R-134a在水平直管和螺旋管内凝结换热特性的实验研究

对替代制冷剂R-134a在水平直管和螺旋管内的凝结换热特性进行了实验研究。在三个不同的冷凝温度（35℃、40℃和45℃）、制冷剂R-1Ma的质量流量变化范围为100-400kg／（m^2·s）和制冷剂的干度范围为0．1-0．8的条件下，实验得到了R-134a在水平直管和螺旋管内的凝结换热系数随R—134a的质量流量和干度的变化关系，并将水平直管和螺旋管内的凝结换热特性数据进行了对比。实验结果表明，R-134a在螺旋管内的凝结换热系数比直管的大4％。13．8％。     

R-134a在三种不同放置方式螺旋管内凝结换热的实验研究

对替代制冷剂R-134a在螺旋管内的对流凝结换热特性进行了实验研究.在制冷剂R-134a的质量流量变化范围为100～400kg/(m2s) 和冷却水的平均温度分别为12℃和22℃的条件下,实验得到了在三种不同放置方式螺旋管内(水平,垂直和倾斜)R-134a的对流凝结换热特性数据.实验结果表明,螺旋管的不同放置方式对R-134a在螺旋管内的凝结换热特性具有重要的影响.通过与已有研究结果的比较,简要分析了螺旋管不同放置方式对R-134a对流凝结换热特性的影响机理.     

具有蒸气过热的R-134a在螺旋管内凝结换热的实验研究

对具有蒸气过热的替代制冷剂R-134a在螺旋管内的对流凝结换热特性进行了实验研究.在制冷剂R-134a的质量流量变化范围为100(400kg/(m2s)和蒸气过热度为3.8℃和8.4℃的条件下,得到了螺旋管内具有蒸气过热的R-134a对流凝结换热特性的实验数据.实验结果表明,蒸气过热对R-134a在螺旋管内的凝结换热特性具有明显的影响.通过与已有研究结果的比较,简要分析了蒸气过热对R-134a在螺旋管内凝结换热特性的影响机理.     

水平光滑细管内R1234ze冷凝换热特性实验研究

对流体R1234ze在内径2 mm 的水平光滑圆管内的冷凝换热特性进行了实验研究,设定流体饱和温度为35 ℃、40 ℃,质量流量为100~400 kg/(m2?s),热流密度为4~22 kW/m2。实验获得了R1234ze在不同工况下的冷凝换热系数和摩擦压降。发现R1234ze的冷凝换热系数范围在1.5到8 kW/(m2?K)之间,且随干度的增加而增加,随质量流量的增大而增大,随饱和温度的升高而降低,比在相同工况下R134a 、R32的换热系数分别平均低约22%和31%。R1234ze的摩擦压降随质量流量增加而增大,随饱和温度的升高而降低,高于相同工况下R32的摩擦压降。并将本次实验值与其它经典换热模型和压降模型进行了对比分析,发现Baird等人的模型对本次实验的换热系数预测较好,对其它文献中的相似数据点预测也较好。Müller- Heck模型对摩擦压降预测最好。     

采用R22、R134a两种制冷剂的毛细管特性对比研究

建立单相区和两相区毛细管模型,模型计算的毛细管质量流量与实验测量的质量流量差别小于(10%.研究了R134a代替R22后毛细管的质量流量随冷凝温度、过冷度、毛细管内直径和毛细管长度的变化规律,结果表明,两种制冷剂有相同的变化规律,但是在相同的条件下,R22的质量流量比R134a大,比如冷凝温度为45℃时,R22的质量流量比R134a高31.1%.     

