Nusselt’s analysis of film condensation makes the following assumptions:
- The film of the liquid formed flows under the action of gravity.
- The condensate flow is laminar, and the fluid properties are constant.
- The liquid film is in good thermal contact with the cooling surface and therefore, the temperature at the inside of the film is taken equal to the surface temperature Ts. Further, the temperature at the liquid-vapor interface is equal to the saturation temperature Tsat at the prevailing pressure.
- Viscous shear and gravitational forces are assumed to act on the fluid; thus normal viscous force and inertia forces are neglected.
- The shear stress at the liquid-vapor interface is negligible. This means there is no velocity gradient at the liquid-vapor interface.
- The heat transfer across the condensate layer is by pure conduction and temperature distribution is linear.
- The condensing vapor is entirely clean and free from gases, air, and non-condensing impurities.
- Radiation between vapor and liquid film; horizontal component of velocity at any point in the liquid film; and curvature of the film are considered negligibly small.