NASA / CR-2003-212 197 

A Design Methodology for Rapid 

Implementation of Active Control 

Systems Across Lean Direct 

Injection Combustor Platforms 

William T. Baumann, William R. Saunders, and Uri Vandsburger 

Virginia Polytechnic Institute and State University, Blacksburg, Virginia 

April 2003 

https://ntrs.nasa.gov/search.jsp?R=20030054519 2018-05-15T16:39:46+00:00Z
The NASA STI Program Office . . . in Profile 

Since its founding, NASA has been dedicated to 

the advancement of aeronautics and space 

science. The NASA Scientific and Technical 

Information (STI) Program Office plays a key part 

in helping NASA maintain this important role. 

The NASA STI Program Office is operated by 

Langley Research Center, the Lead Center for 

NASA's scientific and technical information. The 

NASA STI Program Office provides access to the 

NASA STI Database, the largest collection of 

aeronautical and space science STI in the world. 

The Program Office is also NASA's institutional 

mechanism for disseminating the results of its 

research and development activities. These results 

are published by NASA in the NASA STI Report 

Series, which includes the following report types: 

TECHNICAL PUBLICATION. Reports of 

completed research or a major significant 

phase of research that present the results of 

NASA programs and include extensive data 

or theoretical analysis. Includes compilations 

of significant scientific and technical data and 

information deemed to be of continuing 

reference value. NASA's counterpart of peerreviewed formal professional papers but 

has less stringent limitations on manuscript 

length and extent of graphic presentations. 

TECHNICAL MEMORANDUM. Scientific 

and technical findings that are preliminary or 

of specialized interest, e.g., quick release 

reports, working papers, and bibliographies 

that contain minimal annotation. Does not 

contain extensive analysis. 

CONTRACTOR REPORT. Scientific and 

technical findings by NASA-sponsored 

contractors and grantees. 

CONFERENCE PUBLICATION. Collected 

papers from scientific and technical 

conferences, symposia, seminars, or other 

meetings sponsored or cosponsored by 

NASA. 

SPECIAL PUBLICATION. Scientific, 

technical, or historical information from 

NASA programs, projects, and missions, 

often concerned with subjects having 

substantial public interest. 

9 

TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientific 

and technical material pertinent to NASA's 

mission. 

Specialized services that complement the STI 

Program Office's diverse offerings include 

creating custom thesauri, building customized 

databases, organizing and publishing research 

results . . . even providing videos. 

For more information about the NASA STI 

Program Office, see the following: 

Access the NASA STI Program Home Page 

at http:llwww.sti.nasa.gov 

E-mail your question via the Internet to 

help@sti.nasa.gov 

Fax your question to the NASA Access 

Help Desk at 301-621-4134 

Telephone the NASA Access Help Desk at 

301-6214390 

Write to: 

NASA Access Help Desk 

NASA Center for Aerospace Information 

7121 Standard Drive 

Hanover, MD 21076 
NASA/CR-2003-212197 

A Design Methodology for Rapid 

Implementation of Active Control 

Systems Across Lean Direct 

Injection Combustor Platforms 

William T. Baumann, William R. Saunders, and Uri Vandsburger 

Virginia Polytechnic Institute and State University., Blacksburg, Virginia 

Prepared under Grant NAGS2687 

National Aeronautics and 

Space Administration 

Glenn Research Center 

April 2003 
This report contains preliminary 

findings, subject to revision as 

analysis proceeds. 

d 

I I 

The Propulsion and Power Program at 

NASA Glenn Research Center sponsored this work. 

I I 

Available from 

I 

I NASA Center for Aerospace Information 

7121 Standard Drive 

Hanover, MD 21076 

* 

National Technical Information Service 

5285 Port Royal Road 

Springfield, VA 22100 

Available electronically at httm: / /Pltrs.erc.nasa.eov 
0 

A Design Methodology for Rapid Implementation of Active Control 

Systems Across Lean Direct Injection Combustor Platforms 

William T. Baumann, William R. Saunders, and Uri Vandsburger 

Virginia Polytechnic Institute and State University 

Blacksburg, Virginia 24061 

Abstract 

The Virginia Active Combustion Control Group (VACCG) completed the first 9 months 

of work under the subject NRA Grant NASA Glenn Project (NAG3-2687) in October, 2002. 

This brief summary is intended to provide a synopsis of the progress that was made during the 

2002 fiscal year. It is noted that the research program actually began at Virginia Tech in early 

December, 2001, due to a delay in the funding. Therefore, the work reported here spans research 

conducted at the university between January, 2002 and October 15, 2002, not the entire twelve 

month period of the 2002 fiscal year. 

