Droplet impact dynamics and transient heat transfer of a micro spray system for power electronics devices

• Instantaneous droplet impact dynamics was visualized and measured.
• The initial surface temperature on droplet impact dynamics was clearly noted.
• Instantaneous heat transfer during water spray impacting a surface was explored.

We present a study on the instantaneous heat transfer and droplet impact dynamics caused by multiple streams of water impinging on a polished surface with a constant heat flux (0.1–0.9 W/cm2) heating applicable to power electronics’ thermal configuration design. A multiple spray was produced by a commercial piezoelectric atomization plate (power = 1.5 W and frequency 104 kHz) with three different nozzle arrays of dj = 7 μm, 10 μm and 35 μm and a corresponding mass flow rate of 4.42 × 10−5 kg/s, 1.11 × 10−4 kg/s and 1.15 × 10−4 kg/s, respectively. A heater consisting of an ultra-thin layer (∼200 nm) of Indium Tin Oxide (ITO) combined with quartz glass (0.3 mm thickness) substrate was used to characterize the cooling history and droplet impact hydrodynamics. Through optical visualization from a bottom view, the transient impact droplets’ morphology and the, surface temperature distribution, were measured and extracted to obtain the evolved film thickness. The effects of nozzle diameter, in addition to the spray height and the initial surface temperature on heat transfer for very short periods of time (<1 s), were studied. Furthermore, the resultant transient (∼1 s) cooling performance and heat transfer coefficient were secured and discussed.