Taylor bubble-train flows and heat transfer in the context of Pulsating Heat Pipes

Understanding the performance of Pulsating Heat Pipes (PHPs) requires spatio-temporally coupled, flow and heat transfer information during the self-sustained thermally driven flow of oscillating Taylor bubbles. Detailed local hydrodynamic characteristics are needed to predict its thermal performance, which has remained elusive. Net heat transfer in PHP is contributed by (a) pulsating/oscillating flow (b) distribution of different liquid slugs and bubbles, and, (c) phase-change process; however, its contribution is minimal. In fact, the former two flow conditions are largely responsible for heat transfer in PHPs; such flow conditions can be generated without phase-change and can also be studied independently to observe their explicit effects on PHP heat transfer. With this motivation, systematic experimental investigation of heat transfer is performed during (a) isolated Taylor bubble flow (b) continuous Taylor bubble flow and (c) pulsating Taylor bubble flow, at various frequencies (1 Hz to 3 Hz, as applicable for PHPs) inside a heated square mini-channel of cross-section size 3 mm × 3 mm. This study clearly reveals important insights into the PHP operation. Oscillating Taylor bubbles create significant disturbances in their wake which leads to local augmentation of sensible heat transfer. The implications of bubble length, wake characteristics, oscillating frequency and bubble slip velocity on the heat transfer augmentation and, in turn on thermal performance of PHPs can be clearly delineated from this study. The study also brings out the nuances in the estimation of true bubble slip under time varying Taylor bubble flows.