溴化锂吸收式制冷系统水平管外降膜蒸发强化传热传质性能研究

发布时间：2018-04-17 16:33

本文选题：吸收式制冷 + 数值模拟　；参考：《长安大学》2017年硕士论文

【摘要】：溴化锂吸收式制冷机组具有节电、环境友好且可以利用生产余热等优势,近年来对溴化锂吸收式制冷机组的性能研究十分广泛。蒸发器作为溴化锂吸收式制冷的制冷装置,蒸发器的传热传质效率是影响溴化锂吸收式制冷机组性能的重要因素之一。溴化锂吸收式制冷机组内的蒸发器形式为降膜式,在真空环境下,水溶液在水平管外降膜流动,包含气液两相流,流动过程同时伴随着复杂的传热传质现象。本文从研究水平管管外降膜蒸发流动的传热传质角度出发,主要对管外降膜蒸发换热模型及换热机理以及管外降膜蒸发换热影响因素进行了研究。本文分析了横向对流对水平管降膜蒸发的影响,建立了数学模型,并通过对基本控制方程的求解,得到在横向对流影响下的液膜厚度δ、沿管壁液膜流速u、垂直管壁液膜流速v及总速度U随圆周角θ、喷淋密度Г以及y方向的距离的无量纲量η的变化规律,并重点分析了横向对流速度v对总速度的影响,结果表明:液膜厚度δ随圆周角增加先减小后增加,在圆周角为90°时达到最小值,其平均值随喷淋密度的增加而增加;沿管壁液膜流速u,随圆周角的增加先增加后减小,在圆周角为90°时达到最大值,其平均值随喷淋密度及y方向无量纲长度的增加而增加;垂直管壁液流速流速v随圆周角的增加先减小后增加,在圆周角为90°时达到最小值,其平均值随喷淋密度及y方向无量纲长度的增加而增加;液膜总速度U的变化规律与沿管壁液膜流速u相同,U的数值大小受到垂直管壁液膜流速v的一定影响。同时,本文提出了借用无量纲量对基本控制方程进行化简求解的方法,从而得到传热传质过程的无量纲温度及无量纲质量分数的求解方法。本文使用数值模拟软件Fluent对本文建立的基础物理模型(喷淋密度Г为0.1kg/(m·s),管径φ为10mm,布液高度h为10mm,管间距d为10mm,时间t为0.32s时的水平管降膜蒸发流动模型)的流动、传质、传热进行模拟与分析,并分对不同喷淋密度、不同管径、不同布液高度及不同管间距对流动、传质、传热的影响进行模拟与分析。借助前处理软件Gambit对物理模型进行网格划分,采用后处理软件CFD-Post对模拟结果进行处理与绘图,使用origin软件进行数据处理。在模拟前,针对本文研究的多相流问题,选定VOF多相流模型及编译自定义函数(User Defined Function,以后简写为UDF),采用分离式求解器求解控制方程,设定溶液的物性参数,设置工作压力为870Pa,将基于蒸发原理的两相传热传质的UDF文件嵌入模型,选用PISO算法对迭代进行求解。模拟后得到的分析结果为:(1)基础模型取不同的喷淋密度,随喷淋密度的增加,液膜厚度增加,液膜速度增加,液膜传热及传质效果下降,本文模拟在喷淋密度为0.1kg/(m·s)时达到最佳传热传质效果;(2)对基础模型取不同的管径,随管径的增加,液膜厚度减小,液膜速度增加,但液膜稳定性降低,导致液膜传热及传质效果先增强后下降,本文模拟在管径为16mm时达到最佳传热传质效果;(3)对基础模型取不同的布液高度,随布液高度的增加,液膜厚度减小,液膜速度增加,液膜的传热及传质效果先增强后下降,本文模拟在布液高度为10mm时达到最佳传热传质效果;(4)对基础模型取不同的管间距,随管间距的增加,液膜厚度减小,液膜速度增加,液膜的传热及传质效果先增强后下降,本文模拟在管间距为10mm时达到最佳传热传质效果;(5)由以上几点可以得出,液膜的传热及传质是耦合的,其变化趋势相同。在液膜稳定铺展的条件下,液膜的厚度越薄、速度越低,其传热、传质效果越好。所得到的分析结果与之前的理论分析及前人的实验结果相似。
[Abstract]:With power saving LiBr absorption chiller, environmental friendly and can make use of waste heat production and other advantages, in recent years, research on performance of lithium bromide absorption chiller evaporator widely. As the lithium bromide absorption refrigeration device of refrigeration, heat and mass transfer efficiency of the evaporator is one of the important factors affecting the performance of LiBr absorption chiller. The lithium bromide absorption evaporator form refrigeration unit in the falling film, in a vacuum environment, water solution of falling film flow outside the horizontal tube, including gas-liquid two-phase flow, flow process accompanied by the phenomenon of heat and mass transfer complex. This paper from the research level tube falling film evaporation heat transfer of flow of the main tube falling film evaporation the heat transfer model and heat transfer mechanism and tube falling film evaporation heat transfer influence factors were studied. This paper analyzes the transverse convection of horizontal tube falling film evaporation The influence of the mathematical model was established, and by solving the basic equations, obtained the film thickness in the delta transverse convection under the influence of the liquid velocity along the tube wall u, vertical wall liquid velocity V and the total rate of U with the circular angle theta, no variation of nondimensional ETA gamma spray density and the Y direction in the distance, and analyzes the influence of V on the total speed, transverse convection velocity results show that the film thickness decreased with the increase of the delta angle increased in the circumferential angle of 90 degrees reached the minimum value, the average value increases with the spray density along the tube wall; liquid velocity increased with the increase of u the circular angle increases first and then decreases in the circumferential angle is 90 degrees maximum, the average value with the increase of spray density and Y direction of dimensionless length increases; vertical