Journal of Thermal Science and Technology (Mar 2018)
Dynamic response of wall-stagnating lean methane-air premixed flame to equivalence ratio oscillation
Abstract
In order to clarify the response characteristics of a lean methane-air premixed flame to equivalence ratio oscillation from actual measured values of the burning velocity, we developed a new burner with a wall-stagnating flow that allowed measurement of the flow field by using particle image velocimetry (PIV). To create fluctuations in the equivalence ratio only in the direction of flow without varying the velocity field, fuel and air flow rates were controlled by alternately vibrating two sets of loudspeakers. Burning velocity Su was calculated from the measured unburnt gas velocity ug and flame moving velocity uf. ug at the front edge of the flame was measured by PIV. uf was obtained using a high-speed video camera. For an oscillation with a mean equivalence ratio of 0.85, amplitude of 0.05 and frequency f ranging from 5 to 50 Hz, the following results were obtained: (1) The oscillation amplitude of the flame position changed quasi-steadily in the low frequency range, decreasing with increasing f in the frequency range greater than 40 Hz. (2) The oscillation amplitude of the burning velocity of dynamic flame ΔSud obtained by PIV measurement increased with respect to f, and reached maximum at 40 Hz. The maximum value became greater than that of the static flame ΔSus, over the same equivalence ratio range. This tendency was similar to the result obtained by approximating the flow field by a potential flow. (3) The frequency characteristics of the oscillation amplitude ratio of the burning velocity ΔS (= ΔSud/ΔSus) qualitatively correspond to the results ΔS(p) (= ΔSud(p)/ΔSus(p)) obtained by approximating the flow field by the potential flow. However, ΔS is quantitatively larger than ΔS(p), and the difference between ΔS and ΔS(p) is large in the high frequency range. This is because the approximation of the flow field by the potential flow underestimates the oscillation amplitude of the unburnt gas velocity, and the degree of the underestimation becomes noticeable in the high frequency range.
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