A discrete element approach for simulating the compression of fibrous biomass

Due to the growing world population, the agricultural sector needs more productive and more energy-efficient agricultural machinery in order to adequately address the growing demand for food and biomass. Therefore, the sector significantly invests in the optimization of agricultural machinery.

Historically, the optimization of agricultural machinery was done by trial and error. Design improvements of agricultural machinery are still often based on the experience and the insights of engineers and farmers. To test whether an adjustment has a positive effect, a prototype is developed and validated during field tests. This optimization method, however, has some disadvantages. Developing and constructing a prototype is costly. Moreover, prototypes can only be validated in field conditions during the growing season. These two factors oblige agricultural machinery manufacturers to opt for small adaptations with a high success rate. The current generation of agricultural machinery is, therefore, the result of decades of evolution.

Now that computing power increases, another opportunity to improve the design of agricultural machinery presents itself. Models and simulations facilitate the optimization of machines. However, an accurate virtual crop model is missing. A simulation model that accurately describes the interactions between individual crop stems and the interactions between crop stems and machine components could be used to improve the design of stem processing agricultural machinery. In this thesis, such a simulation model was developed for the processing of crop stems in a baler.

It has previously been shown that the Discrete Element Method (DEM) can be used to simulate and optimize particulate processes. For this, two requirements need to be met. Realistic particle geometries and realistic deformation models are required to obtain accurate simulation results. The virtual crop stems, therefore, need to be compressible and bendable. Also the frictional and tensional properties need to be realistic.

A first step in the development of the DEM simulation model included an analysis of the bending behaviour of crop stems. It was observed that there are two phases during bending: ovalisation and buckling. The forces that occur during ovalisation result in a flattening of the cross-section of the stem and this reduces the bending resistance. This process continues until the maximum force has been reached and the stem buckles. Buckling is associated with a strong reduction of the resistance to bending. The influence of the stem diameter, the thickness of the stem wall and the presence of a core-rind structure were examined for wheat and barley stems. All were found to affect the bending behaviour significantly.

The acquired knowledge was used to develop a data based bending model for flexible particles (i.e. crop stems) in DEM. The influences of the stem length, the support distance and the number of segments which make up the virtual stem, were examined. The same data based method was also used for developing a compression model for virtual stems in DEM. For this purpose, the interactions between individual stems and the interaction between a stem and a plate were studied and modelled. The models were successfully validated by comparing bulk compression simulations and measurements. For this purpose 250 stems were compressed in a box by the movement of a plunger.

To study the influence of friction on the processing of stems, measurements were performed on the stem level. The measured coefficients of friction were significantly lower than those found in the literature, which have been measured on a bulk level. The influence of friction on bulk compression was evaluated and it was found that a small change in the coefficient of friction at stem level has a significant effect on the bulk behaviour.

The last stem parameter that was studied was the tensional stiffness. Stem measurements were again performed for this purpose. The force was found to increase linearly with increasing deformation up to the point where the stem broke. A linear tensional model was therefore implemented in DEM. Afterwards, the influence of the tensional resistance on the bulk deformation behaviour was examined. The effects of the tensional model parameters were found to be very limited. Therefore, the model parameters of the tensional model were selected in such a way that the computation time was minimized.

The effect of strain rate on the force-deformation behaviour at the stem level was studied using a pendulum device. However, no significant effects could be observed for the tests at low and high speed with the used set-up.

When the stem properties were measured and after they were modelled in DEM, the influence of the stem variability (e.g. the variability in physical and mechanical properties) on the bulk deformation behaviour was determined. To this end, simulations were performed with different degrees of variability. As a validation, bulk compression tests were performed. It was observed that a limited number of stem measurements can be sufficient to obtain accurate DEM simulations. As more stems are measured and as the stem database becomes larger, the accuracy of the simulations increases. However, the accuracy gained by measuring additional stems decreases with an increasing crop database. A statistical method was therefore presented to determine the minimum number of stem measurements needed to obtain accurate DEM simulations.

When the behaviour of crop stems was fully characterized and modelled and after insights were obtained on the influence of stem variability, DEM simulations were performed regarding the processing of crop stems by the rotor of a large square baler. First, a method was developed to create virtual swaths. Scalability was demonstrated with these swaths. This reduced the computation time of thesimulations. Again, friction was found to have a significant impact on the crop processing as a higher coefficient of friction led to a higher energy consumption. When stems are damaged, less energy is required for their processing. Also, the feed rate was found to have an influence. The energy demand increased as more stems were processed simultaneously. Finally, the shape of the swath was also found to have a major impact on the required torque. An evenly filled swath was found to require less processing energy than an unevenly filled swath.

In a final step, the filling of the pre-compression chamber was also simulated and successfully validated with stationary measurements. An increasing swath mass, the presence of the trip sensors (determining when the the pre-compression chamber is full) and a reduced rotor speed were found to have a negative impact on the required energy. The crop flow in the simulations was visually compared to the crop flow in the measurements. A high-speed camera was used for this purpose. The crop flows were found to be similar. However, a quantitative analysis should be performed to confirm this.

The simulation model is now ready for the optimization of the design of the pick-up and feeding sections of the agricultural baler. The knowledge that was gained in this dissertation is more broadly applicable and could, for example, also be used for optimizing sections of the combine and the forage harvester. However, more research is required to accurately simulate nodes, leafs and ears. The influence of the strain rate and the number of stem measurements required to obtain accurate DEM simulations of crops should also be studied in more detail. Modelling the cutting and breaking of stems would have a positive effect on the accuracy and the applicability of the simulations. Since in many processes air currents have a major impact, a coupling with a CFD software is also necessary.