Indirect Combustion Noise Generation Research Articles

Computational analysis of the indirect combustion noise generation mechanism in a nozzle guided vane in transonic operating conditions

The combustion noise in modern engines is mainly originating from two types of mechanisms. First, chemical reactions in the combustion chamber leads to an unsteady heat release which is responsible of the direct combustion noise. Second, hot and cold blobs of air coming from the combustion chamber are advected and accelerated through turbine stages, giving rise to entropy noise (or indirect combustion noise). In the present work, numerical characterization of indirect combustion noise of a Nozzle Guide Vane passage was assessed using three-dimensional Large Eddy Simulations. The present work offers an overview to the analytical, computational and experimental studies of the topic. Numerical simulations are conducted to reproduce the effects of incoming planar entropy waves from the combustion chamber and to characterize the generated acoustic power. The dynamic features of the flow are addressed by the means of frequency domain and modal analyses techniques such as Fourier Decomposition and Proper Orthogonal Decomposition. Finally, the predicted entropy noise from numerical calculations is compared with the analytical results of an actuator disk model for a stator stage. The present paper proves that the generated indirect combustion noise can be significant for transonic operating conditions. The blade acoustic response is characterized by the excitation of a latent dynamics at the forcing frequency of the planar entropy waves, and it increases as the amplitude of the incoming disturbances increases.

The physical processes of indirect combustion noise generation

It is known that a uniform compressible flow supports three types of disturbances. One is entropy wave. These waves are made up of temperature and density fluctuations only. The other two are acoustic waves and vorticity waves. When entropy waves are convected into a nonuniform flow region, they are no longer independent waves. Their presence leads to pressure fluctuations, which result in noise radiation commonly referred to as indirect combustion noise. In the past, because entropy waves and acoustic waves are not independent waves in a nonuniform flow region, the mechanism responsible for the generation of indirect combustion noise had been attributed to mode coupling. That is, there is a coupling of entropy and acoustic modes. This leads to pressure fluctuations and hence noise radiation. The principal objective of the present investigation is to seek a more physical explanation of how indirect combustion noise is generated.

Experimental Assessment of Noise Generation and Transmission in a High-Pressure Transonic Turbine Stage

Noise originating from the core of an aero-engine is challenging to quantify since the understanding of its generation and propagation is less advanced than that for noise sources of other engine components. To overcome the difficulties associated with dynamic measurements in the crowded core region, dedicated experiments have been set up in order to investigate mainly two processes: the propagation of direct combustion noise through the subsequent turbine stage, and the generation of indirect combustion noise by the passage of inhomogeneities of entropy and vorticity through the turbine stage. In the current work, a transonic turbine stage was exposed to isolated and well-characterized acoustic, entropic, and vortical disturbances. The incoming and outgoing sound fields were analyzed in detail by two large arrays of microphones. The mean flow field and the disturbances were carefully mapped by several aerodynamic and thermal probes. The results include transmission and reflection characteristics of the turbine stage, the latter was found to be much lower than commonly assumed. The modal decomposition of the acoustic field in the upstream and downstream section shows additional modes besides the expected rotor–stator interaction modes. At the frequency of entropy or vorticity excitation, respectively, a significant increase of the overall sound power level was observed.

Compositional inhomogeneities as a source of indirect combustion noise

The generation of indirect combustion noise by compositional inhomogeneities is examined theoretically. For this, the compact-nozzle theory of Marble & Candel (J. Sound Vib., vol. 55 (2), 1977, pp. 225–243) is extended to a multi-component gas mixture, and the chemical potential function is introduced as an additional acoustic source mechanism. Transfer functions for subcritical and supercritical nozzle flows are derived, and the contribution of compositional noise is compared to entropy noise and direct noise by considering an idealized nozzle downstream of the combustor exit. It is shown that compositional noise is dependent on the local mixture composition and can exceed entropy noise for fuel-lean conditions and supercritical nozzle flows. This suggests that the compositional indirect noise requires potential consideration with the implementation of low-emission combustors.

Assessment of the Indirect Combustion Noise Generated in a Transonic High-Pressure Turbine Stage

Indirect combustion noise, generated by the acceleration and distortion of entropy waves through the turbine stages, has been shown to be the dominant noise source of gas turbines at low-frequencies and to impact the thermoacoustic behavior of the combustor. In the present work, indirect combustion noise generation is evaluated in the realistic, fully 3D transonic high-pressure turbine stage MT1 using large eddy simulations (LESs). An analysis of the basic flow and the different turbine noise generation mechanisms is performed for two configurations: one with a steady inflow and a second with a pulsed inlet, where a plane entropy wave train at a given frequency is injected before propagating across the stage generating indirect noise. The noise is evaluated through the dynamic mode decomposition (DMD) of the flow field. It is compared with the previous 2D simulations of a similar stator/rotor configuration, as well as with the compact theory of Cumpsty and Marble. Results show that the upstream propagating entropy noise is reduced due to the choked turbine nozzle guide vane. Downstream acoustic waves are found to be of similar strength to the 2D case, highlighting the potential impact of indirect combustion noise on the overall noise signature of the engine.

