Reports: DNI953312-DNI9: Role of Elastic Instabilities around Confined Cylinders on Excess Pressure Drop and Oil Displacement
printer friendly1) Past Results: Role of Geometry and Rheology on Phase Space and Excess Pressure Drop
The instability first occurs as a temporal and spatial varying flow behind a highly confined cylinder and develops into an upstream vortex that is both spatially and temporally varying. In published work,2 an examination of the viscoelastic Mach number (Ma), and Elasticity number (El) have been used to create a phase space over which the instabilities occur. In our results, we see the downstream instability first occur at Ma =1 and the upstream instabilities’ onset occurs at Ma > 5. The Macrit of the upstream behavior is dependent on the degree of confinement and blockage caused by the cylinder. We observe for Ma > 1, the dimensionless pressure drop of viscoelastic fluids is larger than Newtonian fluids due to the instabilities; this value appears to be constant for all Ma >1 but varies with El
2) Current work: Mechanism of instabilities onset
The the downstream perturbations occur at Ma > 1, which is the point at which the speed of the local flow exceeds that of the viscoelastic shear waves. This strain on dispersed components creates additional elastic stresses causing the secondary flow. Based on our results above, we believed that the upstream instability was not a separate phenomenon but simply the growth of the downstream instability with increasing Ma. In order to prove these theory, we have created a linear array of 10 cylinders in microchannels. We have varied the blockage and linear porosity of the cylinders, and tracked the development of the instability with increasing Ma. As expected, we first see the instability form on the downstream face of the most downstream cylinder at Ma slightly less than 1. As Ma increases, the instability moves upstream to a fixed point. At large enough Ma, the instability reaches the upstream face of the first cylinder, and the upstream vortex forms. The result is consistent for all linear porosities. These results indicate that our proposed mechanism was indeed correct, and the instabilities grow from the down stream face due to increasing perturbations in the flow.
Interestingly, when examining the dimensionless pressure drop in these channels, we see that as El increases, dimensionless pressure drop decreases. This indicates that although the elastic instabilities do affect flow patterns, they seem to have a minimal effect on excess pressure drop when compared to the simple increase in pressure caused by increasing viscosity. This has led us to wonder what the mechanism for increased oil recovery is when using polymer solutions in EOR. This work is currently under review with Physics of Fluids.
3) Current and Future work: Role of flow patterns on oil displacement
Based on the above results, we have begun to conduct the final research goal associated with this grant, which will continue beyond the funding provided. Seeing the decrease in excess pressure drop with increasing El, we are left to wonder what mechanism is increasing enhanced oil recovery using viscoelastic fluids. We believe that the increase in turbulent/chaotic flow caused by the instabilities allows more residual oil to be displaced because the turbulent flow can better penetrate small pores
We are currently examining this by studying large arrays with various porosities and layout.. By first filling channels with oil and letting them rest for 24 hours, and then using viscoelastic fluids to displace oil at Ma both above and below the critical Ma for instability onset, we can begin to understand the role of flow on displacement. Using both pressure drop measurements and flow visualization, we have begun to see that indeed there is greatly increased clearance of residual oil without large increases in excess pressure drop once the instabilities are observed to occur. These results confirm that the instabilities play a major role in enhancing the ability of the viscoelastic fluid to displace residual oil through flow not pressure.
4) Final Work and Outcome
We are currently finishing our work on oil displacement. We need to examine a wider phase space and take accurate pressure measurements. Once finished, we will turn this work into one more papers.
5) Impact of Research
The current results on phase space and pressure drop have been published2 and presented at the 2014 annual meeting of the Society of Rheology. The work on linear cylinder arrays has been submitted to Physics of Fluids for publication and presented at the 2015 Society of Rheology meeting. We will finish the displacement work in the near future and put forth a publication in that area as well.
1. Kenney, S., K. Poper, G. Chapagain, and G. Christopher, Large Deborah number flows around confined microfluidic cylinders. Rheologica Acta, 2013. 52(5): p. 485-497. 2. Shi, X., Kenney, S., G. Chapagain, and G. Christopher, Mechanisms of onset for moderate Mach number instabilities of viscoelastic flows around confined cylinders. Rheologica Acta, 2015. Online.
