Experimental study of the effects of horizontal and vertical roughnesses of heater surface on bubble dynamic and heat transfer coefficient in pool boiling

In this study the roughness effects of heater surface in vertical and horizontal directions (0.55 mu m and 0.82 mu m in the vertical direction and 0.58 mu m and 0.84 mu m in the horizontal direction) on bubble dynamic and heat transfer coefficient in pool boiling were investigated. The experiments were carried out on the copper cylindrical surface with an initial average roughness of 0.11 mu m in deionized water. The experimental results revealed that with increasing the surface's roughness, the nucleation sites increased and led to enhance in the heat transfer. The maximum increase of the nucleation sites (67.5%) occurred at the vertical direction (0.82 mu m). However, due to the generation of the bubbles with lower departure diameter, it could not achieve the maximum heat transfer coefficient. Therefore, the horizontal direction (0.84 mu m) managed to achieve the maximum heat transfer coefficient through 17% of increase in the heat transfer coefficient. In addition, isopropanol solution could aid in promoting the bubble departure frequency. Increasing the roughness played an important role in decreasing the temperature difference and subsequently increasing the critical heat flux. Finally, horizontal direction, roughness of 0.84 mu m and 15 V.% of isopropanol solution were determined as optimum conditions. Moreover, the experimental data were validated with conventional models.