Flame stretch—A connecting principle for blow-off data

Abstract

A new physical basis for the phenomenon of blow-off of aerated burner flames is proposed. This follows from the observation that combinations of velocity and velocity gradient at the base of this type of burner flame are similar to those which have been shown to result in extinction of a combustion wave. In such cases extinction results from excessive stretch of the combustion wave. Previously unpublished data on the burning velocity and stability of ethylene air flames over a range of temperatures and pressures together with similar data on other gases already available in the literature, are used to demonstrate that the phenomenon of blow-off may be accounted for on the basis of excessive flame stretch in the stabilizing region and that a single equation gb = (0.23 ρ cpSu2k)[1 − (1 − X6.4)α] correlates almost all of the data. This equation is valid for the following range of experimental parameters: atmosphere: air and inert: mixture composition: 0·7 to 1·36 of stoichiometric fuel: ambient pressure: 200 mm of mercury to atmospheric: mixture temperature: atmospheric to 693 K: flow state: laminar, transitional and turbulent Deviations from the equation are discussed; these arise when there are marked preferential diffusion effects. In such cases the relevant value of the burning velocity is not known and therefore the stretch factor cannot be calculated. It is shown that the well known correlation between mixture composition and the boundary velocity gradient at the blow-off condition, which is normally associated with the physical picture presented in the boundary velocity gradient theory of blow-off, is a special case of the flame stretch theory of blow-off. Suggestions are made for applying the similarity criterion known as the Karlowitz stretch factor, to various burner stability problems which are of particular importance to the gas industry at the present time.