Increasing the stability of high contraction ratio flow of Boger fluids by pre-deformation

In this paper we investigate the stabilizing effect of pre-deforming the flow of a PEO/PEG Boger fluid before entering a microfabricated 83:1:83 planar contraction–expansion. Applying pre-deformation has a dramatic effect on the size and stability of the recirculation zones upstream of the contraction. This pre-deformation is generated by a cylinder placed in front of the contraction. Transverse flow past an unbounded cylinder is a combination of both shear and elongational flow, at the cylinder walls and the downstream wake respectively. The deformation of the polymer by the shear and elongational flow temporarily changes the rheological properties of the solution before it relaxes back to the original state. Deformation rates up to γ˙ of 12000 s−1 are attained in the microfluidic device, resulting in the following ranges of dimensionless numbers for experiments performed: 0.003 < Re < 0.019, 8 < Wi < 60 and El = 2758. Streak images and μ-PIV measurements are used to quantify vortex length and velocity field. The distance between cylinder and contraction is chosen such, that at the lowest flow rate the deformation caused by the cylinder is remembered by the fluid before entering the contraction. Applying pre-deformation gives rise to a new type of vortex evolution which is different from the standard contraction case. Moreover a change in the divergence of streamlines is observed. At low deformation rates (Wi < 16) a distinct type of vortices occur which undergo a transition to lip vortices at higher deformation rates (Wi > 16). Pre-deforming the flow is found to reduce vortex length up to 20% and to reduce the divergence of streamlines and vortex growth with increasing Weissenberg number significantly.

► Boger fluid is pre-deformed before entering a micro contraction–expansion.
► Vortex size and velocity are quantified with streak images and μ-PIV.
► Pre-deformation has a dramatic effect on the size and stability of upstream vortices.