Session: 17-01-01: Research Posters

Paper Number: 150318

150318 - Structures for Controlling Liquid-Vapor Counterflow in Pool Boiling 

Two phase immersion cooling is a useful thermal management approach for electronics.  In pool boiling, the effect of liquid-vapor counterflow impacts the critical heat flux performance.  Above the boiling surface, vapor columns form.  The vapor release entrains liquid away from the boiling surface and prevents liquid in flow.  At higher heat fluxes, the vapor growth is aggressive enough to completely blanket the surface.  Smaller heated areas may allow liquid to rewet the surface from the sides of the boiling area, enhancing critical heat flux, while larger boiling areas are more restricted by liquid-vapor counterflow. Studies show that separating the vapor release columns and liquid entry paths can increase critical heat flux by allowing liquid to rewet the surface under the vapor columns.

We describe a method of creating separation between vapor columns using an insulating cover.  Thin glass rails serve as the insulation that covers portions of the heated area.  The insulation hinders boiling, locally stopping vapor growth.  Vapor columns then form within the gaps of the insulation, leaving small channels in between for liquid rewetting.  Glass is chosen for the insulating cover based on its low thermal conductivity, low cost and ease of processing.  Capillary wicks are added to the insulating cover via UV laser ablation to improve liquid retention and supply to the boiling surface.

We have tested the effect of this approach in pool boiling in water using aluminum nitride surfaces textured with pin fin wicks.  Critical heat flux, super heat and heat transfer coefficient are measured.  For a 10 mm x 10 mm heated area, the critical heat flux performance is increased 22% by dividing the area into 4 windows.  An average critical heat of 255W/cm²  is observed with a heat transfer coefficient of 12W/cm²k.  Under these conditions vapor columns combine, but small liquid channels are observed above insulating areas.  The effect of window and rail configuration, width, and texturing are explored to understand the effect on performance.  A range of heated areas is also investigated.  The results analyzed using correlations available in literature for heat transfer performance as a function of heated area and surface capillary behavior.

The use of insulating covers to separate vapor columns provides a potentially low-cost and easily implemented method of increasing boiling performance over large areas.  Insulating covers can be placed on any boiling surface with a relatively uniform height.  Additionally, similar implementations may be useful in separating the effect of multiple heat sources in complex electronics packaging.

Presenting Author: Roman Giglio University of California Merced

Presenting Author Biography: Roman Giglio is a PhD candidate studying mechanical engineering at UC Merced. His research interests lie in two-phase cooling, electronics cooling, thermal energy storage, and pool boiling.

Authors:

Roman Giglio University of California Merced
James Palko University of California Merced