翅片管式换热器表面结霜特性的数值分析和实验研究

换热器表面结霜会增加传热热阻和流动阻力，为了合理确定融霜周期，必须对其结霜特性做深入地了解。本文采用了数值求解方法预测了翅片管式换热器的结霜量和霜层密度的变化过程。翅片管结霜是一个瞬变问题，为了计算方便，将其简化为准稳态过程，即在时间步长内，认为该过程是稳定的，然后把所得的霜层厚度以及霜层的表面温度作为下一时间步长内传热传质的边界条件。在研究中考虑了霜层阻力引起风量下降这一因素，计算结果与翅片管式换热器结霜的实际工况相符。     

翅片管式换热器表面结霜特性的数值分析和实验研究

换热器表面结霜会增加传热热阻和流动阻力，为合理确定融霜周期，必须对其结霜特性作深入的了解。文中采用了数值求解方法预测了平翅翅片管式换热器的结霜量和霜层密度的变化过程。翅片管结霜是一个瞬变问题，为了计算方便，将其简化为准稳态过程，即在时间步长内，认为该过程是稳定的，然后把所得的霜层厚度以及霜层的表面温度作为下一时间步长内传热传质的边界条件。在研究中考虑了霜层阻力引起风量下降这一因素，模型的计算结果与翅片管式换热器结霜的实际工况相符。     

低温翅片管换热器结霜试验研究

为了解翅片管换热器在低温工况下的传热性能以及结霜情况,进行了翅片管换热器的结霜试验.在翅片管换热器设置8个不同的观测点进行霜层厚度数据的采集,得到8条霜层厚度曲线.分析曲线后表明在不考虑环境风速的情况下,结霜量主要与空气相对湿度、流体在管内的形态等因素有关,同时发现翅片管换热器结霜的重点区域是换热器上部和入口处,并从理论上分析了结霜过程.     

电动汽车热泵系统结霜、除霜特性比较研究

热泵技术由于较高的制热效率,近年来被逐步应用于电动汽车,用来提高冬季续航里程。在低温环境下,热泵系统车外换热器表面容易出现结霜的现象,严重影响系统的性能和可靠性。针对热泵系统结霜问题,笔者根据实车要求设计搭建了一套汽车热泵空调系统台架,通过试验对比研究了横流式与纵流式车外换热器的结霜化霜特性,总结了判定结霜和终止结霜的方法。试验结果表明：在低温环境下10 min左右车外换热器表面一半的面积被霜层覆盖;采用逆循环的方式大约3 min可以将霜层除尽;纵流式车外换热器的抗结霜性能更好,但横流式换热器的制热效率更高;可以将车外换热器的出口温度变化作为判定系统结霜和终止结霜的主要依据。     

疏水表面改性在换热器抑霜上的实验研究

对分体式空调室外换热器翅片管表面进行了疏水改性处理并搭建了可视化的结霜测试平台,在干、湿球2℃/1℃环境工况下测试了改性后和常规换热器翅片表面结霜、除霜及融霜过程的性能。实验表明疏水纳米涂层翅片表现出一定的抑霜效果:其液滴冻结时间延后、霜层薄且疏松、除霜周期延长、化霜时间缩短,但抑霜效果会在结霜后期减弱且融霜后翅片上存在残留液滴。而对于未处理的亲水裸铝翅片:其霜层冻结快、霜层较为致密、结霜程度较严重,但融霜后排液效果好。     

高压电场对蒸发器表面结霜影响的研究

为研究高压电场下，自然对流蒸发器表面结霜的过程及电压等参数对该结霜过程的影响，对其在肋片式换热器的结霜过程进行了微细观可视化研究。实验中，在冷表面Tw=-14℃，湿空气温度为15℃至20℃，相对湿度为20％至90％，静电场电压OkV至40kV范围内，发现，受高压不均匀电场的作用，换热器铜管表面的结霜分布不均匀，高压电场有明显抑制结霜的作用。     

融霜时风机运行状态对换热器性能影响的实验与分析

在结霜工况下,蒸发器霜层将影响换热器的换能性能,需定期对换热器进行融霜,以提高其换热效果。本文对一种换热器在融霜时风机的运行与不运行的两种状态的性能进行了实验研究,分析并比较了换热器在两种状态下空气侧压力降及制冷量的变化情况。     

