Effect of mixed convection on the external thermal resistance of single-core and multicore bundled cables in air

The external thermal resistance of single-core and multicore bundled cables exposed to wind is normally calculated from the forced convective heat transfer coefficient and the radiative heat transfer coefficient. However, when the wind velocity is low, the additional heat transfer due to natural convection should be taken into account. When the wind velocity is 0.1 m/s, the calculated total heat transfer coefficient based on this so-called mixed convection is up to 30% higher than that based on pure forced convection, hence the external thermal resistance is up to 30% lower than that for pure forced convection. The relationships between this increase, δ, in the total heat transfer coefficient and the wind velocity, the temperature rise of the surface of the cable and the diameter of the cores are explored for single-core, two-core, three-core and four-core bundled cables. The minimum wind velocity required to reduce δ to 1% is given for each type of cable for practical conditions