Quenching mechanism of spray cooling and the effect of system pressure

The present contribution addresses an open question about spray cooling: when does quenching occur? We performed water spray cooling tests with heated iron disks at system pressures ranging from 0.1 MPa to 0.5 MPa. Sample heating was carried out under high vacuum and then spray cooling in an argon environment, minimising the effect of surface oxidisation. The temperature for the onset of quenching was found to increase with the system pressure. We demonstrated that the quenching point could be predicted reasonably well by a mechanistic model that considers the force balance at the interface between the droplet and the vapour film underneath it. The influence of droplet impact velocity was also discussed. From the temperature histories obtained by the cooling tests, the rate of heat transfer in the film boiling regime was evaluated and found to increase with the system pressure. This was due to the increase in the saturation temperature which then led to the reduction of vapour film thickness, as corroborated by a Leidenfrost droplet model. Using the spray parameters, we quantified the amount of heat removed by a single droplet and compared it with theoretical models. Our analysis suggested a potential existence of partial droplet–surface contact even in the film boiling regime.