Experimental study of forced convection and subcooled flow boiling heat transfer in a vertical annulus using different novel functionalized ZnO nanoparticles

- Effect of ZnO-Nano-particles synthesis methods on thermal performance of prepared Nano-fluids is analyzed.
- Different synthesis methods result in variable thermal performance.
- Positive effects on thermal performance of Nano-fluids in comparison with base fluid are seen.
- Decreasing sub-cooling temperature and increasing heat flux and velocity enhance the heat transfer coefficient.

In this study the effect of different ZnO-nanoparticles synthesis methods on thermal performance of prepared nanofluids has been investigated. The study has been accomplished in two parts. At first, four types of ZnO nanoparticles were synthesized by sol-gel hydrothermal method. The surfaces of these nanoparticles were functionalized and then the characterization tests were performed. At the second step, forced convective and subcooled flow boiling heat transfer coefficients of the synthesized water-based nanofluids were experimentally studied. The experiments were conducted in a closed loop in which the working fluid passed through a vertical cylindrical annulus. The experiments were performed at different levels of operating variables such as subcooling temperature (30-50 °C), volume flow rate (2-4 L/min), heat flux (8-110 kW/m2) and nanoparticle concentration (0.005-0.02 vol%). The results indicated that nanorod-nanotube ZnO nanoparticles which were surface modified by silica gel and UV radiation had the best thermal performance.