Numerical study of sporadic combustion waves in straight channels of different diameters

The dynamics of flames propagating in straight channels filled with a stationary low-Lewis-number premixed gas mixture is studied numerically. A method for determining the propagation velocity of a sporadic combustion wave consisted of separate flame spots is proposed. Dependencies of the sporadic combustion wave propagation velocity, the residual fuel concentration and the number of flame spots on the channel size and the value of radiation heat losses are obtained. Analysis of numerical results show that for the channels of diameter exceeding some value the number of separate cup-like fragments constituting sporadic combustion wave is proportional to the channel cross-sectional area. At smaller diameters, the number of flame spots changes insignificantly and is one or two. It is shown that one of the universal characteristics of the sporadic combustion wave depending only on mixture properties but independent on system geometry is the area necessary to accommodate one reacting spot. Flame velocity which is another fundamental combustion characteristic is found to be almost independent on channel size starting from some critical diameter. This diameter, however, depends on mixture properties or radiative heat loss intensity and corresponds to the sporadic flame containing from several to ten reacting spots. Thus, the main properties of sporadic combustion waves in wide channels can be determined by numerical modeling of the flame propagation in the relatively narrow channels in which the flame consists of 1–10 cup-like fragments.