Fractal structures arising from interfacial instabilities in bio-oil atomization

Abbas Ghasemi,Sangsig Yun,Xianguo Li

doi:10.1038/s41598-020-80059-w

Abbas Ghasemi, Sangsig Yun + Show 1 more

Open Access

https://doi.org/10.1038/s41598-020-80059-w

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Abstract

The intriguing multi-scale fractal patterns ubiquitously observed in nature similarly emerge as fascinating structures in two-phase fluid flows of bio-oil breakup and atomization processes. High-resolution microscopy of the two-phase flows under 15 flow conditions (cases of different flow rates of the liquid and co-flowing air streams as well as different degrees of liquid preheating) reveal that the geometrical complexities evolve under the competing/combined action of the instability mechanisms such as Kelvin–Helmholtz, Rayleigh–Taylor and Rayleigh–Plateau leading into the transition from break-up to atomization. A thorough analysis of the higher order moments of statistics evaluated based on the probability density functions from 15,000 fractal dimension samples suggest that a single-value analysis is not sufficient to describe the complex reshaping mechanisms in two-phase flows. Consistently positive skewness of the statistics reveal the role of abrupt two-phase mechanisms such as liquid column rupture, ligament disintegration, liquid sheet bursting and droplet distortions in a hierarchical geometrical entanglement. Further, large kurtosis values at increased flow inertia are found associated with turbulence-induced intermittent geometrical reshaping. Interestingly, the proposed power-law correlation reveals that the global droplet size obtained from laser-diffraction measurements declines as the two-phase geometrical complexity increases.

Highlights

The intriguing multi-scale fractal patterns ubiquitously observed in nature emerge as fascinating structures in two-phase fluid flows of bio-oil breakup and atomization processes
It should be noted that the onset and growth of the instability mechanisms, which can be predicted by theory[42], are highly affected by the mean velocity as well as turbulence levels introduced at the inlet
Instantaneous snapshots obtained from the longdistance microscopy are provided to unveil the complex two-phase dynamics and instability mechanisms for 15 different flow conditions

Summary

Introduction

The intriguing multi-scale fractal patterns ubiquitously observed in nature emerge as fascinating structures in two-phase fluid flows of bio-oil breakup and atomization processes. One of the most complex yet fascinating classes of fluid dynamics are the two-phase flows in which a competing effect from g ravity[36] and inertial forces against resistances arising from the differences in physical properties such as viscosity, surface tension and density produces a broad range of complex interfacial geometries Such complexities arise from the onset of instability mechanisms such as Kelvin–Helmholtz (KH), Rayleigh–Taylor (RT) and Rayleigh–Plateau (RP) where initial linear growth of the perturbations transition into non-linear state and generate geometrically sophisticated structures in the form of liquid sheets, ligaments and distorted droplets of various length s cales[37,38]. Due to the complex physiochemical properties of pyrolysis oils, resulting in their resistance to break-up and atomization[62,63,64], the current fundamental knowledge of their underlying two-phase physics need to be improved

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