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。     

三种常见的翅片型式对翅片管式换热器性能的影响

针对市场上常见的波纹片型、百叶窗片型以及桥片型翅片管式换热器,为了确定翅片型式对空气侧换热和压降性能的影响,对每种片型的3种翅片间距的换热器进行试验研究。结果表明:在干空气工况下,同一风速时波纹片型换热器热阻最大,百叶窗片型换热器热阻次之,桥片型换热器热阻最小,而换热器空气侧压降与热阻的变化趋势相反;在湿空气工况下,同一风速时,3种片型换热器的热阻变化趋势同干空气工况一致,百叶窗片型和桥片型的换热器空气侧压降接近,波纹片型的换热器空气侧压降相对较低。     

三排变片距翅片盘管换热器结霜特性实验研究

为改善空气源热泵室外翅片盘管换热器在低环境温度下沿空气流动方向结霜不均匀、首排结霜量较大进而导致热泵除霜间隔较短、制热能力下降等问题,对不同翅片片距组合的变片距翅片盘管换热器在低环温工况下运行及结霜的情况进行实验研究。结果表明：变片距翅片盘管换热器在低环温条件下可有效延长迎风面管排发生结霜堵塞的时间、对于结霜生长速度和结霜质量也有抑制作用。变片距翅片盘管换热器在结霜中后期阶段换热功率更高,在合理的翅片间距组合下,变片距翅片盘管换热器可以在不损失过多换热功率的情况下延长换热器迎风面管排结霜堵塞的时间,如样品4的平均换热功率比样品1低6.02%,而除霜间隔延长了37 min。     

翅片管式换热器换热与压降特性的实验研究进展--实验研究

概述目前国内外空调制冷行业中普遍采用的几种不同翅片类型(平直翅片、波纹形、条缝形翅片、百叶窗形翅片)的换热特性和压降特性的实验研究进展，通过对这几种翅片类型换热及压降特性的介绍和分析，提出了趣片管式换热器研究中的一些不足。     

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。     

翅片类型对热泵空调结霜特性的影响

通过实验研究了室外换热器采用平直、波纹、波纹/开缝翅片对热泵空调器动态结霜性能的影响。结果表明,在结霜工况下,平直、波纹、波纹/开缝三种翅片类型的系统制热量、COP和压缩机功率的峰值、平均值依次增大,其中,波纹/开缝翅片的各项平均值比平直翅片分别减小了14.57%、8.26%和7.11%。波纹/开缝翅片的性能参数最早开始衰减且衰减幅度也最大,其次是波纹翅片,平直翅片最晚开始衰减且幅度最小。结霜时,水蒸气变为霜的相变过程对传热具有强化作用,同时霜层减小了翅片间隙使风阻增大、室外风机流量减小,这对传热具有恶化作用。结霜时性能参数的曲线存在上升段和衰减段,与霜层微观成长特性一致:结霜初期,冰晶由粒状变为柱状,尚未显著阻塞翅片间隙,对传热的强化作用较显著,曲线处于上升段;结霜中后期,冰晶先沿半径成长,后又在柱状晶间隙堆垛,对传热的恶化作用较显著,曲线处于衰减段。     

新型组合翅片换热器数值研究

通过对传统多排管翅式换热器的研究分析,提出第1排为平片且后部为开缝翅片的新型组合翅片换热器,并对传统和新型组合翅片2种形式换热器内的空气流动和换热进行数值分析。结果表明,新型组合翅片换热器的综合换热性能比传统翅片的提升5％～8％。     

星型翅片管换热器结霜过程的数值模拟

采用数值模拟的方法对星型翅片管换热器的结霜过程进行了研究。得到了霜层厚度和霜层密度等物性参数的变化规律,分析了其空气侧结霜过程中的传热传质特性,研究了翅片高度和翅片个数两个参数对结霜过程的影响。结果表明,结霜现象会造成换热器换热性能的恶化,并且翅片个数越多、翅片高度越小,传热恶化更严重。     

空气源热泵蒸发器表面霜层生长特性实验研究

在室外环境空气温度-15℃~5℃、相对湿度65%~90%范围内,对一台空气源热泵室外换热器表面霜层生长特性及热泵系统动态性能进行了实验研究。实验结果表明,翅片表面霜层厚度呈分段增长模式,在结霜初始段,霜层主要由粒状冰晶组成,并逐渐形成柱状冰晶,其厚度增长较快;在第二阶段为柱状冰晶主要在其半径方向生长,霜层厚度增长速度减慢;而在第三阶段,柱状冰晶主要在其高度方向生长,逐渐形成针状冰晶,霜层厚度增长速度迅速增大至第二阶段的3~5.8倍。对实验结果的分析表明,热泵机组性能恶化主要是由于蒸发器表面温度下降、霜晶形态变化引起霜层厚度快速增长及空气流动阻力增加导致风机流量下降三个因素之间形成的恶性循环所致,其中换热器表面温度下降引起的霜晶形态变化起到了至关重要的作用。     

圆孔和半圆孔交叉翅片管的传热与流阻性能研究

本文提出一类可用于结霜和灰尘较多场合的新型翅片——圆孔和半圆孔交叉翅片。为了考察这类翅片的性能特征,对9种不同几何参数的单排肋管式换热器样件在吸风式直流热风洞内进行了实验。这些几何参数包括圆孔、半圆孔的直径和位置。通过多元线性回归法将实验数据整理为以雷诺数和换热器几何参数为自变量的j和f关联式。结果表明开孔参数对换热性能有明显影响,而对阻力影响不显著。与平片相比,样片3的平均当量换热系数提高18.3％,节约金属耗材12％。     

Heat transfer characteristics of spirally-coiled circular fin-tube heat exchangers operating under frosting conditions

The objective of this study is to investigate the heat transfer characteristics of spirally-coiled circular fin-tube heat exchangers under frosting conditions. The heat transfer rate, pressure drop, frost thickness, and Nusselt number of the heat exchanger were measured and analyzed by varying the fin pitch and number of tube rows under frosting conditions. In addition, the Nusselt number of the spirally-coiled circular fin-tube exchanger was compared with those of flat plate fin-tube heat exchangers with discrete fins. An empirical correlation of the Nusselt number was developed as a function of the Reynolds number, dimensionless fin pitch normalized by the hydraulic diameter, i.e., D_h/F_p, Fourier number, and number of tube rows. The measured Nusselt number was consistent with the predicted value with mean and average deviations of 3.5% and 0.3%, respectively.     

