  The problem of deflagration-to-detonation transition in an unconfined
environment is revisited. With a freely expanding self-accelerating
hydrogen-oxygen flame as an example, it is shown that
deflagration-to-detonation transition is indeed possible provided the flame is
large enough. The transition occurs prior to merging of the flame with the
flame-supported precursor shock. The pre-transition flame does not reach the
threshold of CJ-deflagration. Numerical simulations employed are based on the
recently developed pseudo-spectral method with time and space adaptation.