两种不同工质在微通道内沸腾换热特性的实验研究

为研究流体物性、流动和换热过程的状态参量对微通道内沸腾换热特性的影响规律,本文采用去离子水和无水乙醇在当量直径为0.293 mm的矩形微通道进行了不同质量流量和热流密度条件下的沸腾换热实验研究,通过对实验数据的计算和处理,分析总结了流体的热物性、质量流量、热流密度、干度和Bo数等参量对沸腾换热系数的影响规律。结果表明:沸腾换热系数随着热流密度、干度和Bo数的增大而降低,核态沸腾占主导地位;相同的质量流量和热流密度条件下,去离子水的沸腾换热系数明显高于无水乙醇的沸腾换热系数,并且前者的换热系数随质量流量的增大而增大,而后者变化不明显。根据考虑了通道尺寸效应及流体物性参量总结出的换热系数关联式进行了计算,计算结果对去离子水和无水乙醇的平均绝对误差分别为14.2%和16.6%,可认为该关联式适用于微通道内沸腾换热系数的预测。     

R134a水平微细管内流动沸腾换热的实验研究

本文对R134a在水平微细管内的流动沸腾进行了实验研究。实验测试段选用了内径为1 mm、2 mm、3 mm共3种不同的水平光滑不锈钢管,实验的饱和温度为5~30℃,热流密度为2~70 k W/m2,流量范围为200~1500 kg/(m2·s)。实验结果表明:相同条件下,干涸前2 mm管较3 mm管换热系数平均增幅为11.6%,1 mm管较2 mm管换热增幅为26.3%,1 mm管径换热系数比3 mm管径平均增大40.8%。随着管径的减小,换热系数在更低的干度开始减小,质量流速和强制对流蒸发作用对换热系数的影响变小,热流密度的影响依然显著;塞状流和弹状流区域减小,泡状流和环状流区域增大。     

R290在水平微细圆管内沸腾换热特性研究

本文研究了R290在内径为1 mm、2 mm和4 mm水平微细圆管内的沸腾流动换热特性,在饱和温度为15℃条件下,质量流速为50～600 kg/(m~2·s)、干度为0～1、热流密度为5～20 k W/m~2时,对沸腾传热系数的影响进行了分析。通过实验发现,增大质量流速对传热系数具有增强作用,质量流速对传热系数的影响在低干度区域比高干度区域小。在热流密度方面,传热系数随着热流密度的增大而增大,且在1 mm和2 mm管内观察到了临界干度对传热系数的影响,这时传热系数有断崖式下降的趋势。在管径对于传热系数的影响方面,通过对不同管径换热特性的横向对比,发现在一定工况下传热系数随着管径的减小有所上升。此外本文还对R290已有的部分关联式进行了适配性验证。     

5 mm小管径内R290流动沸腾换热特性

本文对R290在5mm小管径内的流动沸腾换热特性进行实验研究,重点研究热流密度、质量流率及饱和温度对沸腾换热表面传热系数的影响。实验工况为:热流密度10～60 k W/m2、饱和温度15～25℃、质量流率50～200 kg/(m2·s)、干度0. 1～0. 9。结果表明:增加热流密度可实现强化换热,提高表面传热系数,使干涸现象提前发生,并加剧干涸;质量流率在低干度区间对表面传热系数的影响较小,在中干度和高干度区间表面传热系数与质量流率分别呈正相关;当热流密度较低时,在中干度区间,增大饱和温度会使表面传热系数降低;而在较高的热流密度下,增大饱和温度明显引起表面传热系数的上升。     

R32在小管径水平光管内的流动沸腾换热与压降特性试验研究

对R32在?5 mm的水平光管内的流动沸腾换热与压降特性进行试验研究和理论分析。试验的蒸发温度为5℃,质量流量范围为100~500 kg/(m2·s),热流密度为8~24 kW/m2。结果表明,沸腾换热系数在1~8 kW/(m2·K)之间,压降在1~4 kPa/m之间。沸腾换热系数随着干度增大而增大,质量流量的增大和热流密度的增大都有利于换热系数的增加。质量流量的变化对压降的影响比较明显。与R32在?7 mm管内流动传热性能相比,换热系数提高了30%左右。将得到的沸腾换热系数和压降试验数据与多个模型的预测结果进行比较,发现多数换热经验关联式的预测误差较大,仅有Fuji-Nagata关联式的预测值与试验值较为接近;压降的预测误差相对较小。     

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.     

