This paper attempts to present some registers of sound pressure levels during the operation of large diesel engines (10 MW). During these registers we have found the preparation, occurrence and ending of events of thermoacoustic instability. They appear after a loosing of chaos period or a reduction in fluctuations in some frequencies. The most interesting phenomena were registered at low frequencies. However, they were accompanied by variations in sound emissions at medium and high frequencies. As there has been very little published data concerning these phenomena at real scale, it is imperative to point out that every quasi-stationary state we have measured during these episodes has lasted some minutes, significantly much more time than that of lab scale results.
Thermoacoustic instabilities are a growing concern when working with large machines. A lot of work has been developed about gas turbines, but very few experiences are reported measuring this phenomenon in real scales engines.
Noise in diesel engines is highly consistent with the variation of the pressure inside each cylinder along the time. As combustion has random features, the cylinder pressure and the emitted noise are dominated by randomness over a wide range of frequencies where noise is produced [
When working for determining the acoustic power of large engines (10 MW), we detected some unusual patterns in several third octave bands (TOB) recordings. We went through the analysis and we found we have measured some thermoacoustic instabilities episodes during the tests.
Thermoacoustic oscillations cause increased damage on the engines. The high sound pressure levels associated with the oscillations that occur during thermoacoustic instability, impose an additional load on the wall of the combustion chamber. In addition, non-stationary flow increases heat transfer to the coating and local overheating may occur. Even the electronic systems that control combustion could fail due to high levels of vibration or temperature, leading loss of the system control.
According to Schemel et al. (2004) when the flow is homogeneous, the noise in a combustion chamber is an overlapping of the solution of three independent wave equations: the waves traveling upflow and downflow at sound speed, the turbulent convective waves and the entropy convective waves. If the flow ceases to be homogeneous, the entropy convective waves will begin to emit (radiate) sound [
Combustion instabilities arise due to complex feedback interactions between pressure and heat release oscillations. When these oscillations are sufficiently in phase, a large amplification of the initial perturbation is expected. Thus, the instabilities of the combustion refer to the feedback of a coherent phase oscillation at a fixed frequency.
The pressure oscillations occurring in the combustion chamber appear as fluctuations in the output flow of the injector. As a consequence, fluctuations in the incoming air flow occur in turn. Then, fluctuations in the release of heat would also appear. The frequency of these oscillations depends on the main cause of them. Low frequency oscillations (from 4 Hz to 70 Hz) are mainly due to instabilities in the flame front progressing in a heterogeneous way throughout the combustion chamber. They are related to low frequency emissions. From 70 Hz to 700 Hz stationary waves with different phase angles appear [
Polifke et al. (2001) point out that the feedback between the combustion chamber acoustics and the entropy waves would be important, especially for lower modes and even at higher frequencies than those normally associated with convection waves [
The main issues affecting the occurrence of interferences are fluctuations and heterogeneities in fuel concentration, temperature regions, rate of heat release and also localized phenomena at the inlet and the fuel injection point. Depending on the characteristic times of the convection and acoustic phenomena, the entropy waves and the acoustic of the combustion chamber could couple in a constructive or destructive way.
It should be noted that the rotational frequencies of the machines in our case of study are within the proper range for flame front fluctuations.
The most important mechanisms responsible for instability in combustion are relatedtothe injection of propellants, the formation of liquid droplets and the combustion process itself. If a high temperature region passes through a supersonic nozzle (i.e. where Ma > 1), the interaction with the non-uniform flow region developed produces an acoustic wave which propagates upstream. Then, the action of the acoustic wave in the combustion processes can generate new regions of non-uniform temperature, with the usual consequences on the stability of the combustion. Therefore, there is a feedback loop within the chamber which maintains the oscillatory phenomena and which, under certain conditions, may lead to a condition of instability. The evolution of eddies can excite oscillations, either by purely fluid interactions or by influencing combustion processes [
The oscillations in pressure and velocity in the gas phase (acoustic disturbances) can influence the rate of vaporization of the liquid droplets if the period of oscillation corresponds to one of the characteristic times of the vaporization, namely [
According to Schuermans (2003), fluctuations in fuel concentration are the main (but not the only) cause of the interaction between the heat release and the sound field [
The “soul” of the problem is that all the ideal processes under which combustion is studied in small machines are no longer valid in large ones. The hypotheses about instantaneous ignition, homogeneity of the mixture and all phenomena occurring inside of each cylinder are no longer applicable.
