阀开度和过热度两种电子膨胀阀调节方式得出,后者制热性能更优。     

出口过热度对CO_2微通道蒸发器性能的影响

本文理论分析了影响CO_2微通道蒸发器制冷能力的关键因素,搭建CO_2微通道蒸发器测试实验台,采用CO_2电子膨胀阀实现过热度调节,研究了蒸发器出口过热度对制冷能力以及出风温度的影响。研究表明:相比于其他制冷剂,CO_2出口过热度对蒸发器制冷能力的影响较大,蒸发器的性能随出口过热度的减小而提升,随着过热度的减小,制冷能力的提升存在3个具有显著差异的阶段,系统中蒸发器的性能随过热度减小最多提升57.9%。此外,采用了红外线热像仪拍摄和均布热电偶的方法,得到出风温度和蒸发器表面温度的分布规律,结果表明出风温度随过热度的减小有更好的均匀性。     

Compensation of flow maldistribution in fin-and-tube evaporators for residential air-conditioning

Compensation of flow maldistribution in multi-channel fin-and-tube evaporators for residential air-conditioning is investigated by numerical modeling. The considered sources of maldistribution are distribution of the liquid and vapor phases in the distributor and non-uniform airflow distribution. Fin-and-tube heat exchangers usually have a predefined circuitry, however, the evaporator model is simplified to have straight tubes, in order to perform a generic investigation. The compensation of flow maldistribution is performed by control of the superheat in the individual channels. Furthermore, the effect of combinations of individual maldistribution sources is investigated for different evaporator sizes and outdoor temperatures. It is shown that a decrease in cooling capacity and coefficient of performance by flow maldistribution can be compensated by the control of individual channel superheat. Alternatively, a larger evaporator may be used.     

热力膨胀阀与电子膨胀阀对分液性能影响的比较

本文通过比较蒸发器各回路之间的温差，对分液器在热力膨胀阀和电子膨胀阀控制下的分液性能做了实验研究。结果表明在电子膨胀阀控制下分液器的工作性能更好，而且还可以保持较小的蒸发器传热温差。     

Thermostatic valve control using a non-azeotropic refrigerant, isobutane/propane mixtureRégulation d'un détendeur thermostatique utilisant un mélange d'isobutane et de propane comme frigorigène non azéotropique

This article describes the evaluation and comparison of a conventional R12 cross-charged thermostatic valve and an electronic expansion valve using a non-azeotropic refrigerant mixture (NARM); isobutane/propane mixture (CARE30). The superheat temperature setting on an expansion valve needs to compensate for the temperature glide associated with a non-azeotropic refrigerant as these can be of similar magnitude. It is also advisable to increase the superheat setting to make allowance for change in refrigerant composition as a result of preferential refrigerant/oil solubility. The majority of refrigeration systems operate at fixed evaporating temperatures, hence, once superheat setting is trimmed during commissioning, then there should be no further problems associated with evaporation of a non-azeotropic refrigerant provided the system is leak-tight. An R12 expansion valve with a factory superheat setting of 5°C tested over a wide range of evaporating temperatures proved satisfactory in operation with CARE30 after increasing the superheat temperature screw setting equivalent to 5°C.     

Dynamic simulation of electronic expansion valve controlled refrigeration system under different heat transfer conditions

This work presents a new dynamic lumped parameter model that is able to simulate the oscillation of electronic expansion valve (EEV) controlled refrigeration system due to unsuitable control or abrupt change of external parameters. Based on bubble dynamics theory, the heat transfer characteristic is used to divide the evaporator into the stable and unstable conditions, and a matching switch criteria is presented. Together with the model of main components, the dynamic behavior of oscillation of system parameters is simulated successfully. Model validation against experimental data demonstrates the capabilities of the modeling approach in predicting the instability of system control loop. This study provides the basis for further developing advanced control algorithm which will ensure that an EEV-controlled system refrigeration system has better stability.     

An experimental and numerical study of hunting in thermostatic-expansion-valve-controlled evaporators

The dynamic behaviour of a thermostatic-expansion–valve (TEV)-controlled dry-evaporator is studied experimentally and numerically. Although the linear model of the TEV together with the distributed model of the evaporator is able to predict the stable dynamic response of the system adequately, it fails to reproduce the hunting behaviour that is observed under certain operating conditions. A scrutiny of the experimental data reveals the possible existence of hysteresis in the system. The distributed model including the experimentally determined input-output characteristics of the TEV is able to reproduce the main features of the hunting oscillations well. The amplitude and frequency of these oscillations depend on the static superheat setting, the heat load of the evaporator and the time constant of the TEV bulb.     

Modeling evaluation of a direct-expansion solar-assisted heat pump water heater using R410A

A direct-expansion solar-assisted heat pump (DX-SAHP) system by using R410A as refrigerant is described, which can supply domestic hot water during the whole year. Based on the distributed parameter and homogeneous flow models of collector/evaporator and condenser, the lumped parameter models of compressor and electronic expansion valve, and the refrigerant charge model, a numerical model is developed to estimate the thermal performance of the system. Given the structure parameters, meteorological parameters, initial and final water temperatures, for a fixed superheat degree, the effects of the refrigerant charge quantity on the performance parameters of the system are analyzed, such as compressor power, heat gain of collector, heating time, collector efficiency and system COP. Furthermore, for a fixed refrigerant charge quantity, the effects of various parameters, including solar radiation, ambient temperature, compressor speed and initial water temperature, have been simulated and analyzed on the thermal performance of the system.     

重力供液系统中过热度对传热面积影响的研究

本文主要对重力供液系统中蒸发器出口制冷剂的过热度进行分析,通过visual basic软件仿真计算出全部用气化潜热制冷需要的蒸发器面积A1,和有过热度后多出来的面积A2。将A1＋A2与蒸发器现有面积进行对比,发现A1＋A2比蒸发器现有面积小,说明现有蒸发器的面积比实际需要的面积要大,因此我们可以减小蒸发器的面积来达到优化蒸发器的目的。     

The influences of the operating characteristics of an Electronic Expansion Valve (EEV) on the operational stability of an EEV controlled direct expansion air conditioning system

This paper reports on a study on the influences of the operating characteristics of a proportional–integral (PI) controlled Electronic Expansion Valve (EEV) on the operational stability of a direct expansion (DX) air conditioning (A/C) system. Using the classical control theory, EEV's PI settings and time constant of EEV's temperature sensor were analyzed. The theoretical analysis results using the classical control theory were further verified experimentally using an experimental DX A/C system. The study results showed that a larger proportional or integral gain would lead to a high chance for the EEV–evaporator control loop to become unstable, while slowing down the rate of degree of superheat (DS) signal transfer by increasing EEV's time constants may help mitigate system's operational instability. The results confirmed that the operating characteristics of an expansion valve in a refrigeration system could impact its operational stability.