An Experimental Investigation on Bubble-Induced Heat Transfer Enhancement

Improving heat transfer in fluid flows is beneficial for numerous practical applications. In systems where natural convection occurs, various methods have been employed to accomplish this. We present results on the global heat transfer enhancement in a homogenous bubbly flow. Experiments were performed with and without bubbles at different temperature gradients and varying volume fractions, ranging from 0.95% to 9.9%. The Rayleigh number RaH was varied from 2 ×109 to 3.3×1010. In single-phase flow, heat transfer depends on the thermal boundary layer; however, in two-phase flow, bubble-induced diffusion dominates the heat transfer. We observed that in bubbly flow, the Nusselt number is independent of the Rayleigh number and it solely depends on the volume fraction that evolves α0.41. Heat transfer increased by 37.3 times at a 9.9% gas volume fraction compared to single-phase flow. We performed Particle Image Velocimetry (PIV) at 0.9% gas volume fraction and at different temperature gradients to visualize the change in the velocity field of the fluid and bubbles, thereby increasing our understanding of the mechanism of heat transfer enhancement with bubbles. The transition velocity region becomes wider near the hot wall compared to the cold wall region, with a higher temperature gradient, and the recirculation near the wall becomes weaker. The maximum upward velocity approached the cold wall.