Mechanism Analysis of Falling Film Evaporation Two-Phase Flow in Narrow Channels

In order to further understand the heat and mass transfer mechanism of falling film evaporation flow in narrow channel, a set of narrow channel falling film evaporation experimental system is established. This paper focuses on heat transfer characteristics, pressure loss and two-phase flow pattern identification. Bubble flow, restricted bubble flow and dry area were observed in all four channels. The appearance of flow pattern is related to flow rate and channel width. Under the condition of the same channel width, the initial heat flux of subcooled boiling gradually increases with the increase of flow rate, and this change trend is close to the linear trend. Under the same flow rate, the initial heat flux of subcooled boiling increases with the decrease of channel width, which is due to the faster flow rate of working medium in the narrow channel, resulting in the shortening of heating time. The increase of bubble generation frequency directly leads to the increase of wall heat transfer coefficient and the decrease of bubble separation diameter. With mathematical analysis, it can be seen that under the condition of small flow, the reduction of channel size leads to the reduction of total wall heat transfer coefficient. The reduction of channel size can not enhance heat transfer; With the increasing volume flow rate, the range of hydrodynamic control area becomes larger and the index becomes smaller. When the flow rate is large enough, the total heat transfer coefficient increases greatly with the decrease of channel size. The theoretical values are in good agreement with the experimental data