The VACCG team is comprised of engineers at Virginia Tech who specialize in the 

subject areas of combustion physics, chemical kinetics, dynamics & controls, and signal 

processing. Currently, the team’s work on this NM research grant is designed to determine key 

factors that injluence combustion control performance through a blend of theoretical and 

experimental investigations targeting design and demonstration of active control for five 

different combustors at university, industry, and government facilities. To validate the accuracy 

of our conclusions about control effectiveness, a sequence of experimental verifications on 

increasingly complex lean, direct-injection combustors is planned. During the first year of effort, 

work has focused on two different laboratory-scale combustors at Virginia Tech that will allow 

access for a wide variety of measurements. As the grant work proceeds to future years, one key 

goal will be to obtain certain knowledge about a particular combustor process using a minimum 

of sophisticated measurements, due to the practical limitations of measurements on full-scale 

combustors. In the second year of this grant, results obtained in the first year will be validated 

on test combustors to be identified in the first quarter of that year. In the third year, it is 

proposed to validate the results at more realistic pressure and power levels by utilizing the 

facilities at the Glenn Research Center. 

There were numerous accomplishments and results stemming from the completion of 

their grant’s first year of funding. During the work period (1/1/2002 through lO/l5/2002), two 

students were funded full-time on the effort with an additional 0.5 student funded on the costsharing budget identified in the contract. The summary report below will provide brief 

descriptions of the key results for the first-year work tasks identified below. 

NASNCR-2003-212197 1 
In trod u ction 

The overall goals of this research grant, as summarized in the proposal are repeated below: 

The purpose of this work is to develop a control system design methodology that can be used to 

rapidly design and deploy an active control system to suppress any existing combustion 

instabilities on a lean direct injection combustor. To this end, we proposed to do the following: 

1. Develop a minimal set of tests to characterize fuel-modulation actuators with regard 

to their usefulness for active control of combustion. 

2. Develop a minimal set of tests for combustors that characterize an existing instability 

and the impact of fuel modulation. 

3. Develop a methodology to determine the size, number and location of actuators to 

suppress an existing instability from the test data gathered in items 1 and 2. 

4. Determine a suitable control structure and adaptive tuning technique that, together 

with the actuators selected in item 3, will suppress an existing instability. 

5. Validate the above items on combustors of increasing complexity. 

There were seven work tasks that were offered as part of the three-year work approach that was 

proposed to reach these five end-goals. Work tasks 1 and 2 were began during the first six 

months of the grant, with work tasks 3 and 4 being initiated at the beginning of the third quarter 

of the work period. (Note that the third quarter of the work period began in July, 2002 and ended 

in September, 2002.) The key work items and corresponding results for the work tasks are 

presented below, delineated by work task. 

Work Task Results 

1) Kick-Off Meeting and Investigations 

The NRA grant work began in early January, 2002, after funding had been established for two 

graduate students. At this time, two graduate students, one MS student (Jonathan DeCastro) and 

one PhD student (Wajid Chishty) began working on actuator and combustor design issues, 

respectively. A series of phone calls were held with Joe Saus, one of the NASA Glenn technical 

grant monitors in mid-January timeframe. The overall scope of the grant work plan was 

discussed and it was deemed appropriate to move forward as discussed in the proposal. 

Two other meetings of note were arranged during the first two quarters of the work period. First, 

Mr. Joe Saus and Dr. Clarence Chang visited the Virginia Tech campus in mid-April, 2002 for 

two days of meetings with the VACCG faculty and students. During that visit, a tour of the 

VACCG laboratory was conducted also. The interaction between the NASA Glenn and Virginia 

Tech engineers, during the April visit, served as the primary kick-off meeting for the program. 

In addition, the VACCG arranged for one of the graduate students, Mr. Jonathan DeCastro, to 

visit the NASA Glenn facilities in mid-June. The main purpose for this two-week visit was to 

NASNCR-2003-2 121 97 2 
help the VACCG research team to become familiar with actuator characterization test-rigs and 

procedures that the NASA Glenn engineers were conducting on two different actuators 

scheduled for testing in active combustion control systems. The summer visit allowed a good 

dissemination of information about the testing issues that were deemed important by NASA 

engineers. 

In early June, an additional MS graduate student (Noah Schiller) was added to the grant team. 

This addition was in anticipation of the graduation of Jon DeCastro sometime during the Fall 

2002 semester. Jon’s funding was transferred to the cost sharing budget for the Fall semester; 

therefore, Noah and Wajid are the two students currently being paid by the NRA funds. 

Next, work items completed under the three technical work tasks that were in the timeline for the 

first twelve months of the grant are discussed. 

