  It is known that unconfined dust explosions consist of a relatively weak
primary (turbulent) deflagrations followed by a devastating secondary
explosion. The secondary explosion may propagate with a speed of up to 1000 m/s
producing overpressures of over 8-10 atm. Since detonation is the only
established theory that allows a rapid burning producing a high pressure that
can be sustained in open areas, the generally accepted view was that the
mechanism explaining the high rate of combustion in dust explosions is
deflagration to detonation transition. In the present work we propose a
theoretical substantiation of the alternative propagation mechanism explaining
origin of the secondary explosion producing the high speeds of combustion and
high overpressures in unconfined dust explosions. We show that clustering of
dust particles in a turbulent flow gives rise to a significant increase of the
thermal radiation absorption length ahead of the advancing flame front. This
effect ensures that clusters of dust particles are exposed to and heated by the
radiation from hot combustion products of large gaseous explosions sufficiently
long time to become multi-point ignition kernels in a large volume ahead of the
advancing flame front. The ignition times of fuel-air mixture by the
radiatively heated clusters of particles is considerably reduced compared to
the ignition time by the isolated particle. The radiation-induced multi-point
ignitions of a large volume of fuel-air ahead of the primary flame efficiently
increase the total flame area, giving rise to the secondary explosion, which
results in high rates of combustion and overpressures required to account for
the observed level of overpressures and damages in unconfined dust explosions,
such as e.g. the 2005 Buncefield explosion and several vapor cloud explosions
of severity similar to that of the Buncefield incident.
