Measurement of an equivalent friction coefficient to characterise the behaviour of fertilisers in the context of centrifugal spreading

Abstract

In the context of centrifugal fertiliser spreading, numerous mechanical models demonstrate that the acceleration on the spinning disc depends on the friction coefficient of the fertiliser against the vane. Nevertheless, when the dynamic friction coefficient is measured by traditional methods, the values obtained do not reflect fertiliser behaviour differences. The study proposes a new estimation of the friction coefficient termed “equivalent friction coefficient” (EFC). This parameter is deduced from a mechanical model of the fertiliser motion on a spinning disc and from the measurement of the outlet angle of the particles. The outlet angle is measured using a simple imaging system that captures a high number of particle trajectories in the vicinity of the spinning disc in a few seconds. Values obtained for three common fertilisers are compared. For a flat disc rotating at 800 rpm and equipped with radial vanes, the EFC values are, respectively, 0.13, 0.14 and 0.44 for ammonium nitrate, NPK and KCl. The influence of disc configuration, vane profile and rotational speed is addressed. The influence of fertiliser moisture content is also investigated. Results demonstrate that this method can be used to obtain a relative classification of fertilisers. This paves the way for the definition of a normalized method, which will classify fertilisers in terms of real spreading behaviour.