On Flame Ball-to-Deflagration Transition in Hydrogen-air Mixtures

Ultra-lean hydrogen-air combustion is characterized by two phenomena: the difference in upward and downward flame propagation concentration limits and the incomplete combustion. The clear answers on the  two  basic  questions  are  still  absent:  What  is  a  reason  and what  is  a  mechanism  for  their manifestation? Problem statement and the principal research topics of the Flame Ball to Deflagration Transition (FBDT) phenomenon in gaseous hydrogen-air mixtures are presented. The non-empirical concept of the fundamental concentration limits discriminates two basic low-speed laminar combustion patterns - self-propagating locally planar deflagration fronts and drifting locally spherical flame balls. To understand - at what critical conditions and how the baric deflagrations are transforming into iso- baric  flame  balls? - the  photographic  studies  of   the  quasi-2-dim  flames  freely  propagating  outward radially  via  thin  horizontal  channel   were  performed. For  gradual  increase  of  initial  hydrogen concentration from 3 to 12 vol.% the three representative morphological types of combustion (star-like, dendrite-like  and   quasi-homogeneous)  and  two  characteristic  processes  of  reaction  front  bifurcation were revealed. Key elements of the FBDT mechanism both for 2-dim and 3-dim combustion are the following. Locally  spherical  ""leading  centres""  (drifting  flame  balls) are  the  ""elementary  building blocks"" of all ultra-lean flames. System of the drifting flame balls is formed due to primary bifurcation of the pre-flame kernel just after ignition. Subsequent mutual dynamics and overall morphology of the ultra-lean flames are governed by competitive non-local interactions of the individual drifting flame balls and their secondary/tertiary/etc. bifurcations, defined by initial stoichiometry."