On mechanism of enhancement of two-phase flow heat transfer in a narrow channel

In this paper, several different aspects of thermal dynamics of two-phase flow in a narrow channel have been addressed and analysed. According to the flow regime classification of two-phase flow in a narrow channel, only Taylor bubble flow and annular flow with thin liquid film are considered since they are of most interest for practical applications. The hydrodynamics of Taylor bubbles in a narrow channel is simulated by an advanced CFD method for two-phase flow - Lattice-Boltzmann method. The present numerical simulation provides reasonable pictures of Taylor bubble dynamics which agrees qualitatively well with analytical solutions and experimental observations. More importantly, the simulation results show that several vertices near the head and meniscus of the Taylor bubble appear. Experimental study is performed to study the condensation heat transfer inside a square channel. The experimental results show that significant enhancement of heat transfer is achieved in comparison with the predictions by Nusselt's correlation. This indicated clearly that the capillary force is playing an important role in redistribution of the film thickness inside the channel. An instability analysis is also performed to investigate the dynamics of the thin film flow in a narrow channel. The analytical solution for a narrow gap channel shows that there is no growing wave on the film surface from perturbation, all the kinematic waves coming from perturbations on the film surface are convoyed by vapor flow without growing or decreasing since the liquid film is too thin. The wave structure of the interface of two-phase flow could enhance significantly the heat transfer between the liquid film and heated wall.