Experiments to understand bubble base growth mechanism(s) on hydrophobic surfaces under the influence of bulk flow inertia during nucleate boiling regime

Abstract

Wetting characteristics of substrate surface is one of the parameters that strongly influence the phenomena of boiling heat transfer. In this direction, importance of hydrophobic surfaces has been emphasized in the literature due to their potential to offer high boiling heat transfer rates at low heat fluxes. For such surfaces, it has been shown that microlayer ceases to exist with contact line evaporation being the dominant bubble base growth mechanism during nucleate pool boiling regime. The present work investigates the plausible mechanism(s) of bubble growth on hydrophobic surfaces under nucleate flow boiling conditions. Possibility of formation of microlayer even on low wettability surfaces under the influence of bulk flow inertia has been examined. Thin-film interferometry, in conjunction with high-speed videography, has been configured to record the microlayer dynamics and the bubble growth process in tandem. For a direct comparison, experiments under identical flow and bulk subcooled conditions have also been performed on a hydrophilic substrate, which is known to promote microlayer formation during the bubble growth process. Observations made through thin-film interferograms confirmed the formation of microlayer on hydrophobic surfaces. Experiments further revealed that the rate of growth of dry patch is higher on hydrophobic surfaces than that in the case of hydrophilic substrates. Vapor bubbles have been observed to be pinned on the hydrophobic surface whereas the phenomenon of bubble lift-off has been distinctly observed on hydrophilic substrate surface.