Unsteady heat transfer at impinging of a single spray pulse with various durations

•Unsteady heat transfer from a single spray pulse of various durations was experimentally studied in the evaporative cooling regime (Tw = 70°C).•The change in heat transfer coefficient over time is fundamentally different for short (ton 10 ms) duration of pulses.•It has been determined that the greatest value of the thermal efficiency parameter of a single spray is achieved for very short pulses. The results of an experimental study of heat transfer at the inlet of a pulsed multi-nozzle spray onto a vertical surface in the regime of evaporative cooling (Tw = 70°C) are presented. The main attention is paid to the measurement of unsteady heat transfer for a single spray pulse of various durations (ton = 2–50 ms). The experiments were carried out using pure water and its 50% and 96% mixtures with ethanol as the liquid phase. Initially, with short pulses, the heat transfer coefficient increases with increasing its duration and at ton≈ 10 ms it reaches a plateau with a practically constant value despite the fact that the mass of the liquid injected per pulse increases with ton.The same tendency is observed for time-averaged heat transfer and for all studied coolant compositions. The probable cause of this phenomenon is the increase in the liquid film thickness on the heat exchanger surface, which in turn suppresses evaporation processes. The thermal efficiency coefficient η of the pulsed spray is maximal for the shortest pulses, and it decreases significantly with increasing pulse duration.