【作者】 罗林聪；

【导师】 潘继红；

【作者基本信息】 山东大学 ， 热能工程， 2014， 博士

【摘要】 随着能源供应局势的紧张,高效传热强化技术已经成为节约能源及提高能源利用效率的重要途径之一。水平管降膜蒸发器因其传热系数高温差小等优点而被广泛用于海水淡化、化工石油、制冷空调和食品加工等领域。本文提出了采用异形截面管改善流动条件强化降膜蒸发,并对其流动传热特性及强化传热机理进行了深入的数值模拟和实验研究。首先对水平管降膜蒸发的管间流型、滴柱状流下的分离间距(降膜波长)、液膜厚度及液膜破裂等流动机理进行了总结,整理并分析了水平管降膜蒸发传热的经验关联式及降膜蒸发传热的影响因素,通过理论分析给出了本文研究所需的光滑管降膜蒸发的流动和传热数理模型。通过以上分析发现,降膜蒸发一般处于层流状态,其对流换热的热阻主要存在于壁面附近以及液膜的热阻,必须改善壁面附近的流动及减薄液膜以强化传热。光滑异形截面管特殊的弧形壁面能够在增强壁面与膜内液体对流换热的同时减薄液膜,降低过程热阻并强化降膜蒸发。基于VOF (Volume of Fluid)方法详细模拟研究了蛋形管和滴形管两种异形管的强化传热机理。结果表明异形管的特殊弧形壁面的确能够有效地引导管外流体的流动分布,使流体微团受到沿壁面切向向下的重力分力增大,液膜流动加快而变薄。蛋形管周向130。附近其流速在靠近壁面附近可高出圆管30%左右,而在汽液界面也高出4%。膜内液体流动加快,一方面显著增强了流体与壁面的对流换热,另一方面使液膜减薄的同时增强了液膜波动效应,促进了液膜传热,加快了流体内部的热量向汽液界面的传递。蛋形管和滴形管的液膜厚度最小值出现的周向位置较圆管靠后且比其值分别小9.8%和2.0%。分析发现膜内无量纲过余温度θ随布液高度和喷淋密度的增大而降低,随周向角度的增大而增大,同时发现蛋形管和滴形管的θ小于圆管,具有较薄的热边界层厚度,且两异形管的膜内温度梯度均大于圆管膜内梯度。对其进行场协同分析发现蛋形管、滴形管和圆管的膜内对流换热场协同余弦值依次增大,表明蛋形管膜内温度梯度方向与速度矢量方向夹角优于其余两种,也证实上面的数值模拟是可靠的。对比发现蛋形管的传热性能确实最好,滴形管次之,表明异形截面管可有效强化降膜蒸发传热。为进一步优化蛋形管的强化传热,在喷淋密度为0.29kg/(m-s)和布液高度为9mm时模拟分析了六种不同截面形状蛋形管的降膜流动及传热特性。结果表明半轴比增大导致膜内液体流动加快,液膜厚度降低,膜厚最小值点的周向位置靠后。半轴比大于4的蛋形管降膜特性倾向于竖直平板降膜,热边界层得到较充分的发展而变厚,导致无量纲过余温度增大,温度梯度减小,相应的传热性能减弱。比较发现半轴比在2-3时的蛋形管具有较小的无量纲过余温度和较大的温度梯度,传热性能较好。通过拟合数据得到半轴比为2.4的最优蛋形强化管结构。表明通过合理优化水平降膜管的截面形状改善管子表面液膜分布促进传热性能提高的技术路线是正确的。搭建降膜流动冷态实验台,使用高速摄像技术观测了蛋形管及圆管管间降膜流型及流型间转变并测量了降膜流动数据；自制电导探针并测量了不同Re数和管间距时周向液膜厚度。实验发现：蛋形管及圆管管间流型均随Re数增加依次呈现滴状流、滴柱状流、柱状流、柱片状流和片状流等流型;在管间流型在向滴柱状流和向柱状流过渡时,蛋形管的转变Re数稍小于圆管,向片状流过渡时明显小于圆管,而从柱状流向柱片状流过渡时小得更多,约40%。因此蛋形管的柱片状流型区域扩大,即所跨度的Re数范围增大,此区降膜流动工质的流量消耗较片状流时小但其对液膜的冲击效应并未明显减弱,有利于降膜蒸发传热。降膜波长随Re数增加而变短,在Re>400后变化趋于平缓：随管间距的增大而变长,但当管间距大于一定值后其基本不发生变化。相较圆管,滴、柱状流时蛋形管降膜波长平均降低6.66%。测量发现管间距增大使液膜厚度变薄的影响在管子顶部较为明显；液膜厚度随着Re数增加而增大。电导探针实测膜厚值与数值模拟结果吻合较好,表明文中所用的数值模拟方法是可靠性的。通过拟合实验数据得出了蛋形管和圆管各自降膜波长和膜厚最小值的预测关联式,表明管间距对降膜流动特性的影响不能忽略。设计制作了水平管降膜蒸发器并测验了蛋形管和圆管各自构成的单列管和管束的降膜传热性能,在分离总传热系数后得到了管外降膜表面传热系数。实验结果表明蛋形管的特殊弧形壁面结构的确促进了管外降膜蒸发传热,强化传热效果明显,相对于光滑圆管,单列管和管束时蛋形管外的降膜表面传热系数分别增加了13.4-16.1%和11.9%-13.6%。水作为降膜工质,在200<Re<3500的范围内,圆管和蛋形管的降膜表面传热系数h均随Re数增加呈增大的趋势,但增长速度逐渐减缓。同时研究发现表面传热系数随喷淋温度和管间距增加均呈增大的趋势。拟合实验数据得到了传热特性预测关联式,适合光滑圆管及截面形状变化的蛋形管,可以为该类型降膜蒸发器的设计计算提供依据。更多还原

【Abstract】 With the increasing tension of energy shortage, the high-efficiency heat transfer enhancement technologies has become an important approach to save energy and im-provement efficiency of energy utilization. The horizontal falling film evaporators are widely applied in many industrial processes (the desalination and fresh water treat-ment industry, the petrochemical industry, refrigeration and air-conditioning Industry and the food processing industries, etc.) due to their high heat transfer coefficients at low mass flow rates and small temperature differences. In this paper, an new approach for heat transfer enhancement by using shaped tube in falling film evaporation is pro-posed, and detailed research has been conducted both numerically and experimentally to investigate the heat transfer enhancement mechanism.Firstly, Summaries about the hydrodynamics of falling film flow, including in-tertube flow modes, space between two jets or droplets (falling film wavelength), film thickness and film breakdown, have been done. Then empirical formulas for heat transfer coefficient are discussed and the influence factors are analyzed in details. Al-so, the mathematical model of falling film flow and heat transfer is analyzed, which will be adopted in the following numerical simulation. According to the analysis above, the