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The material pulse flow structure within the fluidized dense phase pneumatic conveying of flyash powder was studied using the Electrical Capacitance Tomography (ECT). Decomposition of the bulk density levels within the pipe cross-sectional area were obtained and statistical analysis of bulk density waves were performed. The results suggest that the pulse amplitude and duration obeyed the Weibull and lognormal distributions. In addition, bulk density factor was found to have a linear relationship with pulse amplitude; however, it is independent of pulse duration.

Abstract This study used a 3D coupled CFD–DEM model to assess how slugs tend towards steady state in single slug horizontal pneumatic conveying. Initial slug length, inlet velocity and initial stationary layer fractions were systematically varied for a total of 72 simulations. Previously made observation that slugs tend towards a steady state was confirmed via a theoretical derivation. The derivation shows that slugs move towards their steady state lengths exponentially. This allowed for a calculation of a characteristic time scale which is a measure of how quickly a slug tends towards the steady state. The theoretical estimate which is a function of slug porosity, steady length, velocity and stationary layer fraction has good agreement with simulated results. A link between steady slug length and solids loading ratio was also shown.

Abstract Due to their differences, predictive methods suitable for dilute and dense phase pneumatic conveying are rare in the literature. Conveying trials are often required to characterise a given system, where pressure drop measurements are plotted against gas mass flow rate for various solids flow rates. Empirical curves of constant solids flow are overlaid with measurements and resemble a ‘J’ shape. This paper presents a model for these curves based on the assumption that the pressure drop is a sum of two terms relating to the gas only influence and a combined gas and solids term. The model is validated for slug flow capable materials, where excellent agreement is obtained. However, it is concluded that the procedure is suitable for fluidised dense-phase capable materials as well. The developed model has the potential to significantly reduce the number of conveying trials required to characterise a pneumatic conveying system.

The many advantages of slug flow pneumatic conveying are outweighed by the lack of understanding of the flow mechanisms. For horizontal slug flow, the unique feature is the stationary layer of material found between the travelling slugs, which was recently shown to be characterised by two constants. This paper looks to utilise the vast data available in the literature, which is representative of the entire mode of flow, and relates the stationary layer and slug velocity to predict the two constants from only these inputs. It was found that, even for the vast range of materials and systems considered, slug flow encompasses a narrow bound of the two constants. Furthermore, an empirical approach that was developed to relate the layer fraction and particle velocity was found to provide good agreement to measurements and may be of use in other investigations that require an additional equation for modelling.

This paper considers the particle exchange between a slug and its stationary layers to propose a limiting case for slug stability. It is shown that a stable slug must have a length that is greater than the product of its velocity and the time taken to accelerate particles from the stationary layer into the slug. Theoretical analysis and experimental results are utilised to derive an expression for the acceleration time – resulting in an inequality relating slug length and velocity, layer fraction and pipeline diameter. Single slug, horizontal conveying trials of five bulk materials are used for validation, after which the inequality is generalised to a function of only operating conditions using expressions and observations from the literature. The paper concludes with a case study demonstrating the ability of the resulting method to predict the area of the operating conditions space for which slugging can occur.

Abstract This paper focusses specifically on the topic of slug velocity and the definitions and origins of the various velocities observed within horizontal slug flow pneumatic conveying. Testing was conducted and measurement of slug particle velocity and wave front and rear velocities confirmed existing observations in the literature that the slug wave travels faster than the individual particles. Examining the relationships between the measured velocities further, a model based on the conservation of mass in a slug provided a relationship between slug velocity terms, length, porosity and the stationary layers. For constant slug length conditions the new model was shown to be equivalent to the gas-liquid analogy model of Konrad [1]. The propagation velocity in the analogy of Konrad was shown to be the relative velocity term, which reflects the perceived change in velocity from the dynamic changes in slug length occurring due to the particle exchanges between the slug and stationary layers. Furthermore, the porosity gradient between the slug and stationary layers was shown to impact both the velocity and layer fraction. The validation of the model showed very good agreement to the measurements and demonstrated that the slug particle and wave velocities should not be used interchangeably as is often done in the literature.

Abstract Pressure drop in fluidized dense phase pneumatic conveying involves frictional interactions among gas, particle and pipe wall. There have been numerous correlations proposed by different researchers for predicting the pressure drop in fluidized dense phase conveying. In this paper steady state flow equations have been written for different phases and these equations are solved by assuming certain factors for different conveying materials. For writing the flow equations, a single gas phase and certain number of solids phases (which are chosen based on the particle size distribution of the conveying material) have been considered. Experimental data have been used as initial conditions at the exit of the pipeline in order to solve for the value of the flow parameters at the inlet of the pipeline. Experimental data have also been used to find the maximum possible conveying distance or maximum possible conveying pipeline diameter by imposing certain limiting conditions of conveying. Scaling equations for the solids mass flow rate and the air mass flow rate have been used to predict the pressure drop for different pipeline diameters and pipeline lengths.

Abstract This study used a 3D coupled CFD-DEM model to study velocity and porosity relationships within horizontal dense phase pneumatic conveying. Inlet velocities, initial layer fractions and initial slug lengths were varied for a total of 72 single slug simulations. The linear relationship between particle velocity and slug velocity was examined and the origins and contributions of individual variables were looked at in detail. It was found that the slug velocity to particle velocity relationship can be defined using slug to bulk density ratio and propagation velocity. It was also found that within a single slug system, slugs tend to their ‘preferred’ steady-state length which is a function of inlet velocity and stationary layer ahead of the slug. Another finding is that hydraulic jump can be used as an analogy to the interaction between slug front and the stationary layer. This analogy can be used to define a boundary for slug flow as well as help predict slug porosity.

The complex dynamics of slug flow generally result in steady-state simplifications regarding both measurements and modelling. Consequently, little is known about the transient nature of individual slugs and their interactions in systems of multiple slugs. Using gas conservation, this paper theoretically investigates the stability of individual slugs in a system of multiple slugs. For a simplified case of multiple slugs of identical velocities, a proof by contradiction shows that it is impossible for such a system to be stable. For general cases, which contain more unknowns than equations, a combined qualitative and quantitative approach is undertaken. For the case of two general slugs, the approach strongly suggests the impossibility of individually stable slugs. For the most complicated general case of n slugs, the approach also indicates that individually stable slugs are unlikely. These findings significantly impact existing modelling and measurement methods that assume individually stable slugs.