Experimental investigation to quantify the influence of single artificial indentations on vapour bubble dynamics and the associated heat transfer rates

Abstract

Dependence of single vapour bubble dynamics and the associated heat transfer phenomena on the type of artificial indentations created on the heated substrate has been studied. Experiments under nucleate boiling regime have been carried out for conical indentations of varying cone angles (2γ = 30, 60 and 90o) and cylindrical cavity keeping the depth of features constant (hc = 500 μm). Rainbow schlieren deflectometry has been employed for simultaneous mapping of bubble dynamics and thermal field in a completely non-intrusive and real time manner. Direct comparison of the rainbow schlieren images shows a relatively large partition of thermal energy through natural convection for conical indentation of small cone angle and cylindrical cavity. For large-angled conical indentation, increased availability of thermal energy for bubble growth process results in the formation of relatively larger-sized vapour bubble at a relatively slower rate. In addition, due to the relative differences in the amount of vapour entrapped inside the cavities between any two bubbling cycles, the consistency of bubbling features was found to be much higher for larger cone angled cavity. The evidence for increased partition of thermal energy transfer through natural convection has been seen in the form of an increase in the portion of superheat layer in the vicinity of vapour bubble for conical cavity with small cone angles from the whole-field temperature reconstructions. For ΔTsup = 10 K, the heat transfer performance of surfaces with conical cavity with large cone angle (2γ = 90o) is significantly much better than the other conical and cylindrical cavities.