Abstract The purpose of this paper is to make some comments on the importance of heat transfer and interfacial drag modelling on the prediction of coarse mixing behaviour. This is done by presenting some calculations obtained from the CHYMES coarse mixing model and comparing them with those recently published by Amarasooriya and Theofanous who used the PM-ALPHA code . These calculations are concerned with the behaviour of mono-sized melt particles poured into saturated water. Melt fragmentation is not modelled. This comparison shows that both of the models predict very similar melt volume fraction and velocity fields, and that much of the melt is in a region where the continuous fluid phase is the vapour. The calculations show how the differences in the chosen modelling of interfacial drag and heat transfer affect the predictions. It is shown that the choice of modelling in both of these areas changes the detailed predictions for the water and steam fields significantly.