Enes Kaya explores the origins, principles, effectiveness, and limitations of physical scale modelling in chute design, comparing it to other techniques such as continuum mechanics and discrete element method modelling.
The design and optimisation of transfer chutes is a critical aspect of the materials handling industry. Properly designed transfer chutes ensure that the desired material throughput rates are achieved, that the material stream is controlled and that dusting, and wear are minimised.
Typically, the design of transfer chutes leverages multiple approaches including empirical methods, including simulation and physical scale modelling.
Related stories:
- Finalists revealed for 2024 Australian Bulk Handling Awards
- Ask an engineer: How can I build a better silo?
- Bulk solids handling design using DEM modelling
Jenike & Johanson is excited to combine these techniques to further the science of bulk material flow with the recent acquisition of Bulk Solids Modelling (BSM). This will enhance the company’s capability to solve the toughest bulk solids handling challenges for clients globally.
Early beginnings of physical scale modelling
Physical scale modelling has its roots in civil and hydraulic engineering, where scale models of dams, spillways, and other structures were used to study fluid flow and structural integrity.
During the 1990s this technique was adapted by BSM for use in materials handling to evaluate chute designs. Engineers were able to observe and test material flow, chute angles, and wear points before full-scale implementation.
Principles of physical scale modelling
The principles behind physical scale modelling are the application of similarity laws ensuring that the model built accurately represents the real system but in a scaled-down form.
The two types of similarity considered are:
- Geometric similarity: Ensuring all dimensions of the model are proportionally reduced by a consistent scaling factor. For example, a 1:10 scale model would have all linear dimensions reduced by a factor of 10.
- Dynamic similarity: Ensuring that the ratio between the major forces in the system remains constant as the scale of the system changes. These ratios are expressed as dimensionless terms such as the Froude Number.
How physical scale modelling works
In physical scale modelling, a scaled-down version of the transfer chute system is constructed, typically using acrylic to allow observation of material flow. The process involves several steps.
- Construction: Building the model to the appropriate scale, ensuring all geometric proportions are maintained.
- Material selection: A proprietary blend of surrogate material is used to emulate the behaviour of that in the actual system.
- Flow simulation: Introducing the scaled material into the model under conditions that replicate the full-scale application.
Testing involves observing material flow, identifying potential issues such as buildup or excessive wear, flow of material onto the receiving belt, and making iterative adjustments to the design. Through repeated testing and modifications, the design is then optimised for full-scale implementation.
Effectiveness and proven success of physical scale modelling
Physical scale modelling has proven effective for several reasons:
- Visualisation: It provides a tangible, visual representation of material flow, facilitating the identification and correction of design flaws.
- Empirical validation: There is an extensive history of success using dynamic scale modelling techniques. Many chutes that have been designed and validated using scale modelling are used in mining operations globally, demonstrating its reliability.
- Cost savings: Early identification of design issues prevents costly changes during full-scale construction and operation.
Comparison with other techniques
All modelling methods have both strengths and weaknesses.
- Continuum mechanics: This approach uses mathematical models to describe material flow as a continuous medium. It excels in fluid dynamics applications but may oversimplify the behaviour of granular materials common in transfer chutes. Continuum mechanics offers detailed stress-strain analysis but can struggle with particle flow behaviour.
- Discrete element method (DEM): DEM simulates material flows by calculating the interactions of a discrete set of model particles to represent the real material using Newtons’ laws of motion and particle contact models. It has the advantages that it can be conducted in the virtual space prior to fabrication or modification.
DEM output provides critical information about the flow of material such as surface forces and velocities. DEM typically requires significant computational resources. DEM simulations must incorporate valid calibration of the material, correct contact model selection, and input parameters. - Scale modelling: A physical scale model is an analogue system which uses a much larger number of particles than a DEM simulation. Once a model is constructed, simulations can be conducted very rapidly. Newtons’ laws are automatically obeyed in this system and particle interactions are set by the selection of the test material. Scale modelling provides an alternative method of validation that is independent of a DEM simulation. Disadvantages of the methodology are that there may be a significant lead time involved in constructing and fabrication the scaled model and that it requires a dedicated space to run the modelling program.
Conclusion
Physical scale modelling remains a valuable tool in the design of transfer chutes, providing unique benefits in visualisation and empirical validation of the designs. Its historical success and practical insights make it a reliable tool for transfer chute design evaluation.
Combining physical modelling with continuum mechanics and advanced computational techniques such as DEM can lead to more robust and efficient evaluation of designs. By leveraging the strengths of both traditional and modern methods, engineers at Jenike & Johanson can achieve the most accurate, cost-effective, and efficient solutions for material handling challenges.