Simplified physical models and analyses were presented in order to describe heat transfer and breakdown of subcooled liquid films falling down the outside of a vertical tube. On the basis of the analogy of heat transfer in the boundary layer

Nusselt number correlations for convection heat transfer of normal and subnormal films (the latter are films approaching breakdown)were obtained by fitting the test data. The film breakdown patterns were observed in our experiments. The thermocapillary force and the flow regimes seemed to play a dominant role in the film breakdown. Simplified physical models based on the analogy of flooding mechanisms in annular two-phase flow were proposed to describe critical phenomena of film breakdown and some important dimensionless groups could be derived. Ma

Nu; was defined as the breakdown criterion number Bd. Its correlations for temporary and permanent breakdown were obtained by fitting the test data. As a first approximation

the simplified physical models are reasonable and the correlations can be used for predicting breakdown conditions. The experiments were performed under the following parameters: r = 0.207-1.181kg/(m·s)

Tin = 19-37℃

and the maximum heat flux was up to 8.67 × 104W/m2.