Combined natural and forced convective cooling of underground eletric cables

Experimental results have been obtained for a wide range of physical, flow, and thermal parameters in three models of underground electric cable systems. The results show that the laminar friction factors depend on cable configuration and have an upper limit of f ċ Re = 30. Laminar Nusselt numbers are independent of the skid wire roughness ratio and the Reynolds number but increase with increasing Rayleigh numbers and axial distance from the inlet. The range of laminar Nusselt numbers was 6 to 18. In the turbulent region, both the friction factors and Nusselt numbers are increasing functions of the skid wire roughness ratio. For roughness ratios of 0.0216 and 0.0293, the asymptotic friction factor values were 0.013 and 0.021, respectively. The Nusselt numbers at Re = 3,000 are about 40 and 60 for the above roughness ratios.