Numerical analysis of a vertical double-pipe single-flow heat exchanger applied in an active cooling system for high-power LED street lights

Abstract The present study examines the use of a vertical double-pipe single-flow heat exchanger as part of an active air cooling system for a 150 W LED street light. The air is circulated inside the lamppost by an internal fan to form a closed-loop system. The heat is dissipated to the surrounding air by natural convection, reaching Rayleigh numbers up to Ra  = 6.5 × 10 10 . Experiments with a 5 m high prototype were conducted, and the data were used to validate the numerical model. The experimental results show that the LED excess temperature can be lowered to about 42 °C. A two-dimensional axisymmetric numerical simulation was performed to study the influence of various parameters, including pipe length, material conductivity, flow direction, pipe diameter ratio, and mass flow rate, on the heat transfer rate. The findings show that the additional heat loss created by extending the lamppost largely depends on the flow rate. When extending the lamppost from 3 to 5 m at a high mass flow rate of 0.014 kg/s, the heat loss increases by 34.1% to 120.2 W. The numerical study was also used to visualize the hydrodynamic boundary layers on the surface of the lamppost and the temperature contours in and outside of the heat exchanger.