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CFD-DPM Analysis of Velocity and Pressure Distribution for Selected Agricultural Cereal Grains for Straight Pipe in Pneumatic Conveyor

This study utilizes CFD-DPM simulations to demonstrate that the pneumatic conveying performance of five cereal grains (teff, wheat, maize, sorghum, and barley) in a straight pipe varies significantly based on their specific particle size and shape characteristics rather than density, with teff exhibiting the highest resistance and sorghum the lowest.

Original authors: Lemi Demissie

Published 2026-09-11

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Lemi Demissie

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a vast network of invisible rivers, not of water, but of air, rushing through metal tubes to carry the world's food. This is the reality of pneumatic conveying, a method used in modern agriculture to move bulk grains like wheat, corn, and barley from silos to processing plants. Instead of using belts or buckets, these systems rely on powerful fans to push or pull grains through pipes, offering a cleaner and more flexible way to transport massive quantities of material. However, just as a river flows differently over smooth stones than it does over jagged rocks, air moves differently depending on the shape and size of the grains it carries. While engineers have long understood the basics of moving bulk materials, the specific behavior of different cereal grains—especially those with unique physical traits—remains a complex puzzle. If the air moves too slowly, the grain falls and clogs the pipe; if it moves too fast, the grain breaks or the system wastes energy. Understanding exactly how different grains interact with the air inside a straight pipe is essential for designing efficient systems that minimize waste and energy use.

In a recent study, a researcher from Dilla University in Ethiopia set out to solve a specific piece of this puzzle by comparing five distinct cereal grains: teff, wheat, maize, sorghum, and barley. The goal was to see how these grains behave when pushed through the same straight pipe under identical conditions. To do this, the researcher did not build a physical pipe and run thousands of tons of grain through it. Instead, he used a sophisticated computer simulation, a digital twin of a real-world system. This method, known as computational fluid dynamics coupled with a discrete phase model, allowed him to visualize the invisible forces at play. The computer calculated how millions of individual grain particles would move, collide with the pipe walls, and interact with the rushing air, tracking their speed, the pressure they created, and the time it took them to travel the length of the pipe.

The study focused on a straight pipe with a diameter of 71 millimeters and a length of 4 meters. The simulation tested the grains at an air speed of 25 meters per second, a standard velocity for such systems. The results revealed a striking truth: even though the grains were moving through the exact same pipe at the same speed, they did not behave the same way. The most surprising finding centered on teff, a tiny grain native to Ethiopia that has gained global popularity for its nutritional value. Despite having a density similar to other grains, teff proved to be the most difficult to move. The simulation showed that teff created the highest pressure drop, meaning the air had to work the hardest to push it through. In the horizontal section of the pipe, teff generated a pressure drop of 1,087.88 Pascals, and in the vertical section, it reached 1,184.11 Pascals. This resistance was significantly higher than that of the other grains.

The reason for this struggle lies in the grain's physical form. Teff is exceptionally small, with a mean diameter of just 0.86 millimeters. Because the particles are so tiny, there are far more of them in a given weight of grain, creating a massive total surface area that rubs against the air. This creates intense friction and drag, forcing the air to lose more energy to keep the grains suspended. In contrast, maize, with its much larger particles of 7.91 millimeters, moved with the least resistance, requiring the lowest pressure to transport. Sorghum also performed very well, requiring the least energy of all the grains tested, with a pressure drop of only 813.21 Pascals in the horizontal pipe. This efficiency was attributed to its shape; sorghum grains are nearly spherical, allowing them to roll and slide through the air with minimal turbulence.

The study also highlighted that the shape of a grain matters just as much as its size. Barley, which has the lowest density of the group, surprisingly required more energy to move than the heavier maize. This was because barley grains are irregular and angular, with a low "sphericity" score. These jagged shapes disrupt the smooth flow of air, creating turbulence and increasing drag, much like how a flat stone skips differently than a round one on water. Wheat fell somewhere in the middle, showing intermediate resistance that matched its moderate size and shape. The simulations confirmed that while the density of the grains was relatively similar, the differences in particle size and shape were the dominant factors determining how much energy the system needed.

When the researcher looked at the stability of the flow, the data showed that teff and barley shared the most stable performance regarding horizontal velocity changes, both exhibiting a very low variability of 3.4%. However, this consistency came at the cost of high energy demand for teff. Sorghum, while the most energy-efficient, showed the highest variability in its vertical pressure drop (13.3%), suggesting that its flow might be more sensitive to small changes in the system. The study also examined teff flour, which is even finer than the whole grain, and found that its tiny particles created even stronger interactions with the air, further increasing resistance.

Ultimately, this research demonstrates that a single design for a grain conveyor cannot work perfectly for every type of cereal. The assumption that all grains behave similarly is incorrect. The study concludes that engineers must tailor their systems to the specific physical characteristics of the grain they intend to move. For a grain like teff, with its tiny size and high resistance, the system requires more powerful fans and careful pressure management to avoid clogging and energy waste. For larger, rounder grains like sorghum, the system can be more relaxed. By understanding these nuances, designers can build more efficient, cost-effective, and reliable systems that respect the unique nature of the food they carry, ensuring that everything from the smallest teff seed to the largest corn kernel reaches its destination with minimal loss.

