The following cinematic sequences are produced along with spatially separated plots of the combustion response function and the forced Rayleigh index for a number of drive frequencies. These two operations mutually contribute to create the overall shape of the combustion response curve.
LIST OF TABLES
INTRODUCTION
Introduction to Combustion Instabilities
In continuous combustion systems such as those seen in gas turbine engines, most major engine manufacturers have used some form of lean, premixed combustion to reduce the production of nitrogen oxides. They later became problematic in other systems such as gas turbine engines as the power density in those devices increased.
A Conceptual Model and Rayleigh’s Criterion
This part of system behavior is not nearly as well understood and involves many more factors. This result implies that the feedback loop illustrated in Figure 1.2 produces a self-induced drive of the system.
Previous Work
PLIF measurements have been used to localize the reaction zone and as a measure of heat release in numerous combustion experiments. The PLIF signal intensity itself was not used as a measure of heat release rate. 1998) demonstrated the use of OH PLIF in combination with CH2O PLIF to obtain an improved measure of heat release rate.
Objectives
The motivation for this was twofold; first, it was to get a sense of the burner's general response over a wide range of operating conditions. These observations, and the guidance they provide for updating the model, generate important insight into the mechanisms involved in producing the net combustion response of the combustor.
Overview
Acoustic Forcing Chamber
A 61 cm extension tube connects to the top of the test section and extends upwards to the acoustic drive cross. The acoustic drive cross is used to connect two 12 inch Cerwin-Vega drivers to the room.
Stagnation-Plane Flat-Flame Burner
The flow then enters the upper part of the nozzle assembly, which contains the aerodynamic contraction. By using a bypass control scheme as described (versus direct flow control), control of the fuel/air equivalence ratio and flow to the burner are effectively decoupled.
Laser Source
The combustible mixture is supplied to the burner from the mixing and fuel supply plates. The flow rate to the burner is independently controlled by a needle valve built into its bottom.
Light-Gathering Equipment
Long-term alignment stability is typically better than ±20 pm for the centerline wavelength at the cavity fundamental. For all experiments involving fluorescence at wavelengths shorter than 390 nm, the camera was paired with a Cerco 2085 UV lens manufactured by EADS-Sodern.
Data Acquisition System
The output of each multiplexer forms one of the signal channels that is selectively conditioned/amplified, analyzed and recorded. b) Acoustic Drive Control – The instrument's closed circuit controls the frequency and amplitude of the standing wave in the chamber using feedback from pressure transducers strategically placed in the test section. PLIF images at specific acoustic phase angles while still maintaining an average laser firing rate of 10 Hz. d) ICCD Enable Control (Camera Gate Enable) - The CGE signal is generated by the laser timing system and the “skip” register.
Overview and Motivation
AFC-100Drive Amplifier
Experimental Setup
The FFT data is used together with the room calibration data (discussed in Appendix D) to determine the phase of the acoustic drive in relation to the reference signal provided by the AFC-100 to the lock-in analyzer. At the same time, the computer issues a command to the AFC-100 to output a signal FFT.
The Test Envelope
The acoustic machine level was kept constant at 110.8 dB (SPL) for all bulk chemiluminescence experiments.
Experimental Results
- The Combustion Response Function
- Combustion Response Variation with Free Parameters
- Role of the Transport Delay
- Phase Branches
- The Forced Rayleigh Index
- Phase Branch Bifurcation Revisited
Clusters can be seen on the ascending side of the magnitude between 20 Hz and 40 Hz. The peak in the combustion response function centered around 375 Hz corresponds to the Helmholtz resonance of the nozzle and combustion cavity. As a starting point, a fixed value of 4.0 is chosen for the nozzle outlet velocity ratio, which is in the middle of the operating range.
As a result, generating the forced Rayleigh index from the collected experimental data becomes a triviality.
SPATIALLY RESOLVED EXPERIMENTAL WORK
PLIF AND IMAGE CHEMILUMINESCENCE
Overview
This is where the time-delay-like behavior of the combustion response gives way to the Helmholtz response of the burner nozzle, and then finally to the high-frequency in-phase response of the system. The purpose of carefully combing this transition is to gain some understanding of the mechanism (or mechanisms) involved in the conversion. The last base drive frequency (375 Hz) is centered on the Helmholtz response of the burner nozzle.
The two additional driving frequencies of 240 Hz and 243 Hz were added to the test matrix near the end of the experimental work.
PLIF Motivation
Some of the radicals in the excited state fluoresce and produce a signal that can be imaged by an intensified CCD camera positioned perpendicular to the laser sheet. The spatial resolution of such a system is of the order of the projected camera pixel area multiplied by the laser sheet thickness. Typical fluorescence lifetimes for the measured species are in the order of tens of nanoseconds.
As such, chemiluminescence is integrated over the depth of the flame, but is not projected onto the focal plane properly (except for that light originating in the relevant object plane.) In recent years, computational post-processing techniques have been applied to a .
