Reconstruction of the heat release response of partially premixed flames

The forced response of a swirl-stabilized, partially premixed flame to inlet velocity and equivalence ratio oscillations was experimentally investigated in a model lean-premixed gas turbine combustor. Three different forcing mechanisms were studied: the response of a premixed flame to velocity oscillations, the response of a partially premixed flame to equivalence ratio oscillations, and the response of a partially premixed flame to velocity and equivalence ratio oscillations. The overall heat release response of the flame was determined from measurements of the CH* chemiluminescence emission intensity from the entire flame, while the response of the spatially distributed heat release was determined from phase-synchronized chemiluminescence images. In addition, simultaneous measurements were made of the inlet velocity and equivalence ratio oscillations using the two-microphone method and the IR absorption technique, respectively. The results show that in the linear regime, the response of a partially premixed flame to simultaneous velocity and equivalence ratio oscillations can be reconstructed from independent measurements of the flame's response to velocity oscillations and to equivalence ratio oscillations using a vector summation method. This is the first experimental demonstration of a two-input one-output model of a swirl-stabilized partially premixed flame. It suggests that the response of a partially premixed flame is governed by four physical parameters, i.e., the oscillation frequency, the amplitude of velocity oscillation, the amplitude of equivalence ratio oscillation, and the phase difference between the two oscillations. As a result, the heat release response of a partially premixed flame can be amplified or damped, depending on the phase difference between the velocity and equivalence ratio oscillations at the combustor inlet.