Detonation Hazard Classification Based On The Critical Orifice Plate Diameter For Detonation Propagation

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Cross, Mitchell
Combustion , Detonation Propagation , DDT
Accidental explosions in the chemical, oil & gas industry are a serious problem. The ultimate objective of this thesis is to establish a new parameter/process that can be used to categorize the relative detonation explosion hazard of a fuel that can be measured in a smaller, more practical apparatus. Experiments were carried out in an apparatus consisting of a 6.1 m long (composed of 2 equal length sections), 100 mm inner-diameter tube. A detonation was initiated in the first half of the tube using an acetylene-oxygen driver ignited by a weak spark. The second half of the tube contained orifice plates equally spaced at the tube diameter. Six different orifice plate diameters between 38.1 mm (1.5˝) and 76.2 mm (3˝), in increments of 6.4 mm (1/4˝) were used in the study. The average combustion front velocity was obtained from time-of-arrival measurements deduced from ionization probe signals. The critical (minimum) orifice plate diameter required for successful transmission of a detonation from a smooth tube was measured for different stoichiometric fuel-air mixtures. The ratio of the critical orifice plate diameter (d) and the mixture detonation cell size (λ) varies strongly with the orifice plate blockage ratio (BR), with a value approaching unity (d/λ→1) with decreasing blockage ratio. It is proposed that the critical orifice plate diameter could be used to categorize the detonation hazard potential of single or multi-component fuels. Additional experiments were performed in same apparatus to measure the Deflagration to Detonation Transition (DDT) and detonation propagation limits (distinguished by the method of initiation). Both the propagation and the DDT limits narrow with increasing BR, more significantly for the DDT limits. The narrowing of the limits with increasing BR also corresponds to a larger deviation from the d/λ=1 detonation limit criterion. These results indicate that the beneficial effects of the orifice plate in providing a reflection surface diminishes with increased BR due to the increased effect of shock wave weakening by the diffraction process. Novel use of soot foils showed that the structure of the detonation wave is very non-uniform and highly unsteady, characterized by local detonation initiation and failure.
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