波纹板强化换热装置中带水膜饱和烟气对流传热的实验研究

为了探讨糖厂的烟气水膜除尘耦合换热技术实施的可行性,建立了带水膜湿烟气与空气之间的换热实验装置,进行带水膜湿烟气的对流传热实验研究。在该换热装置中,烟气在矩形通道中流动并喷入雾化热水,通过换热面加热另一侧的空气,同时,在烟气侧通道安装波纹板强化单元,以强化热质传递。实验确定最佳液气比,通过改变波纹板峰高、波纹段高、段数及烟气流速,研究带水膜饱和烟气的对流传热系数。结果表明:饱和烟气对空气的换热量显著;最佳液气比为3.3~4.2 L/m~3;烟气流速和波纹板添加段数的增加显著增大对流传热系数。回归了最佳液气比下的传热准则关联式。     

混合气体管内对流凝结传热研究

研究了混合气体在垂直圆管内的对流凝结传热。利用修正的膜模型与Nusselt凝结理论建立了换热数学模型，预测了壁面温度对膜厚度和界面温度的影响，计算了凝结液膜厚度，并与报相热阻法进行比较，研究结果表明该模型更接近实验果，提出了混合气体对流凝结换热与Nusselt凝结的不同。     

水平管外降膜蒸发传热特性的数值模拟

为深入研究液膜内的微观传热机理,对水平管外降膜蒸发的传热特性进行了数值模拟,获得了液膜厚度、液膜流动速度和传热系数等热力参数在液膜内的分布特性。通过与实验数据的对比验证了数学模型的准确性。研究结果表明:在饱和蒸发温度62℃、传热温差2.8℃、管外径25.4mm和液膜入口速度0.071~0.15 m/s条件下,沿圆周方向,液膜厚度减小,传热系数增加,直至达到液膜热力发展区,膜厚和传热系数趋于稳定;受液膜内温度变化的影响,液膜内的粘度、表面张力和导热系数的变化对液膜传热特性产生显著影响。     

自由表面摩擦和蒸发对过冷下降液膜传热的影响

从理论上对下降液膜在自由表面上存在反向剪切力和蒸发散热情况下的换热特性进行了分析，得到了膜厚、换热系数的无量纲关系式，讨论了剪切力、液膜雷诺数、壁面热流、蒸发率对流动和传热的影响。     

水平管外降膜蒸发微观传热特性

为了深入探究水平管降膜蒸发的微观传热特性,采用基于VOF法的计算流体模型对水平管外降膜蒸发进行数值模拟,通过求解控制方程得到液膜内的温度场和速度场。分析了不同入口边界温度和Re数下管外薄液膜内热边界层、无量纲温度和局部传热系数的微观传热特性变化规律,定量给出了热发展区与充分热发展区的边界位置。模拟结果表明:液膜入口温度越高,液膜热发展区覆盖的圆周角度越小;液膜内的热发展区覆盖的角度随Re数的增大而增加是平均传热系数随Re数增大的原因;管外圆周方向无量纲温度分布证明了液膜中的传热包含导热和对流传热;管外液膜内纯导热系数与局部传热系数的差值随倾斜角的增加而减少是由于对流效应沿管圆周方向减弱引起的。     

基于VOF模型的膜状冷凝传热分析

基于流体体积函数法(volume of fluid,VOF)建立垂直平行平板通道内膜状冷凝传热预测数值模型,膜状冷凝传热传质过程模拟通过在VOF模型守恒方程中施加基于界面能量平衡方法的源项实现。通过数值分析研究发现,在壁面的顶部,冷凝液膜最薄,存在层流区域;冷凝液向下流动,一系列不规则的波纹随之出现;影响冷凝传热的主要因素是蒸汽的流速、液膜厚度及流动状态等。     

竖直矩形窄缝两相同向分相流动沸腾传热模型

首次对竖直矩形窄缝内的汽液分相流动区提出一维两相同向分相流动沸腾传热模型 ,并进行了数值计算 ,得到不同质量流速下液膜厚度变化和沸腾传热系数等结果。沸腾传热系数的模型预测值初步与已有实验关联式进行了比较 ,两者基本吻合 ,偏差在± 1 4% ;从而证实了液膜导热是竖直矩形窄缝内汽液分相流动区沸腾传热的主导机理。     

