STUDY ON LAMINAR-FILM CONDENSATION OF SATURATED STEAM ON A VERTICAL FLAT-PLATE FOR CONSIDERATION OF VARIOUS PHYSICAL FACTORS INCLUDING VARIABLE THERMOPHYSICAL PROPERTIES

The extended theory of the steady state laminar film condensation proc ess of pure saturated vapour at atmospheric pressure on an isothermal vertical flat plate is established. Its equations provide a complete a ccount of the physical process for consideration of various physical f actors including variable thermophysical properties, except for surfac e tension at the liquid-vapour film interface. First, similarity consi derations are proposed to transform the governing system of partial di fferential equations and its boundary conditions into the correspondin g dimensionless system. Then, the dimensionless new system is computed numerically in two steps: First neglecting shear force at the interfa ce, so that the initial values of the boundary conditions W(xl,s) and W(yl,s) are obtained. Then, the calculations of a problem of the three -point boundary-value for coupling the equations of liquid film with t hose of vapour film are carried out. Furthermore, the correlations for heat transfer coefficient and mass flow rate are proposed by analysis of heat and mass transfer and it is found that the heat transfer coef ficient is function of dimensionless temperature gradient (dtheta/deta (l))eta(l) = 0, and that the condensate mass flow rate is function of the mass flow rate parameter (eta(ldelta)W(xl,s)-4W(yl,s)) of liquid. In addition, the corresponding heat and mass transfer correlations exp ressed by subcooled temperature DELTAt are developed. According to Nus selt's theory four different assumptions are set up for an investigati on of the effects of the film condensation of saturated vapour, so tha t the validity of Nusselt's theory can be further clarified. Quantitat ive comparisons from the results of the heat transfer coefficient and mass flow rate of the condensate indicate that the effect of variable thermophysical properties on the heat and mass transfer is appreciable . The effect of thermal convection in the condensate film is obviously larger than those of shear force at liquid-vapour interface, and the effect of the inertia in the condensate film is very small. Finally, i t is also shown that Nusselt's theory, in using Drew reference tempera ture, will decrease the heat transfer coefficient by at most 5.11%, an d will increase the mass flow rate of the condensate by at most 2.45%, provided that the effect of the surface tension is not taken into acc ount.