Body forces in particle suspensions in turbulence

Abstract

The work contained within this thesis concerns the behaviour of poly-dispersed and electrically charged particles in turbulent flows. Fundamental investigation has considered effects of momentum two-way coupling between particles and turbulence within such scenarios, and practical investigation has examined the potential utilisation of charge on fuel droplets to improve pre-combustion spray dynamics internal to marine Diesel combustion engines.

A spectral formulation is derived for the mesoscopic Eulerian transport equation describing momentum and kinetic energy transport for mono-dispersed electrically charged particles suspended at isotropic or near-isotropic conditions. This spectral transport equation specifically takes into account momentum contributions from the random uncorrelated particle velocity field.

An in-house pseudo-spectral direct numerical simulation (DNS) code has been extended and used to numerically investigate high order spectral statistics associated with momentum and kinetic energy transport equations for homogeneous and isotropic turbulence with particulate suspensions. Key results show that poly-dispersity can reduce the attenuation level of turbulent kinetic energy relative to mono-dispersed suspensions at similar mass loading ratios. In addition, two-way coupled charged particle suspensions were found to exhibit in their mesoscopic spectra, large scale augmentation of kinetic energy and small scale attenuation of kinetic energy, analogous to the behaviour of turbulence spectra in the presence of polymer chain additives. Furthermore, in the presence of gravity, two-way coupled charged particle suspensions were found under certain conditions, to fall with a velocity slower than their own Stokes settling velocity.

A droplet charge-diameter distribution model suitable for attributing electrical charge to poly-dispersed droplets in electrostatically atomized dielectric liquid sprays, has successfully been validated against experimental data. A methodology has been developed that uses the charge-diameter distribution model to simulate and successfully predict characteristics of electrostatically atomized dielectric sprays for low pressure spray systems.

A computational methodology has also been developed, suitable for predicting the characteristics of high pressure electrostatically charged sprays. This methodology has also been validated against existing experimental data, and is found to be reliable at predicting secondary atomization processes for both uncharged and charged spray plumes at elevated injection conditions.

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