Performance of SiO2-water nanofluids for single bubble-based nucleate pool boiling heat transfer

Abstract

Experiments on single bubble-based pool boiling have been performed under saturated bulk conditions to understand the influence of suspended nanoparticles on bubble dynamics and the associated temperature gradients field. Water and two concentrations of silica-water nanofluid (0.005 and 0.01% (V/V)) have been subjected to isolated nucleate pool boiling on a ITO-coated borofloat heater by supplying constant heat flux. Experimental measurements have been made in a purely non-intrusive manner by employing refractive index-based rainbow schlieren deflectometry technique. Bubble dynamic parameters, such as bubble departure diameter and departure frequency, wait and growth times etc. and the temperature gradients in the bulk fluid have been mapped simultaneously in the form of schlieren images. Results on bubble dynamic parameters revealed that these parameters show more variability and assume skewed normal distribution in the case of nanofluids whereas water experiments showed insignificant variations in these parameters. Of the significant changes, the bubble departure diameter reduced, departure frequency and growth rate increased with increasing concentration of nanofluids. Schlieren images corresponding to 0.005 and 0.01% (V/V) nanofluids showed more spreading out of the superheat layer adjacent to the heater substrate than that observed in the case of water experiments. Time-averaged values of natural convection and evaporative heat transfer rates associated with single bubble-based boiling indicated that both these rates decrease in the presence of dispersed silica nanoparticles. Reduction in the overall heat transfer rate in the case of nanofluids has been explained on the basis of direct experimental measurements. In this direction, the observed phenomenon has been attributed to the role played by the suspended nanoparticles in diffusing the thermal gradients (reflected in the form of the broadening of the superheat layer) adjacent to the heater substrate along with substantially increasing the wait time of bubble inception in the case of nanofluids as compared to water-based experiments conducted under same wall superheat conditions.