Uncovering turbulence of dust particles in the Hartmann tube through the Image-Subtraction Method

The present work investigates the dynamics of dust clouds in space and time when dispersed inside the modified Hartmann tube commonly used for explosibility screening and Minimum Ignition Energy (MIE) measurement. This study focuses on the fluid dynamics of the dust cloud in the space between the electrodes where the ignition occurs since fundamental properties of the dust motion, such as the cloud turbulence (intensity and variation), are known to affect both the ignition sensitivity and explosion severity significantly. An imaging re-elaboration method based on an algorithm (Image-Subtraction Method, ISM) is presented and adopted in the basics of the present research. To clarify the cloud dynamics, a novel approach is proposed here, using LabVIEW® specific algorithms, namely Particle Analysis and optical flow detection methods, which allow the tracking of the motion and the velocity vectors of dust clusters identified in the cloud flow. Concurrently, measuring the intensity of concentration changes between the electrodes (luminance change of the video frames in time and space) and cloud velocity, which likely represents the turbulence, is possible. Different types of dust (iron, starch, silica) were used at different dispersion conditions (dispersion pressure and dust amount). The cloud motion was recorded, and videos were analyzed through LabVIEW® to explore the parameters affecting dust turbulence (powder-specific gravity, particle size distribution, and air blast intensity). The outcomes of this work will help characterize the flow of a dust cloud inside a tube before its ignition and better define the optimal testing conditions for MIE determination. • The Image Subtraction Method is used to study the dust cloud dynamics during flammability measurement. • The effects of blast pressure, concentration, density, and particle size were determined. • Ignition delay times should be varied over wide turbulence ranges during MIE testing. • Optimum test conditions are obtained for a homogeneous cloud at reduced turbulence. • A good indicator for defining optimal conditions of tests may be the Stokes number.