Noise of high-performance aircraft at afterburner

The noise from a high-performance aircraft at afterburner is investigated. The main objective is to determine whether the dominant noise components are the same or similar to those of a hot supersonic laboratory jet. For this purpose, measured noise data from F-22A Raptors are analyzed. It is found, based on both spectral and directivity data, that there is a new dominant noise component in addition to the usual turbulent mixing noise. The characteristic features of the new noise component are identified. Measured data indicates that the new noise component is observed only when the rate of fuel burn of the engine is increased significantly above that of the intermediate power setting. This suggests that the new noise component is combustion related. The possibility that it is indirect combustion noise generated by the passage of hot spots from the afterburner through the nozzle of the jet is investigated. Because flow and temperature data were not measured in the F-22A engine tests, to provide support to the proposition, numerical simulations of indirect combustion noise generation due to the passing of an entropy wave pulse (a hot spot) through a military-style nozzle are carried out. Sound generation is observed at the front and at the back of the pulse. This creates a fast and a slow acoustic wave as the sound radiates out from the nozzle exit. Quantitative estimates of the principal directions of acoustic radiation due to the emitted fast and slow acoustic waves are made. It is found that there are reasonably good agreements with measured data. To estimate the intensity level (IL) of the radiated indirect combustion noise, a time-periodic entropy wave train of 15 percent temperature fluctuation is used as a model of the hot spots coming out of the afterburner. This yields an IL of 175.5dB. This is a fairly intense noise source, well capable of causing the radiation of the new jet noise component.

One-dimensional characteristic boundary conditions using nonlinear invariants

A new treatment of boundary conditions using the flow decomposition in characteristics is derived for inviscid one-dimensional flows using nonlinear invariants. The new set of equations is equivalent to the relations of Thompson (1987) [5] but it has the advantage to provide a physical interpretation of the characteristics for nonlinear perturbations. This interpretation is a major advantage to deal with the nonlinear injection of waves in the domain. In particular, the limitation of the standard relations to the injection of waves without nonlinear interactions on the boundary is addressed. Associated errors are evaluated analytically and numerically and a clear improvement of the results is demonstrated with the new expressions. To avoid a drift of the mean values, relaxation terms are usually added in the relations and the boundary conditions become almost non-reflecting. The consequences of these relaxation terms on outgoing and ingoing waves are widely investigated in the present paper and a nonlinear correction is proposed to recover perfectly non-reflecting conditions without drift. To end, simulations are performed on the generation of indirect combustion noise through a nozzle to illustrate the advantages of the new formulation.

Numerical Analysis of Indirect Combustion Noise Generation Within a Subsonic Nozzle

Indirect noise is analyzed by numerical simulations of the Entropy Wave Generator experiment, which has been set up to study the fundamental mechanism of indirect combustion noise generation. Previous studies of this test case show the major importance of modeling the impedance terminations of the test rig to obtain accurate results. So far, characteristic boundary conditions have been used to model the impedance for numerical calculations. Within this paper, different proposals for setting up the characteristic boundary conditions are reviewed. In addition, a higher accuracy of the impedance model is achieved by applying time-domain impedance boundary conditions. As part of the Entropy Wave Generator experiment, indirect noise is generated by electrical heating wires. It has been found in this study that the modeling of these heat sources is crucial for obtaining accurate results. An advanced heat source model is proposed that is in excellent agreement with the experimental data. Both the advanced impedance and heat source model account for a better agreement between experimental and numerical data than reported in literature to date.

Indirect combustion noise of auxiliary power units

Recent advances in noise suppression technology have significantly reduced jet and fan noise from commercial jet engines. This leads many investigators in the aeroacoustics community to suggest that core noise could well be the next aircraft noise barrier. Core noise consists of turbine noise and combustion noise. There is direct combustion noise generated by the combustion processes, and there is indirect combustion noise generated by the passage of combustion hot spots, or entropy waves, through constrictions in an engine. The present work focuses on indirect combustion noise. Indirect combustion noise has now been found in laboratory experiments. The primary objective of this work is to investigate whether indirect combustion noise is also generated in jet and other engines. In a jet engine, there are numerous noise sources. This makes the identification of indirect combustion noise a formidable task. Here, our effort concentrates exclusively on auxiliary power units (APUs). This choice is motivated by the fact that APUs are relatively simple engines with only a few noise sources. It is, therefore, expected that the chance of success is higher. Accordingly, a theoretical model study of the generation of indirect combustion noise in an Auxiliary Power Unit (APU) is carried out. The cross-sectional areas of an APU from the combustor to the turbine exit are scaled off to form an equivalent nozzle. A principal function of a turbine in an APU is to extract mechanical energy from the flow stream through the exertion of a resistive force. Therefore, the turbine is modeled by adding a negative body force to the momentum equation. This model is used to predict the ranges of frequencies over which there is a high probability for indirect combustion noise generation. Experimental spectra of internal pressure fluctuations and far-field noise of an RE220 APU are examined to identify anomalous peaks. These peaks are possible indirection combustion noise. In the case of the APU RE220, such peaks are identified. The frequency ranges of these peaks are found to overlap those predicted by the model theory. Based on this agreement, a tentative conclusion is drawn that there is good reason to believe that APUs do generate measurable indirect combustion noise.This paper is dedicated to the memory of Prof. Phil Doak for his numerous contributions to Aeroacoustics and the Journal of Sound and Vibration.