96 Influence of Variable Properties and Increasing Flow Rate with Time on Convection Heat Transfer in a Tube

Turbulent convection heat transfer with monotonously time-increasing mass flux in a tube with consideration of variable thermophysical properties was investigated numerically. The turbulent heat transfer model was based on the equation of eddy shear stress and for unsteady process. The numerical calculations were compared with experimental data. The calculation results confirm that when the flow rate continuously increasing with time, considering either the variable properties or constant properties, one can identify three major cases of variation of turbulent transfer with time, namely, quasi-steady regime, regime of frozen turbulence, and intermediate regime. The limit between the regimes depends on relationship of characteristic time of variation of flow and turbulent transfer. It is shown that the basic features of the processes discussed in the present paper are the same as for the fluids with constant properties investigated earlier. However variable thermophysical properties increase the transient effects especially at high temperature factors.