Local two-phase heat transfer from arrays of confined and submerged impinging jets

Abstract

Local heat transfer distributions at high spatial resolution are obtained under two-phase transport conditions in confined and submerged impingement from arrays of miniature jets. The dielectric liquid HFE-7100 is investigated to enable direct cooling of electronic components. Three round orifice geometries with the same total orifice open area are investigated, including a single orifice of 3.75mm diameter, a 3×3 array of 1.25mm diameter orifices, and a 5×5 array of 0.75mm diameter orifices. A thin-foil heat source backed by a magnesium-fluoride window is fabricated to allow detailed mapping of the heated surface temperature via infrared (IR) thermography. The rigorous experimental calibration procedures employed, and correction for heat spreading within the thermally conductive IR-transparent window, yield low-uncertainty local heat transfer coefficient distributions. Each of the three orifice geometries is characterized at volumetric flow rates of 450ml/min, 900ml/min, and 1800ml/min, resulting in a Reynolds number range of 1920–39400. Pressure drop across the confined jets is measured for all experimental cases. The test facility and measurement techniques employed are validated against heat transfer and pressure drop correlations in the literature for single-phase jet impingement from a single round orifice. Spatially resolved temperature contour maps, along with local heat transfer coefficient and boiling curves, are presented as a function of applied heat flux. Boiling is shown to coexist with single-phase convection under the impinging liquid jets. Two-phase enhancement is exhibited at large radial distances from the single jet axis, and in regions between neighboring jets within the arrays. The arrays of jets result in higher area-averaged heat transfer than a single jet at a fixed flow rate; however, the arrays display larger relative nonuniformity in local two-phase heat transfer coefficient and surface temperature. While the 5×5 array resulted in a higher (and the 3×3 a lower) pressure drop than the single jet, all orifices displayed pressure drop that is independent of the applied heat flux and vapor generation.