2) Actuator Performance Characterization Methods 

The fundamental goal is to identify parameters that can be associated with the ability of a 

particular actuator/fuel injection system to control combustion instabilities of a specified sound 

power level or unsteady heat release rate. The major factors under consideration are actuator 

bandwidth, modulation percentage and type (proportional or on-off), and atomization 

characteristics. It is important to distinguish here that the “actuator” in this task refers to both the 

actuating mechanism, as well as the modulating component of the fuel injection system. Two 

different actuated fuel injection systems are under study by the VACCG, as part of this task. The 

first system is a piston-check valve configuration, while the second system is a throttling valve 

configuration. Both fuel injection system designs rely on a piezoceramic actuation mechanism 

for the modulation control. 

A large amount of work has been completed under this task heading and will be discussed below. 

However, it is also relevant to note that two of the grant Principle Investigators (W. Saunders and 

U. Vandsburger) are also managing a senior design project, consisting of fifteen senior students, 

which is focused on the characteristics and ultimate scaling of the two existing fuel injection 

system designs. This marks the third year that the PI’S have devoted their energies to this type of 

design class project and the overall benefit to the NASA grant program goals continue to be very 

positive. For this report, however, only the graduate student work items will be reviewed. If the 

NASA technical monitors wish to receive documentation about this year’s senior design team 

goals and results, those documents can be provided under separate cover. The fuel injection 

system characterization work began with the fabrication and testing of all the system 

components, such as the actuator, the valve, the atomizer, and the other components of the fuel 

injection system (e.g. accumulator, pumps, etc.). In addition, there has been a significant effort 

devoted to modeling the unsteady pressures that are expected to exist at the atomizer input 

location, in anticipation of future correlation with laser measurements of the nozzle (atomizer) 

sprays exiting under the action of modulation. The key points of this work will be discussed 

next. 

NASA/CR-2003-212197 3 
I. Piston and Check Valve System Testing 

The first proportional fuel injection system design completed in-house by the VACCG was based 

on a piston check-valve (Figure 1) operation that produced unsteady pressure pulses used to 

modulate fuel flow through a simplex atomizer. Certain data have already been reported to 

NASA about the use of that particular design and will be summarized here only. During openloop testing, it was determined that this type of design provided approximately 40% pressure 

modulation and was capable of reducing the lightly damped, noise driven response of an 

instability in an ethanol combustor by an amount of approximately 5-7 dB. Subsequently, the 

same fuel injection system was used to implement active control on the first-generation kerosene 

LDI combustor built at the very beginning of this program. A representative pressure spectrum 

from that test is shown later in the section discussing unsteady combustor performance 

characterization. 

I 

Figure 1 : Piston and Check Valve Hardware 

11. Throttling Valve System Fabrication and Testing 

In order to reduce the piston check-valve scaling requirements on actuator force, a second 

modulated fuel injection system design was investigated by VACCG members. Within the 

context of this NRA program, it will certainly be beneficial to rapid implementation of ACC 

systems if the scaling of the actuator is eliminated as a design variable. The overall system 

design was completed with the assistance of the 200 1-2002 academic year senior design class 

curriculum. Initial testing of the system showed that an off-the-shelf valve selected for the 

throttling valve function was not properly designed for compatibility with the extremely small 

displacements from the piezoceramic stack actuator (on the order of microns of displacement). 

Therefore, one of the MS student work tasks was to identify a first-generation valve design that 

would provide the fast opening flow profile needed for this application. The valve was designed, 

machined, and fabricated completely in-house at Virginia Tech. A photograph of the resulting 

throttling valve hardware is shown in Figure 2. 

NASNCR-2003-212197 4 
Figure 2: Throttling Valve Hardware 

500 

450 

In very recent weeks, we have begun to test the throttling valve with a newly purchased 500 

micron piezoceramic stack actuator. The analog amplifier for the stack actuator is currently 

limiting the actuator bandwidth and output power. A switching amplifier will be tested in 

coming weeks to determine what bandwidth can be achieved at full modulation power. For now, 

the present performance of the throttling valve is indicated in Figure 3 below. 

I I I I I 

I ....................... :......................;.......................:.....................L. ..................... 

300 

,990 

200 3 

:\ 

:\ : ,, I\ I, 

i: !\ ; s. :\ '< 

: \; 

...................... ....... ................................ ........... -- : i .......a ! i..! , f.7 ...................... .:. . ..i.. ....... +- ,I 

/ j '-, ,' 

.<I i ', -.,/ ., \\ ' %/ 

:d 

............................ < ................................................. ; ........................ ; ...................... 

I I I I I 

0005 OM 0 015 o az 0 025 

Figure 3: 

NASNCR-2003-2 12 197 5 

Pressure Modulation Measured at the Atomizer Inlet 
The data was obtained with a “bypass” line installed and a mean upstream pressure of 425 psi. 

