Year

2001

Degree Name

Doctor of Philosophy

Department

Faculty of Engineering

Abstract

Pneumatic conveying is being selected for an increasing number of industrial applications and products and is playing a more vital and integral role in the transportation of solid materials such as plastic pellets, grain and chemicals. However, despite all the minimum conveying velocity research (one of the operating boundaries for pneumatic conveying) that has been undertaken for several decades, the wide scatter and contradictions in the predictions of the minimum conveying velocity for dilute phase pneumatic conveying exist yet, determination of the operating boundaries for pneumatic conveying (mainly maximum conveying velocity for dense phase and minimum conveying velocity for dilute phase) still has been one of the most important tasks to be solved for the design, optimising and upgrade of pneumatic conveying systems as a consequence of that the mechanisms involved in the formation of boundaries between dilute-phase and dense-phase pneumatic conveying through a horizontal pipeline have not been well explored.

Saltation velocity was investigated initially in this thesis and then the emphasis was placed on the transition between dilute-phase and dense-phase. With careful observations, it is found that pneumatic conveying of granular solid materials through a horizontal pipeline can exhibit five different flow modes (as the air velocity is decreased): fully suspended flow; strand flow; stable or unstable strand flow over a stationary layer for low solid mass flow rates; stable or unstable strand flow over a slowly moving bed for high solid mass flow rates; low-velocity slug-flow. The pressure fluctuations within the unstable zone result from the flow mode alternation between a strand flow over a stationary layer (or slowly moving bed) and slug flow starting at the inlet due to a decrease in air velocity. The first slug moves quickly at a relatively high velocity and picks up a relatively thick stationary layer in front of it but only deposits a small amount of the material behind it. The increase in slug length and large increase in pressure cause severe pressure fluctuations and pipeline vibrations. Two different flow modes may exist simultaneously in the conveying pipeline: strand flow over a stationary layer or slowly moving bed near the feed point followed by the dilute-phase (suspension) flow of particles. For the latter, material erodes away from the end of the stationary layer or slowly moving bed and is conveyed in the form of small dunes (or pulsating strand flow).

Based on the mass balance, force balance, momentum balance and the unstable flow forming mechanism, a theoretical three-layer model for the prediction of the transition zone boundaries has been established. With stability analysis, the boundaries of the transition zone in the state diagram have been identified, and have been found to agree very well with experimental data. According to the model established, the discussion on the influence of design parameters of particle and bulk properties of the material being conveyed and pipe wall properties on boundaries in the state diagram has been conducted.

The discussion on the operating boundaries for pneumatic conveying of granular materials has been extended to conveying of powder materials and a principle for classification of granular materials and powder materials, which have different flow mode in PCC, has been proposed.

The research also has been carried out on the pressure drop prediction for pneumatic conveying of granular materials in the form of low-velocity slug-flow in order to have a perfect PCC state diagram. A new approach for the direct measurement of stress transmission factor has been developed in this thesis. The effect of the weight of the granular material in the slug on pressure drop is taken in account according to the experimental test results. The model for pressure drop prediction also includes a modified equation for the frontal force of the moving slug - allowing for momentum balance of accelerating particles and the additional force from the stationary layer to resist the movement. The modelling predictions agree very well with test results obtained on poly pellets conveyed through 98 mm and 60.3 mm ID horizontal stainless steel pipelines, each 21 m in length.

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