Boiling heat transfer enhancement in mini/microtubes via polyhydroxyethylmethacrylate (pHEMA) coatings on inner microtube walls at high mass fluxes

Abstract

In this experimental study, flow boiling in mini/microtubes was investigated with surface enhancements provided by polyhydroxyethylmethacrylate (pHEMA) coatings (of ∼30 nm thickness) on inner microtube walls. Flow boiling heat transfer experiments were conducted on microtubes (with inner diameters of 249, 507 and 998 µm) having inner surfaces of pHEMA coatings, which increase heat transfer surface area, enable liquid replenishment upon bubble departure, provide additional nucleation sites, and serve for extending critical heat flux (CHF) enhancing boiling heat transfer. The de-ionized water was utilized as the working fluid in this study. pHEMA nanofilms of thickness ∼30 nm on the microtube walls were coated through an initiated chemical vapor deposition technique. Experimental results obtained from the coated microtubes were compared to their plain surface counterparts at two mass flux values (10 000 and 13 000 kg m−2 s−1). In comparison to the plain surface microtubes, the coated surfaces demonstrated an increase up to 24% and 109% in CHF and heat transfer coefficients, respectively. These promising results support the use of pHEMA coated microtubes/channels as a surface enhancement technique for microscale cooling applications.