引入蒸汽对垂直管内降膜蒸发传热及流动的影响

对垂直管内引入饱和蒸汽的湍流降膜蒸发传热及流动进行实验研究。研究结果表明:管内引入饱和蒸汽,可以在不提高液膜传热温差的情况下,提高传热系数12％～25％,降低液膜厚度10％～20％,并得出相应的计算式。     

载气蒸发传热及其流型的研究

载气蒸发是在沸腾的液体中引入一种惰性气体,成为液体的汽化中心,使液体的汽化在气—液界面上进行。载气的引入,强化了传热,降低了壁面液体的过热度。本文对空气一水物系,在φ32×3长为1300mm的垂直管中,进行了载气蒸发过程的传热研究和流型观测,并对载气蒸发传热机理进行了分析。     

载气蒸发的传热性能

本研究以空气-水为物系,在减压条件下,于Φ22×3.5 mm,长1500 mm的垂直管中进行了载气蒸发传热实验。研究了平均传热膜系数和局部传热膜系数随液速、载气流量及操作温度的变化规律,并分析了载气强化沸腾传热的机理。在此基础上获得了载气蒸发传热膜系数的关联式。     

露点蒸发海水淡化过程传热研究

露点蒸发淡化技术是一种新的海水和苦咸水淡化方法,它是基于载气增湿和去湿的原理,同时回收冷凝去湿的热量,传热效率受混合气侧的传热控制。蒸汽冷凝传热中,不凝气的存在严重恶化其传热过程。过去的研究大多只涉及极少量或微量不凝气问题,而实际应用中含有大量的不凝气。本工作是在不凝气含量很高的条件下进行的冷凝传热研究。实验工作系通过空气和蒸汽直接混合形成饱和混合气进行冷凝,考察了混合气温度、流速、冷却水温度和流速对冷凝传热过程的影响。实验混合气温度达92～68℃,流速达1.41～9.79m/s,冷却水温度50～70℃,冷却水流速0.55～1.66m/s。实验表明,适当提高混合气温度和流速,冷凝传热的混合气侧的平均传热系数可保持在较高的实用水平,即1000W/m.2℃甚至更高。同时对含不凝气的冷凝传热过程进行了分析,得出了混合气侧局部传热系数的关联式,并对其设备提出了合理设计的原则意见。     

第十讲 载气蒸发传热的研究

本文提出一种新的蒸发法—载气蒸发法,以一种惰性气体为载气,用水和低亚硫酸钠、丙烯酰胺水溶液等物料,研究了载气量、总传热温差和溶液浓度等对载气蒸发传热系数的影响。在此基础上进一步推算并研究了它们对沸腾传热膜系数的影响。     

内插螺旋线强化糖浆立式降膜蒸发传热

搭建了新型立式单管蒸汽-糖浆降膜蒸发实验平台,进行连续4 h加与不加螺旋线的降膜蒸发运行对比实验,研究管内插入特定结构和工艺参数的螺旋线对变粘性糖浆降膜蒸发传热性能的影响.结果表明,内插螺旋线可明显改善糖浆降膜蒸发传热效果,相同条件下,内插螺旋线时总传热系数比空管提高了25.9%~82.9%,且随时间延长其下降速率明显低于空管;从管内蒸发侧和管外冷凝侧传热角度分析降膜蒸发传热性能,传热温差增大,管内蒸发侧传热性能变好,管外冷凝侧传热性能变差,增大传热温差改变物料物性(粘度减小)可强化传热效果.在非结垢影响的传热时间内,对管内蒸发侧传热系数进行处理,得到了管内综合传热系数与传热温差的实验关联式.     

通气环套内垂直圆柱表面核沸腾传热研究

研究了带有同心套管的垂直圆柱形加热表面在环隙中通入空气时的核沸腾传热性能。结果表明，通入空气使沸腾传热系数有所提高，并且加热表面温度有所降低。通入空气也会增强自然对流传热，而且这种增强幅度比沸腾时更大。在实验条件下，通气可使沸腾传热系数提高39．63％-80．50％，壁面温度可降低1．39-2．58℃，自然对流时通气可提高传热系数380．09％-482．52％。为此对上述实验结果用扰动增强和“界面汽化热阱”效应增强的机理来进行了分析。实验还在恒定热流率下考察了气速对沸腾传热系数的影响。结果表明，沸腾传热系数随气速提高先增加后降低。用Mashima和Ishii的两相流模型进行了计算，结果说明了这种气速影响的原因是由于由泡状流向弹状流流型的转变。     

