Experimental investigation of mixed convection in a horizontal tube for a liquid metal flow

An experimental study of the heat transfer on mixed convection in a turbulent liquid metal flow with azimuthally
inhomogeneous heat flux distribution is presented. A horizontal nickel tube is heated in different sections to
investigate the influence of heat flux orientation relative to gravity on heat transfer in a liquid metal flow. Four
semi-circumferential heat flux distributions (top, bottom, right and bottom right half) are examined. These
configurations emulate the azimuthally inhomogeneous heat flux profile that is characteristic of receiver tubes in
concentrated solar power plants. The working fluid is a near eutectic alloy of gallium, indium, and tin (GaInSn,
Pr = 0.03, ϑ$_{melt}$ = 11°C). The P´eclet number is varied within a range of 2.4 × 10$^2$ < Pe < 3 × 10$^3$, while the Rayleigh number is 1.6 × 10$^3$ < Ra$_q$ < 8.2 × 10$^3$. For all four heat flux distributions, the correlation for the azimuthally averaged Nusselt number 〈Nu〉 can be confirmed. The temperature difference along the circumfer-
ence of the tube reveals mixed convection for heating directions opposing the direction of gravitational accel-
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eration for Pe < 7.2 × 10$^2$. Based on this quotient, the applicability of the Rayleigh number, the Buhr parameter,
the Buoyancy parameter, and the Richardson number as criteria for the determination of mixed convection in
liquid metals is discussed. New correlations for the critical Rayleigh number and limits for the Buhr parameter
are established.