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Polymeric solutions as a subcategory of non-Newtonian fluids are prepared by the dissolution of polymeric additives into a solvent and find applications in many areas. Their advantage over new generation fluids such as nanofluids is that they have no side effects such as agglomeration and sedimentation of particles, which is common for nanofluids. This study presents an experimental study on nucleate pool boiling of aqueous Xanthan gum solutions of different concentrations. The results revealed that heat transfer coefficients of prepared polymeric solutions were lower than those of pure water. Heat transfer coefficients decreased with concentration until the concentration reached a certain amount (500 mg/L). Beyond this value, there was no change in heat transfer performance with concentration. Nevertheless, a monotonically decreasing trend was observed for high concentrations (⩾4000 mg/L), which is associated with sharp changes in surface tension and boiling patterns beyond a certain concentration. In visualization studies, different pool boiling patterns were recorded particularly for high concentrations, which bolster the heat transfer results. RAMAN tests were also conducted to check for the stability of the solutions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]