Critical Morphological Phenomena During Ultra-lean Hydrogen-air Combustion in Closed Horizontal Hele-Shaw Cell

Free quasi-two-dimensional outward propagation of the ultra-lean hydrogen-air flames was studied in a horizontal, closed flat channel in order to minimize the influences of gravity and natural convection. Experiments were carried out with a sequential change of initial hydrogen concentration in the premixed gaseous  hydrogen-air  mixtures  in  the  range  from  3  to  12  vol.  %  H2   under  normal  pressure  and temperature   conditions.   Two   types   of   critical   (in   term   of   concentration   threshold   behavior) morphological phenomena were observed - formation of a pre-flame kernel and primary bifurcation of the pre-flame kernel and the higher order (secondary, tertiary, etc.) bifurcations of the individual locally spherical and restricted in space flame fronts. For the given initial ambient conditions (channel thickness, initial gas mixture pressure and temperature) variation of initial mixture stoichiometry results in a few substantial changes in overall flame shape. These changes were recorded at the specific concentration limits,  which   delineate  three  characteristic  macroscopic  morphological  forms  (morphotypes)  of  the ultra-lean  hydrogen-air  flame's  ""trails"",  -  ""ray-like"",  ""dendritic"",  and  ""quasi-uniform"".   Transitions between the revealed basic flame morphotypes took place in different ways. The ""pre-flame kernel-to- rays"" and ""rays-to-dendrites"" transitions were abrupt and resembled the first order transitions in physics. -to-quasi-uniform morphology"" were significantly blurred and can be regarded as analogue to the second order transitions.