Flow boiling enhancement on a horizontal heater using carbon nanotube coatings

In this study we measure the flow boiling heat flux on a horizontal heater that is heated from below. The horizontal heater consists of either a bare silicon wafer or a silicon wafer that is coated with multi walled carbon nanotubes (MWCNT). The silicon wafer is clamped on a constant heat flux type calorimeter consisting of a vertical copper cylinder with embedded cartridge heaters and K-type thermocouples. De-ionized (DI) water was used as the test fluid. The calorimeter apparatus is housed in a test section with glass walls for visual observation. The liquid is pumped from a constant temperature bath to maintain a fixed subcooling during the experiments under steady state conditions. g curves (wall heat flux as a function of heater temperature) were obtained for different flow rates and liquid subcooling. Experiments were performed for two different flow rates and two different liquid subcoolings. Flow boiling heat flux was enhanced by as much as 180% at boiling incipience for silicon substrates coated with carbon nanotubes. MWCNT was less effective in enhancing heat flux as the flow rate and liquid subcooling was increased. This anomalous behavior was explained using flow boiling models reported in the literature.