Concerns about damaging the Earth's ozone layer as a result of high levels of nitrogen oxides (known collectively as NOx) from high-altitude, high-speed aircraft have prompted the study of lean premixed prevaporized (LPP) combustion in aircraft engines. LPP combustion reduces NOx emissions principally by reducing the peak flame temperatures inside an engine. Recent advances in LPP technologies have realized exceptional reductions in pollutant emissions (single-digit ppm NOx for example). However, LPP combustion also presents major challenges: combustion instability and dynamic coupling effects between fluctuations in heat-release rate, dynamic pressure, and fuel pressure. These challenges are formidable and can literally shake an engine apart if uncontrolled.
To better understand this phenomenon so that it can be controlled, we obtained real-time laser absorption measurements of the fuel vapor concentration (and equivalence ratio) simultaneously with the dynamic pressure, flame luminosity, and time-averaged gaseous emissions measurements in a research-type jet-A-fueled LPP combustor. The measurements were obtained in NASA Glenn Research Center's CE-5B optically accessible flame tube facility. A schematic of the experiment and the optical diagnostic system is shown in the following figure. The CE-5B facility provides inlet air temperatures and pressures similar to the actual operating conditions of real aircraft engines. The laser absorption measurements were performed using an infrared 3.39-µm HeNe laser in conjunction with a visible HeNe laser for liquid droplet scattering compensation (ref. 1).
Experimental facility located in test cell CE-5B (Stand 2) in Glenn's Engine Research Building. The laser absorption diagnostic system is portable and is mounted on two small optical breadboards shown by the dashed boxes. Italicized text denotes real-time data-producing components of the experiment. The dynamic pressure data are acquired with piezoresistive microphone transducers mounted in a semi-infinite tube arrangement for optimal pressure-signal fidelity. The time-averaged gaseous emissions measurements are obtained using a standard suite of online gas bench analyzers.
Data from a "generic" LPP injector operating at 16.7 atm, 754 K, and an equivalence ratio of 0.542. Left: Time-resolved data collected at a rate of 12 kHz. Equivalence mean, 0.532; equivalence standard deviation, 0.240. Right: Probability density function (PDF) of the equivalence ratio at this operating condition. Note that this injector is far from perfect in that it has a very wide PDF.
The second figure shows data from a "generic" LPP injector developed specifically for this study. The left side of the figure shows the real-time equivalence ratio, the dynamic combustion pressure, and the combustion flame luminosity over a period of 40 msec. The right side shows a histogram, or probability density function (PDF), of the equivalence ratio for this injector, which indicates how much "time" an injector spends at a given equivalence ratio. Well-designed injectors have very narrow PDF's, which translate to lower NOx emissions and quieter operation. The following figure shows the frequency spectrum of the fluctuations in the dynamic combustion pressure. These fluctuations drive fluctuations in both the infrared and visible laser signals (at 220 Hz). The final figure shows the amplitude and phase relationship between the combustion pressure, the fuel line pressure, and the flame luminosity for a low inlet temperature condition. The frequency spectrum shows a very strong correlation between all the measured quantities and indicates that the dynamic combustion pressure modulates the fuel line pressure and, hence, the flow rate.
Power spectral density plot of signals acquired in the previous figure, showing the data in frequency space. The spectrum shows a moderate acoustic coupling at 220 Hz that corresponds to the fundamental (organ pipe) mode of the combustion rig.
These are the first measurements of their kind that show a direct correlation between the quantitative fuel vapor fluctuations, dynamic pressure, fuel pressure, and flame luminosity at actual operating conditions. The measurements show that all the measured time-varying quantities are highly correlated in time and frequency--indicating that there is a basis for actively controlling combustion instabilities and noise normally associated with LPP combustion systems. Furthermore, the real-time data provide additional valuable information, such as the measured PDF of the equivalence ratio, which allows us to predict the emissions performance of new injector designs without resorting to expensive testing.
Example of strong acoustic coupling between dynamic combustion pressure, fuel line pressure, and bulk flame luminosity. The upper graph shows the time-varying signals over a 40-msec period. Note the luminosity in this case is almost pulselike and is out of phase with the pressure. The lower graph shows these signals in frequency space. The acoustic coupling between all the signals is clearly evident at about 200 Hz and even has substantially higher frequency harmonics.
Find out more about research in Glenn's Combustion Branch.
Glenn contact: Dr. Quang-Viet Nguyen, 216-433-3574, Quang-Viet.Nguyen@grc.nasa.gov
Author: Dr. Quang-Viet Nguyen
Headquarters program office: OAT
Programs/Projects: UEET
Last updated July 2, 2001, by Nancy.L.Obryan@nasa.gov
Responsible NASA Official:
Gynelle.C.Steele@nasa.gov
216-433-8258
Point of contact for NASA Glenn's Research & Technology reports:
Cynthia.L.Dreibelbis@nasa.gov
216-433-2912
SGT, Inc.
Web page curator:
Nancy.L.Obryan@nasa.gov
216-433-5793
Wyle Information Systems, LLC
