Study of bubble growth and microchannel flow boiling heat transfer characteristics using dynamic contact angle model

In high heat flux electronic cooling applications, a moderate pressure drop with minimal sub-cooling at the entrance is desired to reduce the pumping power. With these flow conditions, saturated critical heat flux (CHF) is encountered. The present work focuses on the understanding of bubble lift-off dynamics and the influence of flow and geometrical parameters and fluid properties on saturated critical heat flux (CHF) in microchannel flow boiling. A three-dimensional DGLSM (Dual-Grid Level Set Method) based numerical model is proposed using dynamic contact angle model considering water, FC72, R113, R245fa as the working fluids. The continuity, momentum and energy equations are solved for the two phases using Finite Volume Method. A bubble growth model is proposed to predict the bubble lift-off diameter within +/- 2% accuracy. The contact diameter to bubble diameter ratio is found to increase with time, which is correlated within 5.5% accuracy. The study further explores the influence of latent heat of vapourization, density ratio of the two phases, Reynolds number, Weber number, Laplace number and channel length to diameter ratio on CHF in microchannel flow boiling. It is found that the decrease in latent heat of vaporization by 2.6% increases the bubble growth and Nusselt number by 3.5% and 2.8%, respectively, while a decrease in density ratio of 10% increases the bubble growth and Nusselt number by 15.8% and 12.9%, respectively. For cooling application, it is therefore suggested that a fluid with high value of latent heat of vaporization, high density ratio of the two phases, low Weber number and low Laplace number with high Reynolds number should be employed keeping low value of channel length to diameter ratio to avoid early onset of critical heat flux.