Holger Nawroth

Recurrence Analysis of Combustion Noise

Recurrence analysis of phase space trajectories reconstructed from scalar time series—a relatively new technique in the field of combustion dynamics—is proposed for analyzing combustion noise data. A demonstration based on the implementation of this analysis on combustion noise data acquired in experiments on an open bluff-body stabilized turbulent premixed flame is presented in this article. A combined analysis methodology involving conventional techniques: spectral analysis and proper orthogonal decomposition, together with recurrence analysis is found to be effective in identifying features embedded in combustion noise signals. In particular, the new perspective provides insights into the temporal transitions of pressure fluctuations between noise and periodic dynamics. The recurrence analysis technique, which is found to be instrumental in extracting the dynamical makeup of combustion noise signals in this work, is foreseen to be greatly beneficial for the analysis of experimental and numerical result...

Experimental and Numerical Investigation of a Turbulent Premixed Flame in an Anechoic Environment

The turbulent jet emanating from an unconfined, premixed burner is investigated by numerical simulation using Large Eddy Simulation (LES) and experimentally by means of optical (OH chemiluminescence), acoustic (microphone) and laser-optical measurement techniques (Laser Doppler Anemometry, Particle Image Velocimetry) for non-reacting and reacting flow, respectively. While 2D-LDA data of the non-reacting flow field are in good agreement with calculated results, 2D-PIV data of the reacting flow field, burning a methane-air mixture, differ significantly from the LES data. This is caused by a falsely chosen seeding injection location, which is apparently located too far downstream. In addition, a large percentage of the total air mass flow has to be used for proper seeding injection and, therefore, is not well mixed with the fuel from the actual fuel injection located further upstream. Consequently, this leads to a non-premixed flame which differs from the partially premixed flame obtained in case using LES. The strong seeding jet itself impacts on the velocity distribution, as well. The emitted noise spectrum has a tonal shape with peaks at the burner’s resonance frequency for the non-reacting flow which changes to broadband noise and is raised in amplitude in case of the reacting flow.

Flow Investigation and Acoustic Measurements of an Unconfined Turbulent Premixed Jet Flame

A turbulent jet emanating from an unconfined, premixed burner is investigated by using large eddy simulation (LES) and direct numerical simulation (DNS) as well as experimentally by means of optical (OH* chemiluminescence), acoustic (microphone), and laser-optical measurement techniques (Particle Image Velocimetry). Comparison of the results obtained through experiments, LES, and DNS indicate a reasonable agreement. In order to analyze the impact of mesh refinement on the resolved flame properties and acoustic radiations, computational grids with varying resolutions are used for the LES. As large coherent flow motion exists in the considered flow case, due to an over-predicted diffusion the flame length calculated with LES is underestimated. On the other hand, DNS exhibits a similar intensity distribution for OH* as the experiment and, hence, the flame length is predicted accurately by DNS. The emitted noise spectrum has a tonal shape with peaks at the burner’s resonance frequency for the non-reacting flow which changes to broadband noise and, in general, is raised in amplitude for reacting flows. In addition, it is shown that an increase in Reynolds number, preheat temperature, or a decrease in equivalence ratio close to stoichiometric ratios yields more noise being emanated from the burner. The latter indicates the fact that direct combustion noise is linked to interactions of turbulent fluctuations with the flame front. When using an equivalence ratio closer to stoichiometric ratio, a thinner reaction zone is expected and the intrinsic interaction between the flame and turbulent flow is more pronounced.

Experimental Study of Noise Generation by a Turbulent Premixed Flame

We focus on investigating temporal dynamics of combustion noise. Specifically, the dynamics of direct combustion noise, emitted from a premixed turbulent jet flame are studied for the first time with the help of recurrence analysis. The analysis showed that combustion noise is characterized by irregularly spaced periodic windows. Comparisons are carried out to bring out differences in the time trace of acoustic pressure for different equivalence ratios.

