FLAME PROPAGATION OF DUST AND GAS-AIR MIXTURES IN A TUBE

The flame propagation of hybrid (nicotinic acid/methane-air) mixtures has been studied using the open-tube method. During the flame propagation four phases can be distinguished and two phases, the first and the third, have been selected as useful for the evaluation of the burning velocity. The first phase has a spherical shape propagation mode while during the third phase the flame propagation can be considered as pseudo stationary and planar. In both phases, the role of the pre-ignition turbulence and of the turbulence induced by the flame itself has been addressed. The main issues which arise when estimating the burning velocity of hybrid mixtures have been evaluated and suggestions for future work are addressed. In the first phase the flame propagation is laminar but mixing between methane and nicotinic acid is prevented. In this case, in order to guarantee a homogeneous dispersion of the dust as well as a good mixing with the flammable gas, the injection of the dust/gas mixture should be realized from the reservoir. In the third phase, the turbulence induced by the flame propagation plays a major role and the determination of the turbulence level is required in order to evaluate the burning velocity. Introduction Explosion severity of dust-air and dust/gas-air mixtures strongly depends on the mode of flame propagation. Several studies have been devoted to reveal the mechanisms driving the flame propagation of dust-air mixtures. The widely accepted idea is that the key mechanisms are heat transfer by radiation and/or conduction, heterogeneous reaction, production of volatile components, heat transfer by convection and conduction [1-3]. All these mechanisms have to be considered when quantifying the burning velocity. However, the role of heat transfer by radiation is often negligible [4,5] and for small particle sizes the controlling mechanism is generally the combustion of volatiles [6]. In order to fully characterise the flame propagation and then the explosion behaviour of dust-air mixtures, the knowledge of the flame burning velocity is required. The method for the evaluation of the dust-air mixtures burning velocity is different from that of gas-air mixtures since the dust dispersion has to be accomplished. In the literature, three methods for measuring the laminar burning velocity of dust-air mixtures have been implemented: burner method [4,7-10], contained explosions method [4,11-13] or tube method [1,2,4,5,14-26]. In this latter, also called the “open tube method”, the propagation of the flame front occurs in a partially open and transparent tube and so the evaluation of the burning velocity is direct and is performed by following the images of the flame front by means of a video camera. The dust dispersion is ensured by injecting a mixture of dust-air into the tube from a pressurized reservoir [16,24]. Krause and Kasch [22] studied the dust flame propagation in a cylindrical fluidized bed. From all these studies it appears that the most relevant concern is the dust dispersion. Dust dispersion is guaranteed by injecting the dust mixtures into the tube or the closed vessel thus generating pre-ignition turbulence. As a consequence, measurements of the burning velocity are strongly dependent on the turbulent flow conditions. Apart from pre-ignition turbulence, turbulence is generated by the flow induced by the expanding burnt gases which eventually affects the flame propagation. Considerable effort has then been devoted to the evaluation of the turbulence induced by the dust-air mixture injection into the tube. Schneider and Proust [18] visualized both the propagating flame and the turbulence intensity using a high speed digital video camera and Laser Doppler Anemometry (LDA). Wang et al. [24] measured the RMS of the turbulent velocity fluctuations in the horizontal and vertical directions, generated in the tube after dust injection, as a function of the ignition delay time elapsing between the time of injection and the time of ignition. They were able to measure the dust concentration at different heights along the tube showing that the dust concentration is not uniform along the tube height. This aspect is more complicated when dealing with the study of the flame propagation of hybrid mixtures (dust/gas-air), where it is important to control not only the concentration of the dust and its dispersion along the tube length, but also the fact that the gas to dust concentrations ratio is uniform everywhere. In the literature, few works have been devoted to the study of the flame propagation of hybrid mixtures [27-31]. Bradley et al. [27-29] studied the laminar burning velocities of methane-air-graphite mixtures and of fine coal dusts by using a burner. Methane and air were fed in a pre-mixing chamber; then this mixture was divided into two streams, one passing through a fluidized bed to entrain the graphite. They demonstrate that in the coal dust explosion the pyrolysis/devolatilization step is very fast and that the combustion occurs substantially in the gas phase. Moreover, they underlined that the presence of the char does not change the gas phase composition and kinetics. Liu et al. [30] and Chen et al. [31] studied the flame propagation of methane and coal dust in a vertical combustion chamber to measure the flame speed. A premixed methane-air mixture is injected in the tube to disperse the dust in the chamber. They found that in the presence of methane, even at concentration lower than LFL, the flame speed and the flame front temperature are higher than that of coal dust flame. They observed an initial flame with poor light imputable to the methane flame and only subsequently the coal dust starts participating to combustion. The combustion of coal dust is further complicated by the presence of flammable methane, this prevents a deep understanding of the detailed nature of coal dust/methane-air combustion propagation process. However, the authors did not deal with the issue of dispersion of the gas and the dust as well as how the turbulence influences the flame propagation of the hybrid mixture. In this paper, tests have been performed with methane-air, nicotinic acid-air and nicotinic acid/methane-air mixtures at various dust concentrations (up to 200 g m) and methane contents (up to 8 % v/v). Firstly, the feasibility of the tube method for the measurement of the burning velocity of hybrid mixtures has been evaluated by highlighting the role of the dust dispersion and mixing with the flammable gas. Moreover, the role of turbulence, both preignition turbulence and induced turbulence, on the values of the burning velocities has been discussed. Eventually, some suggestions have been done to improve the tube method when dealing with the measurements of burning velocity of hybrid mixtures. Experimental apparatus The flame propagation experiments have been performed in a vertical tube of 1 m height with a square cross section of 0.07 x 0.07 m (V = 4.9 L). The tube has two opposite walls made of glass and two opposite wall made of stainless steel. 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