A Numerical Study of Heat Transfer in Bubbly Flows

2019-08-13T12:33:00Z (GMT) by Pramod R Bhuvankar
Two-phase flow and heat transfer has a wide variety of applications ranging from nuclear power plants to computer chip cooling. The efficient designs of these systems require a clear understanding of the mechanisms by which two-phase flows enhance heat transfer. With the rapid growth in computing power, Computational Fluid Dynamics is becoming an increasingly reliable predictive tool to understand the physics underlying two-phase flow and heat transfer. We identify the two chief phenomena
affecting heat transfer in two-phase flows as being the improved convective effect in bubbly flows, and the phase change phenomenon. We examine three key aspects of
bubbly flows in the present work namely: a) The flow of bubbles near vertical walls, b) the heat transfer associated with a non-condensable bubble rising near a vertical wall, and c) the heat transfer associated with boiling and condensation involving bubbles.

The first part involves studying the rise velocity of a layer of bubbles rising near a vertical wall. We derive a scaling between the rise velocity based Reynold’s number and the Archimedes number. The second part involves examining the flow pattern around a single bubble rising under the buoyancy effect in a shear flow near a heated wall, and how it affects the heat transfer from the wall. We study the dependence of the fractional improvement in Nusselt number at the wall on various non-dimensional parameters such as the Archimedes number, the Laplace number and the shear rate. Our study shows the existence of an optimum dimensionless shear rate for heat transfer enhancement and a strong dependence between the flow pattern around the bubble and its associated heat transfer enhancement. The third part involves building a numerical model to study flow boiling in micro-channels. We validate the proposed model with two benchmark problems and two experimental studies. The validated numerical tool is then used to understand the effect of varying the micro-channel inlet flow rate on its heat transfer characteristics. This numerical tool is further developed to include a stagnant micro-layer model that can simulate nucleate boiling. We then use it to study the flow boiling characteristics of a line of bubbles undergoing boiling and lift-off in a shear flow. In the end, based on existing literature in the field, we propose future tasks to be undertaken in the area of numerical two-phase flow.