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Monitoring and control of a pulse detonation engine using a diode-laser fuel concentration and temperature sensor
Proceedings of the Combustion Institute, Volume 29, 2002/pp. 161–166
MONITORING AND CONTROL OF A PULSE DETONATION ENGINE USING A DIODE-LASER FUEL CONCENTRATION AND TEMPERATURE SENSOR L. MA, S. T. SANDERS, J. B. JEFFRIES and R. K. HANSON High Temperature Gasdynamics Laboratory Department of Mechanical Engineering Stanford University Stanford, CA 94305, USA
Fuel measurements are needed to accurately tailor fuel charges in pulse detonation engines (PDEs) to improve engine performance and to validate PDE models and computations. Here, we report simultaneous concentration and temperature measurements of C2H4 fuel in a PDE using a newly developed diode-laser absorption sensor. These measurements enable characterization of the fuel loading and ignition timing of the engine. Based on these characterizations, a real-time control system that optimizes fuel consumption and maximizes specific impulse in the engine has been realized. Similar measurements of C2H4 concentration and temperature were used to characterize pulse-to-pulse interference resulting from loading fresh fuel/oxygen reactants into hot combustion products. The sensor was used in a simple control scheme to minimize such interference, illustrating its potential role in control systems to maximize the engine’s operation rate. During these studies, the sensor demonstrated two valuable improvements over traditional absorption spectroscopic techniques: (1) increased robustness and accuracy and (2) simultaneous measurements of concentration and temperature. These improvements are enabled by broad wavelength scanning of the Q-branch spectra of C2H4 near 1.62 lm. The success achieved in these small-scale tests provides strong support for expanded use of diode-laser sensors in propulsion applications.
Introduction Recent pulse detonation engine (PDE) research aimed at achieving a better understanding of PDE physics, engine operation, and engine control [1] has frequently used gaseous C2H4 fuel, because it is easily detonable and because liquid fuels envisaged for practical PDEs are expected to pyrolyze to C2H4 or C2H4-like species before detonation events. A robust C2H4 sensor is therefore needed to characterize the PDE fueling process and related engine dynamics, to provide fuel distribution data for model validation, and to develop control systems for improved engine performance. Previously, a C2H4 sensor based on 3.39 lm HeNe laser (fixed-wavelength) absorption was used to monitor C2H4 concentration in a model PDE [2]. However, the accuracy of this sensor was compromised by the hostile measurement environment in PDEs [3]. Our new diode-laser C2H4 sensor is aimed at overcoming this deficiency. A broad wavelength-scan sensing technique is developed here to enable simultaneous measurements of C2H4 concentration and temperature, to ensure the sensor’s accuracy and reliability in harsh measurement environments such as PDEs, and to carry out detailed characterization of the fueling process in our laboratory-scale PDE. Reliable real-time control to optimize fuel consumption and maximize specific impulse has been demonstrated in the Stanford
PDE by combining this sensor with a lock-in technique to synchronously detect laser absorption. The pulse-to-pulse interference resulting from loading fresh fuel/oxygen reactants into hot combustion products has also been studied. Finally, sample results indicate the sensor’s potential for an active control system using real-time measurements to minimize pulse-to-pulse interference and thus maximize the engine’s operation rate. This paper first describes the sensing technique and the C2H4 spectroscopic data upon which the sensor is based. The fueling process and active control studies in the Stanford PDE are then discussed.
Description of the C2H4 Sensor Optical absorption sensors exploit the uniqueness of molecular spectra to enable accurate identification and detection of the target species with nonintrusive, rapid-response measurements. The absorption spectrum of C2H4 is rich with a variety of distinct spectral features [4]. Although the strongest absorption occurs in the mid-IR, in the spectral range where tunable diode lasers are commercially available (below 2 lm) the strongest C2H4 absorption occurs at the first overtone of the fundamental stretch of C-H, that is, between 5700 and
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Fig. 1. Absorption spectrum of C2H4 near 3.39 lm simulated by HITRAN 2000. Simulation conditions: T ⳱ 300 K, P ⳱ 1 atm, 25% C2H4, and 75% O2.
Fig. 2. Absorption spectrum of C2H4 near 1.62 lm for a mixture of 25% C2H4 and 75% N2 at 296 K and 1 atm, measured by temperature tuning a DFB diode laser.
