Pool boiling heat transfer of refrigerant R-134a on TiO2 nano wire arrays surface

Abstract

An experimental study was conducted to investigate the pool boiling heat transfer of refrigerant R134a liquid at saturation temperature 6°C on nine mm diameter three circular flat copper surfaces fabricated with nano wire (NW) arrays. Vertically oriented titanium dioxide (TiO2) nano wire arrays in different thickness were fabricated on 100nm TiO2 thin film coated circular flat copper surfaces. Nano wire and thin film were synthesized by glancing angle deposition (GLAD) system using e-beam evaporator (EBE). The three nanowires coated copper surfaces were 150nm NW, 300nm NW and 450nm NW. These copper surfaces were utilized in pool boiling tests with refrigerant R134a liquid at 6°C saturation temperature and their performances were compared to that of an uncoated surface. Result showed that there was reduction of wall superheat with improvement of heat transfer coefficient (HTC) for all nano arrays surfaces. The best, moderate and minimum heat transfer performance was found from 450nm, 300nm and 150nm nanowire arrays respectively. AFM and FE-SEM imaging estimations of the surface proposes that as improvement of heat transfer coefficient (HTC) was because of an increment in the dynamic nucleation site density. Effective surface modification process can augment the pool boiling heat transfer property of a surface using refrigerant, especially in the shell side of flooded evaporators. The techniques of surface modification would greatly contribute to the energy savings.