The valve was being operated about an offset of 160 um with a swing of 2 150 um. At this time, 

the fluctuating pressure amplitude at the nozzle is partially due to a fluctuation in the supply 

pressure. We are currently working to finalize the performance of the throttling valve system so 

that active combustion control testing can begin. 

3 160 

a !e. 

140- 3 

u) 

in 

2! 120- 

I00 

a 

111. Fuel Delivery System Acoustic Modeling 

/--A - 

,-----”\, r- 

_- tJ r_’ - 

I----<- \-- i- - -. -1 

Data1 M ode1 1 

I I I I I I 

One of the fbndamental necessities for a rapid ACC implementation scheme is the understanding 

of the unsteady, modulated flow fiom the nozzle. To begin addressing this facet of the grant 

studies, a series of acoustic modeling efforts have been completed. For this report, a brief 

synopsis will be presented. However, a MS Thesis will be available at the conclusion of this 

calendar year for any NASA personnel who are interested in all of the modeling details. 

I I 1 J 

Four different models were examined during a systematic investigation that was designed to 

ensure understanding of increasingly complex acoustic dynamics in the fuel injectors. The first 

system investigated was a simple closed-tube driven by the piezoceramic-actuated piston (see 

Figure 1). A comparison of the model and measured SPL values inside the downstream section 

of the tubing is shown in Figure 4. 

2oo I 

Figure 4: Piston - Closed Boundary Model and Data 

NASNCR-2003-212197 6 
. 

As shown in Figure 4, the resonance frequencies of the fuel system piping and piston-stack 

assembly are predicted quite well. The amplitude is still not as accurate as we wish to predict; 

however, there are several good hypotheses for this discrepancy and we believe that this will 

improve as more measurements are acquired for analysis. 

The second model also relied on the pistodstack acoustic actuation method but the piping system 

was terminated with a simplex atomizer. This change provided a straightforward way to assess 

the types of acoustic impedance that might be attributed to the atomizer hardware. The 

comparison between model and measured SPL values for this test system, shown in Figure 5, 

indicates that the atomizer behaves in large part as any junction impedance might behave, i.e. as 

a small orifice might behave acoustically. Again, the amplitude is not perfectly matched but the 

frequencies are predicted well. 

1.0 Atomizer Data and Model, 9-foot, p = 0, Vair = 6e007 m3, P = 230 psi 

180 I f I 

2 160 

a 

z 

2 140 a 

2 120 

a 

u) 

fn 1 1 __ Data 1 

100 1 I I I I I J 

0 100 200 300 400 500 600 700 

200 I 1 I 

I 

-200 

0 100 200 300 400 500 600 700 

Frequency (Hz) 

Figure 5: Piston - 1 .O Atomizer Boundary Model and Data 

Figure 6 shows the comparison for the third acoustic modeling effort, which utilized the 

throttling valve as the acoustic source term and a closed pipe boundary condition was repeated 

for this new actuation method. Again, the resonance frequencies are matched nicely but this 

time the amplitude is more accurate than before at some frequencies. The differences in 

amplitude are attributed to a number of factors that will be discussed in the corresponding MS 

thesis discussion, 

NASNCR-2003-2 12 197 7 
Closed Tube Downstream Data & Model, 12-f00t, stack offset = 50 pm, P = 230 psi 

I , r I 

140 s 

n 

2 130 

fn 

a 

li 110 

100 I 1 I I I I I 

0 100 200 300 400 500 600 700 

0 100 200 300 400 500 600 700 

Frequency (Hz) 

Figure 6: Throttle Valve - Closed Boundary Model and Data 

The final acoustic model, currently under investigation, is associated with the throttling valve as 

the acoustic source and the atomizer end-condition for the piping. One example of this work-inprogress is shown in Figure 7. It is important to recognize several important issues for the 

acoustic modeling of this system. First, the throttling valve can be operated at a variety of offset 

positions. This means that the valve head, which serves as the acoustic source, couples the front 

and rear portions of the piping quite differently as the offset values are changed. In addition, the 

pressure drop across the atomizer changes with offset position, thereby changing the details of 

the acoustic impedance that is used to represent the atomizer flow passages. In addition, these 

changes will also be going on during one complete cycle of the modulated valve position. This 

means that the acoustic problem is truly nonstationary and thereby precludes precise modeling 

using the simplified one-dimensional approximations used for the modeling effort so far. 

Nevertheless, these frequency response functions serve a useful purpose in bounding our 

expectations about the possible dynamic response that will be expected to play a key role in 

modulating the atomizer flow sheets for combustion control. 

. 

