Gas-liquid falling film flow characteristics on surface of multi-row plane finned-tube heat exchanger: a 3D numerical study

doi:10.11949/0438-1157.20200880

Abstract

Abstract:

A three-dimensional transient CFD model of multi-row plane finned-tube heat exchanger considering gravity, surface tension and gas-liquid interfacial friction source terms was established firstly based on the VOF method. And then gas-liquid falling film flow characteristics on the surface of the heat exchanger was investigated to provide a theoretical basis for the study of heat and mass transfer enhancement in the closed-type heat source tower. The simulation results of liquid film thickness were agreement with the experimental data in the reference, showing that the established CFD model was reliable. The flow characteristics of liquid film on the fin surface with contact angle of 30° were studied under different gas and liquid Reynolds numbers. The results showed that the critical film Reynolds number and critical spray density of complete film flow on the finned-tube surface are 239 and 0.06 kg/(m·s), respectively. Its mean liquid film thickness is 16.8%—35.1% higher than Nusselt's theoretical solution within the liquid film Reynolds number of 239 to 995. In order to prevent the liquid film break-up and drop falling off from deteriorating the heat transfer process of the equipment, the Reynolds numbers of counter-current and co-current air flow should be less than 2190.7 and 3286.0, respectively. The main reason is that the high Reynolds number of gas phase will lead to the rapid increase of gas-liquid interfacial friction. In a word, the co-current gas-liquid flow is more beneficial to achieve the thin complete film flow at higher air Reynolds number.

Key words: falling film flow, gas-liquid two-phase flow, VOF model, interfacial force, finned-tube heat exchanger, numerical simulation

摘要：

基于VOF模型建立了考虑重力、表面张力及界面摩擦力源项的多排平直翅片管换热器表面气液两相降膜流动三维瞬态CFD模型。不同气流速度下液膜厚度模拟结果与文献中试验值吻合较好，最大偏差小于5%，表明所建立CFD模型是可靠的。通过研究壁面接触角为30°时不同气液Reynolds数下液膜流动特性，结果表明：翅片管表面满膜流的临界Reynolds数Re_l为239，临界喷淋密度为0.06 kg/(m·s)；在239 ≤ Re_l ≤ 995内，其平均液膜厚度较Nusselt理论解高16.8%～35.1%；气液逆流和顺流时气相Reynolds数Re_g应分别小于2190.7和3286.0，其主要原因在于过高的Re_g会导致气液界面摩擦力快速增大，从而引发液膜破裂和液滴脱落等现象恶化设备性能。总之，气液顺流更有利于在较高气相Reynolds数下实现翅片管表面的较薄满膜流动。

关键词: 降膜流动, 气液两相流, VOF模型, 界面力, 翅片管换热器, 数值模拟

CLC Number:

TK 124

ZHANG Yi, ZHANG Guanmin, LIU Lei, LIANG Kai, QU Xiaohang, TIAN Maocheng. Gas-liquid falling film flow characteristics on surface of multi-row plane finned-tube heat exchanger: a 3D numerical study[J]. CIESC Journal, 2021, 72(S1): 278-294.

张毅, 张冠敏, 刘磊, 梁凯, 屈晓航, 田茂诚. 多排平直翅片管换热器表面气液降膜流动特性的三维数值模拟[J]. 化工学报, 2021, 72(S1): 278-294.

Figures/Tables 27

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