CFD to Improve the Repeatability and Accuracy of Dust Explosion Tests in the 20-liters Sphere

Abstract

The experimental characterization of combustible dusts is usually associated to several uncertainty factors that rely on the physical properties of the powder and the operating conditions of the standardized tests (e.g. the ignition time, the dispersion nozzle and pressure). These parameters affect the repeatability of the flammability tests and the precision, or even the accuracy, of their results. In this regard, an explanatory study has been focused on the description of the dispersion process of a combustible dust inside a 20-liters sphere by joining two complementary approaches. At first, a Computational Fluid Dynamics simulation (CFD – Star CCM+) based on an Euler-Lagrange scheme was set according to the geometry and operating parameters that have been established for this equipment through international standards. The predictive results that were determined with this approach were compared with a Particle Image Velocimetry analysis and in-situ measurements. Explosions tests were also performed to validate our analyses.
The results showed how the particle size distribution of the dust affects its dispersion trajectories inside the sphere due to the inertial effects and the drag force exerted by the fluid. Furthermore, the analysis of the velocity field and turbulence intensity reveals that the dispersion process can be divided into three different stages, each related to a different repeatability. Tests performed with the classical rebound nozzle have clearly evidenced their effect on the internal distribution of the gas flow and the segregation levels of the disperse dust.
These facts suggest that the homogeneity assumption that is usually considered for the dust dispersion is obviously invalid. As a consequence, the operating conditions should be adapted to the physical properties of each combustible powder in order to improve both tests accuracy and precision. For this purpose, CFD simulations have been proven to be a useful tool to identify the most suitable conditions to perform this analysis and obtain the most conservative information about the reactivity of the dust as well.