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.