6200 cmⳮ1. We have previously utilized a fixedwavelength 3.39 lm HeNe laser for sensitive detection of C2H4 using the strong fundamental band (Fig. 1), but fixed-wavelength schemes are not generally applicable to hostile environments such as PDEs where interference absorption and noise are common problems. Instead, we now favor use of the overtone spectral region near 1.62 lm (Fig. 2), where tunable diode lasers are available and may be exploited to ameliorate interference and noise effects. In our previous work using a 3.39 lm HeNe laser to monitor C2H4 concentration in PDEs [2], temperature was required for a quantitative measurement of C2H4 and was inferred from a separate sensor, usually a water vapor absorption measurement [5,6]. In addition to the complexity of this monitoring system, several other problems existed. First, from the inset in Fig. 1, we can see that the HeNe wavelength lies very near an absorption minimum in this spectrum, which limits the sensitivity of the measurement. The fixed wavelength of HeNe lasers prohibits choosing an alternative spectral feature with stronger absorption. Second, interference from
other species can easily confuse the HeNe-based C2H4 sensor. Nearly all hydrocarbon molecules absorb the HeNe light, and many have a larger absorption cross section; for example, potential contaminants such as butane and propane absorb the 3.39 lm HeNe radiation about 60 times as strongly as C2H4 [7]. Third, during the fueling process, the water concentration is usually low, which makes it difficult to determine temperature accurately. Tunable lasers offer the opportunity of selecting an optimum absorption feature to obtain strong absorption and to avoid spectral interference. In addition, scan-wavelength techniques can be made insensitive to laser attenuation by broadband absorbers and scatterers (such as soot particles and fuel droplets), flow-induced beam steering, and spectroscopic uncertainties associated with collisional line broadening, as discussed previously [8]. All these advantages are crucial for measurements in the harsh PDE environment. However, the absorption transitions of C2H4 are almost always blended by collisional broadening at atmospheric pressure, a common trait of polyatomic molecular spectra. Fortunately, the overtone and combination bands of C2H4 in the 1.6 lm region [9] have a few isolated features, as shown in Fig. 2. Initially, we chose the feature near 6158 cmⳮ1 (cross-hatched) [10]. The C2H4 concentration is inferred from the integrated area of the feature using Beer-Lambert’s law [5]. This choice was appropriate for room-temperature C2H4 concentration measurements, but not for combustion applications, because the absorption strength decreases and other interfering C2H4 hot lines increase as temperature increases; hence, measurements above 500 K are not possible. An alternative strategy was therefore sought. These problems are solved by scanning the wavelength of a laser absorption sensor over the 6143– 6151 cmⳮ1 Q-branch region shown in Fig. 2. In addition to the advantages discussed above, the C2H4 absorption in this region is much stronger and decreases more slowly with increasing temperature than that of nearby features. The set of overlapped transitions produces a relatively broad feature, which remains less sensitive to interference from other species. Finally, the temperature can be determined simultaneously, from the shape of this feature. We extended the current-tuning range of our distributed feedback (DFB) diode laser by exceeding the manufacturer’s recommended current limit, thus enabling determination of the temperature at kilohertz rates by fast absorption scans of the Q branch. Single-sweep (2 ms) measurements of the C2H4 Q branch at different temperatures are shown in Fig. 3. Note the variation in the shape of this feature with temperature. To speed the spectral fitting needed to determine temperature, we selected the ratio of two peaks illustrated in Fig. 3 for this purpose. The ratio at different temperatures is shown in Fig. 4, fit to a
DIODE-LASER FUEL SENSOR FOR PULSE DETONATION ENGINE
Fig. 3. Q branch of C2H4 near 1.62 lm at different temperatures, measured in a static cell with 20 cm absorption path length, pure C2H4, and 1 atm pressure.
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metered with flow orifices and premixed before injection into the PDE. The firing sequence begins with simultaneous opening of the fuel and oxidizer valves, subsequent filling of the PDE, closing of the valves, electric spark ignition, and a delay time interval (before repeating the cycle) for the burned gases to exhaust. The C2H4 sensor is applied to the head and tail ends of the engine simultaneously. Information obtained at these two measurement stations is sufficient to characterize and control the C2H4 distribution in the engine when C2H4/O2 input flow rates are well behaved. However, more measurement stations can readily be incorporated by adding additional beam splitters and detectors. The DFB laser’s current-tuning range is extended to obtain an 8 cmⳮ1 scanning range, sufficient to record C2H4’s Q branch spectral feature near 6150 cmⳮ1. Transmitted DFB laser beams are monitored by InGaAs detectors. Detector voltages are digitized using 12bit resolution for quantitative absorption measurements or detected with a lock-in amplifier synchronized with the diode-laser wavelength sweep rate for control experiments. When the lock-in signal exceeds the set point, the PDE firing sequence is initiated.
Characterization of the Fueling Process Fig. 4. Ratio of the two peaks indicated in Fig. 2 to monitor temperature.
Fig. 5. Schematic of C2H4 sensor applied to Stanford PDE for monitoring and control.
Boltzmann distribution model. This simple scheme has proven very effective in monitoring C2H4 temperature in PDE flows. Experimental Method Figure 5 provides a schematic of the experimental setup in the Stanford PDE, previously described in Refs. [2,3,6]. The fuel (C2H4) and oxidizer (O2) are
Characterization of the fuel distribution is of fundamental importance for the development of PDEs. To validate PDE models and computations, uniform fuel charges are desired and the quantitative fuel/ oxidizer stoichiometry is needed. For the purpose of improving engine performance, fuel charges must be intelligently tailored to avoid wasted fuel, achieve the desired specific impulse, and create an easily detonable fuel distribution. In the Stanford PDE, we demonstrated quantitative fuel concentration measurements and active fuel-loading control. These measurements provide the fundamental fuel distribution information to understand engine dynamics and detonation physics of PDEs. For example, this information allows us to test our measured head-end pressure data against the simulations performed by Kailasanath at the Naval Research Laboratory and enables meaningful comparisons between simulations and experiments about PDE performance [11]. The C2H4 concentration data recorded at the tail measurement station in the Stanford PDE are shown in Fig. 6 for two C2H4/O2 engine fills, one with and one without ignition. Such data contain a wealth of important information. For instance, the fuel concentration does not rise instantaneously, owing to the finite valve opening time and to diffusion processes that occur as the fuel flows through the PDE tube. Obviously, we could use such measurements to select valves and tailor their control signals
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Active Control of the Fueling Process
Fig. 6. Record of fuel concentration at tail-end measurement station with and without ignition.
