亲水涂层对翅片管式换热器空气侧性能的影响

为了分析亲水涂层对翅片管式换热器空气侧性能的影响,对干/湿工况下亲水涂层翅片和普通翅片管式换热器空气侧的压降和热阻进行对比试验。结果表明：相对于普通翅片管式换热器,在干工况下,亲水涂层翅片管式换热器空气侧压降略增大,空气侧热阻增大10%左右;在湿工况下,亲水涂层翅片管式换热器空气侧压降减小,空气侧热阻增大4%左右;亲水涂层翅片管式换热器空气侧热阻基本不受空气湿度的影响。     

低气压下翅片管换热器空气侧传质特性实验研究

本文实验研究了2个具有不同翅片间距的平翅片管式换热器在低环境气压以及析湿工况下空气侧的传质特性,在环境气压为40~100 kPa,换热器入口空气风速为0.5~4 m/s,入口空气相对湿度为50%~90%,入口空气干球温度为27 ℃,水流速为1.65 m/s的实验工况下,分析了环境气压、换热器迎面风速、翅片间距及入口空气相对湿度对换热器空气侧传质特性的影响。结果表明：低气压环境下,迎面风速、翅片间距以及相对湿度对换热器空气侧传质因子的影响趋势与常压下的研究一致;研究工况下,环境压力从40 kPa升至100 kPa时,空气侧传质因子下降幅度为18.2%~23.6%;在较低迎面风速和环境气压下,翅片间距和换热器入口空气相对湿度对换热器空气侧传质特性的影响更加显著。     

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。     

管径变化对蒸发器性能影响的仿真与实验研究

家用空调换热器采用的管径不同,产生的传热效果等性能有差异。采用空气焓差法,对具有相同制冷量的5 mm管径换热器和7 mm管径换热器进行了蒸发工况的实验,并建立了不同管径换热器的仿真计算模型,分析了管径的变化对蒸发器制冷剂侧和空气侧的换热和压降的影响。对比实验与计算结果,发现:1)5 mm换热器空气侧表面传热系数提高了17%;2)在相同制冷量下,5 mm换热器的制冷剂质量流量减少了4.6%,质量流速增大了89.4%,同时由于管壁热流密度的增大,引起了蒸干点的提前;3)以制冷剂达到相同干度时的换热系数作为基准,随着干度的增加,5 mm管的管内换热系数增大到7 mm管的1.43~1.86倍;同时制冷剂的摩擦压降、加速压降和局部压降均为7 mm换热器的3倍,压降引起了蒸发温度降低1.1℃。     

大直径圆孔翅片管结霜过程动态特性的实验研究

在入口空气温度28℃,恒温恒湿条件下,以三对称大直径圆孔翅片为研究对象,在循环式风洞中对其结霜工况下的动态性能进行了实验研究,考察了入口空气相对湿度和空气流速对换热器性能的影响,给出了换热器换热量、翅片表面传热系数和空气侧压降在结霜过程中的动态变化规律,研究结果表明:入口空气温度28℃时结霜工况下,翅片表面未覆盖满霜层时,在雷诺数Re=3602～5509,进口相对湿度φ=60％～80％范围内,不同的环境参数对结霜影响的差别较大.实验结果表明,空气相对湿度对SK型翅片的传热与流阻性能的影响大于空气流速的影响.实验结果可为优化环境参数,使制冷系统达到节能匹配提供参考.     

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。     

不同方式地下埋管换热器的实验研究

针对不同方式地下埋管换热器建立实验系统,并分别对砂石回填的单U型和双U型井埋管进行夏季排热和冬季取热实验.整理分析了冬、夏季节不同运行工况下的实验数据,在此基础上,以单位管长换热量为评价指标对不同埋管方式的换热性能进行分析比较,得出排热工况和取热工况下,单U型埋管单位管长换热量高于双U型.同时对埋管换热器的流量和进口水温对其换热性能的影响进行了研究,结果表明,随着流量的增大,排热工况与取热工况下,单位管长换热量均增加;随着进口水温的升高,单位管长排热量增大,而单位管长取热量减小.     

板式蒸发冷却空冷器的阻力和传热特性实验研究

通过改变风量、水量和增湿供水量,对板式蒸发空冷器阻力及传热特性进行实验研究,得到干工况下空气侧和热水侧的阻力及对流换热系数关联式.实验结果表明,增湿工况下,空气侧阻力几乎不受影响,换热系数随增湿供水量增加而增大,约为干工况时的6倍.     

