Characteristics of water droplet impaction behaviour on a polished steel heated surface: part II

This article presents the results and analysis of the droplet impaction on a hot stainless steel surface which is in two parts. Part I of this study reported the results of analysis of high speed visualisations of droplet impaction phenomena on a stainless steel hot surface, including the characteristics of reatomized droplets that are produced at high impact Weber number. Part II, herewith, provides the results and analysis of the droplet impaction heat transfer on a hot surface. The 3.0 mm diam flat stainless steel surface was heated to temperatures of 140°C < Tw < 400°C, and water sprays were produced from a droplet generation system based on an 80 mm diam rotary cup. Droplet sizes in the range 20 µm < D < 160 µm were used, impacting with velocities 5 ms-1 < U < 18 ms-1 . Previous research relevant to the topic is reviewed. The main purpose of this investigation was to independently analyse the effects of hydrodynamic parameters, such as droplet size, velocity, frequency, or mass flux, at different surface temperature conditions. The heat transfer results are presented and discussed in terms of overall heat flux results, although the contribution of the natural convection and radiative heat transfer (i.e., at the "no spray" condition) is also presented separately for better understanding of the results. The heat transfer effectiveness is also discussed and presented, indicating the cooling efficiency of the impacting sprays. Heat transfer effectiveness, ef, is found to decrease with increase in mass flux, which is similar in trend to that reported by other researchers. For a given mass flux, the maximum values of ef occur at Weber number 300 < We < 500 (39-62%). For a given mass flux and Weber number, the effect of increasing droplet size is to decrease ef