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Flows Unveiled: Multimodal Measurement in Opaque Two-Phase Flows

Periodic Reporting for period 2 - OpaqueFlows (Flows Unveiled: Multimodal Measurement in Opaque Two-Phase Flows)

Reporting period: 2018-11-01 to 2020-04-30

Flows are everywhere around us. Both in nature and industry flows often involve multiple phases (gas, liquid or solid). This can be a wave on a water surface, but often there is a continuous phase that contains small droplets, bubbles or solid particles. Examples of these so-called dispersed multiphase flows are sand in rivers, oil droplets in contaminated water, catalyst particles in chemical reactors, or red blood cells in blood. Being able to predict these flows is essential for more efficient and sustainable production processes, such as the processing of raw materials, food production or the production of energy.

While we know the exact equations that describe these flows, we cannot solve them analytically. Computer simulations are possible, but demand enormous computational efforts, even for relatively simple problems. The only solution for applied flow studies is therefore a prediction based on simplified models. However, these models lack a solid underpinning, predominantly due to a lack in good experimental data: these flows are difficult to characterize, because they turn opaque even at modest volume loads. Current optical techniques (based on lasers and cameras) will thus no longer work.
By making use of non-optical imaging techniques, generally borrowed from a medical setting, we can see in these opaque flows. For instance, imaging based on ultrasound, magnetic resonance, and X-rays will allow us to determine velocities and concentrations – we can clarify these opaque flows.

Through the study of a number of fundamental flows (such as laminar and turbulent pipe flow), we can unravel the physical mechanisms piece-by-piece. The main goal is to be able to predict the average effect of the countless particles, rather than their individual contributions. With this approach, we can formulate accurate, but efficient models. After validation, these can be used to address the numerous challenges in the field of multiphase flows from society.
Three benchmark flows were selected to simultaneously further develop the measurement techniques, as well as investigate the physics of the flows.

The first benchmark flow is particle-laden pipe flow, with a focus on the transition to turbulence. In a 1-cm diameter pipe, we demonstrated that the transition to turbulence of a particle-laden flow can follow a completely different scenario compared to single-phase flow. Beyond a volume fraction around 14-15%, we observe that the classical scenario (featuring puffs, leading to an intermittent flow) is replaced by a transition where the entire gradually flow becomes turbulent. This discovery was published in Physical Review Letters (Hogendoorn and Poelma, 2018). This result could only be obtained through the use of ultrasound imaging velocimetry, a non-optical flow measurement technique further refined in this ERC project. This measurement technique was also extended to very high frame rates, which improves the accuracy of turbulence statistics (Hogendoorn and Poelma, Int. Symp on Particle Image Velocimetry 2019). Currently, a PhD and a postdoc are investigating the role of particle size and volume fraction on the structure of transitional flow. They combine pressure measurements, optical flow measurement (for dilute cases) and ultrasound-based flow measurements. A manuscript is in preparation detailing the change in flow structure for various particle sizes. A second pipe flow facility was used to study the inertial migration. After confirming some reference cases, we found unique behaviour for relatively large particles (larger than 1/10 of the pipe diameter). This is currently being investigated in detail to try to clarify the underlying physics.

The second benchmark is particle-laden Taylor-Couette flow. We investigated the role of particle concentration on the transition between (low-order) transitions (Couette flow to Taylor Vortex flow, etc.). A combination of torque measurement, (optical) flow visualisation and ultrasound-based velocimetry provides a very detailed insight in these flow. Apart from a different critical Reynolds number for the transition (as compared to single-phase flow), we observe a number of new remarkable features. Some of these were also very recently reported by others, but some are news - e.g. persistent azimuthal structures that have not been reported before. Preliminary work was presented (Dash and Poelma, Int. Symp. on Particle Image Velocimetry 2019) and a manuscript is in preparation.

For the third benchmark, we chose partial cavitation in a venturi. Originally part of a Marie Curie ITN project, this flow is a perfect candidate for the multi-modal approach that is at the heart of this ERC project. We observed the cavitation process using high-speed imaging (Jahangir et al., 2018), particle image velocimetry, magnetic resonance velocimetry (John et al., APS-DFD 2019; Exp. Fluids 2020) and X-ray computed tomography (Jahangir et al., Int. J. Multiphase Flow 2019). This approach also brought to light that a common strategy of estimating the local void fraction using optical techniques is not reliable (Dash et al., Meas. Science & Tech., 2018). The combined data set (mean velocities, mean local void fraction) is hosted by the 4TU Data Centre and freely available; it is already being used to validate numerical simulations.
For all three benchmarks, we intend to continue with more detailed studies. The particle-laden pipe flow will be tested using magnetic resonance velocimetry, as complimentary information to the ultrasound data. High-resolution PIV will be used to investigate the mechanics of partial cavitation in the near-wall region of the venturi (as this cannot be resolved using magnetic resonance velocimetry or ultrasound).
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