NASNCR-2003-212197 8 
Val= with 1.0 Atomizer Data and Model, stack offset = 300 microns, P = 230 psi 

* 

140, I I I I 

J^ 130 

n 

2 

g 120 a 

g 110 

n 

v) 

v) 

.- 

1 

100 I I 4 I I I I 

0 100 200 300 400 500 600 700 

200, I I I 

-400 I I L I I I 

0 100 200 300 400 500 600 700 

Frequency (Hz) 

Figure 7: Throttle Valve - Atomizer Boundary Condition Model and Data 

IV. There are two other important ‘‘ characterization” investigations underway for the 

modulated fuel injection systems. The first study is concerned with quantifying the degradation 

in fuel spray modulation that results from certain sources of compressibility in the fuel lines that 

are being modulated. Time constants are being measured for varying inlet pressures in an 

attempt to create a simplified “linearized” model of the compressibility effects. This information 

will identify whether the compressibility (caused by solubility of gases in the fuel or air bubbles 

resulting from other sources such as cavitation or aeration in the fuel tank) is responsible for 

destroying the bandwidth of the fuel injection systems under any circumstances. In addition, we 

are attempting to identie certain test procedures that can lead to similar quantitative assessments 

of compressibility effects on any arbitrary combustor fuel delivery system that must be 

modulated for active combustion control purposes. The results of this study will be summarized 

at the end of the Fall 2002 semester and will be made available to NASA at that time. 

The final fuel system investigation that should be mentioned is concerned with determining the 

influence of the atomizer design and flow number on the fuel modulation results. At this time, a 

PDPA system is being prepared for shipping to Virginia Tech, where it will be used to measure 

fuel sprays under actuation by the piston or throttling valve injection system. The acoustic 

modeling and measurements, discussed earlier will be used to guide the fuel spray velocity 

experiments. The ultimate goal is to develop methods that rely on simplified acoustic models of 

any fuel delivery system to predict the fuel line pressure at the inlet to the atomizer, and 

measured behaviors of a particular nozzle design, to quickly assess how the atomizer spray will 

interact with the combustor generated unsteady pressures encountered by the nozzle sprays. 

NASNCR-2003-212197 9 
3) Combustor Characterization Methods 

The operating conditions of the combustor determine how a pulse of fuel from an injector will 

burn both spatially and temporally, and it is the heat release of the burning fuel that ultimately 

governs control effectiveness. Key issues that determine performance include mixing, the time 

history of burning due to the flame shape, and the degree of coupling between the heat release 

mode shape. This task involves the characterization of these dynamic features of the combustor 

plant. For this case of direct liquid-fuel injection, the dynamic mixing response of the 

atomization and vaporization must also be determined and will become a critical focus of this 

work task in coming months. 

and the acoustic mode, which is influenced by the relative positions of the flame and the acoustic 

One of the first requirements under this task was simply the design and fabrication of a liquidfuel, lean direct injection type of combustor that exhibits a combustion instability (thereby 

allowing this study to proceed). The current combustor rig is a vertically mounted structure with 

150 kW thermal capacity. Photograph of the rig is given in Figure 8. 

Figure 8: Year-One LDI Combustor Rig 

It comprises three main sections, the combustor, the burner and the flow conditioning section, 

which is connected to the air and fuel supply lines. The combustor is a 5” ID steel cylinder made 

up of hot and cold sections separated by a W insulation packing made from Fiberfrax gasket 

paper. The hot section, which also acts as the test section, is 26” long and is equipped with three 

90’ apart rectangular (S”x3”) viewing windows made up of W’ thick GE Type 124 fused silica 

quartz plates. The windows have extremely high optical, thermal and physical properties and 

were provided for complete imaging and laser measurements. The cold section of the combustor 

has a length of 24” and is wrapped with W’ copper tubing for water-cooling. The total length of 

48” was selected after a series of tests to give the combustor the self-excitation capability. 

r 

NASNCR-2003-212197 10 
The burner section consists of an air plenum, a centrally located injection nozzle and an axial 

swirler, which sits coaxially around the injector. A 2 %” overlap is provided between the 

combustor and the burner section so that the complete flame is captured by the viewing 

windows. The plenum has been fabricated to accommodate various sizes of swirlers and injector 

nozzles. It is also equipped with two microphone taps for measuring forced velocity 

perturbations, and two water taps to provide cooling to the face plate on which the combustor 

sits. The face plate has a diverging quarl and is replaceable. Both pressure atomizing and air 

assist atomizers can be used. The fuel (and air in case of air assist nozzle) is fed to the injector 

nozzle from the bottom of the rig through YI” stainless steel tubing. Axial location of the injector 

nozzle can be changed by adjusting the position of the lance, which houses the nozzle. A variety 

of axial swirlers can be used with different vane angles and sizes. Their location can also be 

changed along the longitudinal axis of the burner. 

