High pressure §uctuations coupled with unsteady heat release can a¨ect a rocket engine seriously. Especially when the oscillations match eigenmodes such as T1, T1L1 and T2, T2L1, the acoustic pressure amplitude can reach a critical level. This paper deals with the investigation of the nozzle admittance, which is an important value to characterize the in §uence of the nozzle on the pressure inside the combustion chamber. Two di¨erent nozzle geometries are investigated experimentally at high frequencies. A method to decouple the acoustic modes is presented. The results are compared against an existing theory and simulated data.

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The influence of axial position and cavity length of an absorber ring with grazing flow on damping of acoustic amplitudes in a rocket combustion chamber is experimentally investigated under nonreactive ambient temperature conditions. At the perforated inlet, high-bias flow velocities are present, providing strong damping. On the basis of power spectral densities gained from flow noise excitation, damping rates for the first transverse mode are derived using a Lorentzian profile fitting procedure. Results show that an absorber ring located in close proximity to the nozzle leads to enhanced damping, whereas in the case of a ring placed closely to the inlet, damping rates are reduced. Absorption coefficients show that an isolated absorber ring is not capable of acoustic amplification. It is concluded that reduced damping originates from weaker impact of the perforated inlet on the acoustics, resulting from a shielding effect by the absorber ring. The highest damping rate is found for a cavity length below the theoretically predicted optimum length for an absorber ring located close to the nozzle. Nomenclature c = speed of sound, m∕s D = diameter, m F,Ĝ = characteristic wave amplitude, m∕s f = frequency, Hz hIi = acoustic intensity averaged over one acoustic period, W∕m 2 J 1 = Bessel function of first kind and order 1 k = wave number, 1∕m L = length, m Ma = Mach number _ m = mass flow, kg∕s p = pressure, Pâ p = complex amplitude of pressure, Pa R = radius, m s 10 = zeroth root of first-order Bessel function derivative x, r, ϕ = axial, radial, and circumferential position; m, m, rad α = damping rate, rad∕s δ = offset angle with respect to frame of reference, rad λ = wavelength, m ξ = absorption coefficient ρ = density, kg∕m 3 ρ = mean density, kg∕m 3 φ = phase, rad 1T = first transverse mode for a system with absorber ring 1T − = additional transverse mode for a configuration with absorber ring 1T∕2T = first/second transverse mode 1T1L = first transverse and first longitudinal coupled mode 1T2L = first transverse and second longitudinal coupled mode Subscripts A = absorber ring cavity length C = chamber co = cut-on e = effective i = ith measurement location num = numerical Optimal = optimal (highest) damping t = nozzle throat u, d = upstream/downstream λ∕4 = property of λ∕4 absorber I = case I, absorber ring located closely to the face plate II = case II, absorber ring located closely to the nozzle * = complex conjugation Superscripts A, B = excitation states u, d = upstream/downstream x = in positive/negative axial direction * = complex conjugation

It is a known phenomenon that single can combustion test rigs and gas turbines have a different stability behavior. Real gas turbines are often more stable than their test rigs. One main difference between test rigs and real engines is the injection of cooling water into the test rigs to reduce the temperature of the exhaust gas and thus to protect the exhaust valve. A literature survey showed that the presence of a two phase flow can drastically reduce the sonic velocity and consequently change the acoustic properties of a system. The aim of this project is to study the influence of water injection on the acoustic properties of a test rig representing the exhaust system of a gas turbine. The experimental results clearly show that the sonic velocity does not change in the present test rig because the droplets are too big to follow the acoustic fluctuations. The critical dimension-less number in this context is the Stokes number, which is mainly determined by the droplet diameter and the acoustic frequency. Furthermore, the experimental results point out that the injected water increases the acoustic damping. It can be concluded from this study that the influence of water injection on the acoustic properties and therefore on the stability behavior is very sensitive to the injection conditions, especially the droplet diameter.