Pool boiling in deep and shallow vessels and the effect of surface nano-texture and self-rewetting

Abstract

Pool boiling of ethanol and self-rewetting fluids on bare copper surface and copper surface with polymer nanofibers were studied experimentally. No significant effect of the depths of ethanol layer on the heat removal rate was found. That indicates that only the heat transfer in the liquid microlayer near the heater surface is a dominant factor. As a result, one can expect that self-rewetting fluids can significantly affect boiling performance. Accordingly, several alcohol solutions including the self-rewetting ones were investigated as working fluids in the boiling chamber. It was found that at the 0.1% (v/v) concentration, only the high carbon-alcohol, n-heptanol in aqueous solution, improved boiling heat transfer considerably. Furthermore, the experimental study of the effect of surface nano-texture on boiling characteristics was undertaken. For that aim, polyacrylonitrile (PAN) nanofibers were deposited onto the copper heater surface. Measurements of the boiling curve revealed a detrimental effect of such nano-texture in the case of such working fluids as ethanol and self-rewetting n-heptanol solutions. On the other hand, when polystyrene (PS) nanofibers were deposited onto the copper heater surface instead of PAN nanofibers, a significant improvement in boiling heat transfer was observed. The more hydrophobic nature of PS compared to copper is responsible for this effect, i.e. is the reason of the heat transfer enhancement on such a nano-textured surface compared to the pure copper one. In addition, the critical heat flux in the case of n-heptanol solution was found to be reduced considerably on the PS nano-textured surface compared to the pure copper one. This stems from the increased propensity of the heater surface to be covered by vapor, while the rewetting is insufficiently effective at high heat fluxes in presence of PS nanofibers.