Abstract :

Initiation and propagation of dust deflagrations are extremely complex phenomena due to the interaction between solid particles and the gaseous flame front. In comparison with premixed gas deflagration, a dust-oxidizer deflagration depends on the rate of evolution of volatiles, the mixing of these volatiles with the oxidizer surrounding the particles, coupling of the particles and gas phase oxidation as well as radiative energy exchange between the flame and its surroundings. Due to these complications, a comprehensive mathematical theory to predict deflagration mechanisms of dust clouds is at present beyond reach. Although vast amount of testing, both small scale (20 liter explosion vessel) and large scale tests have been done over the last 50 years, most theories that connect the data to models are heavily empirical and the problem has never been analyzed from a fundamental viewpoint. This study will identify the controlling parameters of laminar and turbulent hybrid dust deflagration mechanisms. To study flame propagation in dust clouds a novel premixed-dust-air burner is designed to measure the burning velocity of a hybrid mixture of Pittsburgh seam coal dust, with typical particle sizes in the range of 75 to 90 µm and methane-air. Figure 1 depicts shadowgraph images of a sample of flames tested. The results show that the addition of coal dust in methane-air premixed flame reduces the burning velocity for laminar flames and increases as turbulent intensities are increased. Two competing effects are considered to explain these trends. The first effect is due to volatile release, which increases the overall equivalence ratio and thus, the burning velocity. The second is the heat sink effect that the coal particles take up to release the volatiles. This process reduces the flame temperature and accordingly the burning velocity also. A mathematical model is developed considering these effects and it is seen to successfully predict the change of burning velocity for various cases with different dust concentrations and equivalence ratios of the gas mixture. Furthermore, the implication of this study to coal mine safety is discussed. Ali S. Rangwala is an associate professor at the department of Fire Protection Engineering at Worcester Polytechnic Institute (WPI) (2006 – present). He has a BS in Electrical Engineering, from the Government College of Engineering, Pune, India (2000), an MS in Fire Protection Engineering from the University of Maryland, College Park (2002), and a PhD in Mechanical and Aerospace Engineering from the University of California, San Diego (2006). Professor Rangwala’s research interests include, deflagration of combustible dust clouds, ignition behavior of combustible dust layers, in-situ burning of oil, spread of an oil slick in channels, velocity measuring techniques in fire induced flows, and flame propagation and burning rate behavior of condensed fuel surfaces. He has his own research laboratory (1500 square feet) at WPI, and is currently advising 6 funded graduate students (4 PhD’s and 2 MS thesis). In the last few years, he has published over 15 journal articles and has presented in over 30 conferences. He has papers in journals such as Journal of Hazardous Materials, Combustion and Flame, Combustion Theory and Modeling, Fire Safety Journal and Fire Technology. He teaches three graduate courses: Explosion Protection, Industrial Fire Protection, and Combustion at Worcester Polytechnic Institute, Department of Fire Protection Engineering.