This paper outlines a theoretical approach for predicting the onset of
detonation in unconfined turbulent flames. Two basic assumptions are
made: (1) the gradient mechanism is the inherent mechanism that leads
to deflagration-to-detonation transition (DDT) in unconfined condition
s and (2) the sole mechanism for preparing the gradient in induction t
ime is by turbulent mixing and local flame quenching. The criterion fo
r DDT is derived in terms of the one-dimensional detonation wave thick
ness, the laminar flame speed, and the laminar flame thickness in the
reactive gas. This approach gives a lower-bound criterion for DDT for
conditions where shock preheating, wall effects, and interactions with
obstacles are absent. Regions in parameter space where unconfined DDT
can and cannot occur are determined. Copyright (C) 1997 by The Combus
tion Institute.