Characterization of Enhanced Two-Phase Jet Impingement on Femtosecond Laser Surface Processed (FLSP) Aluminum Surfaces

Two-phase jet impingement is a thermal management technique that offers high heat transfer coefficients necessary for dissipating the high heat fluxes in electronics cooling applications. The use of dielectric fluids during jet impingement allows direct cooling of electronic devices but limits the boiling performance due to unfavorable thermophysical properties. Surface enhancements are therefore of interest to increase the boiling heat transfer performance and enable the use of two-phase jet impingement in high-heat-flux electronics cooling applications. Boiling heat transfer experiments are performed using a jet impingement test chamber and a supporting two-phase flow loop. A single 3 mm-diameter jet orifice is spaced 7.5 mm from a 25.4 mm-diameter circular aluminum heater surface. Femtosecond laser surface processing (FLSP) is used to modify the surface structure of the aluminum heater, which creates micro- and nano-scale features. The boiling performance of the FLSP-modified surface is compared to that of a polished baseline surface during impingement with a 1 m/s subcooled jet as well as under pool boiling conditions using the dielectric fluid HFE-7100. Each surface was characterized by incrementing the heat input up to the maximum nucleate boiling heat flux before a transition to film boiling occurred. The surface superheat at the maximum heat flux reduced by 55% and 53% for jet impingement and pool boiling, respectively. High-speed imaging of the boiling process suggests that the mechanism by which FLSP functionalization reduces the surface superheat is activation of additional nucleation sites.