Transient spray cooling: Similarity of dynamic heat flux for different cryogens, nozzles and substrates

Spray cooling is widely used in industries. Understanding of the complex surface heat transfer characteristics is essential to enhance cooling efficiency. In the present study, surface heat transfer of transient spray cooling with different cryogens, nozzles, and substrates were investigated. Minimum surface temperature reached −46.1 °C, −55.9 °C, and −57.9 °C and maximum surface heat flux values were 294.9, 364.1, and 377.4 kW/m2 for R134a, R407C, and R404A, respectively, on an epoxy resin block. Results proved that R404A has the best cooling capacity. Compared with that of the straight nozzle, qmax of the expansion-chamber nozzle increased from 266.9, 364.1, and 403.9 kW/m2 to 339.3, 442.8, and 445.1 kW/m2 or 16.6%, 18.6%, and 16.7% for R134a, R407C, and R404A, respectively. Transient cooling could be divided into two stages, namely, fast boiling cooling and film evaporation cooling. The similarity of dynamic heat flux with different cryogens, nozzles, and substrates was observed, and the dimensionless correlation was proposed by the spray Biot number, which is defined as the ratio between the internal thermal resistance of the substrate and the surface convective heat transfer resistance. The dimensionless Reynolds number Rel and Fourier number Fol were proposed to represent the maximum heat flux (qmax) and the corresponding time (tmax), respectively. By coupling the Jakob number with Rel and the droplet Weber number (We), dimensionless correlations of maximum heat flux and the corresponding time were proposed, through which it is indicated that We number was the key factor in spray cooling.