水平翅片管外混合气体对流冷凝传热的理论研究

以Nusselt理论为基础,结合修正的膜理论,对含湿混合气体横掠水平翅片管外时的翅片表面对流冷凝传热机理进行研究。建议了分区处理方法,建立了翅片侧壁和光管上的液膜流动和传热模型。得到了管壁温度、烟气进口温度和雷诺数对总凝结液量的影响,以及翅片侧壁液膜的厚度分布。     

水平单管外混合气体对流冷凝换热的理论研究

采用修正的膜模型与Nusselt凝结理论结合的方法,对含湿混合气体自上而下横掠水平管外时的对流冷凝换热机理进行研究,建立了液膜流动和传热模型,进行数值求解并分析了雷诺数、壁面温度及水蒸汽浓度等因素对混合气体冷凝换热的影响。计算结果表明:水管外壁液膜厚度分布很大程度上受气体边界层对液膜剪切力的影响。而局部努谢尔数不同于纯蒸气的的冷凝换热,它受气相热阻的影响很大,其分布状况类似于单相气体管外的对流换热。     

管内段塞流传热模型及实验研究

根据段塞流特点,建立段塞流传热模型,模型计算得到的平均传热系数理论值与实验得到的传热系数实验值符合得很好,误差在10％以内,与其它模型对比,段塞流传热模型更接近实际情况.分析了液体、气体流量对传热系数的影响,传热规律实验结果表明:段塞流下气液流量(流速)与对流换热系数呈线性关系,液体流量是影响传热的主要因素.     

A study on flow and heat transfer characteristics for saturated falling-film evaporation of liquid oxygen in a vertical channel

In this study, a mathematical model for the laminar falling film is presented in order to simulate the evaporation heat transfer characteristics in falling liquid oxygen films. The model takes into account the effect of the interfacial shear. The values of the film thickness, the heat transfer coefficient as well as the interfacial shear are obtained under given conditions by solving the model with an iteration method. The influences of the inlet Reynolds number, channel length and the interfacial shear on the flow and heat transfer characteristics of the falling film evaporation are analyzed in detail. Effects of key factors on the circulation ratio of the inlet fluid mass flow rate to the generated vapor mass flow rate, an important design parameter for reboilers/condensers, are particularly analyzed. In addition, the variations of the average vapor velocity and interfacial film velocity are also discussed. The analysis results could provide theoretical guidance for the simulation and design of downflow reboilers/condensers applied in air separation units.     

Experimental study of gas absorption into turbulent falling films of water and ethylene glycol-water mixtures

Experiments are reported on the effects of liquid viscosity and interfacial shear on the liquid side mass-transfer coefficient for turbulent falling films. CO₂ gas was absorbed into a liquid film formed on the inside of a 2·05 cm ID, 1·98 m long vertical tube. The Reynolds number power dependence of the mass-transfer coefficient was found to depend markedly on viscosity. Transition from wavy laminar to turbulent flow occurred at Reynolds numbers from 1000 to 1200. With co-current gas flow a marked and linear increase in mass-transfer coefficient was found, while for counter-current gas flow the coefficient decreased.     

垂直窄缝通道内环状流沸腾传热特性

本研究基于液膜和蒸汽的质量、动量和能量方程,建立了均匀热流垂直窄缝通道内环状流沸腾传热模型,通过相关文献估算环状流起始点处液膜厚度,利用有限差分法对环状流模型方程组进行数值求解,得到沿流道环状流区域的液膜厚度,并进一步预测了局部沸腾传热系数,结果表明:环状流区域的局部沸腾传热系数随质量流量和干度的增加而增加,与Kenning关联式对比,模型预测沸腾传热系数较关联式计算值偏低。将不同工况下的226组两相环状流实验数据与模型预测结果进行对比,平均绝对误差为18.2%。     

Numerical investigation of effective parameters of falling film evaporation in a vertical-tube evaporator

Wastewater treatment is one of the most effective solutions to manage the problem of water scarcity. Falling film evaporators are excellent technology in wastewater treatment plants. These wastewater evaporators provide high heat transfer, short residence time in the heating zone, and high-purity distilled water. In the present study, the mechanism of turbulent falling film evaporation in a vertical tube has been investigated. A model has been developed for symmetrical two-dimensional pure and saline water flow in a vertical tube under constant wall heat flux. The numerical simulation has been carried out by a commercial computational fluid dynamics code. The evaporation of saturated liquid film is simulated utilizing a two-phase volume of fluid method and Tanasawa phase-change model. The main objective of this study is to evaluate the effects of water salinity, liquid Reynolds number, wall heat flux, and liquid film thickness on the two-phase heat transfer coefficient and vapor volume fraction. The numerical heat transfer coefficients are compared with the obtained results by Chen's empirical correlation. With a MAPE ≤ 11%, this study proves that the numerical method is highly effective at predicting the heat transfer coefficient. Moreover, the empirical coefficient of the Tanasawa model and the minimum thickness of the falling film are determined.     

