| Abstract |
For a description and analysis of the flow we consider the conservation equations of the two phases separately, but in thermal and mechanical equilibrium, coupled by the interface shear forces (two fluid model, drift flux model 1. Coupling may be weak or strong, depending on Froude and Mach numbers of the flow. The fluid is highly compressible, not because the individual phases move at such speeds that their in d i v i dual density changes are significant but because evaporation (phase change) results in large density changes of the system at moderate pressure or temperature changes once flashing occurs. The slip between the phases is caused by unequal wall shear stress, acceleration of the fluid or gravitational forces and is hindered by the inter face interaction. If we denote by y the ratio of the liquid density to the vapor density and by the ratio of the vapor speed to the liquid speed we find that in horizontal flows u = yl" yields the maximum slip ( neglecting acceleration effects ) that can be reached with no i nter face force acting (assuming equal friction coefficients for both phases at the wall ). If one investigates the conditions of thermodynamic flow similarity between different substances in two phase flow, one finds that the latent heat of vaporization is the principal controlling parameter. Thus, a 5cm diameter test section in two phase R-114, at room temperature, corresponds to a 30cm diameter duct in water-steam at boiling cond i t ions at high temperatures such as encountered in geothermal and other power production systems. |