Fig. 7. Active control experiments based on diode-laser fuel sensor to realize a full tube fill in Stanford PDE. (a) Comparison of fuel fill duration; (b) comparison of impulse per cycle between fixed duration case and active control case.
to optimize performance. The transient signal corresponds to plug flow of a specific fuel concentration. If a uniform fuel distribution is desired, then the fuel must be loaded at least to the tail end of the engine, which may result in wasted fuel (as in the case shown in Fig. 6). This sensor is of obvious use to determine the fuel distribution and to subsequently tailor a desired distribution. In the case studied here, we prepared a uniform distribution; however, this uniform distribution may not be the optimum loading (in terms of specific impulse) for this particular engine geometry [12]. Arbitrary fuel distribution can be achieved by adjusting the timing of valves and monitoring the fuel with this sensor, therefore providing well-defined fuel distribution to study related detonation physics, such as the transition to detonation. In a practical engine application, the wasted fuel could be avoided by adjusting the timing of valve operation and ignition to ensure that no fuel has exited the tail end of the PDE prior to arrival of the detonation wave.
To optimize fuel consumption and to control specific impulse, we designed an active control system based on the C2H4 sensor to control the valve operation and ignition timing. Figure 5 illustrates the experimental arrangement in the Stanford PDE to actively control valve operation and ignition timing. We modulated the wavelength over the rapidly varying feature in the C2H4 spectrum shown in Fig. 2 near 6150 cmⳮ1. For rapid time response of the control system, analog detection, rather than digital data acquisition, is used to analyze the detector signal. A lock-in amplifier is employed to process the diode-laser absorption signal synchronously with the diode-laser wavelength modulation. The laser absorption is low when C2H4 concentration is low, so the output of the lock-in amplifier is low; similarly, high C2H4 concentration produces high absorption and lock-in output signal. When the output of the lock-in amplifier reaches a set decision level (it corresponds to the C2H4 concentration reaching a desired value at the tail end), we initiate the firing sequence: the valves are closed, and the igniter is triggered to fire the fuel/oxidizer charge. If the decision level is properly chosen, valves are closed and the engine is fired when the PDE tube is just fully filled with C2H4/O2 mixture. Hence, fuel usage is optimized and constant impulse can be obtained with minimal fuel waste for each cycle. We demonstrated this control system in the PDE for the case of finite-volume supply tanks. As the supply tanks exhaust their contents, the fuel/oxidizer flow rates decline with the supply tank pressure. If the PDE charging time is set to a constant fill-time schedule, the impulse (time integral of force on tube head end, measured with a pressure gauge) decreases for each fill. Fig. 7 compares the fuel fill duration and impulse per PDE cycle for a scheduled case and the actively controlled case. Fig. 7a shows how the fuel fill duration must be increased to accommodate the decreasing fuel supply pressure. As a result, the impulse is nearly constant for each pulse, as shown in Fig. 7b, because of the full tube fill ensured by the active control. In contrast, the impulse from a scheduled fill decreases on each PDE cycle. By changing the tail-end measurement station to another location, a similar control scheme could be used to realize partial fuel fills in the PDE; non-uniform spatial fuel distribution may be achieved with non-constant valve flow rates. All these forms of fuel fill tailoring are best accomplished with a quantitative fuel sensor. As noted, the broad wavelength-scanning technique employed renders the sensor relatively immune to noise imposed by the harsh PDE environment, such as beam steering or window effects. The low noise level immediately after the detonation
DIODE-LASER FUEL SENSOR FOR PULSE DETONATION ENGINE
Fig. 8. Head-end measurements in multiple-pulse experiments for two different valve-opening temperatures in Stanford PDE. (a) C2H4 temperature measurement; (b) C2H4 concentration measurement.
wave arrival, shown in Fig. 6, demonstrates this immunity, which is critical whenever the equivalence ratio of a PDE charge must be known or controlled.
