Investigation of the particle dynamics of a multi-component solid phase in a dilute phase pneumatic conveying system
In order to mitigate the risk of global warming by reducing CO2 emissions, the co-firing technique, burning pulverized coal and granular biomass together in conventional pulverised fuel power station boilers, has been advocated to generate “greener” electricity to satisfy energy demand while continuing to utilize existing rich coal resources. A major problem is controllably distributing fuel mixtures of pulverized coal and granular biomass in a common pipeline, thus saving much investment. This is still under development in many co-firing studies. This research into particle dynamics in pipe flow was undertaken in order to address the problem of controllable distribution in co-firing techniques and gain an improved understanding of pneumatic conveying mechanisms. The objectives of this research were, firstly, to numerically evaluate the influence of various factors on the behaviour of particles of the different materials in a horizontal pipe gas-solid flow, secondly, to develop an extended technique of Laser Doppler Anemometry in order to determine cross-sectional characteristics of the solid phase flow in the horizontal and vertical legs of a pneumatic conveying system, and, thirdly, to develop a novel imaging system for visualizing particle trajectories by using a high definition camcorder on a cross-section illuminated by a white halogen light sheet. Finally, a comparison was made of cross-sectional flow characteristics established by experiments and those simulated by using a commercial Computational Fluid Dynamics code (Fluent) and the coupling calculations of Fluent & EDEM (a commercial code of Discrete Element Method) respectively. Particle dynamic behaviour of the solid phase in a dilute horizontal pipe flow was investigated numerically by using the Discrete Phase Model (DPM) in Fluent 6.2.16. The numerical results indicate that the Saffman force plays an important role in re-suspending particles at the lower pipe boundary and that three critical parameters of the critical air: conveying velocity, the critical particle size and the critical pipe roughness, exist in pneumatic conveying systems. The Stokes number can be used as a similarity criterion to classify the dimensionless mean particle velocity of the different materials in the fully developed region. An extended Laser Doppler Anemometry (LDA) technique has been developed to measure the distributions of particle velocities and particle number over a whole pipe cross section in a dilute pneumatic conveying system. The first extension concentrates on a transform matrix for predicting the refracted laser beams’ crossing point in a pipe according to the shift coordinate of the 3D computer-controlled traverse system on which the probes of the LDA system were mounted. Another part focussed on the proper sampling rate of LDA for measurements on the gas-solid pipe flow with polydispersing particles. A suitable LDA sampling rate should ensure that enough data is recorded in the measurement interval to precisely calculate the particle mean velocity or other statistical values at every sample point. The present study explores the methodology as well as fundamentals of measurements of the local instantaneous density of particles as a primary standard using a laser facility. The extended LDA technique has also been applied to quantitatively investigate particle dynamic behaviour in the horizontal and vertical pipes of a dilute pneumatic conveying system. Three kinds of glass beads were selected to simulate the pulverized coal and biomass pellets transported in a dilute pneumatic conveying system. Detailed information on the cross-sectional spatial distributions of the axial particle velocity and particle number rate is reported. In the horizontal pipe section, experimental data on a series of cross-sections clearly illustrate two uniform fluid patterns of solid phase: an annular structure describing the cross-sectional distribution of the axial particle velocity and a stratified configuration describing particle number rate. In the vertical pipe downstream of an elbow R/D=1.3, a horseshoe-shaped feature, which shows that the axial particle velocity is highest in wall regions of the pipe on the outside of the bend for all three types of glass beads on the section 0D close to the elbow outlet. The developments of cross-sectional distributions of particle number rate indicate that the horseshoe-shaped feature of particle flow pattern is rapidly dispersed for particles with high inertia. A video & image processing system has been built using a high definition camcorder and a light sheet from a source consisting of a halogen lamp. A set of video and image processing algorithms has been developed to extract particle information from each frame in a video. The experimental results suggest that the gas-solid flow in a dilute pneumatic conveying system is always heterogeneous and unsteady. The parameter of particle mass mean size is superior to particle number mean size for statistically describing the unsteady properties of gas-solid pipe flow. It is also demonstrated that the local data of particle number rate or concentration are represented by a stratified structure of the flow pattern over a horizontal pipe cross-section. Finally, comparisons of numerically predicated flow patterns and experimental ones show that there is reasonable agreement at pipe cross-sections located at horizontal positions less than half the product of particle mean velocity and mean free fall time in the pipe from the particle inlet. Further away from the inlet, the numerical results show flow patterns which are increasingly divergent from the experimental results along the pipe in the direction of flow. This discrepancy indicates that particles’ spatial distribution in the pipe is not accurately predicted by the Discrete Phase Model or Fluent coupled with EDEM.