Improvement of tube heat exchanger performance with perforated ring inserts, tube rotation and using nanofluids

Abstract

Many engineering applications, such as waste heat recovery, air-conditioning and refrigeration systems, include heat exchangers. In this work, the performance of a double-tube heat exchanger with a rotating tube, perforated ring inserts and nanofluids as working fluids has been studied. After developing a 3D numerical model, verifying its discretization and validating it with experimental results, an optimal design for maximizing heat transfer while keeping a relatively low pressure drop was found (11 eight-holed rings with a 2.2 pitch ratio). Increasing the cold fluid Reynolds number to 4946 and the inner tube rotational speed to 500 rpm increased the heat transfer coefficient from around 7500 to 9500 W (m2 K)−1. Considering the nanofluids studied, the best performance was found with Cu nanoparticles, followed by Al2O3–Cu and Al2O3. With Cu nanoparticles at 3%, heat transfer coefficients above 12,100 W (m2 K)−1 were obtained, increasing heat exchanger effectiveness from 27 to 51%. Pressure drop levels increased up to 235 Pa, resulting in increasing pumping requirements by around 0.1 kJ kg−1. Hence, only very high flowrates would represent a problem when using the exchanger. Explanations for the underlying physical phenomena that cause the enhancement of heat transfer due to the rotational speed, the perforated rings and the nanofluids were provided. Turbulent kinetic energy contours, flow streamlines, and temperature contours were used to gain insight into the thermal and flow fields, identifying the mechanisms responsible for the enhancement of the heat exchanger effectiveness.