Experimental Study of Salt Melt Heat Transfer in a Vertical Round Channel in Transition and Turbulent Flow Regimes

The article presents the results of experimental studies aimed at determining the dimensionless heat transfer coefficient (the Nusselt criterion) in a round tube during forced upward motion of salt melt. The relevance of this problem is substantiated by the presented review of literature sources, which has shown that there is an insufficient scope of experimental data on heat transfer for the flow of salt melts against the gravitation forces in laminar ant transition regimes in the temperature range 230–350°C. The experiments were carried out in transition and turbulent flow regimes in the range of Reynolds numbers 2600–23 000 with the applied heat fluxes 70–670 kW/m2 and Prandtl numbers 8.9–14.7. A melt of 50% NaNO3–50% KNO3 (in molar fractions) salt was used as working medium. The experimental values of heat transfer coefficients are in good agreement with the data obtained using the well-known relationships, such as the correlations proposed by Petukhov–Kirillov, Sieder–Tate, Gnielinsky, and Hausen. Based on the authors’ own data and an analysis of existing correlations, a new empirical formula is proposed for calculating the Nusselt number in the range of Reynolds numbers from 4000 to 10 000. For the developed turbulent flow regime, the Hausen correlation has shown the best agreement with the experimental values: the maximal mismatch does not exceed 7%, due to which this correlation can be recommended for carrying out calculations in the studied ranges of the parameters.