Schlieren-based simultaneous mapping of bubble dynamics and temperature gradients in nucleate flow boiling regime: Effect of flow rates and degree of subcooling

Abstract

Bubble dynamics and the local thermal field associated with single nucleating bubble under subcooled flow boiling conditions have been investigated in a one side-heated rectangular channel with water as the working fluid. A real-time non-intrusive technique, namely rainbow schlieren deflectometry has been employed to simultaneously map the full bubble cycle and temperature gradients field near the heated surface as well as around the bubble. Influence of varying subcooled conditions and Reynolds number on bubble dynamic parameters, various sub-processes associated with subcooled flow boiling phenomenon and the resultant temperature gradients have been studied. Schlieren images for each data set have first been qualitatively interpreted to develop an understanding of sub-processes such as bubble inception, bubble departure/sliding, bubble lift-off, bubble reattachment and their effects on the thermal and flow fields. Experimental observations recorded in the form of color schlieren images clearly highlighted the possible influence of process parameters on a range of phenomena that include enveloping of the superheated liquid layer during bubble inception, micro-layer formation and scavenging of the superheated liquid layer during bubble lift-off. The bubble dynamic parameters observed under different experimental conditions have been quantified in terms of equivalent bubble diameter, aspect ratio and contact angle and bubble trajectory in the field of view. The equivalent bubble diameter is found to be decreasing with increasing Reynolds number and degree of subcooling. The results show that for all the experimental conditions, the bubble grew in a flattened shape, gradually became more spherical during sliding and elongates after lift-off. Regardless of the experimental conditions, the occurrence of bubble lift-off was found in the aspect ratio range of 0.75–0.85. Local thermal field observed over the full bubble cycle has also been captured and presented in the form of temperature gradients field in quantitative terms.