Numerical investigation on heat transfer enhancement by integrally-molded double-sided spiral finned tubes for waste heat recovery

Abstract

An integrally-molded double-sided spiral finned (IDSF) tube has been proposed in this paper to improve the thermal-hydraulic performance of the tubular air preheater due to the lower heat transfer efficiency, which limits its application in large units above 350 MW during the deep peaking. The physical model of IDSF tubes was established, and numerical simulations of the internal and external flow and heat transfer performance under different structural parameters were conducted. The simulation results show that adding spiral fins inside the tube will decrease the thermal-hydraulic performance inside the tube while adding spiral fins outside the tube with reasonable fin structure and arrangement parameters can increase the thermal-hydraulic performance outside the tube, and thus double-sided thermal-hydraulic performance evaluation factors have been proposed to evaluate the operation economy and compactness of air preheaters based on the Nu, j and f factors inside and outside the tube. Results show that the fin inside the tube has a significant effect on the pressure drop when the fin pitch is smaller than 12 mm, and the tubes with the fin heights of 1.5 mm and 2.0 mm exhibit excellent thermal-hydraulic performance at the fin pitch of 12 mm. The f of the tube with the fin height of 1.5 mm is lower than that of the tube with the fin height of 2.0 mm, and the f of the tube with the fin height of 2.0 mm is about 1.28 times that of the fin height of 1.5 mm. The volume of the tubular air preheater can be reduced by a maximum of 43.1 %∼46.0 %, while the power consumption increases by 14.7 %∼17.6 % compared to the smooth tube. The power consumption of the tubular air preheater can be reduced by a maximum of 19 %∼21 % and its volume reduced by 15.7 %∼16.7 %. This work provides essential theoretical guidance and technical support for the design and application of air preheaters.