Study of the granular fertilizers and the centrifugal spreader using Discrete Element Method (DEM) simulations

Since the beginning of production of mineral fertilizers, the worldwide crop yield has been increased dramatically, supplying the food for the exponentially growing population. In the 90’s of the 20th century however, a major concern about the environmental implications of fertilizer spreading has been developed. Over-doses can harm the environment seriously, as well as the crop itself. As a consequence, the spreadability of fertilizer and the uniformity of the spread pattern has become an important topic in this part of agricultural engineering. In Europe, 90% of the fertilizer spreading applications are performed by centrifugal spreading. The popularity of this technique lies in their low price, easy maintenance, and large working width. However, its design is highly sensitive to machine characteristics as well as fertilizer particle properties and weather conditions (e.g. wind). As a consequence, research about the mechanism of spreading has become inevitable. A lot of effort on the influence of machine characteristics on the spread pattern has been done by Olieslagers (1997). In this thesis, the emphasis will be put on the study of fertilizer particles. These particles interact with the machine parts as well as with each other. Hereto, the “Discrete Element Method“ computational technique is introduced. Basically, DEM calculates all possible interaction between objects using force models based on spring – dashpot systems, and describes their motion using classical dynamics. By changing the physical properties of the individual particles, such a method allows a comprehensive study of the particle flow on a rotating disc. The physical properties of the fertilizer serve as the input parameters of the model. However, due to nature of fertilizer particles, they are hard to determine. Several techniques have been proposed in this thesis to provide either an accurate measurement or an acceptable estimation of these properties. The accuracy of the model will largely depend on value of the input parameter as well as the sensitivity to this parameter. Before trying to simulate the whole spreading process, the interaction of a single particle with the machine parts (e.g. disc, vanes,..) of the spreader is considered. This step is primordial since it allows a decomposition of the physical problem as well as an easy validation of the simulation model with experiments. Having obtained this insight, simulations of a particle flow on the disc (multi particle simulations) can be carried out. Both the performance of these simulations and the experimental validations are intensive tasks, requiring a lot of time. However, they are mandatory in order to get full insight in the parameters that affect the spread pattern and the predictive value of the model. Generally, it was found that the DEM simulations perform qualitatively well. The cause of the deviations (below 25%) between simulations and experiments remains uncertain but is likely to be found in the insufficient knowledge of the initial conditions of the particles (e.g. outflow of the bin) as well as the contact force mechanisms. In the light of the accuracy of the measured particle properties and the model output, a sensitivity study is carried out in this research. The main goal herby is to gain insight in the parameters that influence the spread pattern, and those who don’t. Although done in a qualitative way, such a study can provide valuable information for the manufacturer in the development of new fertilizer kinds or spreader designs. In this research, it was found that friction plays a crucial role in the dynamics of the particles on the disc, and the friction coefficients should be treated as velocity dependent parameters. An important part of the errors made by the model could be explained by this.-General introduction -The Discrete Element Method -Particle measuring methods as input for numerical models -Single particle simulations -Multi particle simulations -Model parameter sensitivity -Case study: DEM simulations of a new concept for lawn spreader -General conclusions and future outlinestatus: publishe