润滑油对R32在水平光管内流动沸腾换热特性及压降的影响

对R32在φ5mm和φ7mm的水平光管内的流动沸腾时,润滑油对换热与压降特性的影响进行了试验研究,试验的质量流量范围为100~500 kg/(m~2·s),润滑油的含量在0~5%之间。结果表明,沸腾换热系数随着质量流量的增大而增大。在低干度区,换热系数随干度的增大而增大,当干度达到0.7~0.8时,换热系数达到最大。随着润滑油含量的增大,局部换热系数在减小。压降随着管径的减小和质量流量的增大而增大。润滑油含量的增大,导致压降的增大。在5mm管内,润滑油含量对换热系数和压降影响比较明显。     

润滑油对管内沸腾换热影响研究综述

润滑油的存在会对管内流动沸腾换热产生影响,不同粘度、浓度的润滑油影响效果不同,不同的换热条件,如干度、质量流量、热流密度的变化也会改变润滑油对换热的影响。因此,本文对相火文献的研究成果进行了回顾,分析和总结了润滑油对管内流动沸腾换热的影响。     

工质R245fa水平管内流动沸腾换热的试验研究

设计了管内沸腾换热系数及总传热系数测定试验台架,对R245fa工质在内径6 mm水平光管内的流动沸腾换热特性进行试验研究。测试质流密度为50~150 kg/(m~2·s),工质蒸发温度为50,60和70℃。结果表明:随着工质含汽率的增加,R245fa的流动沸腾换热系数呈先增大后降低趋势。对于304不锈钢套管式换热器,烟气与R245fa工质进行换热的总传热系数约在40~65 W/(m~2·K)。     

微尺度通道内混合物流动沸腾特性研究

对非共沸混合工质R32／R134a(25％／75％)在微尺度管内的流动沸腾换热特性进行了试验研究。试验结果表明,在较高热流密度下,微尺度管内流动沸腾换热与质量干度和质量流量基本无关,热流密度对换热有着很大的影响,在较宽的热流密度范围内,核态沸腾在换热过程中占据主导地位。和细小管道相比,在相同条件下,微尺度管道内的流动沸腾表面传热系数高于细小管道。     

R410A-油混合物在7mm C形光管内蒸发换热特性及关联式

对R410A-润滑油混合物在7 mm C形水平光管内流动沸腾的换热特性进行试验研究和分析,并基于混合物性开发出7 mm C形水平光管内流动沸腾的换热预测关联式.试验工况为:质流密度200 kg·m-2·s-1、300kg·m-2·s-1、400·kg·m-2·S-1;蒸发温度5℃;测试段干度范围为0.3～0.8;润滑油浓度范围为1%～5%.润滑油的存在引起换热增强,最大增强换热达30.1%.而随着干度及质流密度的增加,润滑油对换热的增强作用减弱.基于混合物性开发的新关联式预测值与90%以上的试验数据的偏差都在士15%以内,平均偏差5.1%,最大偏差为21.9%.     

微通道中液氮的流动沸腾——换热特性分析

对微通道中液氮流动沸腾换热特性进行试验研究和分析。给出典型的沸腾曲线,分析壁温、干度和换热系数沿微通道管程的变化规律,考察热流密度、质量流量和压力对流动沸腾换热的影响。将126个试验数据点与四个换热关联式比较,并对微通道中流动沸腾换热机理进行分析。结果表明,在多数情况下干度和热流密度对沸腾换热系数的影响较小,换热系数主要决定于质量流量和压力,随两者增加而增加,换热以对流蒸发为主导机理。KLIMENKO关联式预测效果最好,TRAN微通道关联式次之,对常规管道得到广泛使用的CHEN关联式和SHAH关联式都远远高估了试验值。基于两相流压降和换热特性分析,推知微通道中的两相流流型不同于常规管道：在低干度情况下,流型以弥散泡状流为主;而在高干度情况下,流型以由雾状汽芯和不规则液膜组成的环状流为主。     

竖直矩形窄通道内水流动沸腾换热特性的研究

建立单面加热垂直矩形窄通道流动沸腾换热试验装置,针对截面250mm×3.5mm的窄缝通道,对水流动沸腾换热特性进行试验研究。通过试验分析可知:(1)随着干度的增加,局部换热系数先增加后减小,有一个最大值,此时处于饱和核沸腾区域,其蒸汽干度也接近于0,同时也接近于沸腾起始点。相应地流体从单相流-泡状-块状流-搅拌-环状流转变。(2)在流动沸腾换热中,热流密度对核态沸腾换热有明显影响,而对流动沸腾液膜蒸发的影响甚小,所以可以认为由热流密度的变化而引起的换热变化,主要表现在核态沸腾。(3)入口温度的变化对单相流动的换热系数有影响,而沸腾换热系数与流型及汽泡的产生及扰动有极大关系,入口温度对流动沸腾局部换热系数基本没有影响。     

