The transient free convection of heat through an 8mm thick perforated plate has been studied. To start each experiment the temperature of water above the plate was rapidly reduced, typically by about 20K, and holes in the plate were opened allowing free flow areas between 0.5 and 2.0 %. Significant heat was then transferred, cooling the region below the plate.
A phenomenological model was used to predict temperature profiles in the jets themselves, and in the stagnant water surrounding them. The predictions were fitted to experimental results by choosing suitable values for an entrainment coefficient.
The entrainment coefficients were significantly lower than the values suggested by Morton. We suggest that this is due to the lower Reynolds numbers at the jet outlets, and hence the lower turbulence intensities.