LIF Theory
- Electronic Spectroscopy
- Collisional Quenching
In the application of LIF, a pulsed UV laser is tuned to a specifically chosen absorption line of the target species. Internal conversion is a process by which an electron in the ground vibrational state of S1 can move sideways across the energy diagram to a higher vibrational mode of the S0 electronic ground state. Nevertheless, these models still require knowledge of the temperature, pressure and species concentration in the investigation volume.
For the unsteady combustion work at hand, it is assumed (albeit questionably) that the temporal variations in the collisional quenching rates are small.
Laser, Delivery Optics and Imaging System .1 Laser Light Source .1 Laser Light Source
- Delivery and Sheet-Forming Optics
- Imaging System
- Laser Sheet Intensity Profilometer
A small amount of laser energy is reflected off the surface and into the photodiode assembly. This is necessary for laser plate intensity profile measurements for image post-processing. A pair of 6.35mm diameter push-pull rods allow the small beam limiters to be moved in and out of the path of the laser sheet.
The faces of the beam stops are cut at 45º and covered with white fluorescent cardboard.
General Data Collection Procedure
Laser blade profile measurements are taken at the beginning and end of each experimental run. d) Load and start the AndorBASIC program that controls camera operation and communicates with the AFC-100. Take 200 images with the left (almost) beam stop engaged. 14) Indicate to the operator that the experiment is complete. It should be noted that all references to captured image groups in the list above include capturing corresponding data sets from the AFC-100.
These data sets include the acoustic waveforms, laser firing time, and laser firing energy associated with each image.
First-Pass Data Reduction
- Data Merging
- Background Image Generation
- Image Phase Averaging
- Report Generation
- Image Data Output
Then, each image of the specified data set is loaded and the acoustic data for that image is processed by FFT. The phase information from the FFT is used to locate the rising edge of the fundamental mode. The phase location at the time of the laser capture is determined and the image is averaged.
Finally, certain statistical data are also recorded in the report file to validate the reduction process.
OH PLIF Results
- OH Excitation Regime
- Correction for Spatial and Temporal Variation in Excitation
- Reduced OH PLIF Data
Here, the lowering corresponds to the coordinate in the horizontal direction, where the origin is on the left side of the frame (the side from which the laser sheet approaches). In the images, the active areas appear as thin features that resemble the shape of the stable flame. In contrast, the phases of the active regions in the two highest frequency cases are relatively constant throughout.
This indicates point dilation in response to the fluctuating pressure field.
CH PLIF Results
- Reduced CH PLIF Data
CH PLIF experiments were performed for the seven fundamental driving frequencies mentioned in the summary. Magnitude graphs are displayed in the left column, while phase graphs are displayed on the right. Magnitude graphs are displayed in the left column, while phase graphs are displayed on the right.
At forcing frequencies of 220 Hz and higher, the amplitudes of the forced Rayleigh index decrease dramatically, similar to the decrease observed in the combustion response function.
CH 2 O PLIF Results
- Reduced CH 2 O PLIF Data
The spatially resolved functions produced by using the CH2O PLIF data as an indicator of heat release are shown in the next two figures. These graphs exhibit a striped appearance (with two bands), similar to what was seen in the CH PLIF images. Around this radial position the bands of the CH PLIF images end, while the bands in the CH2O PLIF continue for a maximum of 3 millimeters.
When the driving frequency exceeds 150 Hz, the spatially resolved imposed Rayleigh index develops the alternating lobe pattern observed earlier in the corresponding CH PLIF data.
Combined OH-CH 2 O PLIF Results
- Resulting Combined OH-CH 2 O PLIF Data
In the case of formaldehyde, each band (upper and lower) has its own independent "head". The lobes have clearly visible minima between them and the phase. However, for the corresponding test cases in the combined OH-CH2O results, the two bands merge into a single one. The thickness of the two bands at these drive frequencies is similar to that seen in the formaldehyde data.
The main difference can be seen in the missing gap in the lower band for the combined data.
Image Chemiluminescence Results
- Reduced Image Chemiluminescence Data
Image chemiluminescence data processing used a subset of the same software routines used in previous PLIF data reduction. In cases of PLIF reduction, the spatial integrations included the dependence of r to account for the axisymmetric nature of the flame. The background value of the image has risen above the threshold set by the ICCD camera.
However, in the bulk chemiluminescence case, the aperture number of the lens was set to f/4, resulting in a much smaller aperture area.
Acetone PLIF Results
- Resulting Acetone PLIF Data
The horizontal axis represents the radius from the center of the burner, and the vertical axis represents the height above the exit plane of the nozzle. The data has been mirrored across the burner centerline to complete the visual appearance. The details of the phase-separated images for all driving frequencies will not be covered here.
The images shown are for the pressure phase angle of 0° (i.e. the zero crossing of the rising edge of the fluctuating pressure).
BURNER MODELING
Motivation for Modeling
Transfer Function Fitting
As shown in chapter 3, an abrupt transition occurs where the character of the response changes dramatically. Beyond the notch is the post-transition region where the Helmholtz response of the burner cavity and nozzle is seen. Finally, at drive frequencies of 300 to 450 Hz, the Helmholtz resonant response of the burner cavity and nozzle is observed.
The magnitude response of the resulting transfer function is plotted in Figure 5-2 for comparison with the experimental data.