Experimental investigation of the surface temperature and water retention effects on the frosting performance of a compact microchannel heat exchanger for heat pump systems

Frost formation on a louvered fin microchannel heat exchanger was experimentally investigated in this paper with the aim of determining the dominant factors affecting the time of frosting and frost growth rate. A novel methodology was developed to measure frost thickness and frost weight at intervals during the frosting period. Frost mass and thickness growth rates, corresponding coil heat transfer, capacity degradations and air pressure drop are measured and discussed. The experimental data showed that at a given air dry bulb temperature, the fin surface temperature and air humidity are the primary parameters that influence the frost growth rates. Water retention and air velocity had a secondary impact on the frosting performance. From digital images of the frost growth it was observed that frost does not nucleate from the water droplets retained in between fins but it developed from the leading edges of the fins.     

Fan supplied heat exchanger fin performance under frosting conditions

In this paper, a validated numerical model for frost growth on heat exchange fins is modified to simulate a fan-supplied finned heat exchanger under refrigeration frosting conditions. It is found that frost growth on refrigeration heat exchangers causes a dramatic drop in the fin heat rate, airflow rate, and fin efficiency while the pressure drop increases. A sensitivity study shows the effects of changing several design parameters including the type of fan.     

Experimental investigation of adverse effect of frost formation on microchannel evaporators,part 1: Effect of fin geometry and environmental effects

This study experimentally investigated the frost growth on louvered folded fins in microchannel heat exchangers when used in outdoor air-source heat pump systems. The effects of surface temperature, fin geometries, and air environmental conditions were studied. The overall aim was to isolate and quantify the effect of geometry from surface temperature effects. Experimental data of frost weight, local frost thickness, air pressure drop across the coils, time of frost–defrost cycles and heat transfer rates were recorded. Data showed that the frosting time and the frost growth rates depended mainly on the local fin surface temperature. Lower fin density was beneficial because it delayed the blockage of the air flow. The fin length and fin depth had minor effects on frosting performance. The air humidity had a fairly significant effect on rate of frost formation while air velocity seemed to have a small effect on the frost growth rate.     

微通道换热器结霜特性及换热性能实验研究

结霜是制约微通道换热器在制冷及空调系统应用的主要因素之一。针对微通道换热器结霜问题,本文基于相变驱动力分析了结霜机理,观察了不同环境因素下冷表面霜层生长形貌,并实验研究了湿空气温度、含湿量、气流速度及冷却液温度对微通道换热器结霜特性及换热性能的影响。结果表明:湿空气含湿量及气流速度是影响微通道换热器结霜的主要因素,结霜时间为15 min,含湿量为5.75 g/(kg干空气)工况下,换热器表面结霜量比含湿量为3.58 g/(kg干空气)时提高了63.87%;气流速度为2.5 m/s工况下,换热器表面结霜量比气流速度为1 m/s时增加了55.4%。随着结霜时间的增长,湿空气温度、冷却液温度越低,含湿量、气流速度越大,换热量下降趋势越明显。     

Performance of finned-tube heat exchangers under frosting conditions: I. Simulation model

This paper describes an analytical model that was developed to predict the performance of finned-tube heat exchangers under frosting conditions. The method models the frost growth mechanism and heat exchanger performance in a comprehensive manner. The results include frost accumulation and its effect on energy transfer in relation to varying humidities, fin densities and ambient conditions.     

通道轮式换热器结霜工况下的换热研究

通道轮式新风换气机作为一种新型的通风换气设备近年来得到了广泛的应用。然而，其室外排风侧结霜是影响其在严寒地区冬季运行的主要问题，结霜严重时会影响该设备的运行性能。因此，有必要对通道轮式新风换气机在结霜工况下的运行规律进行深入的研究以指导其在严寒地区冬季的应用。文章在能量守恒、含湿量守恒的基础上建立了通道轮式换热器排风侧结霜工况下的数学模型，该模型耦合了结霜子模型和热交换子模型。利用该模型分析了不同工况下结霜量对该换热器换热的影响。计算出了不同工况下融霜的时间间隔，为采取有效的除霜控制方法提供了依据。将模拟结果与实验数据进行了比较，两者吻合很好，进一步验证了所建模型的可靠性。     

An experimental study on frosting of laminar air flow on a cold surface with local cooling

This paper presents the heat and mass transfer characteristics of the humid airflow in frosting conditions. A flat plate of aluminum with cooling modules at the central region was used for the simulation of flat surface part of the fin of the heat exchanger. The local surface temperature of the plate and the local thickness and total mass of the frost on the plate were measured to analyze the heat and mass transfer characteristics. In order to analyze the frosting characteristics, an analysis algorithm was developed, which can provide the local air temperature, the frost surface temperature, the sensible and the latent heat flux distributions at the test plate. Also, by integrating the local heat flux distribution, the average heat flux characteristics were analyzed. The present experiment and analysis found that the characteristic of the upstream airflow was very different from that of the downstream airflow.