This paper is organized in four sections. After this introduction, some ways to early detecting thermoacoustic instabilities are presented. Then, our own experimental findings when measuring at several 10 MW enginesare detailed. At last, our conclusions are remarked.
Research about thermoacoustic instabilities has mostly developed around gas turbines and at a laboratory scale. There are not enough published data about real scale cases. There are many proposals for detection of the occurrence of these phenomena. Hereby we present three of them; we think our experimental findings can help to deeper studies on them.
The phenomenon of loss of chaos was studied by Vinneeth et al. (2013) [
The precursor turns out to be an objective measure of the proximity of the combustion chamber to the unstable operating regimes and is independent of the details of the geometry, the composition of the fuel and the stabilization of the flame.
During the preparation of a thermoacoustic instability event, some early “symptoms” should be detected, thus allowing taking actions to avoid the instability occurrence. Ibrahim (2007) proposes the use of a low-cost method which implies having a good identification and characterization of several acoustic modes to be able to follow its temporal evolution and to know about its growing and decreasing tendencies. This background allows to make a good prediction without numerically integrating over time: its detection tool analyzes the behavior of rates of variation and not modes, which is undoubtedly simpler and faster. The method consists in cataloging and linearly estimating of magnitudes of the mechanisms of amplification and attenuation. The application of linear approximations to nonlinear mechanisms allows, however, obtaining a reasonably complete and manageable description for the purposes of the analysis. The author defines an index of stability, so that the imminent occurrence of an instability is anticipated when a certain value is exceeded. In this particular case, the oscillation is expected to occur when the value of the index exceeds one unit [
The goal of the method is to allow fast, low-cost decisions that can be made for a wide variety of design configurations and operating conditions without the complexity of other tools that require computational fluid dynamics. The proposed approach achieves moderate success by being tested on a basis of experimental data available in the literature as well as with new experiments, so it may also be useful to complement other methods already in use.
Lee and Santavicca (2005) carry out an extensive discussion of the applicable methods for experimentally diagnosing, i.e. in operation, if instability is occurring in a combustion chamber [
The effect of entropy waves on flow field instabilities is known since 1965, but their importance was supposed to be restricted only to low frequencies. More recent works (e.g., [
Fluctuations of heat release in the flame can cause acoustic waves that propagate upstream in the feed lines and in turn cause disturbances in the incoming air/fuel mixture. These disturbances can be carried by the mean flow and trigger a fluctuation in the flame controller, closing the instability loop. Several studies have addressed this possible mechanism and are considered of high potential to generate instability phenomena. The acoustic-convective waves are carried by the medium flow, such as eddies detached from the flame stabilizer and/or entropy waves that propagate downstream, and generate acoustic waves that propagate upstream.
There are also other possible sources of oscillatory combustion instability ranging from purely chemical-kinetic phenomena to other only fluid-mechani- cal phenomena. Their contributions vary with modes of oscillation. It is also possible that some of the modes of oscillation are caused by a combination of perturbations (velocity, temperature, velocity of laminar flame, etc.).
Several mechanisms contribute to the occurrence of thermoacoustic instabilities [
A similar result would be found if these vortices affect an obstacle downstream (e.g. the outlet nozzle, a throttling, etc.), even if they carry no unburned mixture or if it is a non-reactive flow or a cold flow, causing the pressure oscillations to intensify. This result is purely acoustic and does not consider the contributions related to heat releasing.
