• Energy Research

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.

This study investigates pneumatic conveying of four different biomass materials, namely cottonseeds, wood pellets, wood chips, and wheat straw. The performance of a previously proposed model for predicting pressure drop is evaluated using biomass materials. Results indicate that the model can predict pressure with an error range of 30 percent. To minimize the number of trial tests required, an optimization algorithm is proposed. The findings show that with a combination of three trial tests, there is a 60 percent probability of selecting the right subset for accurately predicting pressure drop for the entire range of tests. Further investigation of different training subsets suggests that increasing the number of tests from 3 to 7 can improve the probability from 60% to 90%. Moreover, thorough analysis of all three-element subsets in the entire series of tests reveals that when considering air mass flow rate as the input, having air mass flow rates that are not only closer in value but also lower increases the likelihood of selecting the correct subset for predicting pressure drop across the entire range. This advancement can help industries to design and optimize pneumatic conveying systems more effectively, leading to significant energy savings and improved operational performance.

Abstract Two methods were developed to investigate the porosity of moving slugs in situ during horizontal slug flow pneumatic conveying. The first method consists in applying a permeability model in combination with measurements of pressure loss and fluid velocity along the slugs. A review of existing models describing the resistance of porous structures to fluid flow revealed that the semi-empirical model of Ergun is particularly suitable to investigate the porosity profile along moving slugs. The second method consists in a direct determination method involving a slug-catcher able to catch a moving slug in a fraction of a second and simultaneously separate it into three horizontal layers. Those two methods were applied to analyse the porosity of naturally occurring slugs during pneumatic transport of polypropylene pellets. It was found that in contrast to common belief, slugs are slightly fluidised structures that do not display any porosity gradient over the pipe cross-section height. The slug porosity appeared independent of the gas conveying velocity, all slugs displaying an average porosity around 0.41, which is slightly higher than the bulk porosity of 0.38. Most of the slugs displayed a rear that is denser than the front. However, some slugs had a front that is denser than the rear while other slugs displayed a relatively constant porosity over the entire length. Those unique results refuting the commonly used hypothesis that slugs are compact structures give a new incentive to the area of slug flow pneumatic conveying. While bulk solids mechanics can no longer be applied to explain the stresses induced by moving slugs, the validity of other theories that imply that slugs are fluidised structures should be investigated.

A wedged plane-flow hopper and horizontal belt feeder is employed to investigate the flow patterns and stress field redistribution at the hopper and feeder interface. The flow patterns are recorded by a high speed camera in conjunction with coloured material layers. The three-dimensional stress field in the feed zone and its influence on the feeder operation are discussed. The vertical stresses acting on the feeder for initial filling and flow conditions are measured along with longitudinal shear feeder loads. The experimental results are compared with theoretical values derived using relevant feeder load theories. The influences of different filling heights and clearance between the hopper bottom and feeder surface on feeder loads are presented. Numerical simulations using the Discrete Element Method (DEM) are carried out additionally to analyse feeder loads at the hopper and feeder interface, with the results being compared with those obtained experimentally.

Abstract The poorly understood mechanisms of slug flow remain an obstacle for widespread application of this dense phase flow. The parameters that characterise the flow have repeatedly been observed to have a bounded range of operating conditions; however, the ability to reliably predict these boundaries has not achieved the same level of repeatability. This paper presents a model to predict the absolute maximum transport boundaries for slug velocity and the absolute minimum transport boundaries for the layer fraction as a function of the gas mass flow rate. The predicted transport boundaries are supported by measurements, following which, the model is further developed as a prediction tool, where very good agreement with measurements is achieved. As the model provides relationships between all of the key parameters of slug flow, analysis is provided demonstrating the ability of the model to be applied as a convenient design tool for slug flow pneumatic conveying systems.

Slug-flow pneumatic conveying is a full-bore mode of flow within the dense-phase flow regime where bulk materials are transported in the form of slugs at conveying speeds below saltation velocity. The mechanism of slug-flow pneumatic conveying consists of the particles being picked up from the stationary bed in front of a moving slug while the same amount of material is deposited behind the slug. Stress field modeling of the material slug is the first step in developing a prediction model for the pressure drop along a pneumatic conveying line. However, a reliable prediction strongly relies on an accurate assessment of several factors, including the particle properties, pipeline dimensions, and operating conditions. So far, the particle diameter has always been one of the crucial parameters, which is not desirable in regards to the limitations it imposes on the choice of bulk materials. This article focuses on one parameter, the stress transmission coefficient kw, which relates the lateral wall stress with...

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.