wall flow rate of V increased with the circular angle decrease after increase in the circumferential angle of 90 degrees to minimum The average value, with the increase of spray density and Y direction of dimensionless length increases; variation of total film speed of U with the same liquid velocity along the tube wall u, numerical size U affected vertical wall liquid velocity of v. At the same time, is presented in this paper based method to simplify and solve for the control equations nondimensional, resulting in dimensionless temperature and heat transfer process and the solving method of dimensionless mass fraction. Based on the physical model established in this paper using the numerical simulation software Fluent (f 0.1kg/ spray density (M - s), pipe diameter is 10mm, height h 10mm liquid distribution, tube spacing D 10mm, t 0.32s for the horizontal tube falling film evaporation flow model) flow, mass transfer, heat transfer is simulated and analyzed, and the different spray density, different diameter, different height and different liquid distribution tube spacing on the flow, mass transfer, heat transfer effect For simulation and analysis. By pre-processing software Gambit to mesh the physical model, the postprocessing software CFD-Post processing and drawing on the simulation results, Origin software was used for data processing. In the simulation, according to the research of multiphase flow, multiphase flow model and selected VOF (User Defined Function compile custom function later, abbreviated as UDF), the separated solver control equation, the physical parameters of the solution set, set the working pressure of 870Pa, UDF file embedded in the heat and mass transfer model based on the principle of evaporation, using PISO algorithm to solve the iterative analysis. Simulation results are obtained: (1) based model of different spray density, with the increase of spray density, the film thickness increases, the film speed increase, decrease heat transfer and mass transfer in the liquid membrane, the simulation of spray density was 0.1kg/ (M - s) to The best heat transfer effect; (2) on the basis of the model with different diameter, with the increase of the diameter of the film thickness decreases, the film speed increases, but the stability of liquid membrane is reduced, resulting the heat transfer and mass transfer effects first increased and then decreased, this paper simulated the best heat transfer effect is 16mm in diameter; (3) on the basis of the model take the cloth liquid height is different, with the height of liquid film thickness increases, decreases, the film speed increases, heat transfer and mass transfer effect of the film first increased and then decreased, this paper simulated the best heat transfer and the height of 10mm in liquid distribution; (4) on the basis of model with different distance between tubes, with increasing tube the spacing of film thickness decreases, the film speed increases, heat transfer and mass transfer effect of the film first increased and then decreased, the simulation in tube spacing is 10mm to achieve the best heat transfer effect; (5) the above points can be concluded that the heat transfer and transfer film The mass is coupled, and the change trend is the same. Under the condition of stable spreading of liquid film, the thinner the liquid film and the lower the speed, the better the heat and mass transfer effect is. The obtained results are similar to previous theoretical analysis and previous experimental results.

【学位授予单位】：长安大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TB657

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