Heat transfer enhancement in helical micro tube heat exchanger using compound technique

Abstract

The performance of heat exchangers can be altered to perform specific heat-transfer tasks via heat transfer augmentation techniques. These techniques can be divided into two groups that are active and passive. This study involved numerical and experimental investigation of heat transfer enhancement for laminar convective flow using a new type of tube insert, namely a helical microcoil in a heat exchanger with nanofluid. The research used microtube with 1.5 mm diameter, and three types of helical coil, namely circle, oval and elliptical with different diameters and pitches ((10 mm, 14 mm, and 18 mm Different types of nanofluids, Al2O3, CuO, SiO2 and ZnO, were used as work fluids with different nanoparticle diameters (25, 50 and 75 nm) and different volume fractions (1%, 1.5%, and 2%). Water was used as the base fluid. The investigation covered Reynolds number in the range of 200 to 1800. Based on numerical simulation results, for all helical microcoil configurations and different types of nanofluids, experimental work was then developed to determine the optimum geometrical structures and parameters, empirical correlations of Nusselt Number and to formulate the frictional factor. A computational model using the ANSYS-FLUENT 18.0 was developed and evaluated against the experimental findings. It showed that helical microtube curvature swirls were an essential phenomenon to increase heat transfer. It also showed that the heat transfer and friction loss increased as volume fractions of nanofluids and Reynolds number increased while nanoparticle diameter decreased. The numerical and experimental results were compared, where the results showed a good agreement for different parameters. The conclusion is that compared to the straight coil, the helical microtube employment resulted in a clear augmentation in heat transfer with a certain increase in pressure drop. The highest value of the average Nusselt number ratio was 1.35 at Re = 1800 for circle shape with water as coolant. The maximum enhancement in heat transfer using nanofluids was around 11% compared to traditional fluid (water). The best thermal performance factor was 3.15, which was achieved using Al2O3 nanofluid with a volume fraction of 2% at pitch and diameter of 18 mm, and the Reynolds number was 1800.