Whole Field Measurements to Understand the Role of Varying Depths of Nucleation Site on Vapor Bubble Dynamics and Heat Transfer Rates

Abstract

The present work studies the possible effects of varying depths of cavity on bubbling features and associated heat transfer rates in nucleate pool boiling regime. Single bubble has been generated on a substrate with a cylindrical cavity at its centre that acts as the nucleation site. Experiments have been conducted for three cavity depths (250, 500 and 1000μm), while keeping its throat diameter constant at 200 μm. With the bulk fluid at saturated conditions, for each cavity depth, surface superheat level has been varied in the range of △Tsuperheat = 8, 10 and 12oC. A gradient-based visualization technique has been employed to simultaneously map the changes in thermal gradients during the formation of the vapour bubble as well as bubble dynamic parameters. The image sequence obtained has been analysed to elucidate the dependence of bubbling features and various heat transfer processes on cavity depth. With increasing cavity depth, the net effect of reduction in available thermal energy due to increased convection effects and depletion of superheated layer are identified as the dominant heat transfer processes that influence the bubbling features. Furthermore, based on the bubble departure characteristics,cavity with higher depth (1000 μm) showed a much stable bubble formation with minimal variation in the bubble departure frequency as compared to the bubbling features from a cavity with smaller depth (250 μm). Evaporative heat transfer process has been identified as the primary cause for increased inconsistency of bubbling features at high superheat conditions for experiments performed for low cavity depths.