Session: 13-02 Heat Transfer Testing & Instrumentation

Submission Number: 175585

Numerical Investigation of Turbulent Heat Transfer Over Rough Surfaces at Medium–High Prandtl Numbers 

Despite more than a century of research on turbulent flow in rough channels, our understanding of this subject remains incomplete. In particular, there is still a significant knowledge gap regarding heat transfer in rough channels. Recent studies have examined heat transfer in rough channels at low Prandtl numbers; however, the challenges posed by Computational Fluid Dynamics (CFD) in such flows have led to the neglect of several important aspects. Irregular roughness is widespread in many heat exchangers and cooling channels, especially those manufactured through additive techniques. Many of these devices operate with water, corresponding to a medium-to-high Prandtl number range that depends on temperature.

This research focuses on Large Eddy Simulations (LES) of heat transfer in rough channels with irregular roughness at medium-to-high Prandtl numbers. The irregular roughness was artificially generated using a numerical method in MATLAB. The rough tube geometry was then constructed from the STereoLithography (STL) surface file using open-source Fortran code. High-fidelity, wall-resolved LES with fully resolved roughness was employed to solve the three-dimensional Navier–Stokes equations within the rough channel. Simulations were carried out at an average flow Reynolds number of 11,700 and Prandtl numbers of 0.5, 1, 2, and 5.

To better understand the effect of wall roughness on momentum and heat transfer mechanisms, mean temperature and velocity profiles, as well as heat fluxes, are presented. The wall-normal Reynolds stress and heat flux decrease for larger wall roughness heights (Ra), while their magnitudes remain similar across different Ra. The evaluation of the thermal performance factor highlights the enhancement in heat transfer achieved by surface roughness.

Furthermore, analysis of the probability density functions of the instantaneous Stanton number reveals that recirculation zones, induced by adverse pressure gradients, strongly influence heat transfer. This finding is important, as it shows that wall-scaled mean temperature profiles are of larger magnitude than mean velocity profiles both inside and outside the roughness layer. Consequently, the temperature wall-roughness function differs from the momentum wall-roughness function. Finally, the impact of the Prandtl number on average flow profiles and second-order turbulent fluxes is examined, and outer-layer similarity for the temperature profile is investigated.

Presenting Author: Himani Garg Department of Energy Sciences, Lund University

Presenting Author Biography: Dr. Himani Garg is an Assistant Professor of Heat Transfer at the Department of Energy Sciences, Lund University, Sweden. She obtained her PhD in Fluid Mechanics from Université de Lille, France, in 2019. Her research expertise lies in turbulent flows, heat transfer, multiphase systems, and computational fluid dynamics, with a strong focus on additively manufactured surfaces and advanced cooling strategies. She has extensive postdoctoral experience in France, Japan, and Sweden, and has been awarded several competitive grants, including Hains Stiftelse, Crafoord and Kempestiftelserna. Dr. Garg has authored numerous peer-reviewed articles in journals such as Physical Review E, Physics of Fluids, International Journal of Thermofluids, International Journal of Heat and Mass Transfer, International Journal of Hydrogen Energy. She is also an active reviewer for leading international journals in fluid mechanics and heat transfer.

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