A Comprehensive numerical study on using lobed cross-sections in spiral heat exchanger: Fluid flow and heat transfer analysis

Abstract

The present study aims to assess the thermohydraulic characteristics of smooth tri-lobed spiral tube (STST) and twisted tri-lobed spiral tube (TTST) numerically. Water with constant thermophysical properties is considered as the working fluid. The Reynolds number, based on the inlet conditions, varies from 500 to 2500 and a constant temperature of 323.15 K is regarded as the boundary condition of the tube wall. Temperature and flow field inside the tube are simulated utilizing a three-dimensional, stationary finite volume method (FVM) and a computational grid consisting of polyhedral elements. The influence of cross-section rotation for the STST and the effects of twist number, pitch ratio, and different cross-sections for the TTST are evaluated on the thermohydraulic parameters of the heat exchanger. According to the results, the optimized geometric configuration for STST in order to have the best performance occurs when the cross-section of the tube inlet has a rotation angle of −30°, resulting in 11.14% increase in the performance evaluation criteria (PEC) parameter. According to the findings for TTST, the Nusselt number (Nu) and friction factor (f) increase continuously by increasing the number of tube twists. Furthermore, from the PEC point of view, the TTST with a low pitch ratio close to 1 exhibits the best findings. Comparing the optimized results of STST and TTST indicates that the highest value of PEC (1.25) is achieved for TTST with a pitch ratio of 1.25 and a twist of 13 turns, which occurs at the Reynolds number of 1000.