Abstract:
Spray impingement cooling has been shown to be a leading candidate for future high heat flux cooling applications. In general, spray cooling curves consist of three heat flux regimes; single-phase, two-phase and critical heat flux (CHF). CHF is considered the design limit for almost all two-phase cooling applications, as a rapid increase in temperature and decrease in heat flux occurs beyond this point.
Recent studies have shown that the addition of micro-structures on the impingement surface can enhance heat transfer relative to a smooth surface. In the present study, spray cooling curves are obtained for two micro-structured surfaces and are compared to smooth surface results. Micro-structured surfaces consisted of bio-inspired fractal-like geometries, denoted as grooves and 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 dimension remained constant
throughout the surface at 127 and 60 μm, respectively.
Heat flux and wall temperature at the impingement surface were calculated from
temperature data measured at multiple locations below the impingement surface. Results
are presented as heat flux, q" , versus the wall-to-spray temperature difference, ΔT[subscript w], at
each of 5 volume flux, Q", conditions ranging from 0.54 to 2.04 x 10⁻³ m³/m²s. Convection coefficients, h[subscript cv], and spray efficiencies, η, are also presented for each case as a function of q" and ΔT[subscript w] , respectively. Results of the study 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 did not occur at the intermediate volume flux condition of 1.37 x
10⁻³ m³/m²s. At all volume flux states tested, significant enhancements, as high as 50%
in some cases, were observed in the two-phase regime for the fin structure, whereas the
groove structure performed identically to the flat surface in the single-phase regime and
exhibited a large degradation in the two-phase and critical heat flux regimes (~50%).
Critical heat flux for the fin surface compared to the flat surface was slightly lower at low
volume flux conditions, equivalent at the intermediate volume flux, and slightly greater at
high volume flux conditions. Further investigations into the underlying mechanisms
responsible for these results are needed.
