Influence of surface roughness on a spray cooling system with R134a. Part II: Film thickness measurements

Experimental measurements in a spray cooling test rig were carried out for two different heater surface roughnesses and for two different types of nozzles with the dielectric refrigerant R134a. In this paper, results of the sprayed refrigerant film thickness measurements are presented. The influence of the volumetric flow rate, the surface roughness and the type of nozzle (through the spray parameters) on the total average film thickness is analyzed and discussed. In a companion paper, results of the heat transfer measurements are presented. It has been found that there is a relation between the variations of the average Nusselt number and of the film thickness along the spray cooling boiling curve. The heat transfer regimes along that curve are related not only to a variation in the average Nusselt number but also to changes in the film thickness. The qualitative analysis of those variations served to better understand the heat transfer mechanisms occurring during the spray cooling.

► Variations of Nusselt number and film thickness are related in the spray cooling.
► Influence of parameters on the film thickness variations is discussed.
► For high roughness heater, three zones are obtained where Nu and H change in a related way.
► For low surface roughness only one zone is obtained where Nu and H change in a related way.