The results of an experimental investigation of the local heat transfer in a vertical gas(air)cooled tube with mixed turbulent convection under the conditions of codirected forced and free convection at Re(in) = (0.3-5) x 10(4), buoyancy parameter K(in) = (0.8-337) x 10(-5) and heat flux parameter q(in)+ = (0.35-2.4) x 10(-3) are reported. An analysis of local heat transfer along the tube length is performed at different degrees of the effect of buoyancy forces and different heat fluxes. Characteristic regimes of heat transfer with a monotonous variation of the wall temperature and with non-uniform wall temperatures along the tube length are revealed. Correlations are obtained for calculating local heat transfer along the tube in the case of weak and strong effect of buoyancy forces. To obtain correlations for the intermediate region of the effect of buoyancy forces, further investigations are required.

This paper describes the variation of the local heat transfer coefficients in the case of combined flow in a vertical pipe, in which forced and free convection act in the same direction, but heat load is varied. The distribution of heat transfer coefficients along the pipe proved to vary with the heat load. At high loads these coefficients are unstable along the pipe even when the effect of thermogravitational forces is low. In addition, certain heat transfer modes, which were not observed in the case of constant physical properties of the flow, occur in the range in which the effect of these forces is moderate. The authors found a general similarity of the heat transfer behavior at different heat loads. The experimental data for the entire range of heat loads were interpreted in terms of the relevant parameters over a broad range of the effect of the thermogravitational forces.

Experimental data on the local coefficient of heat transfer in pipe flow of gas in the laminar to turbulent transition range are presented. The experiments were performed at elevated pressure, so that free convection exerted a significant effect on the forced turbulent flow (the free and forced convection acted in the same direction). Under these conditions the coefficient of heat transfer along the pipe varied significantly, so that there were local peaks of well temperature even at rather moderate heat flux levels. Correlations for the minimum heat transfer coefficient and for the position of the first local maximum of the pipe wall temperature are derived.

Experimental results on the local heat transfer coefficients for transition-range pipe flow at Re between 2 {center dot} 10{sup 3} and 3 {center dot} 10{sup 4} and heat flux parameters q{sup +}{sub in} of 0.00035, 0.00127 and 0.00236 are presented. The experimental dta are analyzed and compared with familiar correlating expressions. An equation for calculating critical Reynolds numbers and a technique for determining the heat transfer coefficient for transition from laminar to turbulent flow with correction for the variability of the physical properties of the gaseous flow are suggested.