Differences for Cooling Capacity of a Cable Tunnel System: Install One or Three Additional Cable Troughs with Cable Circuit for 100/70 Heat Loads – A Numerical Study
Authors: . Steven , Xiaobing Chen 
Published: January 2025

The purpose of this report was to compare the numerical analysis for different cooling efficiency of a cable tunnel
system when installing one additional cable trough or three cable troughs for cable circuit 100% or 70% heat
load. Cable tunnel system is widely known for it is use for high voltage transmission in an underground. Due to
the cables generating large quantities of heat, water pipes and air ventilation were used in the tunnel to cool down
the transmission cable. To investigate the cooling efficiency of a tunnel, research for benchmark case was carried
out. A Computational Fluid Dynamic (CFD) benchmark analysis about 2D natural convection inside the square
cavity was investigated to further understand the air movement inside the square medium. Ansys Fluent was
carried out for benchmark cases of heat transfer natural convection inside the square cavity and to check whether
Ansys Fluent can be used for any CFD and heat transfer purposes by validating the results with previous studies.
The benchmark case used a 2D square with different active temperature side walls and adiabatic walls for top
and bottom side. From the benchmark simulations with different Rayleigh Number, a number representing
buoyancy-driven of a fluid from 103
to 105 affected air differently and the heat transfer natural convection inside
the square. At different Rayleigh Number air forms a recirculation passing the active walls and adiabatic walls.
After investigated the benchmark, the results were used to validate with previous studies. To validate the results
was by using Nusselt Number, ratio of convective to conductive. The benchmark case results were validated and
Ansys Fluent proven can be used for CFD and heat transfer purposes. Two 3D cable tunnel were modelled with
100-meter length after concluded the benchmark. The two tunnel models were based on the number of cable
troughs installed. The first tunnel was with one cable trough and two cable circuits on the bottom, as for the
second tunnel was with one bottom cable circuit and three cable troughs. Cable tunnel simulations were based
on cable circuit for 100% heat load and 70% heat load with each heat load case was given different water flow
rate of 4.25 L/s and 6.5 L/s. A total of eight scenario cases were carried out to determine which provided a better
cooling efficiency choice. The analysis for cable tunnel cases on how the copper cables cooled down through heat
transfer of natural convection inside the cable trough, which the heat then was removed from the cable circuit by
air and continue to the outside of the tunnel. During the heat transfer processes, conductivity was present as not
all heat were removed by air and water, but continue to through the walls of the tunnel and to the ground soil.
Based on the simulations, a cable tunnel with three cable troughs was proven more cooling efficiency compared
tunnel with one cable trough. Some scenarios did not meet the criteria, due to insufficient heat relocation by air and water. The cooling efficiency was determined by which has the better balance heat relocation by air, water, and ground, also considering the comfort and safety of human inside the tunnel.