TWO-PHASE FLOW HEAT TRANSFER IN A HORIZONTAL STEAM WATER SYSTEM

There are various correlations available for predicting heat transfer coefficients during forced convective two-phase flow. The present work was undertaken to gather heat transfer rate data for the steam-water system and to evaluate (and if necessary, to modify) the available correlations in light of those data.

A boiling water loop, capable of being operated at pressures up to 1300 kPa, was used. The test section was in the form of a U-tube with two straight horizontal sections connected by a 180^° return bend. Using this loop, accurate pressure drop and heat transfer data were gathered over a wide range of mass and heat fluxes.

The data obtained were used to arrive at an improved correlation for the two-phase convective heat transfer coefficient h_pfor steam-water systems at relatively low pressures (800 kPa). The proposed correlation shows h_pis a much stronger function of Lockhart-Martinelli parameter than is indicated by most of the other correlations. Among the existing correlations evaluated, the correlation proposed by Davis and David [ 1964] and the one proposed by Dengler and Addoms (1956) fit our data best.     

BOILING TWO-PHASE FLOW HEAT TRANSFER IN A HORIZONTAL BEND

A paucity of heat transfer rate data for boiling two-phase flow through bends was noted after an extensive literature survey, The present work was undertaken to redress this shortcoming.

A boiling water loop, capable of being operated at pressures up to 1300 kpa, was used. The lest section was in the form of a U-tube with two straight horizontal sections connected by a 180^° return bend. Using this loop, pressure drop and heat transfer data were gathered over a wide range of mass and heat fluxes. steam qualities and system pressures.

The data obtained were used to investigate the variation in heal transfer coefficients around the radial positions of the bend. Correlations for heat transfer coefficients for four different radial positions (top. bollom, inside and outside of the bend) have been presented for the first time. A possible explanation for the observed variations in heat transfer coefficients has also been suggested.     

BOILING TWO-PHASE FLOW HEAT TRANSFER IN A HORIZONTAL BEND

Abstract

A paucity of heat transfer rate data for boiling two-phase flow through bends was noted after an extensive literature survey, The present work was undertaken to redress this shortcoming.

A boiling water loop, capable of being operated at pressures up to 1300 kpa, was used. The lest section was in the form of a U-tube with two straight horizontal sections connected by a 180^° return bend. Using this loop, pressure drop and heat transfer data were gathered over a wide range of mass and heat fluxes. steam qualities and system pressures.

The data obtained were used to investigate the variation in heal transfer coefficients around the radial positions of the bend. Correlations for heat transfer coefficients for four different radial positions (top. bollom, inside and outside of the bend) have been presented for the first time. A possible explanation for the observed variations in heat transfer coefficients has also been suggested.     

NON-EQUILIBRIUM TWO-PHASE FLOW IN A HORIZONTAL T-JUNCTION

The paper discusses the flow of saturated or nearly saturated water in a horizontal sharp edged T-J unction. Equations were developed to predict the vapour mass fraction and the splitting ratio in both the run and branch lines. Experimental measurements obtained showed that the mass of vapour formed in the branch line was much higher than that formed in the run line. The calculated vapour mass fractions in both the run and branch lines were higher than, but close to, the measured data. The measured branch line splitting ratio was higher than that calculated from the developed model. A good agreement was obtained between the experimental results and the model predictions.     

垂直下降沸腾管两相流传热恶化研究

<正>1引言 传热恶化有不同的类型,就工程换热设备设计和运行的实际需要来说,人们最关心的是以下几点：（1）传热恶化发生的位置（壁温飞升起始点）,一般用。cr来表示;（2）传热恶化发生后壁温飞升的最大值,一般用A儿。。来表示;（3）壁温飞升最大值的位置,一般用K。。。来表示;（4）传热恶化发生后的。mi。。本文着重分析讨论后3个问题。     

HEAT TRANSFER IN A SUBCOOLED WATER FILM FALLING ACROSS A HORIZONTAL HEATED TUBE

A numerical simulation and an experimental study were carried out for sensible heat transfer for a subcooled water film falling across a horizontal heated tube. A laminar model and a turbulent model were adopted to calculate the heat transfer coefficient. The whole wetting zone on the tube surface was divided into two zones: the top stagnation zone and the lateral free film flow zone. The initial boundary conditions for the free film flow zone were determined by calculating the fluid and the temperature fields in the stagnation zone. A modified wall function method was used for the turbulent model. The comparisons between the experimental data and the numerical solutions show that the experimental data agree reasonably well with the laminar model solutions. Finally, two simple correlations were proposed for predicting the convective heat transfer of a falling film for engineering applications.     