垂直管内R113降膜蒸发传热试验研究

为研究降膜蒸发换热特性,在进口质量流量为0.0121~0.0225 kg/s,热通量为1.02~3.35k W/m2的试验参数范围,对R113在垂直管内的降膜流动蒸发传热特性进行了试验研究。结果表明:出口蒸汽质量流量与热通量密切相关,随着热通量的增加,出口蒸汽质量流量逐渐增加;换热系数随着进口质量流量的增加呈现先增加而后减小的变化规律,增加进口质量流量在一定范围内能增强换热;在较小进口质量流量下,增大热通量不利于换热,在较大进口质量流量下,传热系数随着热通量增加先增后减,存在某个临界值使得降膜蒸发换热效果最好。     

流动沸腾条件下微通道壁面温度的红外特征

设计并搭建了沸腾换热试验台,采用TH5104红外热像仪测量微通道壁面温度来研究混合制冷工质在微通道内的沸腾换热特性.测量试件是一外径为1.22 mm,内径为0.86 mm,长为200 mm的不锈钢单圆管.实验利用红外热像仪测量并记录下质量流量为1 726～8 635 kg/m2·s,热流密度为65～231 kW/m2时壁面温度的变化情况.实验分析和讨论结果显示:微通道壁面的温度分布沿着轴向变化有明显的规律性;水平微尺度通道内流动沸腾过程中,试件前后段有较大的温差效应,温差的正负与热流密度的大小有关;壁面温度的变化与热流密度、管内工质的流型和换热形式关系密切,流型越复杂,壁面温度变化越剧烈.     

制冷剂二氧化碳流动沸腾过程换热性能分析

制冷剂CO2在蒸发器内的流动换热性能受许多因素的影响，比如：质量流速、热流密度以及蒸发温度等参数。由于CO2特殊的热物性和传输性，使得其蒸发换热和两相流特点有别于传统制冷剂。这也决定了其蒸发换热管适合设计成小管径，而蒸发器的型式以紧凑型为发展方向。     

Pool boiling of R-123/MO and R-134a/POE lubricant mixtures on pored surfaces

In a flooded refrigerant evaporator, where enhanced tubes are frequently used, lubrication oil inevitably circulates with the refrigerant. However, the literature shows that systematic studies on this subject are lacking. In this study, the effects of oil on the pool boiling of pored surfaces having a range of pore diameter (0.1 to 0.3 mm) and pitch (0.75 to 3.0 mm) were investigated using R-134a/polyester oil (POE) and R-123/mineral oil (MO) mixtures. The saturation temperature was 26.7 °C, and the oil concentration was varied up to 10 %. The results showed that the oil reduced the heat transfer coefficient. This was true for all the pored surface as well as the smooth surface. Overall, the samples having a ‘small open area’ yielded a significant degradation at a low heat flux, whereas the samples having a ‘large open area’ yielded a noticeable degradation at a high heat flux. Meanwhile, the heat transfer degradation was larger for R-134a/POE than R-123/MO, and the reason was attributed to the oil effect on the surface tension, which was stronger for R-134a/POE. The flow visualization results showed that, with the increase of oil concentration, the bubble departure diameter decreased. Similarly, the bubble generation frequency and nucleation site density decreased. These changes of the bubble dynamic parameters certainly were responsible for the heat transfer degradation. A model was developed extending that of Pastuszko et al. [27] to predict the heat transfer coefficients as well as bubble dynamic parameters. The model predicted 92 % of the heat transfer coefficients within ± 40 %.

     

An experimental study on convective boiling of R-22 and R-410A in Horizontal smooth and micro-fin tubes

Evaporation heat transfer coefficients and pressure drops were measured for smooth and micro-fin tubes with R-22 and R-410A. Heat transfer measurements were performed for 3.0 m long horizontal tubes with nominal outside diameters of 9.52 and 7.0 mm over an evaporating temperature range of ?15 to 5°C, a mass flux range of 68 to 211 kg/m²s, and a heat flux range of 5 to 15 kW/m². It was observed that the heat transfer coefficient increased with mass flux. Evaporation heat transfer coefficients of R-22 and R-410A increased as the evaporating temperature dropped at a lower heat flux. Generally, R-410A showed the higher heat transfer coefficients than R-22 in the range of low mass flux, high heat flux and high evaporating temperature. Pressure drop increased with a decrease of evaporating temperature and a rise of mass flux. Pressure drop of R-22 was higher than that of R-410A at the same mass flux.     

