Case 1: Ref ≤ 30; the film remains laminar and the free surface of the film remains wave free.
Case 2: 30 < Ref < 1600; the film remains laminar but the waves and ripples appear on the surface.
Case 3: Ref ≥ 1600; the film becomes turbulent and surface becomes wavy.
The corresponding average heat transfer coefficient can be calculated by the following correlation,
(It is same as eq. 6.10 if Ref is taken at the bottom of the wall.) |
: for Case 1 |
 |
: for Case 2 |
 |
: for Case 3 |
The Nusselt number in case-1 is defined as Modified Nusselt number or condensation number (Co).
The above relations may also be used for condensation inside or outside of a vertical tube if the tube diameter is very large in comparison to condensate film thickness. Moreover, the relations are valid for the tilted surfaces also. If the surface make an angle “θ” from the vertical plane the “g” will be replaced by “g.cosθ” in the above equations