Pulse-to-Pulse Interference The control strategy based on fuel concentration at the exhaust end of the PDE provides a practical method of controlling valve closing and ignition timing, thus avoiding wasted fuel and achieving constant specific impulse. But exhaust-end sensing does not provide the information needed to control the timing of the valve opening, as is needed to maximize the repetition rate of the engine. In practical PDEs, the engine designer attempts to minimize the delay between the previous detonation event and the next fuel-loading event. Therefore, the engine’s repetition rate, and in turn the average engine thrust, can be increased. When this delay becomes too short, however, the high-temperature combustion products from the previous detonation still reside in the tube and may cause the freshly injected fuel/oxidizer mixture to autoignite. When the PDE autoignites, it becomes a pulsed deflagration engine that produces much less thrust. In the Stanford PDE, we demonstrated that the C2H4 sensor can minimize autoignition interference resulting from the premature loading of fresh C2H4/ O2 reactants into hot combustion products. Multiple-pulse experiments were performed at different valve-opening temperatures with a C2H4/O2 equivalence ratio of 1.17. The C2H4 temperature and concentration measurements at the head end of the Stanford PDE are shown in Fig. 8 for two different valve-opening temperatures. Fig. 8a shows that if valves are opened when the temperature of the combustion products is 477 K inside the engine (at the measurement station), autoignition occurs and a deflagration results; if the time delay is increased by
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about 10%, such that the temperature of the combustion products drops to 420 K at the measurement station, autoignition does not occur and a healthy detonation is observed. Fig. 8b shows the simultaneous measurements of C2H4 concentration. Adding an inert purge flow into the engine system could of course be used to shorten the needed delay for healthy detonations, but in this experiment we prefer to operate the engine without purging because the relatively slow exhaust of combustion products leads to a long required delay, providing an ideal test bed for studying pulse-to-pulse interference and possible real-time control schemes. The C2H4 sensor can provide reliable measurements from 300 to nearly 900 K. Simulation and measurements of the ignition delay of C2H4 [13,14] indicate that stoichiometric C2H4/O2 mixtures at temperatures of 900 K or lower and atmospheric pressure have ignition delay times of 30 ms or greater, a time constant consistent with likely PDE repetition rates [15]. This implies that the sensor may be useful in controlling valve and ignition timing at repetition rates up to ⬃30 Hz. Thus, the 900 K temperature range for the sensor described here is quite useful for autoignition prevention in C2H4-fueled PDEs. Further optimization of practical PDEs, using both the tail- and head-end signals, could be accomplished by combining control of valve-close timing, ignition, and valve-open timing. The C2H4 sensor’s wide working temperature range, operational simplicity, robustness, and accuracy suggest that similar systems may find important use in largescale PDEs. Summary A newly developed sensor provides quantitative measurements of C2H4 concentration and temperature needed to characterize key aspects of PDE operation and to validate PDE models and computations. This sensor is also well suited for experiments aimed at active control of PDEs. Active control of valve and ignition timing under changing injection conditions has been successfully demonstrated in the Stanford PDE to optimize fuel consumption and to achieve constant impulse for each cycle. Autoignition interference has also been studied and effectively reduced in the engine. The success achieved in these experiments using the C2H4 sensor lays the groundwork for future laser-based control systems in larger scale PDEs. Acknowledgments This research was supported by the Office of Naval Research, with Dr. Gabriel Roy acting as technical monitor, under the ONR MURI on PDEs, and by the U.S. Air Force Office of Scientific Research, Aerospace Sciences Directorate, with Dr. Julian Tishkoff as technical monitor. The
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authors gratefully acknowledge their PDE teammates at Stanford University, D. W. Mattison, E. A. Romo, and K. M. Hinckley, for their assistance during the experiments and for many valuable discussions.
REFERENCES 1. Kailasanath, K., AIAA J. 38:1698 (2000). 2. Sanders, S. T., and Hanson, R. K., “Diode Laser-based Sensor System for Multi-parameter Measurements in Pulse Detonation Engine Flows,” paper 3592, ThirtySixth AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Huntsville, AL, 2000. 3. Sanders, S. T., Mattison, D. W., Ma, L., and Hanson, R. K., “Diode-Laser Sensors for Pulse Detonation Engines,” paper 143, Second Joint Meeting of the Combustion Institute, Oakland, CA, March 2001. 4. Brock, A., Mina-Camilde, N., and Manzanares, I. C., J. Phys. Chem., 98:4800 (1994). 5. Arroyo, M. P., and Hanson, R. K., Appl. Opt., 32:6104 (1993). 6. Sanders, S. T., Baldwin, J. A., Jenkins, T. P., Baer, D. S., and Hanson, R. K., Proc. Combust. Inst., 28:587 (2000). 7. Olson, D. B., Mallard, W. G., and Gardiner, W. C., Appl. Spectrosc. 32:489 (1978).
