Natural convection simulation inside the underground conduit of an electrical power cable

The heat dissipation process inside a power cable conduit buried underground is characterized by the three-dimensional natural convection between horizontal, highly eccentric cylinders. The surface of the outer cylinder (concrete conduit) is assumed adiabatic, whereas a constant heat flux, representing heat generation from the power cable due to electrical resistance, is specified on the surface of the inner cylinder (power cable). Unlike the vertical configuration of eccentric annuli in which the fully developed thermal boundary conditions may be achievable, the boundary conditions at the open end of the present case are troublesome and are coped with by the zonal grid approach. Two cases with the modified Rayleigh numbers of 10⁵ and 10⁶ are investigated. A numerical study reveals that cable surface temperatures increase toward the outlet plane. It is also found that higher temperatures occur in the region near the bottom of the inner cylinder, where the cable contacts the concrete conduit (outer cylinder), due to small local Rayleigh numbers in which the characteristic length is defined in terms of radial distance between the inner and outer cylinders. As a result, the portion of the power cable near the contacting point with the concrete conduit will deteriorate earlier.