FORCED CONVECTION HEAT TRANSFER TO AIR/WATER VAPOR MIXTURES

SUMMARY The applicability of forced convection heat transfer data and empirical correlations based on experiments with dry or nearly dry air to situations involving airlwater vapor mixtures (humid air) is consid- ered. The particular application which motivated the present study is the contingency cooling of helicopter gas turbine engine components using evaporatively cooled (and therefore humid) air. Heat transfer co- efficients were measured using both dry and humid air in the same forced convection cooling scheme and were compared using appropriate nondimen- sional parameters (Nusselt, Prandtl and Reynold's numbers). A forced convection scheme with a complex flow field, two-dimensional arrays of circular jets with crossflow, was utilized with humidity ratios (mass ratio of water vapor to air) up to 0.23. Results of a survey of the literature regarding the dynamic viscosity, thermal conductivity and specific heat of air, steam and airlsteam mixtures are reported. Methods for the determination of gaseous mixture properties from the properties of their pure components are reviewed in detail. Convenient methods for the determination of these properties with good confidence are described and the need for more experimentally determined property data for humid air is discussed.