8. Sanders, S. T., ‘‘Diode-Laser Sensors for Harsh Environments with Applications to Pulse Detonation Engines,’’ Ph.D. thesis, Stanford University, Stanford, CA, 2001. 9. Georges, R., Bach, M., and Herman, M., Mol. Phys., 90:381 (1997). 10. Ma, L., Sanders, S. T., and Hanson, R. K., “Laserbased Fuel Diagnostics for Sensing and Control in Pulse Detonation Engines,” paper 0609, Fortieth AIAA Aerospace Sciences Meetings and Exhibit, Reno, NV, January 2002. 11. Jenkins, T. P., Sanders, S. T., Kailasanath, K., Li, C., and Hanson, R. K., “Diode Laser-based Measurements for Model Validation in Pulse Detonation Flows,” paper APS-CS-5A-3, Proceedings of the Twenty-Fifth JANNAF Airbreathing Propulsion Meeting, Monterey, CA, November 2000. 12. Li, C., Kailasanath, K., and Patnaik, G., paper 0314, Thirty-Eighth AIAA Aerospace Sciences Meetings and Exhibit, 2000. 13. Laskin, A., Wang, H., and Law, C. K., Int. J. Chem. Kinet., in press. 14. Schultz, E., and Shepherd, J., Validation of Detailed Reaction Mechanisms for Detonation Simulation, Explosions Dynamics Laboratory report FM99-54, Graduate Aeronautical Laboratories, California Institute of Technology, Pasadena, CA, 1999. 15. Schauer, F. R., Stutrud, J. S., and Bradley, R. P., AIAA paper 2001–1129.
COMMENTS Yeddidia Neumeier, Georgia Institute of Technology, USA. You have shown that with the same pulse frequency and refilling dynamics, closed loop timing was provided significant higher pressure levels than an open loop timing. Seemingly, the only differences between the two can be the ignition timing. What was the rule that set the timing for the open loop? Was it optimized? Can it be set so that the pressure will not degrade? Author’s Reply. The experiment was designed to demonstrate the feasibility of our sensor for PDE fuel control. The fuel tank was relatively small so that the fuel tank pressure declined measurably after each pulse. Thus, the fuel flow through the orifice is reduced, and a longer filling time is needed for constant fuel loading after each shot. Monitoring the fuel loading easily optimized the ignition and valve timing. Certainly a fuel tank regulator could have been used to maintain constant pressure, but these experiments were designed to demonstrate the general utility of our sensor to deal with irregular fuel flows, for example, due to valve fouling or other fuel delivery problems.
● A. Koichi Hayashi, Aoyama Gafuin University, Japan. Did you get a control (transfer) function to control the PDE system? If so, what is your control function? To get the temperature, are you using two wavelengths? How do you scan the wavelength of ⬃1.62 lm and what is the value of speed? Is it possible to decrease the L value (absorption length) of 3.8 cm? Author’s Reply. The control experiment was simply designed to demonstrate the feasibility of the sensor (see previous question/answer). The temperature is determined from a wavelength scan over the absorption feature, and the scan rate of the present laser leads to a response time of 2 milliseconds, which is sufficient for the current PDE firing rates. The direct absorption signal is linearly proportional to path length. The signal-to-noise ratio from the sensor used here is sufficient for reliable fuel control with less than 1/4 the current signal; thus, the control experiment could have performed with 1/4 the path length.
MONITORING AND CONTROL OF A PULSE DETONATION ENGINE USING A DIODE-LASER FUEL CONCENTRATION AND TEMPERATURE SENSOR L. MA, S. T. SANDERS, J. B. JEFFRIES and R. K. HANSON High Temperature Gasdynamics Laboratory Department of Mechanical Engineering Stanford University Stanford, CA 94305, USA
Fuel measurements are needed to accurately tailor fuel charges in pulse detonation engines (PDEs) to improve engine performance and to validate PDE models and computations. Here, we report simultaneous concentration and temperature measurements of C2H4 fuel in a PDE using a newly developed diode-laser absorption sensor. These measurements enable characterization of the fuel loading and ignition timing of the engine. Based on these characterizations, a real-time control system that optimizes fuel consumption and maximizes specific impulse in the engine has been realized. Similar measurements of C2H4 concentration and temperature were used to characterize pulse-to-pulse interference resulting from loading fresh fuel/oxygen reactants into hot combustion products. The sensor was used in a simple control scheme to minimize such interference, illustrating its potential role in control systems to maximize the engine’s operation rate. During these studies, the sensor demonstrated two valuable improvements over traditional absorption spectroscopic techniques: (1) increased robustness and accuracy and (2) simultaneous measurements of concentration and temperature. These improvements are enabled by broad wavelength scanning of the Q-branch spectra of C2H4 near 1.62 lm. The success achieved in these small-scale tests provides strong support for expanded use of diode-laser sensors in propulsion applications.
Introduction Recent pulse detonation engine (PDE) research aimed at achieving a better understanding of PDE physics, engine operation, and engine control [1] has frequently used gaseous C2H4 fuel, because it is easily detonable and because liquid fuels envisaged for practical PDEs are expected to pyrolyze to C2H4 or C2H4-like species before detonation events. A robust C2H4 sensor is therefore needed to characterize the PDE fueling process and related engine dynamics, to provide fuel distribution data for model validation, and to develop control systems for improved engine performance. Previously, a C2H4 sensor based on 3.39 lm HeNe laser (fixed-wavelength) absorption was used to monitor C2H4 concentration in a model PDE [2]. However, the accuracy of this sensor was compromised by the hostile measurement environment in PDEs [3]. Our new diode-laser C2H4 sensor is aimed at overcoming this deficiency. A broad wavelength-scan sensing technique is developed here to enable simultaneous measurements of C2H4 concentration and temperature, to ensure the sensor’s accuracy and reliability in harsh measurement environments such as PDEs, and to carry out detailed characterization of the fueling process in our laboratory-scale PDE. Reliable real-time control to optimize fuel consumption and maximize specific impulse has been demonstrated in the Stanford
PDE by combining this sensor with a lock-in technique to synchronously detect laser absorption. The pulse-to-pulse interference resulting from loading fresh fuel/oxygen reactants into hot combustion products has also been studied. Finally, sample results indicate the sensor’s potential for an active control system using real-time measurements to minimize pulse-to-pulse interference and thus maximize the engine’s operation rate. This paper first describes the sensing technique and the C2H4 spectroscopic data upon which the sensor is based. The fueling process and active control studies in the Stanford PDE are then discussed.
