Numerical study of the effects of geometric parameters and nanofluid properties on heat transfer and pressure drop in helical tubes

In this research the geometric parameters and nanofluid properties effects on heat transfer and pressure drop in helical tube, by using alumina-water nanofluid as cooling fluid, are numerically investigated. Friction factor and heat transfer coefficient are calculated by considering the effects of nanofluid properties, including nanoparticle diameter, nanofluid temperature, Reynolds number, and volume fraction, on the one hand, and the impact of geometric parameters, including tube diameter, coils diameter and coils pitch, on the other hand. Numerical analysis is performed in the Ansys Fluent 19.2 software using the SST k-omega turbulence model. By increasing the nanofluid volume fraction the heat transfer coefficient and pressure drop in helical coils increase, the same as the nanoparticle diameter reduction. The reduction of nanoparticle diameter causes an enhancement of heat transfer and friction factor, the best results happen in d(p) = 5 nm and phi = 4%, where the it was similar to 40.64% more efficient than base fluid. This amounts for phi = 3%, phi = 2% and phi = 1% are 31.80%, 18.02% and 8.83%, respectively. Finally, the performance evaluation criteria (PEC) is compared for different cases, the maximum value was happen on phi = 4% and d(p) = 5 nm, which it is 1.86 times higher than the base fluid. The results indicate that the thermal efficiency of the heat exchanger improve largely by using helical coils and nanofluids, rather than the base fluid, and direct tubes. In addition, increasing coil pitch and curvature ratio enhance heat transfer and reduce friction factor.