Heat transfer characteristics of an 180° bend pipe of different cross sections using nano-enhanced ionic liquids (NEILs)

Abstract

The fluid flow and heat transfer characteristics of ionic liquids (ILs) and nano-enhanced ionic liquids (NEILs) (i.e., alumina-based [C4mim][NTf2] and [C4mpyrr][NTf2]) through circular and non-circular (rectangular) U-bends have been numerically investigated using finite volume method of ANSYS R 16. Extensive numerical simulations have been performed to study the effects of different pertinent parameters on the heat transfer and fluid flow behaviors. Two different heat transfer correlations have been developed using the Levenberg–Marquest (L–M) method of nonlinear regression analysis by varying the influencing parameters. The Reynolds number ( $$\text{Re}$$ ), weight fraction of nanoparticles ( $$\phi$$ ) and aspect ratio (AR) (only for non-circular U-bend) have been varied in the ranges of 500–2000, 0–2.5% and 0.5–3.0, respectively. It has been observed that the Nusselt number increases with Reynolds number, weight fraction of nanoparticle dispersed in the ILs and the aspect ratio of the non-circular duct. We observe a higher heat transfer rate for the NEILs as compared to the ILs. For a particular type of NEIL and Reynolds number, we also obtain an optimum aspect ratio for a non-circular duct. The fluid flow features of the U-bend revealed two counter-rotating vortices in Y-Z planephysical properties, and quite frequently used in different at a curve angle of 90°. As the aspect ratio (for non-circular U-bend) increases up to 1.5, these counter-rotating vortices move toward the inner wall of the bend to intensify flow recirculation enhancing the heat transfer rate. We found the highest heat transfer rate for a rectangular U-bend (AR = 1.5) as compared to a square as well as a circular U-bend.