Laminar film condensation heat transfer in closed two-phase thermosyphons

An analysis is presented of the effects of vapor pressure variation and shear stress at the liquid-vapor interface on the local heat transfer in the condenser section of closed two-phase thermosiphons. For the vapor flow the complete two-dimensional Navier-Stokes equations are solved using a finite-difference method. The equations for the liquid film flow are simplified taking into consideration the assumption of Nusselt's theory for laminar film condensation. The results show that the influence of pressure variation and shear stress on the heat transfer depends on the Froude number and the condensation rate. For the example presented here, the exact theory yields local and average Nusselt numbers which differ from those predicted by Nusselt's theory by about -4 to 11% and about 1 to 4%, respectively.