水平微肋管内流动蒸发换热特性的实验研究

为了研究微肋管结构尺寸及工况等对管内流动蒸发性能的影响,对4种微肋管和1根9.52 mm光管进行了实验,4根微肋管中管外径为9.52 mm和7 mm的各2根,所用工质为R22.实验中质量流速变化范围为90～400kg&#8226;m^-2&#8226;s^-1,所选工况为：蒸发温度7℃,入口干度15%～20%,出口过热度5～6℃.获得了蒸发换热性能随质量流速的变化,讨论了微肋结构尺寸和管径等对蒸发换热性能的影响.两根9.52mm微肋管的传热系数比光管分别分别提高了130%和180%,而其内表面积只比光管分别增加了40%和70%.     

水平管泡状流相分布特性

<正>气液两相泡状流的主要特征是连续液相中携带散布其中的细小气泡,气泡的存在不仅对气液两相流的传热、传质及阻力特性有很大的影响,而且对两相流动的稳定性也有很大的影响.前人有关泡状流的研究大多偏重于两相流的平均参数,对于两相流局部统计参数如局部空隙率等参数的变化规律是近年来两相流研究的新趋势.对于垂直管内的流动已经积累了相当数量的数据,而同样有广泛应用的水平管内相分布规律还知之甚少.气泡对气液两相流的传热、传质及流动结构影响机理的研究必须以了解相分布及气泡的局部统计参数为前提,同时对相分布特性的深人研究也为气液两相流的数学模型化提供实验依据.本文以空气、水为工质,研究水平管内气液两相流的相分布特性,给出了典型泡状流的时域信号图,研究了相分布随气液两相流量的规律变化,并与前人的有关结果进行了比较.     

HEAT TRANSFER CHARACTERISTICS AT EVAPORATING WATER SURFACE IN LAMINAR STREAM OF SUPERHEATED STEAM

The heat transfer characteristics at a horizontal evaporating water surface exposed to a laminar stream of superheated stenm, which is a radiation participating real gas, have been investigated and compared to those of a hot air flow by means of a real-time laser holographic inter ferometry. Water surface temperatures and temperature profiles within the laminar boundary layer were measured precisely under adiabatic evaporation conditions. Temperature gradients at the water surface were determined and local Nusselt numbers were estimated. Those experimental results were compared to the analytical results obtained by solving the laminar boundary equations. It can be concluded here that both temperature profiles within the laminar boundary layer and heat transfer characteristics at the evaporating water surface in superheated steam stream are quite different from those in hot air flow due to an influence of gas radiation from superheated steam.     

HEAT TRANSFER CHARACTERISTICS AT EVAPORATING WATER SURFACE IN LAMINAR STREAM OF SUPERHEATED STEAM

ABSTRACT

The heat transfer characteristics at a horizontal evaporating water surface exposed to a laminar stream of superheated stenm, which is a radiation participating real gas, have been investigated and compared to those of a hot air flow by means of a real-time laser holographic inter ferometry. Water surface temperatures and temperature profiles within the laminar boundary layer were measured precisely under adiabatic evaporation conditions. Temperature gradients at the water surface were determined and local Nusselt numbers were estimated. Those experimental results were compared to the analytical results obtained by solving the laminar boundary equations. It can be concluded here that both temperature profiles within the laminar boundary layer and heat transfer characteristics at the evaporating water surface in superheated steam stream are quite different from those in hot air flow due to an influence of gas radiation from superheated steam.     

气固两相流中颗粒运动强化器壁对流传热的机理

提出了颗粒碰撞壁面过程中颗粒打破边界层和颗粒与壁面导热的强化传热模型,以分析气固两相流中颗粒强化对流传热的机理,通过模型计算研究了风速、颗粒浓度和粒径等因素对颗粒在壁面的停留特性、颗粒在边界层的扰动及对导热和边界层破坏两种强化机制传热比重的影响．以气固两相流横掠圆管的传热过程为例进行计算,计算与实验结果一致,并获得了具有实用价值的关联计算式．     

CONJUGATED HEAT TRANSFER IN COCURRENT FLOW MULTI-STREAM HEAT EXCHANGERS

Considering the cross-sectional velocity profile to be piecewise-constant in each stream of a multi-stream heat exchanger for cocurrent thermally developing flow, this study analytically solves the related conjugated Graetz problem by using an integral transform method. Further, it obtains an analytical solution in an explicit form to the fluid temperatures that vary two-dimensionally. A numerical example is provided for the case of a triple-tube heat exchanger. The numerical results demonstrate the effects of the thermal conductivity ratios of the fluids and the Péclet number ratios on the temperature distribution in the streams. In addition, in order to show the importance of considering streamwise variations in the overall heat transfer coefficients when designing laminar flow heat exchangers, the amount of exchanged heat calculated by the presented analytical solution is compared with that predicted on the basis of bulk temperatures and constant overall heat transfer coefficients.     

