Mark A Gilbertson

The influence of particle size on the flow of fluidised powders

Fluidised particles can form thin coherent flows that move swiftly over horizontal surfaces. When the particles are cohesionless, Eames and Gilbertson [J. Fluid Mech. 424 (2000) 169] demonstrated that the flow may be understood through vertically integrated momentum and mass balances, and good agreement was found between the model predictions and experimental observations. The behaviour of gas fluidised fine particles (diameter of the order of tens of micrometres) differs significantly from that of larger particles. This is true with respect to their flow which takes place through a series of episodic avalanches and waves rather than as a continuous flow. This behaviour can be explained in terms of a mild cohesive force between the particles. When cohesion is included in the vertically averaged momentum equation, the dynamics of fine particle flows can be reasonably predicted for low and high gas flow rates, when the cohesive forces are respectively weaker than internal frictional forces or drag forces generated by the gas flow. A quasi-static analysis of the combined effect of gas flow and cohesion shows that where cohesive sediments are thin they are not necessarily mobilised, even when fluidised, and this is supported by new experimental observations. A flow of cohesive particles thus has two distinct regions: a thin nose region where the material is not mobilised and a pile of mobile particles behind it. At intermediate gas flow rates comparable to the point of minimum fluidisation, the internal structure of the particle flow dominates how the pile flows along a horizontal surface, and continuum models fail to predict the pile dynamics.

Kimberlite ascent and eruption

Arising from: L. Wilson & J. W. Head III 447, 53–57 (2007)10.1038/nature05692; Wilson & Head replyWilson and Head model kimberlite ascent and eruption by considering the propagation of a volatile-rich dyke. Wilson and Head’s model has features in common with Sparks et al., but it is inconsistent with geological observations and constraints on volatile solubility. Here we show that this may be due to erroneous physical assumptions.

The Optimisation of Pressure Measurements for the Control of Bubbling Fluidised Beds

A common form of chemical reactor is the bubbling fluidised bed where gas is introduced into a free bed of particles at a rate exceeding that necessary to support their weight. The behaviour of the reactor is dependent on the distribution and characteristics of the bubbles within the bed. Information about these can be inferred from pressure measurements within the bed, which can also be used to characterise the dynamics of a bed and therefore to control it and the reaction.It is necessary to understand what the pressure signals are measuring, how many of them are necessary, and where they should be placed for them to be used to characterise the state of the bed. Pressure measurements are made at single points within a bed, and it is not immediately clear whether the measurements are local (owing to some sort of wall effect, for example), characterise the region close to the sensor, or the whole bed. It is shown how the dynamics of the bed can be characterised from the pressure signals, and that these signals are sensitive to the state of the bed. It is also necessary to evaluate where the sensors should be placed and how many of them should be used. Important considerations here are the extent to which the measurements of pressure are axisymmetrical, and whether in some circumstances and positions the local dynamics mask the overall bed dynamics. Axisymmetry is shown to exist under many conditions and that a single transducer can be sufficient. Furthermore, the depth of the bed is also shown to have a significant effect on its dynamics.It is also shown how the characterised pressure measurements can be used to control the state of the fluidised bed and used to enhance the performance of a fluidised bed reactor.