Force changes from heating

It should be realized that the Leidenfrost superheat, A7 LDF = (TLDF - Tsat), is a function not only of pressure but also of droplet size, flow conditions, and force fields. Furthermore, experimental results obtained by Berger (Drew Mueller, 1937) for stagnant ether droplets falling on a horizontal, heated surface indicated a possible effect of surface material and roughness, as the minimum surface temperature necessary for the spheroidal state changes from 226°F (108°C) on a smooth surface of zinc to 240°F (116°C) on that of a rough surface, and from 260°F (127°C) on a smooth surface of iron to 284°F (140°C) on that of a rough surface. 

In all flows involving heat transfer and, therefore, temperature changes, the buoyancy forces arising from the gravitational field will, of course, exist. The term forced convection is only applied to flows in which the effects of these buoyancy forces are negligible. In some flows in which a forced velocity exists, the effects of these buoyancy forces will, however, not be negligible and such flows are termed combined- or mixed free and forced convective flows. The various types of convective heat transfer are illustrated in Fig. 1.5. 

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