Numerical study on thermal performances of bare, circular and rectangular finned pipes for road heating

Abstract

The hydronic road heating system based on embedded finned pipes is a new application that can improve the thermal performances of heat conduction in concrete. When the embedded pipe layouts were studied and determined, the thermal performances of three embedded pipe geometries (bare, circular finned and rectangular finned) were detailed. The variables including supplied fluid temperature and flow velocity were used for different pipe geometries. The results indicated that the pipe layout of 80 mm depth and 200 mm spacing was the most suitable one for heating road among all the pipe layouts (depths of 80, 120 and 150 mm and spacings of 200, 300 and 400 mm). This was because it had the highest average heating rate (2.77 K h−1) for a heating period of 6 h with a supplied fluid temperature of 308.15 K and a flow velocity of 0.9 m s−1. Rectangular finned pipe showed the highest thermal performance among the three embedded pipe geometries, due to its shortest preheating time (1.2 h) and highest preheating rate (6.67 K h−1) at the same conditions. When the road was preheated from the initial road surface temperature (265.15 K) to the critical temperature (273.15 K), the preheating time difference between rectangular finned and bare pipes decreased from 4.81 to 0.66 h when the supplied fluid temperature was increased from 298.15 to 318.15 K, and it decreased from 1.61 to 0.94 h when the flow velocity was increased from 0.05 to 0.9 m s−1. It was also observed that the flow velocity had slight effects on the heating performances of the three pipes when it was higher than 0.2 m s−1.