Numerical simulation of fluid flow and heat transfer characteristics of falling film evaporation outside horizontal tubes

doi:10.11949/j.issn.0438-1157.20180538

Abstract

Abstract:

A three-dimensional model was built up to simulate the process of the refrigerant R410A falling film evaporation outside a horizontal tube. The influence of spray density, heat flux and the horizontal distance from the spray hole to the tube axle on falling film flow and the heat transfer was investigated. The results show that the liquid film thickness and local heat transfer coefficient decrease and tend to be stable along circumferential direction of the tube. The film thickness increases and the local heat transfer coefficient decreases near the bottom of the tube due to the local liquid accumulation. The total heat transfer coefficient decreases with the increase of heat flux and significantly increased with the increase of spray density when the spry density is low. After the liquid film Reynolds number reaches 2000, the total heat transfer coefficient increases slowly and tends to be stable with the increase of spray density. The increase of heat flux will also lead to the rise of the total heat transfer coefficient. As the eccentricity of the cloth liquid increases, the total heat transfer coefficient will rise slightly and tend to be stable, and then the total heat transfer coefficient will drop sharply due to local “dry” and liquid film accumulation areas. With the increase of the spray density, the critical point of the sharp decline in the total heat transfer coefficient will gradually shift to long horizontal distance from the spray hole to the tube axle.

Key words: horizontal tube, falling film evaporation, flow, heat transfer, numerical simulation

摘要：

建立三维模型并模拟了制冷剂R410A在水平管外的降膜流动和蒸发过程，探究了喷淋密度、热通量和布液孔偏离管轴心距离对降膜流动和传热的影响。结果表明：沿管周方向，液膜厚度和传热系数逐渐减小并趋于稳定，至管底处由于局部液体堆积，液膜增厚、传热系数降低；喷淋密度较小时，总传热系数随着热通量增加而降低，随着喷淋密度增加而显著提高；液膜Reynolds数达2000后，总传热系数随喷淋密度增加而缓慢提升并趋于平稳，此时热通量的增加会提升总传热系数；随着布液偏心距的增加，总传热系数先略微上升并趋于平稳，而后由于出现局部“干涸”和液膜堆积区域，总传热系数急剧下降；随喷淋密度的增加，总传热系数急剧下降的临界点会逐渐往大偏心距偏移。

关键词: 水平管, 降膜蒸发, 流动, 传热, 数值模拟

CLC Number:

TB657.5

JIANG Chun, CHEN Zhenqian. Numerical simulation of fluid flow and heat transfer characteristics of falling film evaporation outside horizontal tubes[J]. CIESC Journal, 2018, 69(10): 4224-4230.

蒋淳, 陈振乾. 水平管外降膜蒸发流动和传热特性数值模拟[J]. 化工学报, 2018, 69(10): 4224-4230.

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