On the experimental determination of combustion process driving in an unstable combustor

This paper investigates the accuracy of the typical experimental practices that estimate the unsteady pressure in the flame region of an unstable comb ustor from pressure measurements along the combustor walls. This measuremen t, along with that of the unsteady heat release, is often used to determine information about the flame's driving characteristics. These wall pressure measurements are only meaningful, however, if the unsteady pressure is nea rly one-dimensional in the combustor; that is, if the transverse gradient o f the pressure in the flame region is small. This paper presents computatio nal results of the interior acoustic field in a combustor undergoing longit udinal oscillations in order to quantify the difference between the acousti c pressures at the wall (where they are typically measured) and the flame. These results show that the duct pressure at the flame and the wall typical ly differ in magnitude by five to twenty five percent, and in phase by ten to twenty degrees. It is concluded from these results that wall pressure me asurements provide qualitative information about the fluctuating pressure a t the flame and can, in some cases, be used in conjunction with heat releas e measurements to characterize the flame driving.