Tanperrtura Slgnak 

Computer Dlgltal Camera 

Cornburtor 

Alr Camprmtror A 

Flow Control 

Computmr .Q. 

Fuel Tank Cilmp/Matar 

Figure 9: Schematic of Overall Combustor Test Facility 

The overall combustion facility layout is shown in Figure 9. Kerosene is used as the combustion 

fuel and is delivered from the non-pressurized tank to the combustor by a SUNTEC Model 885X 

gear pump at a maximum pressure of 300 psi. The pump is driven by a % hp, 3450 rpm Dayton 

single-phase electric motor. The fuel feed line comprises: filter, to arrest dust and particle 

flowing into the feed line; pressure regulator, to maintain a maximum specified feed pressure; 

accumulator, to avoid pressure pulsation in the feed line; flow control valves, for both coarse and 

NASNCR-2003-2 12 197 11 
fine metering of the fuel; analog pressure gauges, to monitor feed line pressure at various 

locations; Flow meter, AWC Model JVA-lOK1 with HEF-1 sensor, to monitor the fuel volume 

flow rate; and dynamic pressure transducer, Kistler Model 206, to monitor and record fuel 

pressure fluctuations in the feed line. The air is fed to the system from a 150 scfm, 125 max psi, 

Ingersol Rand compressor. Two Eldridge Model 8710 NH flow meters, one each in axial and 

tangential air supply lines, are provided to monitor air flow to the combustor. Also, pressure 

regulators and flow control valves are provided to regulate and meter the air flow. The system is 

also equipped with a rotameter Atheson Model FM 1050B-HA to monitor the air flow to the air 

assist injector nozzle. 

Steady Combustor Characterization 

Under this subtask, the burning characteristics of the steady combustors are being 

mapped for a range of fuel and air flows and for different swirl numbers. The flame 

characterization will eventually include total chemiluminescence signal (OH)*, distribution of 

chemiluminescence throughout the flame, and flame areaholume distribution. At this time, 

measurements of swirl and flame visualization have been completed. Representative data from 

both measurements are shown in Figure 10 and Figure 11, respectively. 

mnml 

h 

I 

I 

1 

', , L 

Figure 10: Swirl Flow Vectors 

NASNCR-2003-212197 12 
Figure 1 1 : Flame Visualization for LDI Combustor 

20 - Qf=O 58 gph, Ph1=0.5, Yellow Flame 

___ Qf=O 58 gph, Phi=O 4, Blue Flame 

There was a significant effort in designing and fabricating the new swirl system that led to the 

compact blue flame shown in Figure 11. Originally, the swirl hardware was not capable of 

providing this type of flame at any settings. The ability of the flame’s heat release rate to couple 

in to the acoustic dynamics was also improved, as a result. An example of the unsteady pressure 

power spectra measured inside the combustor chimney is shown in Figure 12. The time domain 

and frequency domain data both indicate that this combustor is truly unstable, leading to limit 

cycling response that can be used in the design of ACC systems during the coming months. 

Power Spectrum 1.22 m Combustor with 45 deg Axial Swirler with Water Cooling, WDA ,4060 Nozzle 

lot 

- 

- 

0 io0 200 300 400 500 600 700 800 

Frequency (Hz) 

Figure 12: Power Spectrum of LDI Combustor 

NASNCR-2003-2 12 197 13 
4) First-Generation Methods for Rapid ACC Implementation 

- w.(xs,J - Tsp(s) - 

The fundamental objectives of this task build on the results of the previous tasks to determine a 

methodology for the rapid design and implementation of a control system capable of stabilizing 

an existing thermoacoustic instability without a large amount of extensive trial and error that 

characterizes current practice. The design process will attempt to use an energy viewpoint to 

determine the number, size and location of actuators necessary to stabilize the combustion 

system. The following discussion represents the initial energy-based design concepts that have 

been explored during the first nine months of the NRA grant. 

C(S) 

Analysis 

- 

In its simplest form, a one-dimensional acoustic system excited by heat release is 

governed by the equation 

a2pi a2pi 

at ax 

-- c 2 = (y - l)Q% (x - x,t) 

We can always expand the unforced wave equation into a series of normal modes as 

l 

which results in the uncoupled equations 

I. I 

Figure 13: Simplified Thermoacoustic Model with Control 

This figure explicitly shows the modal influence coefficients for the pressure measurement 

location, w (x,) , the flame location, y~ (xp), and the speaker input position, w (x,). Since in 

general we will not know the value of these constants, we will lump them together with the 

NASA/CR-2003-2 12 197 14 
unknown transfer functions so that w 0 (xm )Jr 0 (Xsp Ysp (s) -+ Tp (4 and 

w (xp)Tp (sh (xp ) -+ Tfl (s) . The modal influence coefficients do not need to be identified 

separately. 

