Particle-laden flow characterisation and prediction

Decontamination of legacy ponds and silos is of great importance and stands as a matter of increasing urgency throughout the nuclear industry. In facilities within the UK, waste suspension flows transport legacy material from historic ponds to other interim locations where they are safely stored. However, at present these processes are not as efficient as they might be due to a lack of understanding of the multiphase flows involved. It is also possible that the current transportive systems may be subject to problems if used for long periods of time, wherein issues such as blockages and poor flow conditions may develop downstream. In order to perform successful POCO operations, it is crucial that knowledge surrounding the behaviour of transportive waste sludges is developed in order to provide accurate predictive capabilities for use in design. This project will establish computational tools, with some complementary experimental work, to study the characterization of particle-laden flows which will support the development of efficient and effective retrieval technology. The project will also provide insight into the issues and problems that current waste processing techniques may encounter.

Particle-laden flow simulation techniques using first principles mathematical modelling have been developed at Leeds in order to explore the fundamentals of multiphase flow dynamics. With an increase in available computing power, these high-fidelity methods provide the means to model multiphase fluid dynamics processes with extreme accuracy, such as particle-particle interaction, turbulence modulation and agglomeration events. These mechanisms are fundamental to understanding the aggregation conditions and properties of waste slurries. Direct numerical simulation will be used throughout, and various flow conditions and geometries (channels, pipes and ducts) relevant to waste management will be examined. Focused regions of homogeneous isotropic turbulence will also be simulated. The solid phase will be modelled using two techniques. To establish relevant bulk properties of large ensembles of particles such as collision, aggregation and deposition rates, Lagrangian particle tracking (LPT) will be used, with four-way momentum coupling and deterministic agglomeration mechanisms. To study the particle- and turbulent eddy-scale dynamics an immersed boundary method (IBM) will be employed.

Often in particle-laden flow simulations, the solid phase is represented by point-spheres. In practice this is likely not the case, with morphologies in nuclear slurry flows ranging from disc-like to needle-like, and including many other complex shapes. In practice there are very few studies that consider the way in which drag and lift coefficients are calculated, and work using the IBM will be performed to develop such correlations, comparing with present experimental studies. Although work has been performed studying large numbers of non-spherical particles using LPT techniques, there is little or no detail surrounding the agglomeration mechanisms behind these, even in theoretical models. There is significant challenge in determining the way in which two or more non-spherical particles interact and produce larger structures. The way in which large particles in flows behave is highly dependent on their shape and fractal dimension, and therefore assumptions that larger particles are spherical leads to inaccuracies in the predicted behaviour. We aim to use the IBM to study non-spherical particle aggregation under a range of conditions relating to turbulence level, particle and fluid material and chemical properties and initial interaction energy. Sensible choices will be made surrounding the range of parameters studied based on those present in typical industrial flows. In particular, the work will focus on behavioural modification techniques implemented through material and chemical property changes that can be used to improve flow, mixing and separation of wastes during retrieval and POCO activities.

Another current key research topic in particle-laden flows is developing understanding of the ways in which particles modulate the local turbulence field, which has consequences for the resulting particle interaction dynamics. For instance, microparticles are capable of dampening the turbulence, especially at high volume fractions and particle-fluid density ratios. This leads to fewer collisions in the turbulent regions of pipes, channels and duct transportive flows. It is not fully understood how these processes occur, and whether current LPT two-way coupling methods are fully accurate for large (> Kolmogorov length scale) particle diameters. As such, work will be carried out to generate further understanding behind the particle-turbulence interaction mechanisms and to test the various coupling regimes presently used.

For spherical and non-spherical particles in flows, the potential for aggregation is not only based on the types of turbulence encountered, but also the chemical and mechanical properties of both phases. As noted, by using certain behavioural modification techniques, such as the addition of additives, we are able to tune the properties of the waste flow in order to achieve the desired outcome. For example, reduced deposition, increased aggregation and improved mixing or separation. The effect of such tuneable parameters will be studied in order to establish these techniques. This work will be supported by two PhD projects which will complement the present study by exploring the application of the above techniques in a waste pipe flow, through both modelling and experimentation.

Academic Lead: Mike Fairweather
Researcher: Lee Mortimer
Location: University of Leeds