水平管外表面降膜流动特性的数值模拟分析
发布时间：2018-08-11 12:32
【摘要】：吸收式制冷机和吸收式热泵不仅能够有效利用余热、提高能源利用率,还能够节约电能、降低建筑能耗,因而具有巨大的发展空间。吸收器是吸收式制冷系统的重要部件,深入研究强化吸收式制冷系统的吸收器结构及其内部传热传质机理,不仅具有重要的学术价值而且还有广阔的应用前景。水平管束降膜吸收器具有传热温差小、传热效率高、设备成本低和流动阻力小等特点,是吸收器的主要和重要形式。本文围绕溴化锂溶液在水平管束降膜吸收器中的流动特性展开,系统地分析了溶液在水平管外表面的瞬态铺展特性和达到稳定后液膜厚度沿周向角的变化趋势,为水平管束降膜吸收机理的研究和吸收器结构强化传热的设计提供重要依据。本文针对水平管束降膜流动的特点,建立了溴化锂溶液和水蒸汽在水平管外降膜流动的物理及数学模型;基于数值模拟软件COMSOL中的层流两相流Level set物理场,采用有限元法,数值模拟了不同吸收器结构和流体流动参数下溴化锂溶液在水平管外表面的降膜流动特性。研究了溶液在水平管外表面的瞬态铺展特性,详细分析了特定时刻溴化锂溶液在水平管外表面的布液形态。结果表明,a)在壁面静态接触角为零的理想水平管外壁,溴化锂溶液能够铺展一层完整的液膜,当水平管外壁的溶液喷淋密度减小和壁面润湿性变差即壁面静态接触角增大时,溶液在水平管外壁表面出现局部干斑,其铺展形态由膜状流转化为滴状流。b)溴化锂溶液在水平管外壁的覆盖率随喷淋密度和壁面润湿性的增加或水平管径的减小而增大;溶液铺展到特定周向角处所需的时间随喷淋密度或液膜雷诺数的减小以及壁面静态接触角的增大而延长。c)布液高度较大和/或喷淋密度减小到一定程度时,溴化锂溶液在水平管外壁的覆盖率随着液膜收缩和新溶液的注入而波动,呈周期性变化;在降膜流动过程中,水平管下半圆周的液膜波动幅度大于上半周的液膜波动幅度,且液膜在水平管外壁的波动幅度随着壁面静态接触角减小而增强。研究了液膜在水平管外表面的稳态特征,获得了达到稳定后周向角20°～160°范围内(间隔10°)液膜厚度和速度的大小,详细分析了液膜厚度和速度沿周向角的分布趋势,探究了吸收器结构和流体流动参数对液膜厚度和速度的影响。结果表明,a)水平管外表面的液膜厚度沿水平管圆周呈先减小后增大的趋势,水平管下半周的液膜平均厚度小于上半周的液膜平均厚度,液膜厚度最小值出现在周向角120°附近;基于模拟结果,并对比前人的模拟和实验结果,对Nusselt液膜厚度沿周向角的分布公式进行了修正。b)液膜厚度随喷淋密度的增加而增大,随水平管径和/或布液高度的增大而减小;当水蒸汽流动方向与液膜铺展方向一致时,达到稳定后液膜厚度随着蒸汽流速的增大而减小,液膜表面速度随着蒸汽流速的增大而有所增大,且上半周的液膜表面速度随着蒸汽流速的变化幅度大于下半周;当水蒸汽流动方向与液膜流动方向相反时,蒸汽流动仅对下半周液膜厚度和液膜表面流速有影响,使下半周液膜厚度随着蒸汽逆向流速的增大而减薄,下半周的液膜表面速度随着蒸汽逆向流速的增大亦有所增大。c)沿管壁的法向方向液膜内部流动速度逐渐增大,汽液交界处的液膜表面速度达到最大值,达到稳定状态后液膜表面速度沿周向角的变化趋势与液膜厚度沿圆周方向的变化趋势相反,液膜表面速度沿周向角呈先增大后减小的变化趋势,液膜表面速度的最大值和液膜厚度最小值都出现在周向角120°附近。在实际的降膜吸收过程中,当喷淋密度较小和/或壁面静态接触角较大时,溶液在水平管外壁的覆盖率较低,降膜吸收的有效面积较小,溶液在干斑区域所需的铺展时间延长;当喷淋密度过大和/或水平管径过小时,铺展过程中溶液在液膜前端的堆积量增大,达到稳定后水平管外壁的液膜较厚,这些都对降膜吸收不利。因此,需要根据实际应用选择合理的吸收器结构参数并优化相对应的喷淋密度,也可通过增强水平管壁面润湿性、调整布液高度等方法改善降膜吸收的性能。
[Abstract]:Absorption chiller and absorption heat pump can not only utilize waste heat effectively, improve energy utilization rate, but also save electric energy and reduce building energy consumption. Absorber is an important component of absorption refrigeration system. The horizontal tube bundle falling film absorber is the main and important form of the absorber because of its small temperature difference, high heat transfer efficiency, low equipment cost and small flow resistance. The transient spreading characteristics of solution on the outer surface of horizontal tube and the variation trend of liquid film thickness along circumferential angle after stabilization are systematically analyzed, which provides important basis for the study of absorption mechanism of falling film of horizontal tube bundle and the design of heat transfer enhancement of absorber structure. Based on the Level set physical field of laminar two-phase flow in COMSOL, the falling film flow characteristics of LiBr solution on the surface of horizontal tube with different absorber structures and fluid flow parameters were simulated by finite element method. The results show that (a) LiBr solution can spread a complete liquid film on the outer wall of an ideal horizontal tube with zero static contact angle on the wall. When the spray density of the solution on the outer wall of the horizontal tube decreases and the wettability of the wall becomes worse, the liquid distribution on the outer surface of the horizontal tube becomes worse. When the static contact angle of the wall increases, a local dry spot appears on