Convective heat transfer estimation of dilute metal oxide nanofluids in VUV surface tuned minichannel using Mach-Zehnder interferometry

Abstract

• Minichannels were surface tuned with 30 min of Vacuum Ultraviolet irradiation. • Test fluids used were dilute concentrations of alumina and silica nanofluids and Deionized water. • Mach-Zehnder interferometry was utilized to extract heat transfer data from the minichannels. • Effect of surface tuning was visible only at low Reynolds Number. • At high Reynolds number, the advective effects became more significant. • Higher concentration of alumina nanofluid showed better heat transfer rates under all flow conditions. Convective heat transfer performances of dilute metal oxide suspensions in D.I water (Deionized water) flowing through a VUV (Vacuum Ultraviolet) surface tuned mini channel with a hydraulic diameter of 3 mm were quantified in this work. The experiment was carried out first on a channel with untreated PDMS coating with a contact angle of 86.2 ± 2.7° and then with a VUV treated PDMS channel having a contact angle of 2.7 ± 2.1°. Mach-Zehnder interferometry, a non-intrusive measurement technique, was employed to conduct fringe analysis in the test channel. Modified Naylor – Duarte method was used to quantify the heat transfer coefficients in the minichannels. Experiments conducted using alumina and silica nanofluids were compared with D.I water in both channels. The experiments were carried out at three different heat inputs (5 W, 10 W, and 15 W) and three other Reynolds numbers (294, 591, and 855). At Re = 294 and 5 W of heat input, the highest volume fraction of alumina nanofluid in VUV treated minichannel recorded a 5.92 ± 1.9% increment in heat transfer rates compared to the same charge in untreated minichannel. For the same flow conditions in VUV treated minichannels, 0.02 vol% alumina nanofluid recorded a 20.11 ± 3.4% increment in heat transfer rates than D.I water, while it was 6.11 ± 2.7% for 0.02 vol% of silica nanofluids. In the VUV treated minichannel at 5 W and Re = 294, the boundary layer thickness showed a reduction of 9.64 ± 1.6% for 0.02 vol% alumina nanofluid, as compared to the untreated channel. But at Re = 855, the percentage enhancement in heat transfer was confined to 1.74% for all test fluids in surface tuned minichannel. It was found that the heat transfer in the VUV treated minichannel was superior compared to that in the untreated minichannel at lower Reynolds number ( Re = 294). The results showed that alumina nanofluids performed significantly better than silica nanofluids at a given concentration in both minichannels. The plausible reasons for the enhancement in heat transfer rates in VUV treated minichannels are also discussed.