Spray cooling at low–moderate heat fluxes: Droplet-scale diagnostics and infrared mapping with a low-GWP dielectric fluid

Spray cooling combines convective and evaporative mechanisms to dissipate heat, yet systematic data in the pre-boiling, single-phase regime remain scarce. This work targets heat fluxes of 5–15 W/cm2, typical of battery packs, LEDs, and AR/VR optics, and contrasts the performance of a low-GWP dielectric (Thermasolv IM6) with deionized water using a pressure-atomized nozzle (PAN) and an air-assist atomized nozzle (AAN). Experiments were conducted at 0.1–0.2 L/min and 25–35 °C inlet temperature, with Phase-Doppler Particle Anemometry and high-speed imaging quantifying droplet size and velocity, and infrared thermography providing pixel-wise heat-flux fields. Both nozzles produced comparable droplet diameters (≈120–170 µm), but AAN generated velocities up to 39 m/s, supplying extra momentum for rapid film thinning and intense evaporation. As a result, AAN with Thermasolv achieved sub-ambient surface temperatures (18.4 °C at 14.7 W/cm2 with a 25 °C inlet), whereas PAN maintained 43.6 °C under the same conditions. Although water provided higher sensible heat removal, its electrical conductivity limits direct application in electronics. Overall, the study demonstrates that pairing an air-assist nozzle with a low-GWP dielectric at modest flow rates improves spray cooling efficiency and offers a sustainable option for low-to-moderate heat flux applications.