We will make the fundamental assumption that near the instability frequency, the flame 

and speaker transfer functions vary slowly and so may be considered constant in amplitude and 

phase. This is in contrast to the acoustic transfer function, which varies rapidly in both 

amplitude and phase at a, and hence will induce rapid variations in the closed loop system near 

the instability frequency. 

Because of instability or lightly-damped, closed-loop acoustics, the response of q will be 

predominantly sinusoidal at the frequency of oscillation, q (t) = A(?) sin(o,,t) , and hence 

(t) = d(t) cos(o,,t) . Thus, the forcing effect of the flame can be written as 

The equation for the controlled, closed-loop system can be written as 

7i'(t)+(2cU,~, -uplr -P,h(t)+(w: -actp -a,h(t)=O 

where a term for the acoustic damping with damping ratio <, has been included. For stability 

we require the net damping to be positive. To insure that noise generated oscillations of the 

lightly-damped closed-loop system do not grow too large, it is further required that a minimum 

damping ratio 1; be achieved. This results in 

2c ucoo - aPj7 - Pc ' 2c deso)osc 

or 

NASNCR-2003-2 12197 15 
I Tsp GWmc 1 II CUWos,. 1 sin(LTsp (j~osc) + LC(jWos, )) 

Pc = < 2< acoo - aPjl - 2& desoosc 

0) os, 

The right hand side of this inequality will be negative if we are adding damping to the system. 

Therefore, the minimum control signal will be achieved by picking a control phase such that the 

sin term is -1. This choice results in 

LC(jo,,) = -90" - LTsp (jrnOsc) 

~osc I 21;acuo -aPY -21;desoosc I. 

I Tsp (jWoSc ) I 

(*) I woos, ) 12 

Note that with this choice of phase angle, a, = 0 and so the application of control does not 

change the natural frequency of the thermoacoustic system. 

To determine the maximum input excitation to the actuator, consider the hypothetical plot 

of limit cycle amplitude versus control magnitude, with phase fixed at the optimal value 

determined above, shown in Figure 14. At the crudest level of approximation, this would 

Limit cycle t 

Figure 14: Limit Cycle Amplitude vs. Control Gain 

require a maximum input to the actuator of I Cs,ab I L, . But to attain any point on the amplitude 

curve shown requires only an input of gL(g) , where g is the control gain and L(g) is the limitcycle amplitude at this gain. Thus, the maximum input to the actuator to move from the 

uncontrolled state to the stabilized state is max(gL(g)) . Since we only know two points along 

the limit cycle amplitude curve, the zero gain point and the stabilizing gain point, assume that the 

curve varies linearly between these points so that 

L, L(g) = --g + L, a 

'stab 

This yields 

NASNCR-2003-212197 16 
so the maximum input level to the actuator does not need to exceed this value to cause the 

system to be stabilized. 

Computing the Stabilizing Gain 

From equation (*) it is seen that the quantities needed to compute CSra6 are the instability 

frequency, o,, , the damping of the thermoacoustic system with no control, 2& ace, - apJ, and 

the gain of the actuator transfer function between input voltage and measured pressure, 

I T,(jo) I. The instability frequency is easy to obtain by observation of the combustor limit 

cycle. The linear damping of the system cannot be determined from observation of the ultimate 

limit cycle, but requires observation of the growth of the instability. At the onset of the 

instability, when the system is operating in the linear regime, the instability amplitude will grow 

as exp(-(26 ace, - apJ)t/2. Thus, by observing the exponential growth curve, the required 

damping can be ascertained. 

To determine the gain and phase of the speaker transfer function near the instability frequency, 

an experiment at a stable combustor condition near the instability condition needs to be 

performed. At this stable condition, the transfer function from control input to measured 

pressure in the frequency range near the lightly-damped modal resonance will have the form 

I 'sp(jor) ILKp(jor) 

s2 + 2&ors +a,' 

By measuring the frequency response at a number of points in the neighborhood of resonance, a 

curve fit can be used to find or, , I T,(jor) I, and LT,(jo,). 

Results on the Rjike Tube 

Experiments on the Rjike tube at an equivalence ratio of 0.55 and a mean flow of 120 cc/s show 

an instability at approximately 180 Hz. A simple phase shift controller can stabilize the system 

and the optimal gain is approximately 0.5 and the optimal phase is approximately 270". 

To attempt to predict this control result using the methodology above, measurements of the 

thermoacoustic damping and speaker transfer function were made. To measure the 

thermoacoustic damping, two approaches were used. In the first, the tube was run at an 

equivalence ratio that was near the equivalence ratio at which the system was unstable and we 

wished to achieve control. The equivalence ratio was then quickly changed to the ratio for 

instability and the growth of the instability was monitored. This resulted in a computed damping 

of - 1.6. The second approach, which would not be applicable in practice, involved using control 

to stabilize the system at the equivalence ratio producing instability and then turning off the 

control and observing the amplitude growth of the instability. This method yielded a computed 

damping of -24. 

