How turbulent fluids flow unevenly in porous heat exchangers

What is it about?

Fluid flow and heat transfer inside porous materials, such as tube bundles in heat exchangers and cooling systems, exhibit intricate behaviors under turbulent conditions. Using Large Eddy Simulations, my co-author Vishal Srikanth and I investigated fluid movement through an in-line array of circular cylinders within the intermediate-porosity regime, where obstacles are moderately spaced. We discovered a novel secondary flow instability where fluid wakes behind the cylinders fail to shed symmetrically. Instead, the vortex paths sway back and forth unevenly over time. When averaged over time, this asymmetrical oscillation creates distinct high-velocity and low-velocity fluid channels across the pore space, significantly altering local drag and surface heat transfer.

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Photo by Tatyana Dobreva on Unsplash

Photo by Tatyana Dobreva on Unsplash

Why is it important?

Conventional models often assume that fluid moving through a geometrically symmetrical porous matrix remains symmetrical on average, but our findings show that intermediate-porosity media naturally break flow symmetry under turbulent speeds. Identifying this secondary instability closes a major gap in microscale turbulence theory and provides a precise foundation for modeling transport in porous materials. From an applied perspective, these insights enable targeted engineering of high-performance thermal devices. We demonstrated that circular obstacles encourage attached flow and smaller vortices, yielding a 5% increase in heat flux and a 7% reduction in drag compared to square obstacles. This offers engineers a clear pathway to optimize microscopic geometry for superior heat transfer with reduced energy losses.

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