Date of Award
Doctor of Philosophy (PhD)
Mechanical and Aerospace Engineering
Heat--Transmission., Two-phase flow., Heat engineering.
Presented here is a numerical investigation of a single highly confined bubble moving through a millimeter scale channel without phase change. The underlying flow is laminar and is driven by a pressure gradient, characteristic of a horizontal channel flow. The bubble is confined in the vertical direction, between a heated upper plate and a lower adiabatic surface. The simulation is accomplished in ANSYS Fluent using the Volume of Fluid method to determine the phase boundary. A Lagrangian formulation of the numerical domain is used to simulate a channel of arbitrary length. The dimensions of the channel are 1.25mm in the confinement direction (height), 20mm in the cross-stream direction (width) and 30mm in the streamwise direction (length). Three bubble diameters, and three Peclet numbers were simulated. Observed in the flow field near the bubble are a complex set of fluid structures which produce the fluid mixing responsible for the heat transfer enhancement in the wake of the bubble. Most significant of these structures are a pair of twin channel-spanning vortices that serve to move cold fluid from the bottom of the channel up to the heated upper surface. Also observed are a pair of lateral jets that exist at the sides of the bubbles. These jets have a secondary enhancement effect and are responsible for lateral motion observed in the bubble. Three regions of wake heat transfer response were identified. Active mixing produced by the near-field structures defined the first region. In the second region, Nusselt number exhibited power-law decay. In the third region the heat transfer rate exhibits an asymptotic return to the precursor value. A two-dimensional reduced-order model of the wake heat transfer is also presented. Boundary conditions are applied that capture the near-field mixing immediately behind the bubble. The model results are contrasted with the wake behavior seen in the full simulation.
Willard, John R., "A numerical investigation of heat transfer and fluid structures in the wake of a single confined bubble" (2017). Dissertations. 138.