涡流分离热气体再加热的CO_2热泵系统的分析

对涡流分离热气体再加热的CO2热泵系统进行热力性能分析,并与相同运行工况下的节流降压CO2热泵系统的性能进行了对比,得出涡流分离热气体再加热的CO2热泵系统存在最优的高压压力,在最优的高压压力下,系统获得最大的制热性能系数。提高分离热气体质量比、中间压力、蒸发温度、涡流管制热效应,降低气体冷却器出口温度,涡流分离热气体再加热的CO2热泵系统的制热性能系数提高。随着热气体质量比的增加和气体冷却器出口温度的升高,涡流分离热气体再加热的CO2热泵系统最优的气体冷却器出口压力也升高。在热气体质量比仅为0.2时,涡流分离热气体再加热的CO2热泵系统相比节流降压CO2热泵系统,最佳的制热性能系数提高11%。随着热气体质量比的增加,差值会进一步增大。气体冷却器出口温度的升高,对涡流分离热气体再加热的CO2热泵系统制热性能系数的影响要小于对节流降压CO2热泵系统的制热性能系数的影响。     

An experimental investigation of heat transfer in forced convective boiling of R134a,R123 and R134a/R123 in a horizontal tube

This paper reports an experimental study on flow boiling of pure refrigerants R134a and R123 and their mixtures in a uniformly heated horizontal tube. The flow pattern was observed through tubular sight glasses with an internal diameter of 10 mm located at the inlet and outlet of the test section. Tests were run at a pressure of 0.6 MPa in the heat flux ranges of 5–50 kW/m², vapor quality 0–100 percent and mass velocity of 150–600 kg/m²s. Both in the nucleate boiling-dominant region at low quality and in the two-phase convective evaporation region at higher quality where nucleation is supposed to be fully suppressed, the heat transfer coefficient for the mixture was lower than that for an equivalent pure component with the same physical properties as the mixture. The reduction of the heat transfer coefficient in mixture is explained by such mechanisms as mass transfer resistance and non-linear variation in physical properties etc. In this study, the contribution of convective evaporation, which is obtained for pure refrigerants under the suppression of nucleate boiling, is multiplied by the composition factor by Singal et al. (1984). On the basis of Chen’s superposition model, a new correlation is presented for heat transfer coefficients of mixture.     

非圆截面小通道内R113的流动沸腾换热特性

针对非圆截面小通道流动沸腾换热研究报道较少的现状,以R113为工质,对4种不同水力直径的正方形、三角形截面小通道内的流动沸腾换热特性进行试验研究,试验参数范围：入口干度,过冷～1.0;质量流速400～ 3 300 kg/(m2?s);热流密度20～150 kW/m2,并将试验结果与相近水力直径的圆通道内流动沸腾试验结果进行了对比分析。试验结果表明：非圆小通道内饱和流动沸腾局部壁面温度与质量流速密切相关,并受热负荷与流动沸腾换热状况的影响;质量流速和壁面热负荷是非圆小通道内流动沸腾换热特性的主要影响因素;与相近水力直径的圆通道内流动沸腾试验数据对比显示,非圆截面小通道具有明显的强化传热作用。     

Measurement and correlation of boiling heat transfer coefficient of R-1234yf in horizontal small tubes

We report experimental data of boiling heat transfer of R-1234yf in horizontal small tubes. The experimental data obtained in the horizontal circular small tubes of 1.5 and 3.0 mm inner diameter, the lengths of 1000 and 2000 mm, the mass flux range from 200–650 kg/m²s, the heat flux range from 5–40 kW/m² and saturation temperature of 10 and 15°C, was used to develop a modified correlation for the heat transfer coefficient. The flow pattern of the experimental data was mapped and analyzed with existing flow pattern maps. The heat transfer coefficient was also compared with some well-known correlations.     

R410A和R22在一种内螺纹强化管管内蒸发性能研究

为了研究单管管内蒸发性能,搭建了管内蒸发性能实验台,用隔膜泵代替了传统压缩机作为系统动力。研究了在冷却水量0.6m3/h,0.8m3/h和1.0m3/h下,9.52mm内螺纹管内10℃蒸发的制冷剂侧换热性能。结果表明,R22和R41OA的总换热系数,换热系数hr和压降均随着制冷剂流量的增加而增加,在小质量流量下,R410A比R22有更好的换热性能,看起来可以替代R22。但当制冷剂流速增大到300～400kg/(s.m2)时,R22的换热系数增加显著,而R410A趋于平缓,所以在大质量流量下,R410A没有R22换热性能好,替代工作仍待研究。