水平管外沸腾强化换热的数值模拟与场协同分析

应用FLUENT软件对制冷剂R134a在光管和横纹槽管水平管外沸腾传热进行三维数值模拟,得到其饱和泡状沸腾过程中体积含气率的分布规律,并比较它们的换热系数。结果表明横纹槽管外侧能够很好地强化沸腾传热。此外,还通过改变边界条件分析质量流量、热流密度的变化对横纹槽管管外沸腾换热系数的影响。最后应用场协同理论,从局部换热角度分析其强化机制。研究表明,横纹槽管水平管外沸腾换热得到强化的原因是其凹槽前后的速度场与温度梯度场之间夹角较小,协同程度更好。     

5 mm微肋管内R404A沸腾换热模型的适配性

本文针对5 mm微肋管内R404A流动沸腾换热进行实验研究,并将研究结果与筛选出的一批换热模型进行适配性验证。实验工况:热流密度5~25 kW/m^2、饱和温度0℃、质量流率200~500 kg/(m^2·s)、干度为0.1~0.9。结果表明:Zhang Xiaoyan等的模型由于工质热物性差异较大,过高的预测了部分数据;Liu Zhongliang等的模型低估了热流密度对传热系数的影响,过低的预测了实验数据;S. M. Kim等的模型不能体现高干度区域传热系数的衰减,整体预测精度不高;K. E. Gungor等的模型能够很好的解释管内传热的过程,同时预测精度较高,平均绝对偏差仅27.46%。乘以修正系数1.372后的模型平均绝对偏差仅为8. 95%,落在30%偏差带上的数据多达98.18%。     

Two-phase flow heat transfer of CO2 vaporization in smooth horizontal minichannels

Experiments were performed on the convective boiling heat transfer in horizontal minichannels with CO₂. The test section is made of stainless steel tubes with inner diameters of 1.5 and 3.0 mm and with lengths of 2000 and 3000 mm, respectively, and it is uniformly heated by applying an electric current directly to the tubes. Local heat transfer coefficients were obtained for a heat flux range of 20–40 kW m⁻², a mass flux range of 200–600 kg m⁻² s⁻¹, saturation temperatures of 10, 0, −5, and −10 °C and quality ranges of up to 1.0. Nucleate boiling heat transfer contribution was predominant, especially at low quality region. The reduction of heat transfer coefficient occurred at a lower vapor quality with a rise of heat flux, mass flux and saturation temperature, and with a smaller inner tube diameter. The experimental heat transfer coefficient of CO₂ is about three times higher than that of R-134a. Laminar flow appears in the minichannel flows. A new boiling heat transfer coefficient correlation that is based on the superposition model for CO₂ was developed with 8.41% mean deviation.     

A research on the dryout characteristics of CO2'S flow boiling heat transfer process in mini-channels

The experimental and theoretical researches have been carried out to get the flow boiling heat transfer characteristics of carbon dioxide (CO₂ or R744) as a refrigerant in horizontal mini-channel. Based on infrared thermal imaging tests and experimental studies on heat transfer coefficients, the heat transfer coefficients and dryout characteristics of CO₂ are analyzed qualitatively and quantitatively in following conditions: Heat flux: 2~35 kW m^-2, Mass flux: 50~1350 kg m^-2 s^-1, saturation temperature: −10~15 °C, mini-channel inner diameter: 1 mm and 2 mm. Primary conclusions can be drawn from the results of the experiments: The increase of heat flux enhances the nucleate boiling heat transfer of the refrigerant inside mini-channel, which leads to the remarkable increase of heat transfer coefficient. But it speeds up the process of dryout. It also has a certain influence on vapor qualities of dryout at both the starting and the ending stage. The effect of mass flux on heat transfer enhancement depends on the dominant heat transfer mode in the tube. With the increase of mass flow rate, the vapor quality at the start of dryout has a decreasing trend. But the heat transfer coefficient increases at the end of dryout process or even after dryout process; the heat transfer coefficient does not vary monotonically with the saturation temperature: when the saturation temperature is high and even close to CO₂'s critical temperature, the heat transfer coefficient increases with the increase of saturation temperature; when the saturation temperature is low, the heat transfer coefficient increases with the decrease of saturation temperature. Besides, during the heat transfer process, the dryout vapor quality falls monotonically with the increase of saturation temperature. It is reasonable to conclude that dryout characteristics have significant influence on heat transfer coefficient. Fang correlation that predicts the heat transfer coefficient of CO₂ is in good agreement with the experimental data, which has a mean absolute deviation of 15.7%, and predicts 71.98% of the entire database within ±20% and 86.84% of the entire database within ±30%.     