the outer surface of the horizontal pipe, and the spreading pattern changes from film flow to droplet flow. When the spray density or Reynolds number of the liquid film decreases and the static contact angle of the wall increases, the liquid distribution height and/or the spray density decrease to a certain extent, the coverage of the LiBr solution fluctuates periodically with the shrinkage of the liquid film and the injection of the new solution. The fluctuation amplitude of the liquid film in the lower half of the circle is larger than that in the upper half of the circle, and the fluctuation amplitude of the liquid film in the outer wall of the horizontal pipe increases with the decrease of the static contact angle of the wall. The distribution trend of liquid film thickness and velocity along circumferential angle is analyzed in detail, and the influence of absorber structure and fluid flow parameters on liquid film thickness and velocity is investigated. Based on the simulation results, the distribution formula of Nusselt liquid film thickness along the circumferential angle is revised by comparing the simulation and experimental results. b) The liquid film thickness increases with the increase of spray density and decreases with the increase of horizontal pipe diameter and / or liquid distribution height. When the water vapor flow direction is consistent with the spreading direction of the liquid film, the thickness of the liquid film decreases with the increase of the steam flow velocity, and the surface velocity of the liquid film increases with the increase of the steam flow velocity. When the flow direction is opposite, the vapor flow only affects the thickness of the liquid film and the surface velocity of the liquid film in the second half of the cycle. The thickness of the liquid film in the second half of the cycle decreases with the increase of the reverse flow velocity of the steam. The surface velocity of the liquid film in the second half of the cycle also increases with the increase of the reverse flow velocity of the steam. The liquid film surface velocity at the vapor-liquid junction reaches the maximum value. After reaching the stable state, the change trend of the liquid film surface velocity along the circumferential angle is opposite to that of the liquid film thickness along the circumferential direction. The liquid film surface velocity increases first and then decreases along the circumferential angle. The maximum value of the liquid film surface velocity and the minimum value of the liquid film thickness are both. In the actual falling film absorption process, when the spray density is small and/or the static contact angle of the wall is large, the coverage of the solution on the outer wall of the horizontal tube is low, the effective area of falling film absorption is small, and the spreading time of the solution in the dry spot area is prolonged. During the spreading process, the accumulation of solution in the front of the liquid film increases, and the liquid film on the outer wall of the horizontal pipe becomes thicker after stabilization, which is disadvantageous to the absorption of falling film. Methods to improve the performance of falling film absorption.