气汽混合流体凝结液膜传热特性研究

液膜厚度对凝结传热具有较大影响,且传热管管型影响着凝结液膜形成及排除。为了通过改变管型降低液膜厚度达到强化传热的目的,对圆管、椭圆管及滴形管等三种管型凝结液膜建立了相应的物理及数学模型,并计算了液膜沿管壁的厚度分布及传热系数;分析了三种管型对液膜传热的影响。结果表明:在气汽混合流体凝结传热过程中,不同管型其凝结液膜厚度差别较大;壁面温度和混合流体速度对液膜传热均有影响;相同条件下滴形管管壁上所形成的液膜,其平均厚度较薄,传热系数较高,因此滴形管传热性能优于其他管型。     

Direct-contact condensation of pure steam on co-current and counter-current stratified liquid flow in a circular pipe

We carried out a set of experiments on the direct-contact condensation of atmospheric steam for subcooled water flowing co-currently and counter-currently in a circular pipe. The condensation heat transfer coefficient was evaluated both for co-current and counter-current steam–water flow cases in a horizontal circular pipe. In the current experiment the dependency of the liquid Nusselt number on the gas Reynolds number is higher in the counter-current than in the co-current experimental data. The dependency of the liquid Nusselt number on the steam Reynolds number is stronger in the rectangular channel than in the circular pipe. The overall heat transfer characteristics are better in the co-current flow than in the counter-current flow with the same injection flow rates of the steam and the water. The present co-current experimental data were used to assess four existing correlations. However, there are few reliable correlations existing to predict co-current experimental data. The comparisons of the present counter-current experimental data with the existing correlations show that Chu’s (Chu, I.C., Yu, S.O., Chun, M.H. 2000. Interfacial condensation heat transfer for counter-current steam–water stratified flow in a circular pipe, J. Korea Nucl. Soc., 32 (2), 142–156) correlation predicts the experimental data well.     

竖直矩形狭缝通道内环状流沸腾换热分析模型

对竖直矩形狭缝通道内有液滴卷吸环状流阶段流动沸腾进行分析。以液膜紊流的动量方程和能量方程为基础，加上相应的边界条件和使控制方程组封闭的经验关系式，建立了环状流的教学模型并进行数值计算，得到了矩形狭缝通道内的液膜厚度分布、沸腾传热系数等结果；将模型预测的换热系数同实验关系式作比较，最大相对误差为17．8％。     

A closed-form solution for laminar film condensation from downward pure vapour flow in vertical tubes

An analytical solution is derived for the film thickness for simplified steady-state governing equations of laminar film condensation from laminar pure vapours flowing downward in vertical tubes. This approach yields an accurate, approximate closed-form non-marching solution for the condensate film thickness. All other relevant quantities such as the heat transfer coefficient, the vapour and liquid velocity profiles, the vapour and liquid mass flow rates, the interfacial shear stress, and the pressure gradient can be easily computed in closed-form from this solution directly at any given axial location. The present solution compares very well to other analytical works that require more complicated iterative techniques with a marching solution approach.     

紧凑传热管束受限空间内沸腾强化换热特性

海水淡化装置以及太阳能或余热吸收式制冷机中的蒸发换热器,采用管排外降膜式蒸发方式,它具有很多优点,但管间距离较大,以致尺寸较大,供液方式较复杂。将传热管束紧凑排列置于饱和状态液体中,将其变为满液式蒸发换热器,利用传热管束间受限空间内早期沸腾强化机理,将中小热负荷条件下的自然对流换热转化为核沸腾换热,在间隙尺寸适宜时,其换热性能可能优于降膜式蒸发换热器。对紧凑传热管束在受限空间内沸腾强化换热进行实验研究,确认了满液式蒸发换热器具有良好的换热性能,在中小热负荷条件下甚至超过降膜式蒸发换热器。