Description of the C2H4 Sensor Optical absorption sensors exploit the uniqueness of molecular spectra to enable accurate identification and detection of the target species with nonintrusive, rapid-response measurements. The absorption spectrum of C2H4 is rich with a variety of distinct spectral features [4]. Although the strongest absorption occurs in the mid-IR, in the spectral range where tunable diode lasers are commercially available (below 2 lm) the strongest C2H4 absorption occurs at the first overtone of the fundamental stretch of C-H, that is, between 5700 and
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Fig. 1. Absorption spectrum of C2H4 near 3.39 lm simulated by HITRAN 2000. Simulation conditions: T ⳱ 300 K, P ⳱ 1 atm, 25% C2H4, and 75% O2.
Fig. 2. Absorption spectrum of C2H4 near 1.62 lm for a mixture of 25% C2H4 and 75% N2 at 296 K and 1 atm, measured by temperature tuning a DFB diode laser.
6200 cmⳮ1. We have previously utilized a fixedwavelength 3.39 lm HeNe laser for sensitive detection of C2H4 using the strong fundamental band (Fig. 1), but fixed-wavelength schemes are not generally applicable to hostile environments such as PDEs where interference absorption and noise are common problems. Instead, we now favor use of the overtone spectral region near 1.62 lm (Fig. 2), where tunable diode lasers are available and may be exploited to ameliorate interference and noise effects. In our previous work using a 3.39 lm HeNe laser to monitor C2H4 concentration in PDEs [2], temperature was required for a quantitative measurement of C2H4 and was inferred from a separate sensor, usually a water vapor absorption measurement [5,6]. In addition to the complexity of this monitoring system, several other problems existed. First, from the inset in Fig. 1, we can see that the HeNe wavelength lies very near an absorption minimum in this spectrum, which limits the sensitivity of the measurement. The fixed wavelength of HeNe lasers prohibits choosing an alternative spectral feature with stronger absorption. Second, interference from
other species can easily confuse the HeNe-based C2H4 sensor. Nearly all hydrocarbon molecules absorb the HeNe light, and many have a larger absorption cross section; for example, potential contaminants such as butane and propane absorb the 3.39 lm HeNe radiation about 60 times as strongly as C2H4 [7]. Third, during the fueling process, the water concentration is usually low, which makes it difficult to determine temperature accurately. Tunable lasers offer the opportunity of selecting an optimum absorption feature to obtain strong absorption and to avoid spectral interference. In addition, scan-wavelength techniques can be made insensitive to laser attenuation by broadband absorbers and scatterers (such as soot particles and fuel droplets), flow-induced beam steering, and spectroscopic uncertainties associated with collisional line broadening, as discussed previously [8]. All these advantages are crucial for measurements in the harsh PDE environment. However, the absorption transitions of C2H4 are almost always blended by collisional broadening at atmospheric pressure, a common trait of polyatomic molecular spectra. Fortunately, the overtone and combination bands of C2H4 in the 1.6 lm region [9] have a few isolated features, as shown in Fig. 2. Initially, we chose the feature near 6158 cmⳮ1 (cross-hatched) [10]. The C2H4 concentration is inferred from the integrated area of the feature using Beer-Lambert’s law [5]. This choice was appropriate for room-temperature C2H4 concentration measurements, but not for combustion applications, because the absorption strength decreases and other interfering C2H4 hot lines increase as temperature increases; hence, measurements above 500 K are not possible. An alternative strategy was therefore sought. These problems are solved by scanning the wavelength of a laser absorption sensor over the 6143– 6151 cmⳮ1 Q-branch region shown in Fig. 2. In addition to the advantages discussed above, the C2H4 absorption in this region is much stronger and decreases more slowly with increasing temperature than that of nearby features. The set of overlapped transitions produces a relatively broad feature, which remains less sensitive to interference from other species. Finally, the temperature can be determined simultaneously, from the shape of this feature. We extended the current-tuning range of our distributed feedback (DFB) diode laser by exceeding the manufacturer’s recommended current limit, thus enabling determination of the temperature at kilohertz rates by fast absorption scans of the Q branch. Single-sweep (2 ms) measurements of the C2H4 Q branch at different temperatures are shown in Fig. 3. Note the variation in the shape of this feature with temperature. To speed the spectral fitting needed to determine temperature, we selected the ratio of two peaks illustrated in Fig. 3 for this purpose. The ratio at different temperatures is shown in Fig. 4, fit to a
DIODE-LASER FUEL SENSOR FOR PULSE DETONATION ENGINE
Fig. 3. Q branch of C2H4 near 1.62 lm at different temperatures, measured in a static cell with 20 cm absorption path length, pure C2H4, and 1 atm pressure.
