High Speed ​​Imaging Accelerates Dust Explosion Investigation

In many industries, dust fires and explosions pose serious risks. Therefore, it is necessary to assess the explosibility and flammability of dusts in industrial installations to recognize the dangers and implement the appropriate safety measures.

What is the MIE Minimum Ignition Energy?

The minimum energy required to ignite an explosive dust-air combination with a high voltage spark discharge is known as the minimum MIE ignition energy of a dust cloud.

Dust cloud MIE measurements are crucial for determining plant equipment grounding and bonding specifications and for predicting the possibility of dust cloud ignition during solids handling operations.

MIKE3 is a dust cloud MIE measuring device similar to other vertical explosion tube devices.

Factors that affect MIE measurements

MIE measurements are affected by several particle characteristics such as morphology, polydispersity and size distribution.

MIE results are also influenced by test environment conditions, including oxidant composition and spark discharge method.

A concept similar to the fire triangle is a dust explosion pentagon that includes five essential causal factors: ignition, containment, mixing, oxidizer, and fuel.

Advanced measurement techniques and MIE

Advanced measurement techniques complement MIE data and help explore the underlying chemistry and physics of dust cloud combustion.

Advanced experiments in vertical blast tube setups use lasers, cameras, and several other sensors to extract high-resolution data from the dust cloud. For example, researchers have used digital on-line holography (DIH) in several experiments to help collect quantitative data on particles inside the MIKE3 vertical tube setup.

Another study combined online digital holography with particle image velocimetry (PIV) which enabled the measurement of macro-scale flow and micro-scale particles in a MIKE3 glass tube.

What is Chemiluminescence?

Chemiluminescence is described as the emission of light from chemically stimulated species returning to their electronic ground state. Species-specific data is obtained from the combustion reaction zone of the dust cloud using chemiluminescence imaging.

Excited-state radicals such as methylidyne (CH*) and hydroxyl (OH*) in hydrocarbon flames help identify the flame front and reaction zone, respectively. As a result, flame chemiluminescence can be used to more accurately assess flame structure in burning dust clouds.

Comparison with previous studies

The experiment described in this study combines broadband, OH* and CH* high-throughput imaging for in situ, non-intrusive measurements of the flame core in the MIKE3 device. It features advanced dust cloud ignition and combustion measurements that are fully compatible with MIKE3 device operation and typical MIE data generation.

Most previous studies of flame spread in comparable configurations and spatial domains lacked sufficient temporal and spatial resolution and were less sensitive. Previous studies have focused on measuring the velocity of the leading edge of the flame rather than the development and motion of the flame core. Therefore, under an industry standard test environment, this study constitutes a thorough evaluation of the flammability of the dust cloud.

How the experiment was conducted

The experimental setup included a UV lens, a high-speed camera, a nozzle cup, a glass tube, an ignition electrode, and a MARK3 device. A dust cloud MIE test device was used to create and ignite the dust clouds.

The components of the MIKE3 device included two tungsten electrodes for generating spark discharges, a vertical flow-limiting glass tube, a mushroom-shaped nozzle for dispersing dust with a jet of compressed air and a cup to contain the dust. The spark ignition energies required for aluminum and niacin were 300 mJ and 20 mJ, respectively. A moving electrode discharge circuit was used to provide these spark energies.

An ultraviolet-sensitive lens mounted on a high-speed camera was used to image the resulting dust flames. Using a 5-volt trigger signal from the solenoid valve that operates the air puff in the MIKE3 device, it was possible to synchronize the high-speed camera and the MIKE3 device to the using a digital delay generator.

Important Study Findings

This study implements high-speed, species-specific imaging (OH* and CH*) for the measurement of spark-ignited dust clouds inside the MIKE3 device.

The researchers focused on the development of the flame core from clouds of aluminum dust and niacin after spar ignition for 10 ms in the central ignition region.

An intensity thresholding algorithm extracts velocity, position, and core size measurements from high-speed image patterns. In addition, samples of aluminum dust and niacin have studied the combustion properties of metals and organic fuels.

A continuous non-uniform reaction zone composed of excited-state species and particle clusters was evidenced by niacin flame nuclei that grew 5-17 mm and moved at velocities of 5 m/s from the central region of the electrode.

An unresolved internal structure with a bright reaction zone was shown by aluminum flame cores comprising distinct burning particles near its boundary. The aluminum flame cores grew from 7 to 10 mm and moved at a speed of 3 m/s from the central region of the electrode.

Future prospects

In future studies, chemiluminescence of other species such as aluminum oxides (AlO) can be incorporated to better understand substance-specific sparking dust clouds.

The current study can be extended to the spread of flame in the overall combustion process instead of focusing only on the early growth and motion of the flame core.

Reference

Christian Schweizer, Chad V. Mashuga, Waruna D. Kulatilaka (2022) Investigating the ignition characteristics of niacin and aluminum dust clouds in an explosion hazard test device using high angle imaging speed. Process safety and environmental protection. https://www.sciencedirect.com/science/article/pii/S0957582022006966

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