Comparison of time-resolved heat transfer characteristics between laminar and turbulent convection with unsteady flow temperatures

Experimental investigations have been carried out to compare the time-resolved characteristics between laminar and turbulent heat transfer for steady boundary-layer-type flows with different unsteady flow temperatures. Time-resolved heat transfer coefficients at two locations were measured with two transient heat transfer measurement methods. One is the frequently used transient heat transfer measurement method with an assumption of time-wise constant heat transfer coefficient, and the other is the Cook-Felderman method without such assumption or restriction. Correction methods are applied for the direct measurements of time varying flow temperature and calculated surface heat flux to eliminate the influences of thermocouple thermal inertia effects. The results show that the temporal behavior of heat transfer coefficients is different between laminar and turbulent convection under unsteady flow temperatures. The change of flow temperature in step or multi-step form produces differences in the laminar heat transfer coefficient, while has very weak influence on the turbulent heat transfer coefficient. The heat transfer coefficient in a thermally transient process will be in steady state when the boundary layer is laminar and the flow temperature does not fluctuate with high frequencies and large amplitudes. However, the heat transfer coefficient varies intensely in the same frequency with the sinusoidally varying flow temperature when the boundary layer is turbulent.