• Energy Research
• 2021-2025close

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 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.

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.

This study introduces a novel methodology to evaluate the behaviour of biomass material by examining the ratio of aeration and deaeration time constants. To this end, a series of tests were conducted on four different materials, namely, cottonseed, wood chips, wood pellets, and wheat straw, in order to investigate their aeration and deaeration behaviours. The study derives the aeration and deaeration pressure drop equations, and discusses the corresponding time constant expression. Subsequently, the four materials were conveyed in 12 m long batch-fed and continuous pneumatic conveying pipelines to examine their behaviour in longer pipelines. The results indicate that the aeration and deaeration time constants increased with an increase in air superficial velocity. However, the ratio of the aeration and deaeration time constants was identified as a unique number, where a value close to 1 indicates a higher likelihood of plug flow. On the basis of the results, cottonseed, with the lowest ratio of time constant, was more likely to form a stable plug flow in both batch-fed and continuous pneumatic conveying. Given the unique properties of biomass and the limited research on the pneumatic conveyance of biomass, this methodology represents a novel approach for predicting modes of flow in materials with complex properties.

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.

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.

Within the field of pneumatic conveying horizontal (Plug-1) and vertical plug flows have been investigated only in the context of cohesive fine powders. This paper considers a series of experiments using fuzzy cottonseeds, which greatly differ in particle and bulk properties from fine powders, to investigate plug formation. In this study, several possible dense phase behaviours were observed, which were consistent in vertical and horizontal orientations and mostly influenced by the batch size of feeding into the rig due to its influence on particle arrangement. Particle arrangement at the plug base or rear was found to be critical for achieving stable plugs, with a requirement of the rear or base batch having the length of more or equal to pipe diameter. This work sheds light on the general features and mechanisms governing horizontal and vertical plug formation.

Abstract This paper presents the results of using an inertial measurement unit (IMU) to study various dynamic relationships in horizontal slug flow pneumatic conveying. The accuracy of the IMU was assessed and compared to particle image velocimetry (PIV) and once good agreement was confirmed it was used to investigate various aspects of slug flow. Relative movement between core particles and slugs tails and heads was assessed using relative pressures and quantified times spent in a slug. It was found that the propagation of particles backwards through a slug is relatively constant. Pressure-velocity relationship was observed that was theorised to be related to variations in stationary layer ahead of the slugs. Observations of further nuanced features of slug motion are also included to demonstrate the capabilities of IMUs in capturing the many dynamic aspects of the flow.