UC Davis

Passive two-phase fluid cooling systems are of major interest for cooling electronics in terrestrial and micro gravity environments due to their compactness and minimization of moving parts. In this thesis, the use of asymmetry for bubble ebullition and growth on a heated surface to passively generate lateral motion of fluid within an open-ended channel is discussed. The asymmetry is achieved by locating reentrant slot cavities on one face of a mm-scale 30/60-degree ratchet. Two such ratcheted walls with cavities in every third ratchet form the vertical walls of an open-ended channel. The ratcheted walls are heated using a serpentine thick-film metallic heater. The open-ended channel is located within a quiescent pool of a dielectric fluid. Visualization studies show that bubbles and slugs tend to move in a preferential direction within the open channel. This direction corresponds to the 30-degree slope face of the ratchet, in which the reentrant cavity is located. Prior studies have alluded to two potential mechanisms for this preferential lateral motion- (1) bubble pump model developed by Kapsenberg et al. [14], which attributes the lateral motion to the momentum imparted by the growing bubble to the surrounding liquid, and (2) asymmetry in the curvature of the slug that spans several ratchet lengths, resulting in a net surface tension force along the 30-degree slope of the ratchet. In order to assess the importance of the bubble pump model, bubble ebullition and growth from the nucleation sites for different heat fluxes and subcooling temperatures are captured using high speed videos and are analyzed using custom image processing of high-speed videos. The major purpose of the image processing is to detect the bubbles in the frames and obtain height and diameter of the bubbles attached to the ratchet in each frame. Active contouring and segmentation techniques are used to detect bubbles in the frames. The velocity imparted by the growing bubble on the ratchet to the surrounding liquid is calculated using this data with the semi empirical model of Kapsenberg et al. This predicted velocity is compared against velocities of detached small (Stokes) bubbles in the field of view obtained using particle (bubble) tracking velocimetry. This comparative analysis has revealed that the semi empirical formula predicts the horizontal velocity imparted by the growing bubble on the ratchet with a deviation of ±15% of the horizontal velocity obtained from the particle tracking velocimetry except for high subcooling mid heat flux condition. Overall, it was observed that around 20-30 mm/s of lateral velocity is imparted in the surrounding fluid by the bubble growing on ratchet. These results validate the bubble pumping model and the semi empirical formula developed by Kapsenberg in the nucleate boiling regime.