高压电场下自然对流蒸发器结霜的试验研究

为研究高压电场下自然对流蒸发器表面结霜的过程及电压等参数对该结霜过程的影响，对其在肋片式换热器的结霜过程进行微细观可视化研究。试验中，在冷表面Tw=-14℃，湿空气温度为15℃至20℃，相对湿度为20％～90％，静电场电压为0-40kV范围内，受高压不均匀电场的作用，换热器铜管表面的结霜分布不均匀，高压电场有明显抑制结霜的作用。     

室外环境参数对空气源热泵翅片管蒸发器动态结霜特性的影响

对一台空气源热泵空调器在不同环境条件下室外换热器的动态结霜性能进行了实验研究，分析了进风温、湿度对热泵空调器结霜量及霜层厚度的影响。实验中考虑了结霜引起的热泵系统蒸发温度及风机流量的变化，采用显微照相法测量翅片表面霜层厚度，结霜量则通过测量蒸发器进出口含湿量的方法来获得。实验结果表明，室外换热器结霜量随时间线性增长，而翅片表面霜层厚度则分为初始段、匀速增长段和快速增长段三个阶段；在结霜循环的最后20％～30％的快速增长段内霜层生长速率大大加快，可达匀速生长段霜层生长速率的2．4，3．3倍。对于不同的工况，蒸发器均在进风温度0～3℃附近时结霜最为严重，且相对湿度对霜层厚度的影响要大于对结霜量的影响。     

微通道换热器结霜特性研究现状与展望

微通道换热器在低温工况下用作蒸发器时存在结霜速度较快的问题,制约了其在制冷系统的应用。针对微通道换热器的结霜现象,本文归纳了影响微通道换热器结霜特性的因素、改善微通道换热器结霜特性的方案和微通道换热器结霜的相关仿真研究,介绍了微通道换热器结霜特性的研究现状和方向,发现目前影响微通道换热器结霜特性的因素主要分为:外部因素(环 境参数、表面温度、凝水),结构因素(换热器布置方向、翅片结构、翅片密度、涂层、结垢)和内部因素(制冷剂分布)。改善微通道换热器结霜特性的研究集中在调整翅片的结构以实现更好的排水性能和更均匀的霜层分布,未来研究的重点在于开发抑霜性能 更好的微通道换热器和建立更高精度的仿真模型。     

Analysis of the heat transfer area distribution in a frosted plain fin-and-tube geometry

The development of frost is a phenomenon that deteriorates thermohydraulic performance on heat exchangers. In this study, several heat-transfer area distributions on a fin-and-tube geometry are proposed and their performances as frost develops are compared using simulation. The frost development at a specific location is determined using a segment analysis. In each segment, a semiempirical model to predict the frost growth based on air temperature, velocity, relative humidity and surface temperature is applied. The analysis considers airflow redistribution among channels, leading to changes in heat transfer and frosting rates with time. Results show that a geometry that allows even flow distribution along the operation time is less sensitive to thermohydraulic deterioration. An area distribution with larger fin spacing and fin length presents an advantage, particularly on the pressure drop, which allows longer operation time between defrost cycles.     

Analysis of early-stage frost formation in natural convection over a horizontal cylinder

Frost growth process on a cold surface consists of two stages: The early-stage or one-dimensional growth of ice columns and multidimensional growth in the form of a porous structure. The transition time which marking these two stages is important for any numerical modeling of frost formation. This paper proposes a mathematical model to predict the transition time and frost properties in natural convection of frost formation over a cooled horizontal cylinder in the first stage of growth period. Comparison is performed among the results of this model and experimental observations reported in the literatures. It is observed that the presented model can be used more efficiently to determine transition time and frost properties in the early-stage of frost formation. Based on the obtained results a new correlation is developed for the duration time of early-stage of frost formation process (transition time) in natural convection.     

