Kostantinos Vontas, PhD Student under a Marie Curie/DTA fellowship, published his most recent work on the International Journal of Heat and Mass Transfer (IF 4.947), entitled “The effect of surface wettability on flow boiling characteristics within microchannels“.

The process of flow boiling within micro-passages plays a very important role in heat transfer processes. However, there is still lack of understanding of the effect of an important controlling parameter, i.e. surface wettability. In this paper, an advanced numerical investigation on the effect of wettability characteristics on single and multiple bubble growth during saturated flow boiling conditions within a microchannel, is performed. The 3D numerical simulations are conducted with the open-source Computational Fluid Dynamics (CFD) toolbox OpenFOAM, utilising a custom user-enhanced Volume of Fluid (VOF) solver. The proposed solver enhancements involve an appropriate treatment for spurious velocities dampening, an improved dynamic contact angle treatment as well as the implementation of a phase-change model in the fluid domain, also accounting for Conjugate Heat Transfer (CHT) with the solid domain. In total, three sets of simulations of hydrophilic and hydrophobic surfaces with constant heat and mass flux were performed. In the first set, a single bubble seed is patched close to the inlet of the microchannel and the Heat Transfer Coefficient (HTC) along the channel interface is measured until the leading edge of the bubble reaches the outlet. The bubble growth and transport process within the channel are analysed, however a minor effect of the wettability characteristics on the HTC is observed. In the second set of simulations, multiple recurring nucleation events at the same position were simulated, observing that in such more realistic cases the effect of wettability in the HTC is more profound. Finally, simulations with multiple nucleation sites and recurring nucleation events were conducted to analyse cases closer to reality. These results show indeed that surface wettability plays a significant role on the HTC, with the hydrophilic and hydrophobic cases performing approximately 43.9% and 17.8% higher compared to the single-phase reference simulations, respectively. Additionally, it is found that the dominant heat transfer mechanisms for the hydrophilic and hydrophobic surface are liquid film evaporation and contact line evaporation, respectively, and that for the proposed simulation parameters liquid film evaporation can be considered as a more efficient heat transfer mechanism compared to contact line evaporation.

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