A non-intrusive study of single vapor bubble formation from an artificial cylindrical cavity under saturated pool boiling conditions: effect of cavity depth

An experimental study to understand the effect of varying depth of cavity on the dynamics of single vapor bubble has been reported. Experiments have been performed under saturated bulk conditions on two substrates containing artificial cylindrical cavities of constant throat diameter (d_throat= 200μm) with a depth of 250 μm and 1000 μm. Rainbow Schlieren deflectometry has been employed to map the dynamic changes in the thermal gradients. Schlieren images have been qualitatively analysed to understand the relative significance of each of the constituting fundamental processes namely bubble inception, bubble growth and it subsequent departure from the nucleation site, scavenging of superheat layer and quenching heat transfer. With decreasing depth of cavity, a significant increase in the strength of the thermal gradients was seen in the form of sharp variations in color contrast from red (zero or negligible thermal gradients) to magenta (regions corresponding to maximum thermal gradients) in the rainbow Schlieren images. The Schlieren images corresponding to instances of bubble departure are subjected to quantitative analysis procedure discussed in the work to determine the bubble dynamic parameters such as equivalent bubble diameter, bubble ebullition frequency, bubble base diameter and aspect ratio. With decreasing cavity size (depth), bubble ebullition frequency and equivalent bubble diameter were found to be increasing. On the other hand, the bubble base diameter and aspect ratio of the vapor bubble showed a decreasing trend. Furthermore, the bubbling features from small cavity were observed to be inconsistent with the estimated bubble dynamic parameters showing a significant variation at each test condition. The inconsistency in the behaviour of cavity with reduced depth can be attributed to the decreased availability of thermal energy due to the formation of relatively large vapor bubbles in quick succession resulting in complete removal of entrapped vapor bubbles. (author)