Thermoacoustics of a turbulent premixed flame

Quantitative analyses of noise induced by turbulent combustion processes are essential for the design of efficient combustors. To understand the noise generating mechanisms detailed thermoacoustic source mechanisms for the acoustic perturbation equations are deduced from the governing equations of compressible reactive fluids. A generic burner configuration operated with a turbulent premixed flame is experimentally and numerically investigated to identify relevant source mechanisms and to show the dependence of the noise radiation on the operating conditions. Besides direct combustion noise mechanisms by heat release fluctuations indirect mechanisms by acceleration of entropy inhomogenities and non-isentropic mixing processes are identified as major noise sources.

Effects of Shear Layer Manipulation on Noise Emissions of a Turbulent Jet Flame

This study presents the current state of research on direct combustion noise emitted from an unconfined, turbulent, premixed jet flame. Unlike previous investigations, which focused on global flow parameters such as Reynolds number or temperature, the current study focuses on the impact of local parameters, namely shear layer thickness. For this purpose, the shear layer does not develop naturally but is manipulated by installing a tripping device upstream of the flame. Through artificially introduced turbulent fluctuations of the velocity within the shear layer, heat release rate fluctuations are enhanced, too. As the latter is the main source of direct combustion noise, sound pressure levels of reacting flows increase. Investigations reveal the impact of tripping location, equivalence ratio, and Reynolds number on noise emissions. Especially the equivalence ratio has a strong influence on emitted sound pressure levels. At lean conditions and low Reynolds numbers, different tripping locations yield similar noise emissions whereas high Reynolds numbers lead to differences in noise emissions between locations. However, higher equivalence ratios result in contrary findings, i.e., distinct differences between tripping locations for low Reynolds numbers, coinciding spectra for high Reynolds numbers.

Recurrence Plots for the Analysis of Combustion Dynamics

Practical applications involving combustion suffer from serious issues such as combustion instabilities and sudden loss of flame: flame flashback and blowout . These phenomena are related to dynamical changes in the combustion system. Here, we summarize our recent studies on the application of recurrence-based methods to identify such dynamical transitions as well as for the characterization of combustion dynamics in laboratory combustors.

Flow Fields of Manipulated Shear Layers of a Turbulent Jet Flame

Noise emitted from a turbulent, premixed flame is investigated experimentally with a focus on the impact of the shear layer. For this purpose, the shear layer is altered by installing a tripping device upstream of the flame. Through artificially introduced turbulent fluctuations of the velocity within the shear layer, heat release rate fluctuations are increased, too. As these are the main source of direct combustion noise, sound pressure levels of reacting flows increase. Investigations reveal the impact of tripping location, equivalence ratio, and Reynolds number on noise emissions. At lean conditions and low Reynolds numbers, different tripping locations yield similar noise emissions whereas high Reynolds numbers lead to differences in noise emissions between locations. Higher equivalence ratios result in contrary findings, i.e., distinct differences between tripping locations for low Reynolds numbers, coinciding spectra for high Reynolds numbers.

Experimental Investigation of an Unconfined Swirl-Stabilized Turbulent Premixed Flame

A turbulent, swirl-stabilized jet emanating from an unconfined, premixed burner is investigated experimentally by means of optical (OH* chemiluminescence), acoustic (microphone), and laser-optical measurement techniques (Particle Image Velocimetry) for various swirl intensities. It is shown that even in case of an unconfined swirl flame, combustion-induced vortex breakdown (CIVB) occurs which, on one hand, contributes to the stabilization of the flame and stable combustion due to the increased recirculation zone and, on the other hand, can promote flashback if the recirculation zone travels upstream or is extended upstream, especially for high swirl intensity flows. Another effect associated to CIVB is the generation of vortex breakdown for low Reynolds number, reacting flows which corresponding non-reacting, isothermal flows at the some operating conditions do not create a recirculation zone at all. After crossing a threshold of injected momentum, i.e. Reynolds number, normalized flow fields become Reynolds number independent. It is found that noise emissions from the burner grow with increase in different parameters: equivalence ratio of the injected and burnt mixture, Reynolds number, and number of vanes on the swirler disc. All three parameters cause an extended area of heat release which is supposed to generate larger pressure oscillations and, hence, more noise.