Unsteady effects of a pair of opposed confined impinging air jets and application to microelectronics cooling

The unsteady laminar flow and heat transfer characteristics for a pair of opposed confined impinging air jets in a channel were studied numerically. The space and time-averaged heat transfer coefficients for a pair of heat sources arranged at different locations on opposite target walls was determined together with the oscillating jet frequency. The present study continues the authors\´ previous investigations, which found that opposite confined jets remain steady at Reynolds numbers that make the parallel (side-by-side) jets highly unsteady. The nature of the unsteadiness depends on the proximity of the jet inlets, the channel dimensions and the jet Reynolds number. The unsteadiness causes the stagnation point locations to sweep back and forth over the impingement region, causing the jets to "wash" a larger surface area on the target wall. The results for the parallel jets indicate that they become unsteady between a Reynolds of 200 and 300. A fixed stagnation "bubble" is formed on the target wall between the two jets, which reduced the heat transfer removal from that region, leading in fact to a quasi-independence of the local heat transfer on flow conditions. By comparison, the opposed jets become unsteady between a Reynolds of 700 and 800, with a weaker "bubble" formation between the jets. The actual study focuses mostly on the flow field unsteadiness and associated heat transfer coefficients when varying the distance between the chips placed on the target walls for a flow at Re = 750. These trends will be further compared to the parallel jets hydrodynamic and thermal fields. The complex vortex patterns resulting from the jet interaction at the higher Reynolds number is investigated and its impact on the chip/microelectronics component cooling are documented