Spot-cooling of discrete electronic packages mounted on a printed wiring board (PWB) is achieved by the impingement of an axisymmetric synthetic jet when the jet actuator is attached to one board and cools target integrated circuit (IC) on the opposite board. Present work demonstrates that even when the jet outflow is confined between two closely-spaced boards, the jet entrains ample volume flow rate of cooler ambient air, induces effective cooling by strong mixing near the heated surface, and ultimately dispenses the heated air to ambient. The cooling performance of the jet module is investigated experimentally in a scaled up model that enables high-resolution thermal and flow measurements. The test setup comprises of two circular parallel plates (D = 158.8mm) where one plate contains an integrated jet actuator and the opposite plate includes a target heater (dh = 86mm). The spacing between the plates is variable between D/12 and D/3. The flow within the gap is mapped using particle image velocimetry (PIV). It is found that confined jet impingement induces a countercurrent radial flow within the gap that includes a layer of cool ambient air entrained along the actuator plane and a layer of heated air that is dispensed along the target surface. Particle pathlines demonstrate significant mixing between the countercurrent streams and strong entrainment into the vortex rings that synthesize the jet. Heat transfer coefficient attains a local maximum away from the stagnation point that can be attributed to strong vorticity diffusion into the thermal boundary layer and enhanced mixing that accompanies the vortex ring impingement. Although the jet Reynolds number does not exceed 3300, the stagnation heat transfer coefficient is about 90 W/m2K for H/D = 0.32.

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