Critical Heat Flux of Graphene Coated Copper Surface at High Pressures

Boiling is an efficient way to transfer heat due to the latent heat of vaporization. Many variables, such as surface properties, fluid properties, and system pressure, will affect the performance of pool boiling. Enhanced pool boiling has extensive applications in chemical, microelectronics, and power industries. Previous research has shown that micro- or nanostructured surfaces and coated surfaces will increase heat transfer coefficients up to one order of magnitude at atmospheric pressure. Graphene as a very good material with superb mechanical and electrical properties also has potential to enhance pool boiling performance. The purpose of this research is to investigate heat transfer enhancement on a graphene coated surface compared to a plane copper surface at atmospheric pressure and increased pressures with deionized water. The effect of the graphene coating on the critical heat flux is also investigated. To carry out the experiments, we designed and fabricated a special experimental facility that will withstand the high pressures (up to 20 bar) and high temperatures. Graphene is coated on a 1 cm² copper surface using spray coating. The boiling vessel is pressurized with nitrogen and the system pressure is controlled by a back pressure regulator. The test fluid is preheated to saturation temperature by two 500 W cartridge heaters. Multiple 150 W cartridge heaters are inserted in a copper cylinder to provide wall superheat for bubbles to nucleate on the studied surface. When the system reaches steady state, a process controller controls these cartridge heaters to increase the heat flux gradually from 0 kW/m² to the critical heat flux. The copper cylinder is insulated with PTFE to minimize heat loss from the side. The gap between the copper cylinder and the insulation surface is carefully sealed with high temperature epoxy to reduce undesired nucleation sites. The wall superheat corresponding to each heat flux is extrapolated using Fourier’s law from three thermocouple readings. The heat transfer coefficient can thus be calculated at each heat flux for the every test fluid at its corresponding pressure. A camera with 3.2 cm field of view at a working distance of 12 cm to 15 cm is used to visualize the bubble formation on the heated surface.