CONDENSING HEAT TRANSFER IN AN ADVANCED TWO-PHASE CLOSED THERMOSYPHON

1 INTRODUCTIONClosed thermosyphon has been developed to enhance heat transfer and recover wasteheat in various process industries [1,2].Stimulated by this success,a new type oftwo-phase closed thermosyphon was designed by inserting respectively two inner tubesinto the thermosyphon,one in the boiling section and the other in the condensing sec-tion.The two-phase flow boiling heat transfer coefficient was calculated successfully onthe basis of Chen's dual-mechanism [3].A boiling heat transfer model for thetwo-phase closed thermosyphon with an inner tube in the boiling section was pro-     

环隙内流动沸腾传热的计算

提出环隙内流动沸腾传热计算的数学模型和计算方法。将传热系数K与流动沸腾传热系数hb的计算结果与实测数据加以比较。对环隙与空管两种结构的流动沸腾传热性能也进行了分析对比。     

DISPERSED FLOW HEAT TRANSFER IN CIRCULAR BENDS

This paper presents an experimental study of dispersed flow heat transfer in 90-degree circular bends. From extensive measurements, two different heat transfer patterns are identified, i.e. heat transfer without and with rewetting. Their intrinsic mechanisms are analysed, based on the present experimental evidence and our previous theoretical studies. Effects of mass flow rate, wall heat flux, system pressure and curvature ratio on heat transfer are also investigated. An empirical criterion is developed to identify the heat transfer pattern in the bend.     

DISPERSED FLOW HEAT TRANSFER IN CIRCULAR BENDS

This paper presents an experimental study of dispersed flow heat transfer in 90-degree circular bends. From extensive measurements, two different heat transfer patterns are identified, i.e. heat transfer without and with rewetting. Their intrinsic mechanisms are analysed, based on the present experimental evidence and our previous theoretical studies. Effects of mass flow rate, wall heat flux, system pressure and curvature ratio on heat transfer are also investigated. An empirical criterion is developed to identify the heat transfer pattern in the bend.     

水平微翅管内环状流两相强制对流蒸发换热的计算模型

对水平微翅管内的环状流流动和换热特性进行了分析.考虑微翅管内环状流液膜中的扰动和二次流的作用,借用粗糙管速度分布和摩擦相似函数建立了水平微翅管内环状流两相强制对流蒸发换热系数的预测模型.理论计算值与实验数据相比较,结果较令人满意.     

HEAT TRANSFER FROM A HORIZONTAL BUBBLING SURFACE TO AN OVERLYING WATER POOL†

An experimenl was performed to study the effects of bubbling from a circular, horizontal, fiat plate on the heat transfer to an overlying water pool. The plate had drilled orifices through which nitrogen was injected into an overlying pool of water at atmospheric pressure. For “deep” pools, the heat transfer coefficient was found to increase only about 20% over a range of superficial gas velocities from 0.6 to 8.5cm/s, A turbulent heat transfer model developed by Konsetov was found to agree well with the data from this experiment. This model and the experimental data suggest that under certain conditions the heat transfer coefficient is similar for both horizontal and vertical surfaces. These conditions are, that the bubbles only contribute to the stirring action in the pool and only when the bubbling pool is considered “deep”. When the pool height fell below 60% of its diameter, the heat transfer coefficient decreased almost linearly with pool height. This suggests that there occurs a reduction in the turbulent fluctuations or in the characteristic length scale based on the average turbulent eddy size as the pool height decreases.     

CONJUGATE HEAT TRANSFER WITH TURBULENT FLOW IN A CIRCULAR TUBE

A steady heat transfer problem has been analyzed as a conjugate problem with turbulent flow in a circular tube. The three kinds of thermal boundary conditions considered here are specified as constant temperature, constant heat flux and constant heat transfer coefficient at the outer surface of the wall.

From the results of numerical calculation for Prandtl numbers in the range 0.01 ≤ Pr ≤ 10 and for Reynolds numbers in the range 10⁴ ≤ Re ≤ 10⁵, it was confirmed that the dimensionless parameter R_c could have significant effects on the heat transfer and the temperature field in the fluid adjacent to the wall.     

新型两相闭式热虹吸管的传热性能

本文设计了一种新型结构的两相闭式热虹吸管(重力热管),通过可视化研究和传热性能研究,探索了新型结构热管中充液率、操作温度及传热量对传热性能的影响。提出了新型热管的流体流动的物理模型,并得到实验验证。结果表明,本文所提出的新型结构热管操作稳定,传热性能良好,临界热通量高。