Study on methods of measuring frost crystal dimensions/structure and frost scraping force using a scanning probe microscope

Frosting on a cooling surface causes serious damage as well as economic losses. Therefore, it is necessary to clarify the mechanism of frosting on a cooling surface both scientifically and technologically. In addition, taking the size of frost crystals into account requires nano-micro-scale investigation. However, there have been few such studies. Thus, the focus of the present study is methods of measuring frost crystal dimensions/structure and frost scraping force in nano-micro-scale fields. In the present study, methods by which to measure frost crystal dimensions/structure, such as diameter, height, number of frost crystals, and interval between adjoining frost crystals, as well as the force required to scrape frost from a cooling surface using a scanning probe microscope (SPM) are proposed, and the effectiveness of these methods is investigated. With the exception of the frost crystal height, the feasibility of these measurements was demonstrated, and examples of the correlation of frost dimensions/structure and frost scraping force could are presented.     

Heat and mass transfer under frosting conditions

The effect of frost formation on heat transfer between a test cylinder and its gaseous environment was studied experimentally. The main parameters discussed in the paper are: the total heat flux, the steady-state convective heat transfer coefficient, and the mass of frost adhering to the test cylinder. The emphasis of the paper is on the thermal conducivity of frost. The data indicate that the diffusion mechanisms of moisture transfer within the frost layer causes the frost density and thermal conductivity to increase with time. Frost thermal conductivity is a function of the local temperature and average density. The can be used by designers of low temperature systems with uninsulated surfaces.     

霜层生长初期冰晶体分布状况实验研究

研究霜层的结构对于理解结霜现象有着重要的意义。利用自行研制的图像放大及采集系统，对霜层生长初期冰晶体的形态进行显微观测，得到了不同生长条件下冰晶体的图像。随后采用数字图像处理方法，将原图像转换为二值图。通过对图像的分析，发现霜层的生长初期冰晶体的分布与充分生长的霜层有所不同，此时霜层靠近冷表面固含率最大；随着高度的增长，固含率以近似线性的方式减小。实验还发现，对霜层生长初期影响最大的两个因素是冷表面的温度和空气的相对湿度。随着冷表面温度的降低，霜层的高度明显增长，冰晶体沉积量增加，而平均密度的变化则不明显；随着空气相对湿度的增加，霜层的高度、平均密度以及冰晶体总的沉积量都有所增加。空气温度对霜层生长的影响不明显。     

Laboratory and field testing on rate of frost heave versus heat extraction

Equipment for performing laboratory freezing tests on soils has been developed to study the relation between the rates of heave and heat extraction. Laboratory freezing tests are performed on specimens over 500 mm tall. In addition, field equipment for automatically recording frost depth and heave has been installed in a pavement. The equipment gives temperature readings for every 50 mm.Temperature gradients at different depths, from laboratory as well as field, are calculated to estimate the rate of liberation of latent heat as a function of time. The calculations indicate that, for the laboratory testing, the heave rate is almost independent of the heat extraction rate for fairly long periods at a constant frost penetration rate. The recorded field data indicate that the heave rate is on average almost constant for relatively long periods of time with a continuous frost penetration, thereby supporting the existence of a maximum heave rate that can not be surpassed persistently. However, heave rate varies greatly if the heat extraction rate often changes between a high and a low level and can for short periods widely exceed the maximum average heave rate possible over long periods.The observed behavior is explained as a consequence of a limited capacity in the redistribution of water to the frost front to supply the heaving.     

A new structure to control frost boiling and frost heave of embankments in cold regions

Frost boiling and frost heave are the main factors that cause road damage in cold regions. A new kind of embankment structure, which consists of geotextile, crushed-rock layer and geomembrane, was designed and investigated both in the laboratory and at the field of Budongquan located at Qinghai–Tibet Plateau. Comparison tests were conducted at the same time. The key of the new structure is the porous crushed-rock layer which has smaller thermal conductivity with function of drainage and blocking of moisture migration induced by freezing. Both the laboratory and field research results show that the frost penetration and thawing depths of the new structural embankment are much smaller than those of the coarse-grained soil embankment. Also, the new embankment structure has lower water content in the upper layer and smaller frost heave and thawing settlement than the latter does. In addition, it has good drainage effect. Water coming from the road surface can be drained away from the embankment through the porous crushed-rock layer. All these states that the new structure consisting of the porous crushed-rock layer is superior in frost damage mitigation to the normal structure used in cold regions.