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High-speed microscale flows across obstacles for enhanced mixing

Zhang, Shigang; (2020) High-speed microscale flows across obstacles for enhanced mixing. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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In microfluidics, mixing remains a challenge due to the diffusion-dominated nature of the flows therein. Inserting obstacles such as cylindrical pins in a micromixer has the potential to significantly enhance mixing. However, it is unclear how the mixing efficiency in confined microchannel flows is affected by the proximity of the channel walls, the relative position of the interface between two fluids to be mixed (i.e. flow ratio) and the cylindrical pin, as well as the pin arrangement. In the present thesis, the microscale flow past single and two pin arrays is investigated for different levels of confinement, flow mixing ratios and pin configurations for Reynolds numbers (Re) up to 340; micro particle image velocimetry (μPIV) is applied to investigate the velocity fields, and micro laser-induced fluorescence (μLIF) to acquire the concentration fields and assess the mixing performance, utilising the intensity of segregation technique. A high-speed μPIV technique was implemented to resolve the unsteady flow field and capture the shedding frequency (typically in the hundreds Hz range) up to Reynolds number of 240. Wall proximity delayed the onset of vortex-shedding from a single pin and increased the shedding frequency. Vortex-shedding was found to enhance mixing effectiveness as expected, particularly when the pin intercepted the interface between the two fluids to be mixed. Adding a second pin suppressed vortex-shedding for spacings between the two pins up to four-pin diameters in a tandem arrangement but not in staggered arrangements. The mixing index increased from 2% to 61% with Re increasing from 39 to 335 in the microchannel with a distance of 4.5 pin-diameters distance between the two pins in tandem. Overall, the thesis has elucidated the fundamental features of high Re mixing at small scales at an exquisite level of detail and resolution.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: High-speed microscale flows across obstacles for enhanced mixing
Event: UCL (University College London)
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Copyright © The Author 2020. Original content in this thesis is licensed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0) Licence (https://creativecommons.org/licenses/by/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request.
Keywords: microfluidics, particle image velocimetry, laser induced fluorescence, mixing enhancement
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Mechanical Engineering
URI: https://discovery.ucl.ac.uk/id/eprint/10103116
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