Author(s):

Sierra A. Tutwiler, Dillon R. Shaver, & Lane B. Carasik.

Abstract:

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 effects 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 performed a Buckingham-Pi analysis that presented the cross-sectional aspect ratio (Ra) and Prandtl number (Pr) as Pi groups for twisted elliptical tube bundles. Using large eddy simulations of twisted elliptical tube bundle unit cells, we confirmed the importance of both Pi 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.

Full Article:

https://www.sciencedirect.com/science/article/pii/S0142727X24002194 

Author(s):

Cody S. Wiggins, Arturo Cabral, Adam Mafi, Jerel Houston & Lane B. Carasik.

Abstract:

A combined positron emission particle tracking (PEPT) and X-ray computed tomography (CT) technique is presented, and its utility is demonstrated through investigation of flow in a pipe with twisted tape swirl insert with varying flow conditions (diameter-based Reynolds numbers 16,300–63,300). A description of this technique is given, as well as data handling practices used to relate geometric information captured by CT to fluid flow data gathered via PEPT. It is found that the CT component is readily capable of capturing the stainless steel insert geometry in this present system, but the use of combined plastic and metal materials leads to artifacts in imaging of the plastic surface. Nonetheless, CT data are related to PEPT flow measurements, and average velocity fields are calculated via a pseudo-framing and interpolation scheme and used to visualize and interrogate key flow phenomena within the system. Radial velocity profiles of the mean flow characteristics are seen to collapse to a nearly common form across all flow conditions considered. Helical vortices are seen propagating through the flow field, generated by bypass flow around the gap between the insert and pipe wall, with additional coherent secondary flow structures seen in the higher Reynolds number cases. These findings enhance the understanding of the mixing mechanisms in these swirl flows and encourage the continued development of PEPT-CT methodologies for 3D flow measurements in optically inaccessible systems.

Full Article:

https://link.springer.com/article/10.1007/s00348-024-03860-7