Whole field measurements to quantify the thermal impact of single vapor bubble under nucleate flow boiling regime

Abstract

The work reports on simultaneous measurement of bubble dynamics and thermal field to understand the local effects of bubble motion on thermal boundary layer behavior and heat transfer characteristics under subcooled flow boiling conditions. Single vapor bubble was generated using a specially designed ITO glass heater to avoid the phenomenological complexities and to obtain reproducible data. Flow boiling experiments were carried out in a vertically oriented rectangular channel with water for varying Reynolds numbers (Re = 600–4800), keeping constant subcooling level and power input. The thermal field around the vapor bubble along with the dynamics has been visualized using a non-intrusive optical technique; rainbow schlieren deflectometry, which was configured with a high-speed color camera to acquire high spatial and temporal resolution. The recorded experimental data have been analyzed qualitatively as well as quantitatively and the obtained results have been presented in the form of whole field temperature distribution, the dynamics of thermal boundary layer, local heat transfer coefficients etc. It was observed that the effect of bubble formation on the wall temperature variation, thermal boundary layer and heat transfer coefficient was relatively higher during the growth period (until the bubble lift-off) compared to the waiting period after the lift-off of the bubble. The experimental results showed that the bubble-induced heat transfer significantly enhance the overall heat transfer rate from the heated surface while the degree of enhancement was observed to be reducing with increasing Reynolds number.