falling film is generally laminar flow. It can be found that the thermal re-sistance mainly exists in the near wall region and also thermal resistance through the liquid film. Therefore, it is an effective enhanced heat transfer technology to keep disturbance to the fluid in the near wall region and thin the film. Smooth shaped tube with a special curved surface could improve the falling film evaporation by enhancing the heat convection between the wall and the liquid and thinning the film thickness.Based on the VOF method, the mechanism of heat transfer enhancement caused by two shaped tubes that are oval-and drop-shaped tubes is studied numerically in details. The results show that the shaped tubes with special curved surfaces can pro-duce an effective guidance to the fluid that flow outside the tubes and accelerate the film flow by inducing a larger gravity component in the flow direction. For example, at the angular positions of30°, the dimensionless velocity of the oval-shaped tube is about30%and4%larger than that of the circular tube neat the wall and the va-por-liquid interface, respectively. The increase of the liquid velocity in the film is helpful to enhance the heat convection between the wall and the liquid. Besides, it can improve the fall of the liquid and thin the film and strengthen the wave in the film, which resulted in enhancing heat transfer from the inner liquid to the vapor-liquid in-terface through the laminar liquid film. The circumferential angle, where the minimum film thicknesses appear, of oval-and drop-shaped tubes is larger, and the value of minimum film thicknesses decrease9.8%and2.0%compared with the cir-cular tube, respectively. The dimensionless excess temperature decreases with the increases of feeder height, flow mass rate and increases with the increase of the angu-lar positions. Compared with the circular tube, the two shaped tubes have a lower dimensionless excess temperature and a thinner thermal boundary layer with a lager temperature gradient. The analysis based on field synergy principle shows that the co-sines of average synergy angle of flow field outside the circular tube, the drop-and oval-shaped tube the tubes increase in its order, which indicates that the increased ve-locity induced by the oval-shaped tube can improve the field synergy performance in the whole film, may also proves the reliability of the numerical simulation. By com-parison, the oval-shaped tube has best heat transfer ability followed by drop-shaped tube, which means tubes with a special shaped section can effectively improve heat transfer on the falling film evaportation.In order to obtain the optimal structure, the falling film flow and heat transfer characteristics of oval-shaped tubes with six different cross-sections are analyzed at a mass flow rate of0.29kg/(m·s) and a liquid feeder height of9mm. With increasing semi-axis ratio, the liquid flow velocity in the film increases and the film thickness decreases. The circumferential position of the minimum film thickness is larger at a bigger semi-axis ratio. The falling film characteristic of the oval-shaped tube with a semi-axis ratio than4is inclined to similar with that on the vertical plate, and has a fully developed and thicker thermal boundary layer leads to weaken heat transfer. Results show the oval-shaped tube with a semi-axis ratio of2to3has a smaller di-mensionless excess temperature and