Technical Summary: CFD-DPM Analysis of Velocity and Pressure Distribution for Selected Agricultural Cereal Grains

Problem Statement
Pneumatic conveying is a critical component of modern agricultural supply chains, yet existing systems often rely on generalized designs that fail to account for the distinct physical characteristics of different cereal grains. While grains like wheat, maize, and sorghum are commonly transported, the unique properties of grains such as teff (Eragrostis tef) present specific challenges. Teff is characterized by an exceptionally fine particle diameter and high bulk density, which can lead to significantly different gas-solid interactions, pressure drops, and energy requirements compared to larger, more spherical grains. Current literature lacks detailed numerical studies evaluating these specific interactions, particularly for teff, leading to potential operational inefficiencies, excessive energy consumption, and material degradation when standard conveying parameters are applied across diverse grain types.

Methodology
This study employs Computational Fluid Dynamics coupled with the Discrete Phase Model (CFD-DPM) to simulate and compare the pneumatic conveying behavior of five cereal grains: teff, wheat, maize, sorghum, and barley. The simulations were conducted in a straight-pipe, dilute-phase pneumatic conveying system with a pipe diameter of 71 mm and a length of 4,000 mm.

Key methodological elements include:

Turbulence Modeling: The Re-Normalization Group (RNG) $k-\epsilon$ turbulence model was utilized to accurately capture high Reynolds number flows, swirl, and boundary-layer separation typical of industrial pneumatic transport.
Particle Tracking: The Discrete Phase Model tracked individual particle trajectories, accounting for drag forces, gravity, and other inter-particle forces. The simulation assumed spherical particles for drag calculations but incorporated specific sphericity factors for each grain type to reflect real-world geometry.
Grid Independence: A grid independence study was performed to ensure that simulation results regarding particle trajectories and flow interactions were not artifacts of mesh resolution. The mesh size was optimized to be 4 to 10 times the mean particle diameter.
Operating Conditions: Simulations were run under identical operating conditions for both horizontal and vertical pipeline configurations, including specific material flow rates (e.g., 0.13 kg/s for grains, 0.044 kg/s for teff flour) and air velocities (ranging from 20 to 28.9 m/s).
Physical Properties: The study utilized specific physical parameters for each grain, including mean true density, mean diameter, and sphericity. For instance, teff was modeled with a mean diameter of 0.86 mm and a density of 1284 kg/m³, while maize had a diameter of 7.91 mm and a density of 1087 kg/m³.

Key Results
The CFD-DPM analysis revealed substantial variations in conveying resistance and flow dynamics among the five grains, even under identical pipeline configurations:

Pressure Drop and Resistance: Teff exhibited the highest pressure drop and conveying resistance in both horizontal and vertical configurations. In horizontal pipes, teff showed a pressure drop of approximately 1087.88 Pa, compared to 813.21 Pa for sorghum (the lowest). In vertical pipes, teff reached 1184.11 Pa, while sorghum remained at 975.23 Pa.
Influence of Particle Size and Shape: The study determined that particle size and shape are the dominant factors influencing conveying behavior, outweighing the effects of particle density.
- Teff: Its exceptionally small particle size (0.86 mm) resulted in a high surface-area-to-mass ratio, leading to stronger gas-solid interactions and greater drag. Its relatively high density further increased the energy required for suspension.
- Barley: Despite having the lowest density, barley exhibited high resistance due to its low sphericity (0.48), which increased turbulence and drag.
- Sorghum: Showed the lowest resistance, attributed to its high sphericity (0.91) and favorable particle geometry, which minimized drag and inter-particle friction.
- Maize: Demonstrated lower resistance than expected relative to its density, primarily due to its large particle size (7.91 mm), which reduced the total surface area interacting with the air.
Velocity Distribution: Air velocity variations were most pronounced for teff, requiring higher velocities to maintain suspension. Conversely, larger grains like maize required lower air velocities for effective transport.
Teff Flour vs. Whole Grain: A specific simulation of teff flour (40 µm particle size) indicated that reducing particle size further significantly increases flow resistance and pressure loss due to the massive increase in total particle surface area and momentum transfer requirements.

Significance and Claims
The paper claims that the findings provide a comparative basis for understanding grain-dependent conveying behavior, challenging the assumption that a single pneumatic conveyor design can be generalized across different cereal grains. The study asserts that:

Design Specificity is Essential: Pneumatic conveying systems must be evaluated and designed according to the specific physical characteristics (particularly particle size and shape) of the grain being transported.
Teff Requires Special Attention: Due to its unique combination of small size and high density, teff poses the greatest conveying challenge and requires distinct operational parameters compared to standard grains like wheat or maize.
Numerical Validation: The study offers a validated numerical framework (CFD-DPM with RNG $k-\epsilon$ ) for predicting pressure drops and velocity distributions, which can assist engineers in optimizing pipeline transport systems to mitigate energy losses and prevent operational failures.

The author concludes that while the study provides significant numerical insights, future work should focus on experimental validation of these predictions and the examination of additional variables such as moisture content and varying pipeline dimensions to further enhance the practical applicability of the model.

Technical Summary: CFD-DPM Analysis of Velocity and Pressure Distribution for Selected Agricultural Cereal Grains

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