Experimental investigation of pool boiling heat transfer on pillar-structured surfaces with different wettability patterns

In this paper, we experimentally investigate the pool boiling heat transfer on pillar-structured surfaces with different wettability patterns and different values of the channel-to-pillar width ratio. Five types of wettability patterns are compared, i.e., homogeneous hydrophilicity (HPi), homogeneous hydrophobicity (HPo), homogeneous near-superhydrophilicity (SHPi), a mixed pattern with hydrophilic bottom and hydrophobic pillar top (MPiPo), and another mixed pattern with hydrophobic bottom and hydrophilic pillar top (MPoPi). The effects of homogenous wettability patterns on the boiling performance of pillar-structured surfaces are basically consistent with those on plain surfaces, but the differences between the HPi and HPo patterns are found to be gradually enlarged with the decrease of the channel-to-pillar width ratio. Moreover, compared with the base HPi pattern, the MPoPi mixed pattern leads to a leftward shift of the boiling curve and an upward shift of the heat transfer coefficient (HTC) curve but gives a lower value of the critical heat flux. Such a tendency of the MPoPi mixed pattern is not affected when the channel-to-pillar width ratio is decreased from 2.0 to 0.5. In contrast, the boiling performance of the MPiPo mixed pattern is found to be significantly affected by the channel-to-pillar width ratio as it performs best among the five types of wettability patterns when the channel-to-pillar width ratio γ⩾1, but it deteriorates the boiling heat transfer at high heat fluxes when the ratio γ is reduced to 0.5.