Effect of surface wettability on bubble dynamics and heat transfer in microchannel flow boiling

Despite extensive studies and modeling of bubble dynamics manipulation in macroscale boiling, the effect of surface wettability on the bubble dynamics of microchannel flow boiling has seldom been investigated. The confinement of the microchannel and fluctuations in the dominant forces lead to unique confined bubble growth and distinctive bubble detachment. Surface wettability can significantly affect the bubble dynamics parameters, thereby influencing the heat transfer performance of microchannel flow boiling. In this study, we conducted subcooled flow boiling experiments and flow visualizations to quantitatively investigate the influence of surface wettability on sliding bubble dynamics, confined bubble growth, and heat transfer characteristics in a microchannel using HFE-7100 as the working fluid. Numerous nucleation sites were activated owing to the lower energy barrier for bubble nucleation on the hydrophobic surface. The elongated bubble shape was flatter, and the bubble size was larger, which could be attributed to the strong bubble adhesion force on the hydrophobic surface. The bubble sliding and growth velocities were much higher on the hydrophilic surface, and bubble acceleration increased the shear stress and pressure gradient surrounding the bubble, producing a more non-axisymmetric oblique triangle profile of the elongated bubble. The bubble growth rate on the hydrophilic surface was approximately three times higher than that on the hydrophobic surface. The heat transfer coefficients (HTCs) on the hydrophobic surface increased by up to 82 % and 25 % during microchannel flow boiling at mass fluxes of 112 and 230 kg·m⁻²·s⁻¹, respectively, because of the activation of numerous bubble nucleation sites. Furthermore, the HTCs increased by up to 56 % for higher mass fluxes owing to the strengthening of microconvection and suppression of annular flow. The nucleate boiling mechanism dominated the microchannel flow boiling heat transfer.