a lager temperature gradient, which means a better heat transfer performance. It is found that the semi-axis ratio of2.4is the best optimal structure for oval-shaped enhanced tube by fitting the numerical data, which declares that the optimization of the cross section of horizontal tubes will be helpful to the film distribution improvement and the heat transfer enhancement of the falling film flow.For studying the falling film flow characteristic, an experimental system is set up. The intertube flow modes, modes transform and the falling film wavelength are ob-served with the help of a high speed camera. The film thicknesses along the circumference under different Reynolds number and intertube spacing are measured by using a home-made conductance probe. Experimental results show that the flow modes on the oval-shaped tube are similar to the circular tube, which is the droplet, droplet-jet, jet, jet-sheet and sheet mode with the increase of film Reynolds number. The mode transition starts early (at lower Reynolds number) by using the oval-shaped tube. The primary difference is an enlargement of the zone in which jet-sheet mode exists; that is, the jet to jet-sheet transition occurs at about40%lower film Reynolds numbers than that of the circular tube. While the jet-sheet to sheet transition occurs at somewhat higher Reynolds numbers. In practice, this means that an evaporating fall-ing film will tend to stay in the advantageous jet-sheet mode down to lower mass flow rates on oval-shaped tubes than on an array of circular tubes. The falling dimension-less wavelength increases obviously with the decrease of Reynolds number below400and firstly increases then tends towards stability with increasing of intertube spacing. The study found that the falling wavelength of the oval-shaped tube is smaller than that of the circular tube by6.66%. The film thickness increases with the increase of Reynolds number and decreases with increasing the intertube spacing with a larger effect at the top of tube. The numerical results are in good agreement with the exper-imental results. The empirical correlations including the intertube spacing for the falling film wavelength and minimum film thickness were obtained for the circular and oval-shaped tube.The falling film evaporator is designed and heat transfer experiments are carried out under the conditions of single row tubes and tube bundle, respectively. The sur-face heat transfer coefficients outside the tubes are obtained by separating the overall heat transfer coefficient. Special curved surface of oval-shaped tube is experimentally proved to be an effective method for heat transfer enhancement. Compared with the circular tube, the surface heat transfer coefficients increase for the oval-shaped tubes of13.4-16.1%and11.9%-13.6%under the conditions of single row tubes and tube bundle, respectively. When water is used as the working medium, within200<Re<3500, the heat transfer rate can be raised by increasing Reynolds number, the growth rate is reduced. The study also found that the heat transfer coefficient increase with their increases of the spray liquid temperature and intertube spacing. Based on the experimental data, the prediction correlations to estimate Nusselt number for cir-cular and oval-shaped tubes has been proposed and can serve as references for the design and calculation of this kind of falling film evaporator.更多还原