The hydrodynamics and heat transfer of single droplets impinging on a heated surface have been used in the previous literature as a basis to assess the full spray impingement processes. In this study the effects of simultaneous impingement of more than one droplet at a time were investigated and compared to single droplet data in order to bridge the gap between the real full spray impingement process and the ideal process of single droplet. For that purpose parcels, groups of droplets with a uniform size, were used as the spray flux.
The effect of the inter-parcel spacing L/D, the parcel’s Weber number, and the surface superheat were tested. The liquid used was water, the inter-parcel spacing was 5-20 L/D, the droplets had a size 200-400 $\mu$m, and reach the wall at We = 70-300, and the surface superheat range was 0-250$\sp\circ$C.
The generated parcels were photographed, during their wall impact, using a high speed camera back lit by a pulsed laser. The heat transfer from the heated surface to the impinging parcels was measured by measuring the surface temperature instantaneously with time, with and without impingement.
In the low superheat regime, a liquid film was shown to deposit on the wall. The film was destabilized the most if impinged by two droplets impinging with We = 200 at (time ratio) TR = 0.5, and when (liquid film thickness) $\delta\sb{f}/d = 1.$ The change of the liquid film thickness produced three different breakup modes.
In the transition and film boiling regimes, the multiple droplet impact ejected more droplets than a single droplet does, however the dispersion of the ejected droplets was smaller than in the single droplet case due to increased coalescence and collision probabilities in the impingement site with the increase of the liquid fraction near the wall. The normal dispersion of the ejected droplets in the transition boiling regime was one order of magnitude higher than that in the film boiling regime. In the film boiling regime tangential dispersion of the droplets is higher than that for the transition boiling regime.