微流道换热器内阻力特性的实验研究

本文是制冷换热器小型化研究的一部分，采用等壁温法，对水在正方形，正三角形微流道内受热单相流动的阻力特性进行了实验研究，通过对实验数据的分析发现，层流的摩擦阻力系数随微流道壁面的温度的升高而下降，且低于经典层流阻力系数公式的预测值，流态由层流向紊流过渡也比常规尺度流道提前，且临界雷诺数随微流道壁面温度的升高而减小。     

圆柱型翅片管换热器变工况传热性能模拟与分析

基于分排参数模型,本文建立了圆柱型翅片管换热器的性能仿真计算模型,对换热器的传热性能进行计算,并进行实验验证。结果表明:换热量的平均相对误差最大,为6.31%;出风干球温度的平均相对误差最小,为0.61%。计算所得各性能参数与实验值吻合良好。根据仿真模型,对不同制冷工况下的换热器进行计算,研究了循环风量、水质量流量、进风干球温度以及进水温度的变化对换热性能的影响。分析换热器的变工况特性可预测其非设计工况下的换热性能,并为换热器的运行工况调节提供依据。     

水冷套管式CO_2气冷器的设计及实验研究

本文设计了一台CO_2套管式气冷器并对其进行了换热特性的实验研究。该气冷器采用逆流三重套管,CO_2在内管流动,冷却水在内外管间流动。实验研究了不同CO_2质量流量、入口压力和冷却水温度对传热系数、换热量和换热器效能系数的影响。实验结果表明,随着CO_2质量流量的增加,传热系数和换热量均呈先增后减的趋势,换热器效能系数逐渐减小;CO_2质量流量不变时,传热系数、换热量和换热器效能系数均随气冷器CO_2入口压力的升高而逐渐增大;随着冷却水温度的升高,传热系数、换热量和换热器效能系数均逐渐减小。     

Performance optimization of a miniature Joule–Thomson cryocooler using numerical model

The performance of a miniature Joule–Thomson cryocooler depends on the effectiveness of the heat exchanger. The heat exchanger used in such cryocooler is Hampson-type recuperative heat exchanger. The design of the efficient heat exchanger is crucial for the optimum performance of the cryocooler.In the present work, the heat exchanger is numerically simulated for the steady state conditions and the results are validated against the experimental data available from the literature. The area correction factor is identified for the calculation of effective heat transfer area which takes into account the effect of helical geometry. In order to get an optimum performance of the cryocoolers, operating parameters like mass flow rate, pressure and design parameters like heat exchanger length, helical diameter of coil, fin dimensions, fin density have to be identified. The present work systematically addresses this aspect of design for miniature J–T cryocooler.     

制冷剂分液器分配特性的实验研究

分液器是制冷系统中重要的辅助设备,分液不均会引起换热器面积不能充分利用,制冷能力下降,蒸发器出风温度不均,膨胀阀阀芯振动等问题,合理选择和设计分液器对制冷系统高效运行有着重要的意义。文中介绍了压降式、离心式和贮液式三种分液器的工作原理,并且通过实验定量地研究了质量流量50~110 kg/h,入口干度0.1~0.3工作条件下,质量流量和入口干度对三种分液器分配特性的影响,引入各支路间质量和干度的标准偏差STDm和STDx评估分液均匀性。实验结果表明,当质量流量从50 kg/h增大到110 kg/h时,三种分液器的STDm减小,质量流量的均匀性得到改善。其中对压降型和离心式的影响较大,STDm分别降低了77%和51%,而对贮液型的影响较小,STDm降低了21%。当入口干度增大时,分液均匀性有所下降。     

The performance of a transcritical CO2 cycle with an internal heat exchanger for hot water heating

The main purpose of this study is to investigate the performance of a transcritical CO₂ cycle with an internal heat exchanger for hot water heating. Performance test and simulation have been carried out for a transcritical CO₂ cycle by varying secondary heat transfer fluid temperatures at evaporator and gas-cooler inlets as well as the discharge pressure. Variations of mass flow rate of refrigerant, compressor power, heating capacity, and co-efficient of performance (COP) with respect to the length of an internal heat exchanger are presented at various operating conditions. Good quantitative agreement between model predictions and experimental results has been found; most parameters have absolute average deviations of less than 4%. As the length of the internal heat exchanger increases, COP is enhanced but heating capacity tends to decrease due to the trade-offs between the effectiveness and pressure drop in the internal heat exchanger.     

