The high power density of emerging electronic devices is driving the transition from remote cooling, which relies on conduction and spreading, to embedded cooling, which extracts dissipated heat on-site. Two-phase microgap coolers employ the forced flow of dielectric fluids undergoing phase change in a heated channel within or between devices. Such coolers must work reliably in all orientations for a variety of applications (e.g., vehicle-based equipment), as well as in microgravity and high-g for aerospace applications, but the lack of acceptable models and correlations for orientation- and gravity-independent operation has limited their use. Reliable criteria for achieving orientation- and gravity-independent flow boiling would enable emerging systems to exploit this thermal management technique and streamline the technology development process. As a first step toward understanding the effect of gravity in two-phase microgap flow and transport, in the present effort the authors have studied the effect of evaporator orientation, mass flux, and heat flux on flow boiling of HFE7100 in a 1.01 mm tall × 13.0 mm wide × 12.7 mm long microgap channel. Orientation-independence, defined as achieving similar critical heat fluxes (CHFs), heat transfer coefficients (HTCs), and flow regimes across orientations, was achieved for mass fluxes of 400 kg/m2 s and greater (corresponding to a Froude number of about 0.8). The present results are compared to published criteria for achieving orientation- and gravity-independence.

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