【学位授予单位】：东南大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TK124;TB657
本文编号：2177001
[Abstract]:Absorption chiller and absorption heat pump can not only utilize waste heat effectively, improve energy utilization rate, but also save electric energy and reduce building energy consumption. Absorber is an important component of absorption refrigeration system. The horizontal tube bundle falling film absorber is the main and important form of the absorber because of its small temperature difference, high heat transfer efficiency, low equipment cost and small flow resistance. The transient spreading characteristics of solution on the outer surface of horizontal tube and the variation trend of liquid film thickness along circumferential angle after stabilization are systematically analyzed, which provides important basis for the study of absorption mechanism of falling film of horizontal tube bundle and the design of heat transfer enhancement of absorber structure. Based on the Level set physical field of laminar two-phase flow in COMSOL, the falling film flow characteristics of LiBr solution on the surface of horizontal tube with different absorber structures and fluid flow parameters were simulated by finite element method. The results show that (a) LiBr solution can spread a complete liquid film on the outer wall of an ideal horizontal tube with zero static contact angle on the wall. When the spray density of the solution on the outer wall of the horizontal tube decreases and the wettability of the wall becomes worse, the liquid distribution on the outer surface of the horizontal tube becomes worse. When the static contact angle of the wall increases, a local dry spot appears on the outer surface of the horizontal pipe, and the spreading pattern changes from film flow to droplet flow. When the spray density or Reynolds number of the liquid film decreases and the static contact angle of the wall increases, the liquid distribution height and/or the spray density decrease to a certain extent, the coverage of the LiBr solution fluctuates periodically with the shrinkage of the liquid film and the injection of the new solution. The fluctuation amplitude of the liquid film in the lower half of the circle is larger than that in the upper half of the circle, and the fluctuation amplitude of the liquid film in the outer wall of the horizontal pipe increases with the decrease of the static contact angle of the wall. The distribution trend of liquid film thickness and velocity along circumferential angle is analyzed in detail, and the influence of absorber structure and fluid flow parameters on liquid film thickness and velocity is investigated. Based on the simulation results, the distribution formula of Nusselt liquid film thickness along the circumferential angle is revised by comparing the simulation and experimental results. b) The liquid film thickness increases with the increase of spray density and decreases with the increase of horizontal pipe diameter and / or liquid distribution height. When the water vapor flow direction is consistent with the spreading direction of the liquid film, the thickness of the liquid film decreases with the increase of the steam flow velocity, and the surface velocity of the liquid film increases with the increase of the steam flow velocity. When the flow direction is opposite, the vapor flow only affects the thickness of the liquid film and the surface velocity of the liquid film in the second half of the cycle. The thickness of the liquid film in the second half of the cycle decreases with the increase of the reverse flow velocity of the steam. The surface velocity of the liquid film in the second half of the cycle also increases with the increase of the reverse flow velocity of the steam. The liquid film surface velocity at the vapor-liquid junction reaches the maximum value. After reaching the stable state, the change trend of the liquid film surface velocity along the circumferential angle is opposite to that of the liquid film thickness along the circumferential direction. The liquid film surface velocity increases first and then decreases along the circumferential angle. The maximum value of the liquid film surface velocity and the minimum value of the liquid film thickness are both. In the actual falling film absorption process, when the spray density is small and/or the static contact angle of the wall is large, the coverage of the solution on the outer wall of the horizontal tube is low, the effective area of falling film absorption is small, and the spreading time of the solution in the dry spot area is prolonged. During the spreading process, the accumulation of solution in the front of the liquid film increases, and the liquid film on the outer wall of the horizontal pipe becomes thicker after stabilization, which is disadvantageous to the absorption of falling film. Methods to improve the performance of falling film absorption.
【学位授予单位】：东南大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TK124;TB657
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