Impact of plate design on the performance of welded type plate heat exchangers for sorption cycles

Numerical and experimental analysis was carried out to examine the heat transfer and pressure drop characteristics of welded type plate heat exchangers for absorption application using Computational Fluid Dynamics (CFD) technique. The simulation results based on CFD are compared with experimental results. A commercial CFD software package (FLUENT) has been used to predict the characteristics of heat transfer, pressure drop and flow distribution within the plate heat exchangers. In this paper, a welded plate heat exchanger with a plate of chevron embossing type was tested by controlling mass flow rate, solution concentration, and inlet/outlet temperatures. The working fluid is H₂O/LiBr solution with the LiBr concentration of 54–62% in mass. The numerical simulation examines the internal flow patterns, temperature distribution and the pressure distribution within the channel of the plate heat exchanger. Three plates of embossing types; chevron embossing, elliptic and round, are proposed and simulated in this paper. The simulation results show reasonably good agreement with the experimental results. Also, the numerical results show that the plate with the elliptical shape gives better performance than the plate of the chevron shape from the viewpoints of heat transfer and pressure drop.     

地下水渗流对垂直埋管换热器换热性能影响的实验研究

地埋管换热器的换热性能是地源热泵空调系统设计的关键问题之一。建立了地源热泵砂箱实验台,实验研究了地下水渗流对地源热泵地埋管换热器换热性能的影响。研究结果表明:地下水渗流可以强化地埋管的换热,且随着渗流速度的增大,强化换热作用越明显。另外,实验结果也表明,U型管周围温度峰值的位置会沿渗流方向向下游偏移,地埋管群布置时应避开该位置,以强化埋管的换热。     

Heat transfer characteristics of accumulator heat exchangers under various geometric and operating conditions

To minimize the possibility of flash gas generation at the inlet of the expansion device in multi air-conditioners, an accumulator heat exchanger is usually adopted. In this study, the heat transfer rate and subcooling difference in an accumulator heat exchanger were measured under various operating and geometric conditions. The effect of the operating conditions on the performance of the accumulator heat exchanger was analyzed for R-22, R-134a, and R-410A. The performance of the accumulator heat exchanger was also analyzed by varying diverse geometric parameters: the inner volume and the ratio of the diameter to height of the accumulator, and the diameter, length, and bending diameter of the inner heat exchanger. In addition, the flow of the refrigerant-oil mixture inside the accumulator was visualized. Based on the measured data, a dimensionless correlation for predicting the subcooling difference across the accumulator heat exchanger was developed.     

Analytical investigation of two different absorption modes: falling film and bubble types

The objectives of this paper are to analyze a combined heat and mass transfer for an ammonia–water absorption process, and to carry out the parametric analysis to evaluate the effects of important variables such as heat and mass transfer areas on the absorption rate for two different absorption modes — falling film and bubble modes. A plate heat exchanger with an offset strip fin (OSF) in the coolant side was used to design the falling film and the bubble absorber. It was found that the local absorption rate of the bubble mode was always higher than that of the falling film model leading to about 48.7% smaller size of the heat exchanger than the falling film mode. For the falling film absorption mode, mass transfer resistance was dominant in the liquid flow while both heat and mass transfer resistances were considerable in the vapor flow. For the bubble absorption mode, mass transfer resistance was dominant in the liquid flow while heat transfer resistance was dominant in the vapor region. Heat transfer coefficients had a more significant effect on the heat exchanger size (absorption rate) in the falling film mode than in the bubble mode, while mass transfer coefficients had a more significant effect in the bubble mode than in the falling film mode.     

Effect of the header pressure drop induced flow maldistribution on the microchannel evaporator performance

This paper presents an experimental and numerical investigation of the flow maldistribution caused by the pressure drop in headers and its impact on the performance of a microchannel evaporator with horizontal headers and vertically oriented tubes. Experimental results show that the flash gas bypass method almost eliminates the quality induced maldistribution. However, refrigerant flow maldistribution caused by the header pressure drop still exists. This is mainly because the pressure drop along the headers results in uneven pressure difference and therefore non-uniform liquid refrigerant mass flow rate across each microchannel tube. A microchannel evaporator model validated by experimental results is employed to quantify header pressure drop induced flow maldistribution. Parametric analysis reveals that such maldistribution impact is significantly reduced by enlarging the outlet header size, increasing heat exchanger aspect ratio, or reducing the microchannel size while other parameters are kept constant. When ratio of outlet header to the total evaporator pressure drop is less than 30%, the cooling capacity reduction is limited below 3%.