Effect of temperature-dependent physical properties on heat transfer

7 Effect of temperature-dependent physical properties on heat transfer 

The theoretical treatments considered so far have been based on the assumption that the thermo-physical properties are constant (i.e. independent of temperature and therefore the velocity profiles do not change over the heat transfer section of the tube. Christiansen and Craig [1962] investigated the effect of temperature-dependent power-law viscosity on the mean values of Nusselt number for streamline flow in tubes with constant wall temperature. They postulated that the flow behaviour index, n was constant and that the variation of the consistency coefficient, m, with temperature could be represented by equation (6.45) giving  

The sign and magnitude of the dimensionless parameter, f = ( /R)(l/7 i — l/To) depends upon the direction of heat transfer and 

On the other hand, Forrest and Wilkinson [1973, 1974] modelled the temperature-dependence of the power-law consistancy coefficient as  

The enhancement of heat transfer at a isothermal tube wall due to temperature-dependent viscosity has been correlated by Kwant et al. [1973] as follows  

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