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metered with flow orifices and premixed before injection into the PDE. The firing sequence begins with simultaneous opening of the fuel and oxidizer valves, subsequent filling of the PDE, closing of the valves, electric spark ignition, and a delay time interval (before repeating the cycle) for the burned gases to exhaust. The C2H4 sensor is applied to the head and tail ends of the engine simultaneously. Information obtained at these two measurement stations is sufficient to characterize and control the C2H4 distribution in the engine when C2H4/O2 input flow rates are well behaved. However, more measurement stations can readily be incorporated by adding additional beam splitters and detectors. The DFB laser’s current-tuning range is extended to obtain an 8 cmⳮ1 scanning range, sufficient to record C2H4’s Q branch spectral feature near 6150 cmⳮ1. Transmitted DFB laser beams are monitored by InGaAs detectors. Detector voltages are digitized using 12bit resolution for quantitative absorption measurements or detected with a lock-in amplifier synchronized with the diode-laser wavelength sweep rate for control experiments. When the lock-in signal exceeds the set point, the PDE firing sequence is initiated.
Characterization of the Fueling Process Fig. 4. Ratio of the two peaks indicated in Fig. 2 to monitor temperature.
Fig. 5. Schematic of C2H4 sensor applied to Stanford PDE for monitoring and control.
Boltzmann distribution model. This simple scheme has proven very effective in monitoring C2H4 temperature in PDE flows. Experimental Method Figure 5 provides a schematic of the experimental setup in the Stanford PDE, previously described in Refs. [2,3,6]. The fuel (C2H4) and oxidizer (O2) are
Characterization of the fuel distribution is of fundamental importance for the development of PDEs. To validate PDE models and computations, uniform fuel charges are desired and the quantitative fuel/ oxidizer stoichiometry is needed. For the purpose of improving engine performance, fuel charges must be intelligently tailored to avoid wasted fuel, achieve the desired specific impulse, and create an easily detonable fuel distribution. In the Stanford PDE, we demonstrated quantitative fuel concentration measurements and active fuel-loading control. These measurements provide the fundamental fuel distribution information to understand engine dynamics and detonation physics of PDEs. For example, this information allows us to test our measured head-end pressure data against the simulations performed by Kailasanath at the Naval Research Laboratory and enables meaningful comparisons between simulations and experiments about PDE performance [11]. The C2H4 concentration data recorded at the tail measurement station in the Stanford PDE are shown in Fig. 6 for two C2H4/O2 engine fills, one with and one without ignition. Such data contain a wealth of important information. For instance, the fuel concentration does not rise instantaneously, owing to the finite valve opening time and to diffusion processes that occur as the fuel flows through the PDE tube. Obviously, we could use such measurements to select valves and tailor their control signals
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Active Control of the Fueling Process
Fig. 6. Record of fuel concentration at tail-end measurement station with and without ignition.
Fig. 7. Active control experiments based on diode-laser fuel sensor to realize a full tube fill in Stanford PDE. (a) Comparison of fuel fill duration; (b) comparison of impulse per cycle between fixed duration case and active control case.
to optimize performance. The transient signal corresponds to plug flow of a specific fuel concentration. If a uniform fuel distribution is desired, then the fuel must be loaded at least to the tail end of the engine, which may result in wasted fuel (as in the case shown in Fig. 6). This sensor is of obvious use to determine the fuel distribution and to subsequently tailor a desired distribution. In the case studied here, we prepared a uniform distribution; however, this uniform distribution may not be the optimum loading (in terms of specific impulse) for this particular engine geometry [12]. Arbitrary fuel distribution can be achieved by adjusting the timing of valves and monitoring the fuel with this sensor, therefore providing well-defined fuel distribution to study related detonation physics, such as the transition to detonation. In a practical engine application, the wasted fuel could be avoided by adjusting the timing of valve operation and ignition to ensure that no fuel has exited the tail end of the PDE prior to arrival of the detonation wave.