糖厂蒸发器的强化传热及多效蒸发的节能优化

通过实验研究了内插旋流片的缩放管对糖汁蒸发传热的影响,对缩放管和不同个数旋流片对强化传热的影响做了比较分析。与传统蒸发器相比,内插旋流片缩放管的蒸发传热系数提高150%~200%;在此基础上,利用内插不同个数的缩放管对糖厂蒸发器进行改造,对多效蒸发系统进行优化,在相同的总传热温差和总传热面积下,传热系数的提高能够增加系统效数,进而使生蒸汽消耗量减少。优化结果表明,生蒸汽消耗量最减少37%左右,从而达到节能目的。     

含不凝气体蒸汽泡直接接触冷凝

通过可视化实验研究了含空气蒸汽泡的冷凝行为和传热特性,气泡中蒸汽质量含量在0.5～0.8之间.在两种不同的过冷水温度下,混合气体气泡分别由直径1.5 mm和3 mm的喷嘴以不同的速度喷入静止的过冷水空间中.通过高速摄像机记录气泡冷凝过程,通过气泡体积变化状况计算气泡热力学参数,进而计算气泡的冷凝传热系数.结果表明,气泡在冷凝过程中不断变形,且在冷凝开始时,气泡出现中空现象;冷水温度越低,冷凝传热系数越低;气泡体积越大,冷凝传热系数越低;喷嘴直径对冷凝传热影响不明显;不凝气体的加入恶化了冷凝传热.对实验中65个气泡传热系数的数值进行了拟合,获得了传热系数关联式.     

水平管油气两相段塞流及其传热特性

海底油气管道的冷却传热过程是结蜡、水合物等海洋石油工业流动保障问题的关键控制因素。采用电容探针与热电偶、热电阻等流动及温度测量手段对不同冷却条件下空气-油段塞流的流动参数和传热参数进行实验测量,分析了空气-油段塞流流动参数对传热特性的影响,并与空气-水对流换热进行对比。结果表明,空气-油段塞流对流传热系数主要受液相折算速度的影响,且冷却液温度越低,管底热流体黏度越大,导致热边界层越厚,传热系数降低;受黏性力及边界层影响,对流传热系数远小于空气-水;沿管壁周向,从管顶到管底的对流传热系数不断增大。提出了适用于冷却条件下的油气段塞流传热关联式和传热模型。     

Evaporative Heat Transfer of Annular Two-phase Flow of Air-water in a Small Vertical Tube

The evaporative heat transfer of the non-boiling annular two-phase flow of air-water in a small vertical tube with uniform wall heat flux was studied both theoretically and experimentally. A simplified two-phase flow boundary layer model was used to calculate the thickness of the water film attached to the wall, and from the liquid film thickness the evaporative heat transfer coefficients of the annular two-phase flow were obtained. Theoretical equations and semi-theoretical equations were proposed for predicting the evaporative heat transfer of the annular two-phase flow of air-water in a small vertical tube. The semi-theoretical prediction agrees well with the experimental data. The mechanism of the heat transfer enhancement is the evaporation of the thin liquid film attached on the wall.     

Evaporative Heat Transfer of Annular Two-phase Flow of Air-water in a Small Vertical Tube

The evaporative heat transfer of the non-boiling annular two-phase flow of air-water in a small vertical tube with uniform wall heat flux was studied both theoretically and experimentally. A simplified two-phase flow boundary layer model was used to calculate the thickness of the water film attached to the wall, and from the liquid film thickness the evaporative heat transfer coefficients of the annular two-phase flow were obtained. Theoretical equations and semi-theoretical equations were proposed for predicting the evaporative heat transfer of the annular two-phase flow of air-water in a small vertical tube. The semi-theoretical prediction agrees well with the experimental data. The mechanism of the heat transfer enhancement is the evaporation of the thin liquid film attached on the wall.     

单组分液体蒸发过程动力学特性

引言 危险性液体或液化气体泄漏后不断向环境中挥发出有毒有害蒸气,不但污染环境,也会威胁人体健康[1].     

Natural convection heat transfer enhancement through latent heat transport in vertical parallel plate channel flows

A numerical analysis is carried out to investigate the effects of latent heat transfer, in connection with the vaporization of a liquid film, on natural convection heat transfer in a vertical parallel plate channel. Major nondimensional groups identified are Gr_T, Gr_M, Pr and Sc. Results for Nusselt and Sherwood numbers are specifically presented for the air-water and air-ethanol systems under various heating conditions to illustrate the heat transfer enhancement through latent heat transfer during the evaporation processes. Considerable enhancement in heat transfer due to the exchange of latent heat was clearly demonstrated.     

