Effects of nozzle exit size and edge shape on transient flash spray cooling performance of a millimeter-scale sweeping nozzle

Spray flash evaporation cooling is a promising solution for transient thermal management of high-power electronics, but unstable spray patterns and uneven liquid film deposition hinder practical use. This measurement explores and uncovers transient heat transfer of a millimeter-scale sweeping nozzle that couples a self-oscillating flow with R134a flash evaporation. Nozzle exit size (d/D = 2.5–4.0, D = 1.0 mm) and edge contour (sharp vs. curved) are varied, and surface temperature and heat flux are captured via embedded thermocouples and an inverse Duhamel method, while high-speed scattering imaging visualizes the spray structure. A sharp-edged nozzle with intermediate exit (d/D ≈ 3.0–3.5) maintains a coherent sweeping motion, yielding a delayed but intense cooling burst (peak heat flux up to 147 W/cm²) and a cascading thermal wave that enhances lateral uniformity. Increasing d/D to 4.0 or using a curved edge collapses the oscillation into a quasi-steady centerline impingement, accelerating thermal response but reducing peak heat flux and uniformity. The curved d/D = 4.0 nozzle further suppresses the film boiling plateau, widening the safe operating window during temperature overshoots. As R134a is already the working fluid in many electric vehicle heat pump systems, this cooling concept can be integrated directly into the existing refrigerant circuit to cool AI computing chips and drive motors without a separate loop. These findings provide a geometry–performance framework for designing sweeping nozzles that balance cooling intensity, response time, and spatial uniformity.