ALBERT

Description: This paper describes computer simulation studies of granular materials under dense conditions where particles are in persistent contact with their neighbours and the elasticity of the material becomes an important rheological parameter. There are two regimes at this limit, one for which the stresses scale with both elastic and inertial properties (called the elastic-inertial regime), and a non-inertial quasi-static regime in which the stresses scale purely elastically (elastic-quasi-static). In these elastic regimes, the forces are generated by internal force chains. Reducing the concentration slightly causes a transition from an elastic to a purely inertial behaviour. This transition occurs so abruptly that a 2% concentration reduction can be accompanied by nearly three orders of magnitude of stress reduction. This indicates that granular flows near this limit are prone to instabilities such as those commonly observed in shear cells. Unexpectedly, there is no path between inertial (rapid) flow and quasi-static flow by varying the shear rate at a fixed concentration; only by reducing the concentration can one cause a transition from quasi-static to inertial flow. The solid concentrations at which this transition occurs as well as the magnitude of the stresses in the elastic regimes are strong functions of the particle surface friction, because the surface friction strongly affects the strength of the force chains. A parametric analysis of the elastic regime generated flowmaps showing the various regimes that might be realized in practice. Many common materials such as sand require such large shear rates to reach the elastic-inertial regime that it is unattainable for all practical purposes; such materials will demonstrate either an elastic-quasi-static behaviour or a pure inertial behaviour depending on the concentration-with many orders of magnitude of stress change between them. Finally, the effects of nonlinear contacts are investigated and an appropriate scaling is proposed that accounts for the nonlinear behaviour in the elastic-quasi-static regime.

Description: The particle pressure is the surface force in a particle/fluid mixture that is exerted solely by the particle phase. This paper presents measurements of the particle pressure on the faces of a two-dimensional gas-fluidized bed and gives insight into the mechanisms by which bubbles generate particle pressure. The particle pressure is measured by a specially designed 'particle pressure transducer'. The results show that, around single bubbles, the most significant particle pressures are generated below and to the sides of the bubble and that these particle pressures steadily increase and reach a maximum value at bubble eruption. The dominant mechanism appears to be defluidization of material in the particle phase that results from the bubble attracting fluidizing gas away from the surrounding material; the surrounding material is no longer supported by the gas flow and can only be supported across interparticle contacts which results in the observed particle pressures. The contribution of particle motion to particle pressure generation is insignificant. The magnitude of the particle pressure below a single bubble in a gas-fluidized bed depends on the bubble size and the density of the solid particles, as might be expected as the amount of gas attracted by the bubble should increase with bubble size and because the weight of defluidized material depends on the density of the solid material. A simple scaling of these quantities is suggested that is otherwise independent of the bed material. In freely bubbling gas-fluidized beds the particle pressures generated behave differently. Overall they are smaller in magnitude and reach their maximum value soon after the bubble passes instead of at eruption. In this situation, it appears that the bubbles interact with one another in such a way that the defluidization effect below a leading bubble is largely counteracted by refluidizing gas exiting the roof of trailing bubbles.

Description: A combined experimental and detailed numerical study was conducted on the interaction between chemically inert solid particles and strained, atmospheric methane/air and propane/air laminar flames, both premixed and non-premixed. Experimentally, the opposed jet configuration was used with the addition of a particle seeder capable of operating in conditions of varying gravity. The particle seeding system was calibrated under both normal and micro gravity and a noticeable gravitational effect was observed. Flame extinction experiments were conducted at normal gravity by seeding inert particles at various number densities and sizes into the reacting gas phase. Experimental data were taken for 20 and 37 (mu) nickel alloy and 25 and 60 (mu) aluminum oxide particles. The experiments were simulated by solving along the stagnation streamline the conservation equations of mass, momentum, energy, and species conservation for both phases, with detailed descriptions of chemical kinetics, molecular transport, and thermal radiation. The experimental data were compared with numerical simulations, and insight was provided into the effects on extinction of the fuel type, equivalence ratio, flame configuration, strain rate. particle type. particle size. particle mass, delivery rate. the orientation of particle injection with respect to the flame and gravity. It was found that for the same injected solid mass, larger particles can result in more effective flame cooling compared to smaller particles, despite the fact that equivalent masses of the larger particles have smaller total surface area to volume ratio. This counter-intuitive finding resulted from the fact that the heat exchange between the two phases is controlled by the synergistic effect of the total contact area and the temperature difference between the two phases. Results also demonstrate that meaningful scaling of interactions between the two phases may not be possible due to the complexity of the couplings between the dynamic and thermal parameters of the problem.