doi:10.3850/978-981-08-7724-8_02-05

ABSTRACT

This paper presents results of an experimental investigation of the deflagration and deflagration-to-detonation transition (DDT) in an obstructed semi-confined flat layer filled with uniform hydrogen-air mixtures. We studied the effects of mixture reactivity and flat layer thickness on the flame propagation regimes in order to evaluate critical conditions for sonic flame propagation and detonation onset. The experiments were performed in a large rectangular box with dimensions 9 × 3 × 0.6 m opened from below and filled with obstacles with a blockage ratio of BR = 0.5-0.53. The hydrogen concentration in its mixtures with air was varied in the range of 13-28 vol.%.

The critical expansion ratio, defined as the ratio of the volume of combustion products over reactants, for mixtures propagating with sonic or supersonic velocity was found to be linearly dependent on the reciprocal layer thickness. The detonation onset in a semiconfined mixture layer occurred if the layer thickness was 13-14 times the detonation cell width. It was also found that the detonation cannot propagate or fails just beyond an obstacle if the orifice size in obstructions is less than 3 times the detonation cell width. In those cases with the critical orifice size, we could visualise a zone of failed detonation on sooted plates installed just after the obstacle then a detonation reinitiation zone with refined cellular structure, followed by re-establishment to normal detonation cell size. This work was done within the framework of a German nuclear safety programme for numerical code validation, in order to provide critical conditions for flame propagation regimes in thin semi-confined layers of hydrogen-air mixtures.

Keywords: Semi-confined deflagration, Flat layer, Turbulent flame acceleration, Deflagration-to-detonation transition.

© 2011 CPD Unit, University of Leeds