Research

Microdroplet Formation

Droplet-based microfluidics has attracted increasing attention over the last few decades owing to its enormous applications in the chemistry and biology fields, mainly involving micro-level droplet manipulations, such as generation, breakup, coalescence and sorting. Droplet generation is the first step in droplet-based microfluidics and precise control over the droplet status is vitally important to meet the downstream manipulation and application demands. Microfluidic droplets have high specific surface areas, low sample consumption, no dispersion and better fluid control; therefore, they can be regarded as the ideal micro-reactors. Several configurations that are most frequently used are cross-junctions, flow focusing, and co-flow streams, all of which exhibit high performance, high throughput production of monodispersed droplets.

References:
[1] Q. Chen, J. Li, Y. Song, et al., Pressure-driven microfluidic droplet formation in Newtonian and shear-thinning fluids in glass flow-focusing microchannels, International Journal of Multiphase Flow. 140 (2021) 103648.Doi: https://doi.org/10.1016/j.ijmultiphaseflow.2021.103648.
[2] Q. Chen, J. Li, Y. Song, et al., Modeling of Newtonian droplet formation in power-law non-Newtonian fluids in a flow-focusing device, Heat and Mass Transfer. 56 (2020) 2711–2723. Doi: https://doi.org/10.1007/s00231-020-02899-6.
[3] Q. Chen, J. Li, Y Song, Q. He, D. M Christopher, X. Li. Newtonian droplet generation in shear-thinning fluids in a flow-focusing microchannel, CIESC Journal. 71(2020) 1510-1519. Doi: https://doi.org/10.11949/0438-1157.20191331.

Hydrogen Safety

The development and revision of safety codes and standards for hydrogen infrastructure require a solid scientific basis, including studies of unignited releases from high-pressure systems for various scenarios. Most hydrogen releases are modeled as axisymmetric jets, but real leaks are more likely to be non-axisymmetric jets issuing from high aspect ratio cracks or slots. In the present study, underexpanded hydrogen jets from square and rectangular nozzles with aspect ratios of 1e16 were numerically modeled for stagnation pressures up to 20 MPa. The near and far flow fields were modeled separately using two sequential computational domains to accurately and efficiently capture the flow characteristics. The numerical models were first validated with experimental data from a previous experimental study and literature data. The mass fraction and velocity distributions show that the centerline decay rates increase as the nozzle aspect ratio increases, but this increase is dependent on the pressure. This means that the canonical decay law of round turbulent jets and plumes no longer applies to the slot nozzle jets for high pressures. The radial profiles collapse onto a Gaussian curve in the major axis plane, but neither collapse nor are they Gaussian in the minor axis plane with peaks away from the jet centerline. Different shock patterns were identified along the major and minor axes and the axis switching phenomenon seen in the literature was also reproduced. The axis switching resulted in significantly wider flattened concentration distributions compared with the axisymmetric jet which may require consideration during safety analyses for non-circular nozzles. A scaling factor taking both the nozzle shape and pressure effects into account was then developed to better scale the centerline decay rates for jets from both the square and rectangular nozzles. The present study demonstrates that the nozzle shape effects on the jet spreading should not be overlooked and proper scaling factors are required to collapse the data and calculate decay rates.

Schematic of the (a) computational domains where the red dashed lines indicate the interface (not to scale) and (b) details of the 3D mesh for the AR8 nozzle where the inset shows the front view of the orifice mesh. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Reference:
X. Li, Q. Chen, M. Chen et al., Modeling of underexpanded hydrogen jets through square and rectangular slot nozzles, International Journal of Hydrogen Energy. 44 (2019) 6353–6365. Doi: https://doi.org/10.1016/j.ijhydene.2019.01.079.

Heat Exchanger

Helical flow is an optimal flow pattern in shell-and-tube heat exchangers, but the poor heat transfer performance has been always the bottleneck. Aiming to improve heat transfer without a tremendous increase of pressure drop, the optimized continuous helical baffled heat exchangers with varying elliptical tube layouts are proposed and investigated by numerical simulation. It concludes that the tube-array angle influences the characteristics of flow and heat transfer significantly. The optimized tube layout can improve heat transfer in a fully-developed section and decrease vortices at the back of tubes to some extent. The mean convective heat transfer coefficient and the pressure drop are respectively 2.10–2.33 times and 1.82–4.46 times higher than that of the original heat exchanger with concentric annular layout, and the comprehensive performance proves to be improved by about 50%. The results show that the optimized elliptical tube layouts with varying tube-array angles can realize the initial objective, especially when the Re number offlow is relatively small. This research is beneficial for further optimization on helical baffled heat exchangers to improve heat transfer as well as comprehensive performance.

Computational domain of continuous helical baffled heat exchanger

Reference:
T. Du, Q. Chen, W. Du et al., Performance of continuous helical baffled heat exchanger with varying elliptical tube layouts, International Journal of Heat and Mass Transfer. 133 (2019) 1165–1175. Doi: https://doi.org/10.1016/j.ijheatmass transfer.2018.12.142.

Microcapsule Production

In progress..

RBC Flow in Porous Media

In progress.

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University of Manchester

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