Elastic Turbulence of Aqueous Polymer Solution in Multi-Stream Micro-Channel Flow.

Yee Cheong Lam,Jiayan Tai

doi:10.3390/mi10020110

Abstract

Viscous liquid flow in micro-channels is typically laminar because of the low Reynolds number constraint. However, by introducing elasticity into the fluids, the flow behavior could change drastically to become turbulent; this elasticity can be realized by dissolving small quantities of polymer molecules into an aqueous solvent. Our recent investigation has directly visualized the extension and relaxation of these polymer molecules in an aqueous solution. This elastic-driven phenomenon is known as ‘elastic turbulence’. Hitherto, existing studies on elastic flow instability are mostly limited to single-stream flows, and a comprehensive statistical analysis of a multi-stream elastic turbulent micro-channel flow is needed to provide additional physical understanding. Here, we investigate the flow field characteristics of elastic turbulence in a 3-stream contraction-expansion micro-channel flow. By applying statistical analyses and flow visualization tools, we show that the flow field bares many similarities to that of inertia-driven turbulence. More interestingly, we observed regions with two different types of power-law dependence in the velocity power spectra at high frequencies. This is a typical characteristic of two-dimensional turbulence and has hitherto not been reported for elastic turbulent micro-channel flows.

Highlights

In fluid dynamics, turbulence is a phenomenon that is associated with highly disordered motion and is conventionally recognized as an inertia-driven phenomenon that occurs at high Reynolds number (Re > 2300 in pipe flows) i.e., high-Re turbulence
Ubulk is defined as Ubulk = Q/wd, where w refers to the channel width (i.e., 1 mm)
The observed peak is due to incomplete relaxation of the extensional stresses that have been stored in the polymer molecules, which had been highly stretched as they flowed past the contraction

Summary

Introduction

Turbulence is a phenomenon that is associated with highly disordered motion and is conventionally recognized as an inertia-driven phenomenon that occurs at high Reynolds number (Re > 2300 in pipe flows) i.e., high-Re turbulence. When an elastic liquid is subjected to high shear and extensional rates in micro-channels, the polymer molecules become stretched and cause a buildup of elastic stresses within the bulk liquid. The elastic forces (i.e., due to stress accumulation and subsequent relaxation) become sufficiently amplified to dominate over inertia and viscous forces, resulting in the onset of flow instabilities. This phenomenon is termed as ‘elastic turbulence’ and was first introduced in the 1960s to describe polymer melt fracture [2], and later on in the 1970s to illustrate instabilities in Couette flows [3]. The phenomenon has been greatly exploited in the last decade to enhance mixing in micro-channels [12,15,18,23,24], which is usually difficult due to the low-Re restriction

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Results

Conclusion