Continuum modelling of granular particle flow with inelastic inter-particle collisions

The kinetic theory of granular flow is a successful model for gas-solid flows. However, inelastic collisions between particles, among other mechanisms, cause agglomeration of particles, which may be the reason why undue sensitivity of the model to any slight inelasticity in inter-particle collisions has been seen previously. In contrast to a dry (i.e. no interstitial gas) granular system, this tendency to agglomerate in a gas driven two-phase system may be countered by the carrier gas turbulence. In this paper, a heuristic model for particle gas turbulence interaction is introduced within the scope of a generalized kinetic theory model which incorporates the carrier fluid effect on particulate stresses. The numerical results for the flow of granular particles in vertical pipes, which considers slightly inelastic inter-particle collisions, are in reasonably good agreement with published experimental data. Even in this relatively simple model, the results indicate that the interactions between the particle phase and gas turbulence need to be appropriately addressed in any kinetic theory based model for gas solid flows

The Influence of geometrical and operational parameters on internal flow characteristics of Internally Mixing Twin-Fluid Y-Jet Atomizers

Internally mixing twin-fluid Y-jet atomizers are widely used in coal fired thermal power plants for start-up, oil-fired thermal power plants and industrial boilers. The flow through internally mixing Y-jet atomizers is numerically modeled using the compressible Navier-Stokes equations; Wall Modeled Large Eddy Simulations (WMLES) is used to resolve the turbulence with Large Eddy Simulations whereas the Prandtl Mixing Length Model is used for modeling the subgrid scale structures, which are affected by geometric and operational parameters. Moreover, the Volume-of-Fluid (VOF) method is used to capture the development and fragmentation of the liquid-gas interface within the Y-jet atomizer. The numerical results are compared with correlations available in open literature for the pressure drop; further results are presented for the multiphase flow regime maps available for vertical pipes. The results show that the mixing point pressure is strongly dependent on the mixing port diameter to airport diameter ratio, specifically for gas to liquid mass flowrate ratio (GLR) in the range 0.1 < GLR < 0.4; the mixing port length moderately affects the mixing point pressure while the angle between mixing and liquid ports is found not to have an appreciable effect. Moreover, it is found that the vertical pipe multiphase flow regime maps in the literature could be applied to the flow through the mixing port of the twin-fluid Y-jet atomizer. The main flow regimes found under the studied operational conditions are annular and wispy annular flow

Mathematical Modelling and Experimental Evaluation of Electrostatic Sensor Arrays for the Flow Measurement of Fine Particles in a Square-shaped Pipe

Abstract—Square-shaped pneumatic conveying pipes are used in some industrial processes such as fuel injection systems in coal-fired power plants and circulating fluidized beds. However, little research has been conducted to characterise the gas–solid two-phase flow in a square-shaped pneumatic conveying pipe. This paper presents mathematical modelling and experimental assessment of novel non-restrictive electrostatic sensor arrays for the measurement of pulverised fuel flow in a square-shaped pipe. The sensor arrays consist of twelve pairs of strip-shaped electrodes, which are uniformly embedded in the four flat pipe walls. An analytical mathematical model of the sensor arrays is established and the induced charge and currents of different electrodes due to a point charge are then derived based on the model. Experimental tests were conducted on a 54 mm square-shaped pipe section of a pneumatic conveyor test rig under a range of flow conditions. The fuel velocity profile over the whole cross-section of the pipe is measured. Mathematical modelling and experimental results demonstrate that the proposed non-restrictive electrostatic sensor arrays are capable of characterising the local pulverised fuel flow in a square-shaped pneumatic conveying pipe. Index Terms—electrostatic sensor, square-shaped pipe, mathematical modelling, velocity profile, pulverised fuel

Numerical Simulation of Particle Collision and Agglomeration in Turbulent Channel Flows

The study described in this thesis concerns the simulation of dispersed and dense particle-laden turbulent channel flows. The research primarily investigates the role of gravity; in terms of its contribution to particle collision, agglomeration and re-distribution. Large eddy simulation is employed to predict the fluid-phase, with solutions coupled with a Lagrangian particle tracking routine to model the particle-phase. In order to establish the validity of the preferred numerical method, results generated from the single-phase and the dilute particle-phase predictions were compared with those based on DNS, with good agreements found. Results obtained for horizontal zero gravity channel flows, show effects of particle size, particle concentration and turbulence on colliding and agglomerating particles. All variables were shown to strongly impact on collision and agglomeration, with the number of events reaching maximum towards the channel walls due to increased particle concentrations and turbulence levels in these regions. The collision and agglomeration is, however, shown to enhance exponentially with the inclusion of gravity and accentuated on the lower wall of the channel. An extension of the investigation into vertical channels of upward and downward flow configurations, also demonstrated the significance of the gravity force on particle collision and agglomeration. The effect of the particles on the flow is small, owing to the low mass-loading. Agglomeration is found to be most favourable for flows of low turbulence; and unlike collisions, dominantly forms in the channel centre. The investigation presented is a novel contribution to literature that provides a fundamental improvement on the understanding of turbulent fluid-particle flows. Particularly, it extends the existing knowledge on cohesive particle behaviours in turbulent flows by examining the effect of gravity on such flows. The contribution finds relevance in many engineering and industrial flow processes and should aide the design of better flow processes