Visualization study of boiling and condensation co-existing phase change heat transfer in a small and closed space with a boiling surface of enhanced structures

A visualization study of boiling and condensation co-existing phase change heat transfer in small and closed spaces with a boiling surface of enhanced structures is experimentally investigated. The enhanced structure is fin arrays (1.4 × 1.4 × 1.4 mm) that directly manufactured on the top end of the copper rod (40 mm in diameter) which is used as the boiling surface of the confined space. A cooling copper block whose bottom end (40 mm in diameter) is used as condensation surface of the confined space and a circular quartz glass tube is used as the side wall to observe the phenomenon. The spacing between the base of the fins and the condensation surface is 33 mm. Liquid level (the water layer thickness in the chamber) is set at 9 mm, 18 mm, and 24 mm, respectively. Experimental results and observation show that within the covered heat flux region (4.6 × 104–9.7 × 104 W/m2) in this paper, boiling heat transfer is depressed while condensation heat transfer is enhanced as the increase of the liquid level. Typical images and possible explanations for the heat transfer characteristics of different liquid levels and heat fluxes are presented. Isolated bubble regime and mushroom cloud as well as bubble coalescence are also found. The comparison between plain surface and enhanced surface shows that boiling heat transfer is significantly enhanced by the fin arrays. The heat transfer resistance analysis of the confined space is also performed and it is disclosed that the heat transfer resistance of the condensation surface is the largest part of the overall heat transfer resistance.