学 术 报 告
Flame Propagation in Hybrid Methane - Coal Dust – Air Flames
Ali S. Rangwala
Department of Fire Protection Engineering,
Worcester Polytechnic Institute, Worcester, MA 01609 USA
Dust deflagrations or flame propagation in a mixture of flammable particles (~10 - 100 µm) and air or gases has gained increasing importance in industrial fire and explosion safety. Every year dust deflagrations in coal mines and chemical plants cause extensive material damage, injury, and loss of life. Current guidance for prevention and suppression originates from experiments performed in simple vessel arrangements, and the parameters used to assess the hazard of flammable dusts are empirically driven. The problem is thus unresolved from a fire and explosion safety perspective. From a scientific viewpoint enhancing our fundamental knowledge of particle combustion lies at the heart of national security priorities such as energy efficiency and pollution control by improving the design of power plants. There has also been a recent push towards studying influence of nano-particulate matter in combustion systems.
In this talk, I will describe the results of laboratory experiments to identify the controlling parameters of laminar and turbulent hybrid dust deflagration mechanisms (Xie et al., Comb. Flame, 159, 2449-2456, 2012, and Rockwell and Rangwala, Comb. Flame, 160, 635-640, 2013). 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 25 to 106 µm and methane-air. Figure 1 depicts shadowgraph images of a sample of flames tested. The results show that adding 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 the coal particles take up to release the volatiles. A mathematical model is developed considering these effects. The implications of the work towards numerical modeling of turbulent particle air flames is discussed
Figure 1: Shadowgraph images of: (a) laminar methane-air flame (b) laminar methane-air-dust flame (c) turbulent methane-air flame, (d) turbulent methane-air-dust flame. Methane-air equivalence ratio, Φ = 0.9 for all cases. Coal-dust particle size = 75 – 90 µm in (b) and (d), and concentration = 70 g/m3. Turbulent intensity u’ = 0.7 m/s for (c) and (d).
Ali S. Rangwala is an associate professor in the department of Fire Protection Engineering at Worcester Polytechnic Institute (WPI). 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 are in the area of industrial fire and explosions. His recent projects 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.