Smart surface in pool boiling: Thermally-induced wetting transition

Abstract

The boiling heat transfer coefficient (HTC) and critical heat flux (CHF) of a TiO2-coated surface (TCS) were investigated in pool boiling with increasing saturated temperature at the pressurized conditions ranged from 1.0 to 4.1bar. TCS increased HTC in comparison with a reference surface coated with SiO2 (SCS) under the pressure ranged from 1.0 to 4.1bars. CHF of TCS was higher than SCS at pressure of 4.1bar while lower below 4.1bar. Measurement of the contact angle of a water droplet on the tested surfaces after heat treatment showed a wettability increase of TCS, a contact angle reduction from 83.1° to 32.7° when the heat treatment temperature changed from 100°C to 200°C. No such change was observed for SCS. This contact angle change after heating suggests that the wetting transition of TCS is a key factor in the enhancement of both HTC and CHF in boiling. TCS is hydrophobic at a low wall temperature and becomes hydrophilic as the wall temperature increases. Hydrophobicity of TCS at low wall temperatures explains the improved HTC over SCS near the boiling inception point and low heat flux regime, and hydrophilicity at high wall temperatures explains the increase of CHF. The transition in the wettability of TiO2 appeared to be involved in CHF enhancement. The thermally-induced wetting transition of TiO2 provides a simple and innovative means for enhancing both HTC and CHF with no additional treatment; as such, we refer to TiO2 as a ‘smart’ surface.