September 5, 2017

Boiling heat transfer

Phase change is the most efficient way to increase heat transfer since latent heat is typically much larger than sensible heat. Moreover, phase change is usually followed by the significant increase of volume of the working fluids necessary to drive turbines for propulsion and generation of electricity. The main challenge for increasing energy savings is to design surface patterns to control heat transfer in nucleate boiling. Boiling is inherently multi-scale in nature as it starts with bubble nucleation at scales down to nanometers, continues with bubble growth and detachment at the micrometer scale and induces flow instabilities up to centimetre scale.

Resolving these scales is critical to predict heat transfer accurately. The modern level of micro fabrication sophistication has created new interests and discoveries on the effect of surface textures and chemistry on boiling. Optimal surfaces for phase change require that the wall be in contact with both liquid and vapour. The scientific challenge comes from the twofold role of surface energy (wettability) on heat transfer. On one hand, heat transfer is enhanced on superhydrophobic surfaces, because these provide pits and cavities that entrap vapour and gas and reduce the system free energy to facilitate nucleation. On the other hand, at high heat fluxes or temperature differences, heat transfer is enhanced on surfaces with high wettability (hydrophilic) because these delay transition to film boiling (dry-out), when the wall is in contact with vapour only and the heat flux dramatically decreases. To solve this dilemma, surfaces characterized by nanoscale and multiscale patterns and wettability contrasts have been recently proposed. These combine nano- and microscale patters, alternating hydrophobic and hydrophilic regions (biphilic substrates).