ON THE INFLUENCE OF HEATING SURFACE STRUCTURE ON BUBBLE DETACHMENT IN SUB-COOLED NUCLEATE BOILING FLOWS

摘要

This research examines the influence of heating surface structure on bubble detachment, which includes bubble departure and bubble lift-off, under sub-cooled nucleate boiling condition, in order to obtain better understanding to the bubble dynamics on horizontal flat heat exchangers. Refrigerant R-134a is chosen as a simulant fluid due to its merits of having smaller surface tension, reduced latent heat, and lower boiling temperature than water. Experiments were run with varying experimental parameters e.g. pressure, inlet sub-cooled level, and flow rate, etc. High speed digital images at frame rates up to 4000 frames/s were obtained, showing characteristics of bubble movement. Bubble radius and center coordinates were calculated via Canny's algorithm for edge detection and Fitzgibbon's algorithm for ellipse fitting. Results were compared against the model proposed by Klausner et al. for prediction of bubble detachment sizes. Good overall agreement was shown, with several minor modifications and suggestions made to the assumptions of the model.