A Numerical Study of Heat Transfer in Bubbly Flows
BhuvankarPramod R
2019
<div>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</div><div>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</div><div>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.</div><div><br></div><div>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 Reynoldâ€™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.<br></div><div><br></div>