New experimental data of CO2 flow boiling in mini tube with micro fins of zero helix angle

Heat transfer and pressure drop characteristics of CO₂ flow boiling in mini tube with micro fins of zero helix angle were experimentally investigated. The working conditions cover mass flux from 100 to 600 kg m⁻² s⁻¹, heat flux from 1.67 to 8.33 kW m⁻², vapor quality from 0 to 0.9 and saturation temperature from 1 to 15 °C. The results show that the heat transfer coefficient increases with increasing vapor quality, but sharply decreases at vapor quality around 0.2~0.4 under most conditions, and the dryout vapor quality decreases with the increasing heat flux and saturation temperature. Pressure drop increases with increasing mass flux and heat flux, or decreasing saturation temperature, and mass flux is the major influence factors. The enhancement ratio of heat transfer coefficient is higher than that of pressure drop, which shows potentials of using such kind tubes to enhance the overall heat transfer performance. A heat transfer coefficient correlation and a pressure drop correlation for 0° helix angle micro-fin tube were developed, and they agree well with the experimental data.     

Condensation of refrigerants in a multiport tube with rectangular minichannels

This study investigated the condensation heat transfer and pressure drop characteristics of refrigerants R134a, R32, R1234ze(E), and R410A in a horizontal multiport tube with rectangular minichannels, in the mass velocity range of 100–400 kg m⁻² s⁻¹ and saturation temperature set at 40 and 60 °C. The effect of mass velocity, vapor quality, saturation temperature, refrigerant properties, and hydraulic diameter of rectangular channels on condensation characteristics is clarified. A new correlation is proposed for predicting the frictional pressure drop for condensation flow in minichannels. A heat transfer model for condensation heat transfer in rectangular minichannels is developed considering the flow patterns and effects of vapor shear stress and surface tension. Then, based on this model, a new heat transfer correlation is proposed. The proposed correlations successfully predict the experimental frictional pressure drop and heat transfer coefficients of the test refrigerants in horizontal rectangular minichannels.     

Boiling heat transfer of R-22, R-134a, and CO2 in horizontal smooth minichannels

This study examined convective boiling heat transfer in horizontal minichannels using R-22, R-134a, and CO₂. The local heat transfer coefficients were obtained for heat fluxes ranging from 10 to 40 kW m⁻², mass fluxes ranging from 200 to 600 kg m⁻² s⁻¹, a saturation temperature of 10 °C, and quality up to 1.0. The test section was made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm, and a length of 2000 mm. The section was heated uniformly by applying an electric current to the tubes directly. Nucleate boiling heat transfer was the main contribution, particularly at the low quality region. An increasing and decreasing heat transfer coefficient occurred at the lower vapor quality with increasing heat flux and mass flux. The mean heat transfer coefficient ratio of R-22:R-134a:CO₂ was approximately 1.0:0.8:2.0. Laminar flow was observed in the minichannels. A new boiling heat transfer coefficient correlation based on the superposition model for refrigerants in minichannels was developed with a mean deviation of 11.21%.     

R717在小管径水平光管内流动沸腾换热及压降特性

氨制冷剂存在可燃性和毒性,因此减少其在制冷系统中的充注量极为重要。小管径换热管通常可以提供更高的表面传热系数,这可以作为提升换热器紧凑性同时减少系统中充注量的有效方法。本文搭建了氨制冷剂管内流动沸腾换热及压降测试实验装置,测试了氨制冷剂在4 mm水平光管内的流动沸腾换热及压降,并分析了干度、质量流速及热流密度对换热及压降特性的影响。结果表明:流动沸腾换热表面传热系数随着干度的增加而增大,同时质量流速和热流密度越高,流动沸腾换热表面传热系数越大。此外,氨制冷剂在管内的两相摩擦压降也随着干度的增加而增大,在固定干度下,质量流速的升高导致压降增大。