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Proceedings of Symposium on Energy Engineering in the 21<sup>st</sup> Century (SEE2000) Volume I-IV

1-56700-132-7 (Print)


Xiaoze Du
North China Electric Power University

Bu-Xuan Wang
Laboratory of Phase Change and Interfacial Transport Phenomena, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China


The Kelvin-Helmholtz instability of phase-change interface during flow film condensation in vertical mini tube has been studied by means of work or energy analysis. According to the interfacial boundary conditions acquired, the film thinning effect and the phase change area enlarging effect by interfacial waves on heat transfer enhancement are analyzed in different diameter tubes. It is indicated that in mini diameter tube, more obvious heat transfer enhancement due to interfacial waves can be expected than in normal-sized tube, and the interfacial waves enhance the heat transfer mainly by film thinning effect.

The revised Nusselt models, which take care the factors neglected by Nusselt, such as variable wall surface temperature and interfacial transport phenomena, are remarkably successful to predict the relevant parameters in traditional industrial fields [1]. It is well-known that, the heat transfer enhancement of condensation affected by phase change interfacial waves is not more than 20% if the shear stress between liquid and vapor phase can be neglected, and the interfacial waves take effect only as Rel>20~30 [2]. However, in many heat transfer conditions, the tube diameter of heat exchangers is reduced to less than 3~5mm, named as mini tube, such as generally appeared in two-phase flow micro system or for conditions of high heat flux. Analysis on flow condensation indicated that, in such mini tubes, the bend effect of condensate film can't be neglected, which void the "plate" approximations in Nusselt theory [3], so the surface tension can take obvious effect on condensation. Also in mini tube, the shear stress on liquid-vapor interface has more important effect than gravity. Surely, the increasing effect of shear stress and surface tension will disturb the condensate film and enhance the instability of the film. It is therefore logical to deduce that the interfacial waves may have more obvious effect on heat transfer in mini tube than in normal scale tube. It is the purpose of the present paper to investigate the effect of interfacial waves on heat transfer during flow film condensation in mini tube.

According to literature [4], the present paper assumes the amplitudes of the waves growing due to the Kelvin-Helmholtz instability. Occurence of finite amplitude capillary waves is responsible for such an appearance. The interface waving and the interfacial boundary conditions are treated by work or energy analysis, based on which, the effect of interfacial capillary waves on heat transfer characteristics of flow film condensation in mini tube are analyzed.