Effect of coolant viscosity on natural convection film boiling

A range of small-scale experiments performed over the last decade have suggested that vapour explosions can be prevented by increasing the coolant viscosity. Whilst various reasons have been suggested, no sound theory exists. As a first step towards developing one, a study of film boiling has been performed. A natural convection film boiling model, based on an integral solution of the boundary layer equations, has been developed. A careful analysis of the assumptions used in its formulation was made to understand its range of validity. Results are compared with those of Naylor, who validated the model against experimental heat flux data. The effect of viscosity is examined and it is found that a tenfold increase in the viscosity has only a modest effect on the vapour film thickness, increasing it by approx. 50% for the case studied. Small-scale experiments show that very large changes in the viscosity are required to have a stabilizing effect. From this study, the effect of coolant viscosity seems to be small unless additives are introduced in such significant proportions that they change the entire system.