Influence of cross-sectional aspect ratio and Prandtl numbers on pressure drop and heat transfer characteristics in twisted elliptical tube bundles

• Published in: The International journal of heat and fluid flow 2024-10, Vol.109, p.109494, Article 109494
• Main Authors: Tutwiler, Sierra A., Shaver, Dillon R., Carasik, Lane B.
• Format: Article
• Language: English
• Subjects: Cross-sectional aspect ratio; Heat transfer and flow characteristics; LES; Prandtl number influence; Twisted elliptical tubes
• ISSN: 0142-727X
• DOI: 10.1016/j.ijheatfluidflow.2024.109494

With the push for clean energy systems such as nuclear reactors, fusion energy systems, and concentrated solar power, novel heat exchanger designs that incorporate the use of heat transfer enhancements (HTEs) are being investigated to optimize performance. One particular heat transfer enhancement is the twisted elliptical tube used to modify standard shell and tube heat exchangers in a novel manner. The twisting geometry of the tube causes increased mixing of the fluid, inducing turbulence at lower Reynolds numbers and allowing for improved heat transfer characteristics. This occurs with marginal to major increases in the frictional pressure drop for higher Prandtl number fluids (i.e. molten salts and heat transfer oils) and lower Prandtl fluids (e.g. gases and liquid metals) intended for advanced power conversion systems. Currently, the existing twisted elliptical tube Nusselt number and friction factor correlations only include the dependence of Reynolds number, Prandtl number, and the modified Froude number or swirl number while not reporting a Prandtl number range. The modified Froude number has been used previously to characterize the affects of the tube diameter and torsional pitch on heat transfer. However, we have found evidence that those two geometric characteristics should be treated separately from one another. In this study, we preformed a Buckingham-Π analysis that presented the cross-sectional aspect ratio (RA) and Prandtl number (Pr) as Π groups for twisted elliptical tube bundles. Using large eddy simulations of twisted elliptical tube bundle unit cells, we confirmed the importance of both Π groups on Nusselt number and friction factor. As we increased the Prandtl number from 0.001 to 1, we observed a corresponding non-linear increase that was not able to be collapsed to one exponential fit. From our findings, we suggest two different functional forms to account for Prandtl number. When linearly increasing the aspect ratio, a non-linear increase in both the Nusselt number and friction factor was observed that is not captured by existing correlations. For the aspect ratio, we observed that the Nusselt number had a logarithmic dependence that was independent of the Prandtl number. •Investigated cross-sectional aspect ratio and Prandtl number dependency.•Observed increase in Nusselt number when increasing the tube aspect ratio.•Observed increase in Nusselt number when increasing the Prandtl number of the fluid.•Separate trends for Nusselt num