空气-水/吸湿溶液热湿传递特性分析（Ⅱ）传质特性分区

选取典型状态点计算了空气与水或溶液进口状态相对位置不同时的热湿处理结果,依据传质效果的不同及全热换热与传质方向的异同,将空气与溶液的热湿传递过程在焓湿图上划分为A～D区,空气与水的过程分为A～C区,分析了各分区中的热湿传递特点及获得较优传质效果的流型,确定了各分区的界线。利用可及处理区域和热湿传递的分区结果分析和优化设计空气的加湿过程和溶液的再生过程,指出应将热量投入给水或溶液来获得更好的加湿效果或再生效果。     

蒸汽喷射升液的管内蒸发

用蒸汽喷射作为提升液体的动力,在1.5m长的管内实现了稳定的升膜蒸发,并对该升膜两相流动及传热进行了实验研究和理论分析.研究结果表明,蒸汽喷射对蒸发传热有一定的强化作用,以蒸汽喷射作为提升动力的经济性可以接受.考察了蒸汽喷射升液膜的流动及蒸发传热机理,建立了环状流传热模型,考虑了膜流动界面波的影响.模型计算值与实验值吻合较好.     

上置加热管筛板蒸发器流体力学及蒸发性能

对筛板上方液层内置有水平加热管束的载气蒸发器的流体力学和蒸发性能进行了实验研究,并对实验结果进行了分析。结果表明,在气速一定时,筛板单板阻力随着液体流量的增大而提高;在液体流量一定,气速较大时筛板单板阻力不随气速而变化;随着液体温度的提高,筛板单板阻力明显减小;在相同条件下,上置管束的筛板单板阻力比普通筛板单板阻力明显增大。对于常温下的空气水体系,在相同条件下,加热管束通入加热蒸汽可使出口空气的湿含量提高8. 7%—12. 6%。随着加热管束通入蒸汽流量的增加,出口空气的湿含量进一步提高。     

低温多效海水淡化混流效组系统优化与分析

建立了多个混流点的混流效组低温多效蒸发(LT-MED)海水淡化数学优化模型,将海水视为盐和水的混合物,其焓表达为温度的函数,降低优化模型的非线性程度,使其易于收敛。以比传热面积为优化目标分析了不同流程和参数对系统的影响。结果表明,在喷淋密度为30~80 g/(m·s)时,多混流点系统的造水比与单混流点系统相比提高了12.1%,比传热面积降低了8.4%,优化流程为5效+2效+2效组合;提高首效加热蒸汽温度可有效的减少换热面积,降低制水成本,对系统的造水比影响不大;增加系统效数,可以有效提高造水比。各效等面积蒸发方案造水比比不等面积方案略有下降,比传热面积略有增加。模拟结果与文献中相关数据吻合良好,可为低温多效蒸发海水淡化系统工程化设计提供技术支持。     

Mixed convection heat and Mass transfer in a vertical channel with film evaporation

Numerical results are presented for effects of latent heat transport associated with film vaporization on laminar mixed convection heat and mass transfer in a vertical channel with a half channel width b = 0.01 m. The influences of the inlet liquid mass flowrate and wall temperature on the film vaporization and the associated heat and mass transfer characteristics are examined for air-water and air-ethanol systems with gas Reynolds number Re_g = 2000. Predicted results obtained by including transport in the liquid film are contrasted with those where liquid film transport is neglected, showing that the assumption of an extremely thin film made in Lin et al. (1988) and Yan and Lin (1989) is only valid for systems having small liquid mass flow rates. Additionally, it is found that the interfacial heat flux is predominantly determined by latent heat transfer connected with film evaporation.     

Combined buoyancy effects of thermal and mass diffusion on laminar forced convection in the thermal entrance region of horizontal square channels

Heat and mass transfer in laminar mixed convection in the thermal entrance region of horizontal square channels is investigated by using the vorticity-velocity formulation of the Naiver-Stokes equations. The numerical results, including the developments of temperature and concentration contours, Nu_z and friction coefficient ratios, fRe / (fRe)₀, are presented for an air-water system. The effects of bottom wall temperature and the relative humidity on the momentum, heat and mass transfer in the flow are examined in detail. Results show that the influences of the evaporation of the water vapor along the wetted wall on the heat and mass transfer and the ratio fRe / (fRe)₀ are rather substantial.