Fluids play a crucial role in industrial processes like cooling, heating, and mixing. Traditionally, most industries would utilize Newtonian fluids—which have a constant viscosity—for such processes. However, many are now adopting viscoelastic fluids, which can behave as both liquids and elastic materials.

These fluids can suppress turbulence in simple flows like straight pipes or channels, leading to reduced wall friction. This “drag reduction effect” has attracted significant interest due to its potential to enhance energy efficiency.

To advance the industrial applications of such fluids, it is critical to understand how these fluids interact with turbulence.

Against this backdrop, Associate Professor Shumpei Hara from the Faculty of Science and Engineering, Doshisha University, Japan, along with Professor Takahiro Tsukahara and Emeritus Professor Yasuo Kawaguchi from Tokyo University of Science, Japan, conducted experiments on viscoelastic fluid flow through a backward-facing step (BFS) to evaluate the fluid dynamics governing turbulent motions.

This study was made available online in the International Journal of Heat and Mass Transfer.

“While fluid motion has a characteristic time scale for recovery and relaxation, viscoelastic fluids have a relaxation time and a variety of phenomena may occur depending on the relationship between these two-time scales. Our primary motive was to clarify the instability and uncover the fundamental characteristics of BFS flow in viscoelastic fluids through experiments,” says Dr. Hara.

The team conducted an experiment in a closed-circuit water loop with a two-dimensional channel with a height of 20 mm and a BFS expansion ratio of 1:2.

In addition, they employed particle image velocimetry and a capillary breakup extensional rheometer to track the flow of a surfactant-enhanced viscoelastic fluid and to measure the relaxation time of the viscoelastic fluids, respectively. Additionally, T-type thermocouples were used to measure heat transfer.

In a BFS flow, turbulence results in a separated shear layer, which is highly unstable due to the hydrodynamic instabilities. These instabilities generate turbulent eddies, producing turbulent kinetic energy as the flow attempts to recover its equilibrium.

However, when viscoelastic fluids are introduced, their unique relaxation time interacts with the natural recovery process of the flow. This interaction led to unexpected fluid behaviors, such as the inertia-viscoelastic meandering motion.

By adjusting the Reynolds number (through flow rate) and Weissenberg number (fluid elasticity), the researchers identified three distinct flow states: low, middle, and high diffusivity states. In the low diffusivity state, the fluid exhibited a high-speed flow without any turbulence or mixing, resulting in a low Reynolds shear stress (which represents how momentum is transferred due to turbulence). Moreover, it had a poor heat transfer rate.

In the middle diffusivity state, the fluid exhibited similar turbulence levels to that of Newtonian fluids, like water, with a moderate Reynolds shear stress and heat transfer. Notably, the observations for high diffusivity states were exceptional. In this state, the main flow of fluid exhibited a wavelike meandering motion, oscillating only vertically, perpendicular to the wall, which significantly boosted the heat transfer efficiency.

The high-diffusivity state induced by the meandering motion significantly enhanced fluid mixing, reducing temperature differences and improving momentum transfer. These effects make this approach highly suitable for industrial applications that demand efficient heat exchange and fluid transport.

This remarkable discovery has potential applications in heat exchangers, chemical reactors, and agitators in the chemical, food, and pharmaceutical industries. Looking ahead, the researchers plan on investigating different viscoelastic fluids to understand their behavior in real-world industrial settings and optimize their properties for increasing energy efficiency.

“Our study paves the way for the development of new turbulence control strategies with energy-saving effects using viscoelastic fluids, contributing to heat transfer and mixing phenomena in manufacturing processes, further improving quality and ensuring its assurance,” concludes Dr. Hara.