The rheology of aerated fine powders theory and application in pneumatic conveying systems

One of the most important technical issues of recent pneumatic conveying technology development is the new approach for better modelling of fine powder flow performance. This presents a significant challenge in ensuring the model adequately reflects the physical nature of the flow, and the predicted results are comparatively more accurate than the conventional empirical based methods. Instead of empirically based methods, this research adopted a fluid rheology based technique to assess the flow performance of fine powders within pneumatic conveyors. The approach stemmed from the liquid-analogy characteristics of fluidised powder materials where the critical property to determine the flow performance of a liquid is its rheological response. This liquid analogy principle can be applied to fine powder pneumatic flows. Therefore, the objective of this research was focused on following specific aspects: theoretical modelling of fine powder pneumatic flow from a rheology perspective, experimental determination of the rheological properties for aerated powders, validations of the proposed conveying models, and transient characteristics of fine powder pneumatic flows. Initially, conveying models based on the rheology of aerated powders were derived for straight pipelines. This was achieved by analysing a continuous flow system using classic fluid mechanics. By inputting the rheological and conveying parameters, models capable of predicting the pressure drops were proposed. Experimental investigations on the rheology of aerated powder materials (alumina, cement and flyash) were subsequently conducted. These investigations were predominantly focused on the hydrostatic yield stress at low aeration and viscous characteristics at high aeration. Two purposely designed and built experimental apparatuses were utilised to examine the rheology of aerated powders accordingly. Empirical power-law and yield power-law rheological correlations were subsequently formulated based on the quantitative results obtained through testing. Rheology based conveying models were finally evaluated and validated by comparing the pressure drop produced from conveying models with the pressure drop measured from a 7-metre long pilot scale pneumatic conveyor. Results showed good agreements between the model predictions and experimental observations for alumina with overall error around 11.7 %. However, for flyash, only the results in low to intermediate dense states (m* <130) were found to be close the experimental results. Transient flow behaviours of the flyash powder were also investigated using a non-invasive imaging device-Electrical Capacitance Tomography (ECT). A 20-metre long pilot scale pneumatic conveyor was designed and set up to conduct various tests with the ECT embedded in the pipeline. Flow patterns reconstructed from the ECT results indicated that the dense flyash pneumatic flows were naturally unsteady and pulsatile. Structural and statistical analyses on the pulse structures were subsequently carried out to unveil the pulsing characteristics under different experimental conditions. Additional transient flow parameters such as the solids flow velocity and solids mass flow rate were also estimated using the ECT results. This study not only enhances the current understanding of the fine powder pneumatic flows, but also provides an alternative approach to estimate the flow performance of fine powders in pneumatic conveying systems.