Critical dimensions of holes and slots for transmission of gas explosions: Some preliminary results for propane/air and cylindrical holes |
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| Institution: | 1. Department of Mechanical Engineering, Stanford University, Stanford, CA, USA;2. Chair of Fluid Dynamics, Faculty of Engineering, University of Duisburg–Essen, Duisburg, Germany;3. Department of Chemical Engineering, Queen’s University, Kingston, ON, Canada;4. Institute of Propulsion Technology, German Aerospace Center (DLR), Cologne, NRW, Germany;5. Department of Mechanical and Aerospace Engineering, University of California, Irvine, Irvine, CA, USA |
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| Abstract: | Critical hole diameters for explosion transmission from a primary virtually closed chamber into an ambient gas cloud were determined. Most of the present experiments were conducted with a 1-l primary chamber. The motivation for the study is two-fold. First, results from this kind of experiment are of direct practical use in further improvement of design and maintenance procedures of “flame-proof” electrical equipment. However, such experiments can also contribute to improvement of the general understanding of the mechanisms of flame propagation in turbulent, premixed gases. The preliminary experimental results presented confirm that the minimum tube diameter for flame transmission depends strongly on the location of the ignition point. The generally accepted limiting values are conservative, in the sense that they can only be approached if the ignition source is located in a narrow zone in the vicinity of the entrance to the transmission hole. This must be taken into account if flame-proof equipment is to be designed on the basis of risk analysis. An observation related to mechanisms of turbulent flame propagation in premixed gases in general was also made. In the case of ignition far away from the transmission hole, i.e. for high gas velocities through the hole at the moment of flame front arrival at the hole, the re-ignition probability for a given hole diameter was in fact higher for off-stoichiometric propane concentrations than for concentrations close to stoichiometry. The average chemical reaction rate in the primary chamber had its peak in the region of stoichiometry, and hence the pressure in the primary chamber at the moment of flame front arrival at the transmission hole entrance, was also at its maximum in that concentration range. Therefore, the average turbulence intensity in the potential re-ignition zone, and hence the rate of entrainment of cold unburnt gas by the hot jet was also at its maximum. Hence, a main reason for the observed effect may be that at stoichiometry more efficient cooling by cold-gas entrainment, compared with at leaner and richer mixtures, more than compensated for the faster chemical reaction. |
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