Abstract
In the present work, we propose a novel enhanced tube with discrete inclined grooves, aiming to enhance heat transfer with minimum consumption of pump power by generating longitudinal swirl flows with multiple vortices in the proposed grooved tube. A numerical study has been conducted to investigate the turbulent flow structures and the effects of geometric parameters on the thermal performance. According to the results, longitudinal swirl flows with multiple vortices are generated in the grooved tube, and the number of induced vortices is proportional to the number of circumferential grooves. The heat transfer and friction factor are enhanced by a factor of approximately 1.23–2.17 and 1.02 to 3.75 over the smooth tube, respectively. To further understand the physical mechanism of the enhanced grooved tube and to assess the effects of geometric parameters, entropy generation analyses have also been performed. The results show that the entropy generation number ratios decrease with increasing the number of circumferential grooves and with the reduced groove pitch ratio, and a number of circumferential grooves of 12 and a groove pitch ratio of 3 are recommended for the proposed grooved tube. In addition, comparisons with previous work show that the proposed grooved tube provides considerably higher overall thermal performance than the transversally grooved tube, internally helical grooved tube and continuous corrugated tube, but lower thermal performance than the discrete corrugated tube at lower Reynolds numbers, indicating that the proposed grooved tube is very promising for heat transfer enhancement in practical applications.
1. Introduction
Heat exchangers, which ensure heat transfer from one medium to another, have wide applications ranging from petroleum, chemical, power generation industries and domestic uses. Enhancement in a heat exchanger's overall performance contributes to reducing the size of heat exchangers and make material and energy savings related to the heat exchange processes [1]. In practical applications, it is usually necessary to develop efficient enhanced heat transfer tubes to improve the efficiency of a heat exchanger, due to the fact that a heat exchanger's overall performance is determined by the heat transfer in the tubes which are the basic heat transfer units for a heat exchanger. Tube inserts like twisted tapes and coiled wires or artificial roughness elements such as ribs, grooves and corrugations on the surface are commonly used heat transfer enhancement techniques for tubes. Among these techniques available, artificial grooved tubes show better overall thermal performance and are widely used in modern heat exchangers, because they are very effective in heat transfer augmentation [2].
5. Conclusions
Turbulent flow and heat transfer in a circular tube with discrete inclined grooves were numerically investigated. Turbulent flow and heat transfer details, and effects of geometric parameters on the heat transfer performance were presented and analyzed. Entropy generation analyses were performed to further understand the physical mechanism of the enhanced grooved tube. Comparison between the proposed grooved tube and previous enhanced tubes was also conducted. Based on the numerical investigation, following conclusions can be drawn:
(1) Longitudinal swirl flows with multiple vortices are induced in the grooved tube. The flow pattern results in a long flow path and relatively intense flow mixing between the wall and the core flow regions. The temperature is more evenly distributed due to better flow mixing induced by the longitudinal swirl flows.
