Visualization of bubble behavior for jet impingement cooling with phase change

Jet impingement cooling with phase change is a technique that has been successfully used to dissipate high heat fluxes (of order of 100 W/cm² and higher) from advanced electronics. This work reports high speed visualization of jet impingement boiling on a copper heat spreader with porous coating. The coolant used in the experiments is a dielectric fluid, HFE-7100, in combination with a 5 × 5 array of round jets with orifice diameter of 0.75 mm. Both heat transfer and pressure drop are measured for different heat fluxes with fixed mass flow rate of 1.05×10^-2 kg/s. Two borescopes in combination with high intensity Xenon light sources are used to illuminate the inside of the boiling chamber, and a high speed camera is used to capture the high speed images of the boiling process. Based on the visualization results, it is observed that the nucleate boiling process begins in the region surrounding the stagnation point and then moves inward towards the stagnation region as the heat flux is increased. Interesting observations are made related to the variation in the pressure drop of the cooler. Specifically, the pressure drop remains constant at 1.18 kPa during the early stages and then peaks to a value of 1.23 kPa at 26 W/cm². The peak in the pressure drop is attributed to the accumulation of a large number of bubbles near the jet orifice producing flow resistance to the incoming liquid jet.