水平内微肋管局部凝结换热性能实验与数值求解

以Ｒ１１为工质，蒸汽凝结压力为１４７－２６５ｋＰａ，质量流率４ｔｙ１５３ｋｇ／ｍ２ｓ，本文对二维内微肋管和三维内微肋管水平管内凝结分层流区局部换热系数进行了系统的实验。与光管比较，二维内微肋管和三维内微肋管局部凝结换热系数分别提高了１４７－７８３％和２６１－９９７％。本文首次从理论分析入手建立了二维内微肋管水平管内凝结分层流区局部换热系数分析模型并进行了数值求解。计算结果与本文实验相当吻合。     

有肋片的倾斜向下表面的凝结换热和表面水平时肋片对液膜稳定性的影响

本文给出了水蒸汽在有肋片的倾斜向下表面的凝结换热实验数据,特别分析了表面水平时肋片对相邻肋片间表面上的液膜的稳定性的影响。分析表明,肋片对液膜有引流作用,可以减小肋根处液体表面的圆角,从而增加肋片间的有效距离,提高液膜的不稳定性,增强水平表面的凝结换热能力,同时肋片本身也有凝结蒸汽的作用,得剑了肋片高度不同的水平向下表面的凝结换热关联式。     

超音速蒸汽浸没射流凝结换热的实验研究

针对入口压力为0.20～0.50 MPa的饱和蒸汽在20～70℃过冷水中超音速浸没射流凝结换热进行了实验研究.结果表明入口蒸汽压力和过冷水温度是影响汽羽形状的主要因素.分子动力学理论得到的凝结换热系数在0.16～1.91 MW/(m2·℃)之间,随着过冷水温度的增加而增加;湍流强度理论得到的凝结换热系数在0.68～1.68 MW/(m2·℃)之间,随着过冷水温度的增加基本不变;对流换热理论得到的凝结换热系数在1.47～2.11 MW/(m2·℃)之间,随着过冷水温度的增加先增大后减小.     

蒸汽喷入过冷液面接触冷凝实验研究

对饱和蒸汽垂直喷入过冷液面直接接触冷凝进行了可视化实验,观察到近液面层汽液两相作用区的凝结现象,研究了系统压力响应特性,提出将蒸汽凝结过程分为"压力平衡"和"压力稳定"两个阶段,根据实验结果回归了不同凝结模式下平均凝结换热系数的实验关联式.通过对温度场的分析、得到了热水层的分布规律.     

流速对混合蒸汽Marangoni凝结换热影响的实验研究

本文在蒸汽压力为47.36 kPa的条件下,通过实验研究了不同蒸汽流速(u=2、4、5 m/s)下纯水和不同酒精浓度水-酒精混合蒸汽沿重力方向流过竖直紫铜平板表面上的凝结换热特性,并实现了实验的可视化,同时分析了不同蒸汽流速下造成Marangoni凝结换热特性差异的原因.实验及分析结果表明,在相同蒸汽浓度、蒸汽压力和表面过冷度条件下,高流速下的凝结换热系数比低流速的大.且蒸汽流速对凝结换热的影响因混合蒸汽酒精浓度的不同而不同,低浓度0.5%和高浓度50%时流速的增加对凝结换热特性的影响较小,而在中间浓度2%时凝结换热强度随流速的增加明显.     

PMMA低温表面液滴凝结过程的实验研究

为了解不同温度下有机玻璃(PMMA)表面蒸汽凝结相变、液滴生长以及传热的过程,对有机玻璃表面的液滴冷凝过程进行了可视化实验观测,将液滴的凝结形态、面积率、接触角及尺寸变化等参数进行了分析并绘制出变化曲线,发现蒸汽在有机玻璃表面凝结时可形成颗粒分明的珠状液滴,凝结时间越长液滴直径越大;底板温度越低,面积率越大,接触角越大。控制以上变量可以有效改善蒸汽凝结效果,有利于提高热量传递效率,为液滴相变(汽-液)特性和换热表面结霜除霜技术提供理论支持。     

R134a过热蒸汽在三维内微肋管内的凝结换热特性

本文对三维内微肋管内进口区段R134a过热蒸汽的凝结换热过程进行了实验研究。结果表明;微肋管内过热蒸汽过热度降低的速率明显高于光管,且主要受质量流率和管望过冷度的影响.本文得到的过热蒸汽凝结换热计算式与实验的偏差在±15％以内。     

冷凝器分液特性实验研究

结合传统的管壳式换热器设计加工出一个冷凝器,冷凝器管箱两端的隔板上开有分液小孔,并通过设计搭建的凝结换热实验台,对冷凝器的换热特性进行了实验研究。通过对实验结果的归纳,结果表明凝结液封稳定的情况下,凝结换热系数随开启漏液管数增加而变大。这是对以后设计高效换热器进行的有益探索。     

