Microscopic Flow of Viscoelastic Wormlike Micellar Solutions through Contraction Geometries




Nodoushan, Emad Jafari

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Viscoelastic fluids are commonly used in many engineering applications such as in polymer processing industry, enhanced oil recovery and inkjet printing. Viscoelastic contraction flow represents the most common flow condition in those applications, but the flow is associated with its inherent problems such as additional pressure drops and flow instabilities. It is well known that formation of secondary flows at the upstream corners of the contraction greatly influences the fluidic behavior and promotes the flow instabilities. Therefore, investigating the flow conditions that induce the secondary flow has been an extensive research subject with the ultimate goal of elucidating the kinematics of viscoelastic contraction flows. Several relevant studies concluded that formation of secondary flow is influenced greatly by the elastic stress associated with the viscoelastic flow. Non-dimensional numbers that estimates the ratio of elastic stress to viscous (Wi and De numbers) or inertial (El number) stress has been used to quantify the effect of elasticity on the viscoelastic flow behavior. Even though a general consensus has been established that the higher elastic stress promoted the formation and enhancement of secondary flow, quite different flow behaviors were reported for the different fluids at the same level of elastic stress. Additionally, comparable secondary flows were also observed at different levels of elastic stresses. It should be noted that these non-dimensional numbers could not solely characterize the behavior of viscoelastic contraction flow. In this study, in addition to the role of elastic stress, the contribution of shear dependent rheology and extensional characteristics of the viscoelastic test fluids to the contraction fluidic behavior has been investigated experimentally. Wormlike micellar solutions (WMS) were chosen as the viscoelastic test fluids in virtue of their advantageous characteristics over polymer solutions and their growing applications. The shear and extensional rheology of the test fluids were characterized, and the velocity and shear deformation profiles associated with their contraction flow were quantitatively measured. Secondary flows in the form of upstream corner vortex were observed at sufficiently high elastic stresses where the viscoelastic Mach number (Ma=√DeRe) was larger than the unity. The length of secondary flows was found to be correlated with both the elastic stress (De number) and the ratio of extensional to shear relaxation times (λE/λ). For the test fluids that showed high yield stress and strong shear thinning rheology, a new type of secondary flows with a quasi-static condition (~zero velocity) was observed at the upstream corner of contraction entry. Furthermore, the role of shear stress and shear viscosity in forming the quasi-static secondary flow has been discussed for the first time by matching the shear rates measured from the quantitative analysis of viscoelastic contraction flow to the rheological behaviors of the test fluids.



Wormlike micelles, Viscoelastic solutions, Microfluidics, Rheology, Sudden contraction


Nodoushan, E. J. (2020). <i>Microscopic flow of viscoelastic wormlike micellar solutions through contraction geometries</i> (Unpublished dissertation). Texas State University, San Marcos, Texas.


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