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Heymes, Frederic
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document
Natural background oriented schlieren and multiscale visualizations of overpressure wave resulting from vapor cloud explosion
Abstract
Considerableresearch effortsare focusedtoforecast consequencesofaerialoverpressureandimpulsedueto fuel-airexplosions.Statisticsindicatethatsometwo-thirds ofthefinanciallossisattributabletoexplosions.Previous workoftheauthors [1] wasdoneonpropane-oxygen stoichiometriccloudsignitedbySEMTEX™ high explosives.Inthis second study,stoichiometricmixtureof methaneandoxygenfilledapolyethyleneenvelope witha totalvolumeofthemixtureof5.5 m3.Different image processing was performed tovisualizesimultaneouslythe propagationofcombustionflameintheenvelopeandthe overpressurein ambient air,outsidetheenvelope.To represent realistic conditions, the shape of the gas envelope was elongated like the dispersion shape from an accidental leak withwind.Anelectric spark ignitedthemixturegas volume ononeside.Bothflamepropagation velocity and overpressureinopen-airwererecordedwithdirect visualization. Near field scene (10 m x 6.9 m i.e. about 1.7 timestheenvelopelengthand6.2timestheenvelope diameter)wasrecordedwithdirectilluminationfromthe flame propagation through the use of a 20 kfps (fps, frame persecond) camera.Far field scene (32.4mx17.3mi.e. about 5 times the envelope length and about 16.8 times the envelope diameter) was recorded with direct lighting of the scenethroughtheuseofa9.4kfpscamera.Themixture envelopeandthecamerawerepositionedinordertouse far-field vegetation ascontrastednaturalbackground.In ordertovisualizetheoverpressurewave,natural BackgroundOrientedSchlieren (BOS) was applied as image processing [2,3]. Because of the light fluctuation due to theexplosion,thereferenceimage foraframe correspondstothepreviousframe. Directvisualization showsthecombustionofthemixture with increasing velocity.Atransitionfromdeflagrationtodetonationis identified while the combustion front reaches the middle of theenvelope. AnalysisoftheBOSimagesshowsthe presence of two overpressurewaves of decreasing velocity outsidetheenvelope. Evolutionoftheglobaldynamicsof thephenomenonisachievedthroughtheuseof synchronization of near and far field in a composite image where RGB image layers are respectively set with the BOS image, the global view and the flame view scaled to fit the far field view.