Enhancemet of flow boiling heat transfer in pHEMA/pPFDA coated microtubes with longitudinal variations in wettability

Abstract

Flow boiling heat transfer was investigated in stainless steel hypodermic microtubes, whose surfaces were enhanced by gradient crosslinked polyhydroxyethylmethacrylate (pHEMA)/polyperfluorodecylacrylate (pPFDA) coatings thereby offering variations in wettability along the surface as well as high porosity. The initiated chemical vapor deposition (iCVD) method was implemented for coating the inner walls of the microtubes with an inner diameter of 502 μm, and deionized water was used as the working fluid. Experimental results were obtained from the coated microtubes, where one end corresponded to the pHEMA (hydrophilic) coated part and the other end was the most hydrophobic location with the pPFDA (hydrophobic) coating so that wettability varied along the length of the microtube. The results of both the hydrophobic and hydrophilic inlet cases were compared to their plain surface counterparts at the mass flux of 9500 kg/m2s. The experimental results showed a remarkable increase in boiling heat transfer with the coatings. The highest heat transfer coefficients were attained for the pHEMA coated (hydrophobic inlet and hydrophilic outlet) outlet case with a maximum heat transfer enhancement ratio of ∼64%. The reason for the enhanced heat transfer with the coated microtubes can be attributed to the increased nucleation site density and bubble release as well as enhanced convection and bubble motion near the surface due to the variation in wettability along the length. The results proved that gradient pHEMA/pPFDA coatings can be utilized as a viable surface enhancement method in microscale cooling applications.

Flow Boiling in Micro-Passages: Developments in Fundamental Aspects and Applications

“…At the highest mass flux the corresponding values were: 104%, 55%, and 19 %. Nedaei et al [72] extended the work by Kaya et al [71] and coated the 0.507 mm diameter tube such that the surface wettability (contact angle) can be different along the tube length, i.e. wettability gradient in the axial direction.…”

Recent Advances of Surface Wettability Effect on Flow Boiling Heat Transfer Performance

“…However, a single homogeneous wetting surface cannot embrace the above conditions. The heterogeneous wetting surface can combine the advantages of strong wetting surface and weak wetting surface, which has excellent heat transfer performance for flow boiling (Nedaei et al 2016). Kousalya et al (2015) reported the horizontal subcooled flow boiling characteristics on the heterogeneous wetting surfaces (superhydrophilic -superhydrophobic) in a rectangular channel with a height of 5.5 mm, as shown in Fig.…”