Experimental study on the flow pattern and local heat transfer characteristics of water flow boiling in a vertical upward narrow rectangular channel heated by steam

Flow boiling was experimentally investigated in a vertical rectangular narrow channel using water as working fluids. The channel was heated by steam to simulate the heating conditions of plate type rising film evaporators. The geometric dimension of the channel is 1400 mm x 240 mm x 2.75 mm. The experiments were carried out with mass flux in the range of 4.04-12.11 kg/(m2 & sdot;s), inlet temperature in the range of 75-95 degrees C and steam heating power in the range of 2.5-6.0 kW. A data reduction method for local heat transfer under steam heating conditions was established and the parameters for local heat transfer characteristics were calculated. The heat transfer mechanism was revealed with the analysis of the flow pattern maps and local heat transfer characteristics. Different types of flow pattern maps were drawn and compared with other researchers' data under adiabatic and electrical heating conditions. The experimental results showed that the variation trend of the local heat transfer heat flux and coefficient varies under different mass fluxes and inlet temperatures and the optimal point of heat transfer ability appears in the churn-annular flow region. The most suitable heating power under a given working condition can be obtained to maximize the utilization of rectangular narrow channel heat exchangers.