Measurements of the speaker transfer function were made for both the cold tube and the hot tube 

running at a stable equivalence ratio. For the cold tube, the values produced by curve fitting 

NASNCR-2003-2 12 197 17 
were or = 2.n (164), < = 0.02, I Typ(jor) I= 47000, and LTs,,(jor) = 170". This leads to a 

predicted optimal control phase of 

which is very close to the measured value. 

LC(jo) = -90" - LT,(jo) = -260" 

I Tp(jW,.) I 

47000 

The results of the stabilizing gain computation are shown in Table 1 below. Clearly the massive 

uncertainty in the thermoacoustic damping creates unacceptable uncertainty in CsrUb . 

Considering the more accurate value due to turning off control at the actual instability condition, 

the computed value of Csrah is close to the experimental value of 0.50. 

2ruco, -4p r des 'stub 

-1.6 0 0.0385 

Table 1: Results of Stabilizing Gain Computation 

47000 1 -24 10 1 0.58 

47000 I -24 I .01 I 1.28 

The obvious critique of the analysis presented thus far is that the testing and methodology has 

been investigated only for a Rijke tube combustor using an acoustic actuator for control. 

However, these ideas will be expanded in the coming months to include the acoustic properties 

of the fuel injection systems described above, as well as the effects of the transition from 

modulated jet sprays to heat release rate. 

Summary and Upcoming Work 

The first nine months of the NRA research has been largely consumed by system fabrication, 

testing, and conceptual investigations that are required to support the ultimate goal of creating 

rapid characterization methods for unknown combustor systems. A new swirl LDI combustor 

has been designed and fabricated. This combustor can be operated in either thermoacoustically 

stable or unstable regimes. Flow rates have been optimized with respect to flame conditions and 

the strength of the instability when it exists. Two different modulating fuel injection systems, 

capable of proportional fuel modulation at flow rates in the range of 0.4 gph to 4.0 gph have been 

designed and are being tested at this time. Acoustic models of the fuel delivery systems for both 

injector designs have been completed and good agreement has been shown between the model 

and test results for most of the tested configurations. Flow measurements will soon begin, 

allowing correlation between upstream fuel line unsteady pressures and the actual modulation 

percentages measured in the nozzle sprays. Energy methods are under consideration, with the 

goal of determining minimum required "actuator" authority for any combustor of interest. 

The next three months will complete the first calendar year of the NRA grant. The VACCG 

grant program is on progress in terms of its technical goals, as well as its budgeted expenditures. 

We look forward to continuing this research effort. 

NASNCR-2003-212197 18 
REPORT DOCUMENTATION PAGE 

1. AGENCY USE ONLY (Leave blank) 

Form Approved 

OMS NO. 0704-0188 

2. REPORT DATE 3. REPORT TYPE AND DATES COVERED 

April 2003 Annual Contractor Report 

4. TITLE AND SUBTITLE 

A Design Methodology for Rapid Implementation of Active Control 

Systems Across Lean Direct Injection Combustor Platforms 

WU-708-87-2340 

NAG3-2687 

6. AUTHOR@) 

5. FUNDING NUMBERS 

William T. Baumann, William R. Saunders, and Uri Vandsburger 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 

Virginia Polytechnic Institute and State University 

Blacksburg, Virginia 24061-0238 

Department of Mechanical Engineering 

a. SPONSORlNGlMONlTORlNG AGENCY NAME(S) AND ADDRESS(ES) 

National Aeronautics and Space Administration 

Washington, DC 20546-0001 

11. SUPPLEMENTARY NOTES 

8. PERFORMING ORGANIZATION 

REPORT NUMBER 

E- 13800 

10. SPONSORlNG/MONlTORlNG 

AGENCY REPORT NUMBER 

NASA CR-2003-212197 

128. DlSTRlBUTlONlAVAlLABlLlTY STATEMENT 

Unclassified - Unlimited 

Subject Categories: 01 and 07 Distribution: Nonstandard 

Available electronically at http://eltrs.ac.nasa.gov 

This publication is available from the NASA Center for Aerospace Information. 301-621-0390. 

12b. DISTRIBUTION CODE 

14. SUBJECT TERMS 

Active combustion control; Thermo-acoustic; Fuel modulation 

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 

OF REPORT OF THIS PAGE OF ABSTRACT 

Unclassified Unclassified Unclassified 

15. NUMBER OF PAGES 

16. PRICE CODE 

24 

20. LlMlTATlON OF ABSTRACT