Fluid and thermal flow in annular side of the double twisted helically coiled tubes heat exchangers under various configurations

Abstract In the existing research, the fluid and thermal flow of different configurations of double-twisted, helically coiled tube heat exchangers ( DTHCTHEs ) was presented numerically. The design addresses the faults of a double-smooth helical tube heat exchanger, which uses a twisted tube as a passive heat transfer technique. The twisted surface changes the flow patterns and turbulence and is expected to augment the characteristics of heat transfer. A 3D CFD model was designed and developed via the ANSYS-FLUENT package to predict the thermal performance and fluid flow of different DTHCTHE . The outcomes of key design considerations, such as different twisted cross-sectional profiles (circular, elliptical, triangular, square, pentagonal, and hexagonal) and twisted pitch ratios (6, 9, and ∞), were analyzed. The effects of adding various nanoparticle materials, such as MWCNTs , Al 2 O 3 , and Ag, at a concentration of 1% (by volume) to distilled water are also presented. The simulations were carried out in counter-flow at Reynold number, Re, values ranging from 5500 ≤ Re i ≤ 17,500 in the inner tube and 3000 ≤ Re o ≤ 18,500 in the annulus. The results indicated that U o A o increases by 98.5%, 64.8%, 50.2%, 41%, and 6.2% at the expense of ΔP o by 147%, 88.8%, 60.8%, 46.3%, and 16.3%, respectively, for triangular, square, pentagonal, hexagonal, and elliptical twisted cross-section profiles compared with the circular profile. Compared with ∞, the twisted pitch ratio enhances Nu by 37%, 26.1%, 27.6%, 17.3%, and 29.8% at the expense of increasing f by 121.2%, 118.6%, 51.6%, 73.7%, and 41.6%, respectively, for triangular, square, pentagonal, hexagonal, and elliptical profiles at a τ of 6. The addition of MWCNTs , Al 2 O 3 , and Ag to water increased Nu by 15.5%, 35%, and 18.3%, respectively, whereas the increase in pressure loss was negligible. The maximum η values reach 1.43, 1.48, 1.52, 1.45, and 1.5 for the triangular, square, pentagonal, hexagonal, and elliptical profiles respectively at τ = 6.