Since hot spots are carried by the medium (usually low-speed) flow, it is assumed that entropy effects (if exist) are to occur at low frequencies. When these hot spots reach the entrance of a strangulated nozzle, the propagation of an upflow acoustic wave is triggered and it can cause an acoustic instability.
A set of measurements were carried out to determine the acoustic power of eight large diesel engines (10 MW each). They were done according to UNE-EN-ISO 3744:2010 Standard [
All the figures in this section have been built using the experimental data registered during the tests; please notice that all graphics relate to only one enginein operation.
We found three kinds of phenomena that are to be called as cases A, B and C.
Case A refers to some high frequency components that became coherent from some time during the test (first presented at
Case B is related to some simultaneous jumps occurring also at high frequencies (first presented at
Case C show reduced variability and increasing sound pressure levels in some high frequency TOB (initially presented at
The first case (Case A) shows a qualitative change at the highest frequency waves at about 11:15: they become coherent as shown by the pattern they exhibit from 10,000 Hz and upper frequencies.It doesn’t happen at lower frequencies (
Another kind of phenomena appears at high frequencies in Case B. No coherence phenomena appear but there are some ascending and descending jumps that occur within some 3 to 5 minutes of difference. Although they are not reflected in the broad band levels, sound pressure levels jumps occur simultaneously in several TOBs (
The third case we found at high frequencies (Case C) is shown in
bility while increasing their value.
This occurs from 2500 Hz and upper frequencies. The sound pressure levels at 2000 Hz are rather constant over the time and have very few fluctuations.
We have also registered interference phenomena during noise measures, as shown in
Once we have identified what was happening at high frequencies, we went on looking for regularities at other frequencies. We found that the root causes in the above-mentioned cases were linked to changes at low frequencies.
The changes in regime of acoustic emissions in TOB of 25 Hz but also 12.5 Hz and 50 Hz were always present during the recorded events.
We had only one measurement where Case A occurred (coherence in high frequencies components). For this particular case, we found that the component in 50 Hz was qualitatively less chaotic in the previous 10 minutes (
Case B was the most frequent during our measurements. It can be seen as a set of jumps in sound pressure levels occurring simultaneously in several TOB. There are at least three of these jumps in
Figures 10-13 show the same kind of phenomena occurring in other events.
The third group of cases we measured (Cases C) had the most unexpected behaviour at low frequencies.
Another interesting example is shown in
very smooth; it lasts more than one hour since the beginning of the phenomenon at low frequencies (25 Hz and 50 Hz), as it can be seen in a closer approach shown at
Finally,
We have been able to measure the occurrence of several events of thermoacoustic instability in 10 MW engines.
These phenomena occur in large machines in which homogeneity of parameters and simultaneity of processes in the combustion chamber are not proper hypothesis. Small differences and/or fluctuations can generate disturbances and fluctuations in combustion parameters. In these conditions, for example, if a coupling occurs with some geometric dimension, the system will go into resonance.
During the preparation of the instability events, processes of loss of chaos in low frequencies, especially in 25 Hz or 50 Hz, were observed. Also, a reduction of the variability of sound pressure levels in high frequencies occurred. After some minutes, the system retrieves the condition of randomness of the combustion.
Each of the “steady” states of operation lasts several minutes, as so the transition from one to another does.
Every quasi-stationary before and after the occurrence of a thermoacoustic instability episode last a few minutes, so active control systems seem to be a suitable solution to address such problems on large machines.
González, A.E., Ottieri, J.C., Kovar, P.G., Croucciée, J.M. and Lisboa, M.R. (2017) Measuring Sound Pressure Levels during Thermoacoustic Instabilities in Large Engines: Case Study. Journal of Modern Physics, 8, 1685-1699. https://doi.org/10.4236/jmp.2017.810099