高效凝结换热可视化实验研究

文中通过对实验数值的分析来研究冷凝传热传质问题,以套管式冷凝器为研究对象,设计出一种对冷凝段凝结换热过程进行可视化研究的实验台。研究的目的在于,寻求蒸汽管内换热由珠状凝结渐变为膜状凝结时的管长与热力参数之间的的特征关系,在相变发生的管长位置设法将凝结液提前排出,以维持较高的对流换热系数。通过研究表明:凝结状态转折点所对应的管长与蒸汽入口速度、流体压降以及蒸汽干度密切相关,并给出了具有指导意义的关联式,且实验值与理论计算值的误差值小于10%。     

蒸汽流速对竖直管外Marangoni凝结换热特性影响的实验研究

本文在不同蒸汽流速下对水-酒精混合蒸汽在竖直管外Marangoni凝结换热特性进行了实验研究.结果表明:在相同蒸汽压力下,混合蒸汽在竖直管外凝结换热系数随蒸汽流速的增加而增加.当蒸汽酒精浓度较低时(1%、2%、5%),凝结换热系数随流速的增加而明显增加.当蒸汽酒精浓度较高时(20%、50%),流速的增加对凝结换热系数影响较小.这主要是由于蒸汽流速在低酒精浓度下对凝结扩散层热阻的影响大于在高酒精浓度下对扩散层热阻的影响.     

Heat Transfer Characteristics for Condensation of R134a in a Vertical Smooth Tube

In this study, condensation of pure refrigerant R134a vapor inside a smooth vertical tube was experimentally investigated. The test section was made of a copper tube with inside diameter of 7.52 mm and length of 1 m. Experimental tests were conducted for mass fluxes in the range of 20–175 kg/m²s with saturation pressure ranging between 5.8 and 7 bar. The effects of mass flux, saturation pressure, and temperature difference between the refrigerant and tube inner wall (ΔT) on the heat transfer performance were analyzed through experimental data. Obtained results showed that average condensation heat transfer coefficient decreases with increasing saturation pressure or temperature difference (ΔT). In addition, for the same temperature difference (ΔT), heat can be removed from the refrigerant at a higher rate at relatively low pressure values. Under the same operating conditions, it was shown that average condensation heat transfer coefficient increases as mass flux increases. Finally, the most widely used heat transfer coefficient correlations for condensation inside smooth tubes were analyzed through the experimental data. The best fit was obtained with Akers et al.'s (1959) correlation with an absolute mean deviation of 22.6%.     

螺旋扁管管外蒸汽冷凝双侧强化传热试验研究

为促进螺旋扁管在冷凝换热装置上的应用,对螺旋椭圆管管外蒸汽冷凝工况下的传热特性进行了试验研究。研究结果表明,螺旋椭圆管在强化管内无相变对流传热的同时也可以强化管外冷凝传热。相同工况下,同圆管相比,所用螺旋椭圆扁管的总传热系数高11%-16%,管内传热系数高约18%,管外冷凝传热系数高约9%。并从二次流减薄传热边界层及冷凝表面利于排除冷凝液的角度,分析了螺旋椭圆扁管的双侧强化传热机理。     

Modeling of vapor condensation in a longitudinally finned minichannel

Heat transfer with vapor condensation inside a longitudinally finned tube is numerically studied. The proposed model considers vapor condensation on two initial flow areas, namely, annular and rivulet. The model allows prediction of pressure difference along the tube length, vapor velocity profiles in the central channel and an interfin groove, and also a velocity profile in the condensate rivulet at the bottom of the interfin channel, local heat transfer coefficients at different fin points, and average heat transfer coefficients over tube section and length. The calculations showed that in the case of vapor condensation in longitudinally finned tubes of a small diameter it is of fundamental importance to divide the flow tube section into a central channel and interfin channels. The governing vapor velocities in these channels may differ by more than an order of magnitude. The reduced vapor velocity, used in engineering calculations, does not reflect the character of dynamic vapor impact on a condensate film on the most part of the heat transfer surface. For tubes with relatively large fins the proposed model describes vapor condensation almost completely,meanwhile, the mass vapor quality by the time of filling of the grooves reaches 0.01–0.05. The highest heat transfer intensification was obtained for “sharp fins” with a high value of the fin head curvature. Comparison of results of calculation by the model with results of the known experiments on water vapor condensation yields a good qualitative and quantitative agreement for low vapor velocities at the channel inlet (under 30 m/s). The wall thermal conductivity coefficient value affects significantly the condensation efficiency.     

氨的添加引发水蒸气凝结换热强化的实验研究

研究了添加极少量氨时,氨-水混合蒸气在水平圆管上的凝结传热特性。结果表明:由于氨的添加引发的Marangoni效应,水蒸气的凝结换热在实验工况范围内基本上都得到了强化。随着表面过冷度的增加,凝结换热系数表现出有峰值点的非线性变化规律。当氨蒸气的浓度为0.38%时,混合蒸气的最大凝结换热系数可达纯水蒸气的1.9倍,从液膜热阻和扩散热阻的角度分析了强化换热的机理。     

