Experimental investigation of the effect of dimensions on nucleate boiling heat transfer in straight tunnel–structured boiling surfaces

Pool boiling experiments were carried out to investigate tunnel width, pore, and height size effects on boiling heat transfer and to determine the performance of surfaces in different ranges of heat fluxes by using distilled water under atmospheric conditions. Also, the behavior of nucleation of vapor bubbles was observed. The nine structured surfaces, which have the same fin thickness (2.0 mm) but different tunnel width and height, and pore diameter, were developed for enhancement of boiling heat transfer. Also, a structured surface having 3.0 mm tunnel width without pores was investigated to observe pore effect. In order to investigate tunnel height, four surfaces with the same pore diameter and tunnel width, but various tunnel height values, were also used. In addition, it was considered that pore structures would help for fluid transition along the channels to increase the bubble frequency (nucleation, growing and leaving periods). Although a surface whose tunnel width is 1.0 mm can be considered best due to higher heat transfer area and providing more active nucleation sites, it was observed that vapor bubbles could hardly release throughout the tunnel, especially at high heat fluxes. Even though pore size is very significant for fluid transition during boiling and pores are needed for suction-evaporation operational mode for surface structure, it was observed that pores decrease the active nucleation sites because pore internal surfaces are not suitable places for nucleation.