Spray Cooling Heat Transfer Enhancement and Degradation Using Fractal-Like Micro-Structured Surfaces

In the present study, spray cooling curves are presented for two micro-structured surfaces and are compared to smooth surface results. The micro-structured surfaces consisted of bio-inspired fractal-like geometries, denoted as grooves or fins, extending in a radial direction from the center to the periphery of a 37.8 mm circular disc. Depending on the location on the surface, dimensions of groove widths and heights varied from 100 to 500 μm, and 30 to 60 μm, respectively. Fin width and height dimensions remained constant over the surface at 127 and 60 μm, respectively. Results are presented as heat flux versus the surface-to-exit spray temperature difference at each of five volume flux conditions ranging from 0.54 to 2.04 × 10⁻³ m³/m²-s. Convection heat transfer coefficients are also presented for each case as a function of heat flux. Results indicate that at low and high volume fluxes, an improvement in heat transfer occurs in the single phase regime for the fin geometry. Enhancement in the single phase regime does not occur at the intermediate volume flux condition. In the two phase regime for the fin structure significant enhancements, up to 50%, are observed. Whereas the groove structure performs similarly to the smooth surface in the single phase regime and exhibits large degradation in the two phase and critical heat flux regimes, up to 50%. Critical heat flux for the fin surface compares well to that of the flat surface, with a slightly increase at high volume flux conditions.