A novel correlation for azimuthal and longitudinal distributions of heat transfer coefficients in developing horizontal pipe flow under transitional mixed convection

Developing flow of FC-72 in a uniformly heated horizontal cylindrical pipe was studied in the regime of transitional mixed convection. Heat transfer coefficients were measured at six cross sections along the heated length, with 60 azimuthal positions being monitored at each section. Tests were conducted at six different flow rates and three imposed heat fluxes. A significantly different thermo-fluid-dynamic behaviour was observed between top and bottom of the pipe. A single Nusselt number correlation was proposed for the entire heat transfer surface, best fitting the 5760 experimental points with only seven parameters. The conceptual evolution to the final expression of the correlation is shown. The average deviation from the measurements is 3%, with a maximum deviation of 17.5%. The consistency shown by the proposed correlation suggests it can be used to increase the design accuracy of heat exchangers and heat sinks that work in the explored range of dimensionless parameters.