Time-of-flight variant to image mixing of granular media in a 3D fluidized bed.

This paper describes a variant of time-of-flight magnetic resonance (MR) imaging that provides a method of measuring the inherent mixing in a fluidized bed without the introduction of tracer particles. The modifications to conventional time-of-flight imaging enable the measurement of the axial mixing of a precisely controlled initial particle distribution, thereby providing measurements suitable for a direct comparison with models of solids mixing in granular systems. The imaging sequence is applied to characterize mixing, over time scales of 25-1000 ms, in a gas-fluidized bed of Myosotis seed particles; mixing over short timescales, inaccessible using conventional tracer techniques, is studied using this technique. The mixing pattern determined by this pulse sequence is used in conjunction with MR velocity images of the motion of the particles to provide new insight into the mechanism of solids mixing in granular systems.

Rapid two-dimensional imaging of bubbles and slugs in a three-dimensional, gas-solid, two-phase flow system using ultrafast magnetic resonance.

Ultrafast magnetic resonance has been applied to measure the geometry of bubbles and slugs in a three-dimensional gas-solid two-phase flow. A bed of particles of diameter 0.5 mm were fluidized with gas velocities in the range of 0.08-0.26 m/s. Bubbles were imaged in transverse as well as vertical planes with an acquisition time of down to 25 ms and a spatial resolution down to 1.7 mm. Owing to the ultrafast character of these measurements, it is not only possible to evaluate correlations, e.g., for the bubble diameter, but also evaluate models of complex hydrodynamic phenomena, such as the splitting and coalescence of bubbles.

The behaviour of Sal'nikov's reaction, P --> A --> B, in a spherical batch reactor with the diffusion of heat and matter.

The behaviour of a simple chemical reaction, occurring with the release of heat in a closed batch reactor, is considered for the situation when matter and heat are transported only by diffusive processes; thus, the reacting fluid has negligible velocity, so that heat transfer is by thermal conduction. The reaction is Sal'nikov's, which consists of two, consecutive first-order steps, producing a product B, from a precursor P, via an active intermediate A, in P --> A --> B. The first of these steps is assumed to be thermoneutral, with zero activation energy, whilst the second is exothermic, with an appreciable activation energy. These features make Sal'nikov's reaction the simplest to display thermokinetic oscillations that characterise many, more complex schemes, e.g. cool flames in hydrocarbon combustion. This study involves identifying the regions of parameter space, in which these oscillations in the temperature and the concentration of the intermediate A occur, by means of numerical simulation. These regions are compared with previous analytical stability analyses in one-dimensional systems. It was found that oscillations occur over a much larger range of conditions in the case considered here, i.e. a reactor with spherical symmetry, than in the simple 1-D case, previously studied by Gray and Scott (P. Gray and S. K. Scott, Chemical Oscillations and Instabilities, Clarendon Press, Oxford, 1990, pp. 264-291). In addition, approximate analytical solutions for the temperature and concentration of A are presented for two limiting cases of non-oscillatory behaviour. These analytical solutions have been verified by comparison with full numerical solutions of the governing equations.

Real-time measurement of bubbling phenomena in a three-dimensional gas-fluidized bed using ultrafast magnetic resonance imaging.

Ultrafast magnetic resonance imaging has been applied for the first time to measure simultaneously both the rise velocities and coalescence of bubbles, and the dynamics of the solid phase in a gas-solid two-phase flow. Here, we consider the hydrodynamics within a gas-fluidized bed of particles of diameter 0.5 mm contained within a column of internal diameter 50 mm; gas velocities in the range of 0.18-0.54 m/s were studied. The data are of sufficient temporal and spatial resolution that bubble size and the evolution of bubble size and velocity following coalescence events are determined.