Deflagration to detonation transitions (DDT) were previously observed in a set of large-scale unconfined vapor cloud explosion (VCE) tests involving ethylene-air mixtures in a moderately congested environment. The test rig congested region was 48 feet long by 12 wide by 6 feet high, with a regular array of vertical circular tubes providing the congestion (pitch-to-diameter ratio of 4.5, area and volume blockage ratios of 22% and 4.1%, respectively). The test rig was configured without any confinement (i.e., no wall or roof sections). The fuel-air mixture completely filled the congested region in these tests, but did not extend beyond it. DDTs were observed at ethylene concentrations ranging from 5.9% (lean) to 9.3% (rich). A comparison has now been made between a DDT criterion established for the FLACS code and this test data. The FLACS DDT criterion utilizes the maximum dimensionless pressure gradient and was established by benchmarking primarily against hydrogen-air tests in confined geometries (i.e., pipes and channels). The comparison with the existing ethylene-air VCE DDT data indicates a different critical value for the maximum dimensionless pressure gradient than that developed based on the confined hydrogen-air tests. Alternative DDT criteria were also explored as part of this work.
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