蒸气压力和流速对低浓度酒精-水混合蒸气微细竖直管外凝结的影响

本文在不同蒸气压力和流速状态下对低浓度的酒精-水混合蒸气在微细竖直圆管外表面Marangoni凝结传热特性进行了可视化实验研究,观测到不同工况下的凝结形态。结果表明,随着过冷度的增加,凝结传热系数急剧升高,凝结传热系数在达到峰值后开始下降,下降曲率相对较缓;随着压力的增加,凝结传热系数在大的过冷度下增幅较大。蒸气流速的影响与压力的影响类似。可视化研究表明蒸气压力和流速大小对凝结速率影响较大。     

Flow condensing heat transfer of R410A,R22, and R32 inside a micro-fin tube

An experimental study of condensation heat transfer characteristics of flow inside horizontal micro-fin tubes is carried out using R410A, R22, and R32 as the test fluids. This study especially focuses on the influence of heat transfer area upon the condensation heat transfer coefficients. The test sections were made of double tubes using the counter-flow type; the refrigerants condensation inside the test tube enabled heat to exchange with cooling water that flows from the annular side. The saturation temperature and pressure of the refrigerants were measured at the inlet and outlet of the test sections to defined state of refrigerants, and the surface temperatures of the tube were measured. A differential pressure transducer directly measured the pressure drops in the test section. The heat transfer coefficients and pressure drops were calculated using the experimental data. The condensation heat transfer coefficient was measured at the saturation temperature of 48°C with mass fluxes of 50–380 kg/(m²s) and heat fluxes of 3–12 kW/m². The values of experimental heat transfer coefficient results are compared with the predicted values from the existing correlations in the literature, and a new condensation heat transfer coefficient correlation is proposed.     

水蒸气在竖直微细管外凝结传热特性研究

针对不同压力和不同流速下的饱和水蒸气在竖直微细圆管(内外径分别为0.571 mm和0.793 mm)外的凝结传热特性分别进行了实验研究,分析了蒸气压力和蒸气流速对凝结传热特性的影响。实验结果表明,凝结传热表面传热系数随着蒸气流速的增加而增加,在较高的蒸气压力下增加的更明显,且大于相同实验条件下的Nusselt理论分析解。在蒸气流速为2 m·s~(-1)时,凝结传热系数随压力的变化不大;在4 m·s~(-1)和6 m·s~(-1)时,随着蒸气压力的升高,凝结表面传热系数明显增大。     

Effect of Coolant Temperature on the Condensation Heat Transfer in Air-Conditioning and Refrigeration Applications

This article directly investigates the effect of a cooling medium's coolant temperature on the condensation of the refrigerant R-134a. The study presents an experimental investigation into condensation heat transfer, vapor quality, and pressure drop of R-134a flowing through a commercial annular helicoidal pipe under the severe climatic conditions of a Kuwait summer. The quality of the refrigerant is calculated using the temperature and pressure obtained from the experiment. Measurements were performed for refrigerant mass fluxes ranging from 50 to 650 kg/m²s, with a cooling water flow Reynolds number range of 950 to 15,000 at a fixed gas saturation temperature of 42°C and cooling wall temperatures of 5°C, 10°C, and 20°C. The data shows that with an increase of refrigerant mass flux, the overall condensation heat transfer coefficients of R-134a increased, and the pressure drops also increased. However, with the increase of mass flux of cooling water, the refrigerant-side heat transfer coefficients decreased. Using low mass flux in a helicoidal tube improves the heat transfer coefficient. Furthermore, selecting low wall temperature for the cooling medium gives a higher refrigerant-side heat transfer coefficient.     

Forced Convective Condensation of R134A Vapor Inside a Vertical Microfin Tube

In this study, condensation of pure refrigerant R134a vapor inside a vertical 18° helical microfin tube was experimentally investigated. Tests were performed at saturation pressure of 5.7–5.9 bar with mass fluxes of 20–100 kg/m²s and heat fluxes of 1.7–5.3 kW/m². The effects of mass flux and the temperature difference between the refrigerant and tube wall (ΔT) on the heat transfer performance were analyzed throughout experimental data. For experiments in which ΔT is more than 2.5°C, the average condensation Nusselt number showed a tendency to be independent from ΔT. Heat transfer enhancement ratio was found to be 1.59–1.71, which is always higher than the heat transfer area enhancement factor (1.55). Fins always act as a turbulence promoter in the given experimental data range. Finally, the most widely used heat transfer coefficient correlations for condensation inside microfin tubes were analyzed through the experimental data. Best fit was obtained with Yu and Koyama's correlation with an absolute mean deviation of 17% and Kedzierski and Goncalves's correlation with an absolute mean deviation of 19%.     

一种内螺纹强化管的冷凝实验研究

实验研究了环保替代制冷工质R410A、R22在水平强化管内冷凝换热特性,探索了热流密度、水流速度对换热特性、压降的影响。实验测试管为内螺纹强化管,长度为5.2 m,外径为9.52 mm。实验结果表明:制冷剂R410A、R22的传热系数和压降随热流密度的增大而增大,同时内螺纹管的换热系数还随管外冷却水流量的增加而升高,压降随冷凝温度的升高而降低,而R410A比R22有更好的换热效率和较小的压降。