EXPERIMENTAL CHARACTERIZATION OF ACOUSTIC WAVE PROPAGATION THROUGH A SUPERSONIC DUCTED FLOW

Abstract

In scramjet combustors, if pressure waves could propagate upstream through subsonic boundary layer flow, it would set up an acoustic feedback mechanism that could lead to selfsustained combustion instability. To investigate the possibility of upstream wave propagation, non-reacting supersonic flow experiments were conducted in a specially-designed supersonic flow duct, which simulated the internal flow path of a dual-mode scramjet combustor. Furthermore, to experimentally simulate combustion instability, large-amplitude pressure oscillations were created by passively exciting the exhaust jet flow using screech mechanism, which resulted in large-amplitude pressure oscillations with dominant frequencies ranging between 2.7kHZ and 4.2kHz. Then, the acoustic signal was tracked along the supersonic flow duct using four high-frequency-response Kistler pressure transducers that were flush-mounted at the combustor and isolator walls. Schlieren visualization was conducted to characterize the internal supersonic flow field, and an analytical approach was used to estimate the turbulent boundary layer growth and displacement thickness. Ten sets of experiments were conducted at various stagnation pressure values ranging from 35psi to 125psi, and four sets of experiments where strong resonances were observed were repeated over ten separate runs for reproducibility. Fast Fourier Transform was used to quantify the changes in pressure oscillation amplitude in each case. The results conclusively show that the downstream disturbances were propagating upstream, and they were being attenuated at different rates depending on flow conditions and duct geometry. Possible reasons for this new phenomenon were examined and discussed.