To optimize fuel consumption and to control specific impulse, we designed an active control system based on the C2H4 sensor to control the valve operation and ignition timing. Figure 5 illustrates the experimental arrangement in the Stanford PDE to actively control valve operation and ignition timing. We modulated the wavelength over the rapidly varying feature in the C2H4 spectrum shown in Fig. 2 near 6150 cmⳮ1. For rapid time response of the control system, analog detection, rather than digital data acquisition, is used to analyze the detector signal. A lock-in amplifier is employed to process the diode-laser absorption signal synchronously with the diode-laser wavelength modulation. The laser absorption is low when C2H4 concentration is low, so the output of the lock-in amplifier is low; similarly, high C2H4 concentration produces high absorption and lock-in output signal. When the output of the lock-in amplifier reaches a set decision level (it corresponds to the C2H4 concentration reaching a desired value at the tail end), we initiate the firing sequence: the valves are closed, and the igniter is triggered to fire the fuel/oxidizer charge. If the decision level is properly chosen, valves are closed and the engine is fired when the PDE tube is just fully filled with C2H4/O2 mixture. Hence, fuel usage is optimized and constant impulse can be obtained with minimal fuel waste for each cycle. We demonstrated this control system in the PDE for the case of finite-volume supply tanks. As the supply tanks exhaust their contents, the fuel/oxidizer flow rates decline with the supply tank pressure. If the PDE charging time is set to a constant fill-time schedule, the impulse (time integral of force on tube head end, measured with a pressure gauge) decreases for each fill. Fig. 7 compares the fuel fill duration and impulse per PDE cycle for a scheduled case and the actively controlled case. Fig. 7a shows how the fuel fill duration must be increased to accommodate the decreasing fuel supply pressure. As a result, the impulse is nearly constant for each pulse, as shown in Fig. 7b, because of the full tube fill ensured by the active control. In contrast, the impulse from a scheduled fill decreases on each PDE cycle. By changing the tail-end measurement station to another location, a similar control scheme could be used to realize partial fuel fills in the PDE; non-uniform spatial fuel distribution may be achieved with non-constant valve flow rates. All these forms of fuel fill tailoring are best accomplished with a quantitative fuel sensor. As noted, the broad wavelength-scanning technique employed renders the sensor relatively immune to noise imposed by the harsh PDE environment, such as beam steering or window effects. The low noise level immediately after the detonation
DIODE-LASER FUEL SENSOR FOR PULSE DETONATION ENGINE
Fig. 8. Head-end measurements in multiple-pulse experiments for two different valve-opening temperatures in Stanford PDE. (a) C2H4 temperature measurement; (b) C2H4 concentration measurement.
wave arrival, shown in Fig. 6, demonstrates this immunity, which is critical whenever the equivalence ratio of a PDE charge must be known or controlled.
Pulse-to-Pulse Interference The control strategy based on fuel concentration at the exhaust end of the PDE provides a practical method of controlling valve closing and ignition timing, thus avoiding wasted fuel and achieving constant specific impulse. But exhaust-end sensing does not provide the information needed to control the timing of the valve opening, as is needed to maximize the repetition rate of the engine. In practical PDEs, the engine designer attempts to minimize the delay between the previous detonation event and the next fuel-loading event. Therefore, the engine’s repetition rate, and in turn the average engine thrust, can be increased. When this delay becomes too short, however, the high-temperature combustion products from the previous detonation still reside in the tube and may cause the freshly injected fuel/oxidizer mixture to autoignite. When the PDE autoignites, it becomes a pulsed deflagration engine that produces much less thrust. In the Stanford PDE, we demonstrated that the C2H4 sensor can minimize autoignition interference resulting from the premature loading of fresh C2H4/ O2 reactants into hot combustion products. Multiple-pulse experiments were performed at different valve-opening temperatures with a C2H4/O2 equivalence ratio of 1.17. The C2H4 temperature and concentration measurements at the head end of the Stanford PDE are shown in Fig. 8 for two different valve-opening temperatures. Fig. 8a shows that if valves are opened when the temperature of the combustion products is 477 K inside the engine (at the measurement station), autoignition occurs and a deflagration results; if the time delay is increased by
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about 10%, such that the temperature of the combustion products drops to 420 K at the measurement station, autoignition does not occur and a healthy detonation is observed. Fig. 8b shows the simultaneous measurements of C2H4 concentration. Adding an inert purge flow into the engine system could of course be used to shorten the needed delay for healthy detonations, but in this experiment we prefer to operate the engine without purging because the relatively slow exhaust of combustion products leads to a long required delay, providing an ideal test bed for studying pulse-to-pulse interference and possible real-time control schemes. The C2H4 sensor can provide reliable measurements from 300 to nearly 900 K. Simulation and measurements of the ignition delay of C2H4 [13,14] indicate that stoichiometric C2H4/O2 mixtures at temperatures of 900 K or lower and atmospheric pressure have ignition delay times of 30 ms or greater, a time constant consistent with likely PDE repetition rates [15]. This implies that the sensor may be useful in controlling valve and ignition timing at repetition rates up to ⬃30 Hz. Thus, the 900 K temperature range for the sensor described here is quite useful for autoignition prevention in C2H4-fueled PDEs. Further optimization of practical PDEs, using both the tail- and head-end signals, could be accomplished by combining control of valve-close timing, ignition, and valve-open timing. The C2H4 sensor’s wide working temperature range, operational simplicity, robustness, and accuracy suggest that similar systems may find important use in largescale PDEs. Summary A newly developed sensor provides quantitative measurements of C2H4 concentration and temperature needed to characterize key aspects of PDE operation and to validate PDE models and computations. This sensor is also well suited for experiments aimed at active control of PDEs. Active control of valve and ignition timing under changing injection conditions has been successfully demonstrated in the Stanford PDE to optimize fuel consumption and to achieve constant impulse for each cycle. Autoignition interference has also been studied and effectively reduced in the engine. The success achieved in these experiments using the C2H4 sensor lays the groundwork for future laser-based control systems in larger scale PDEs. Acknowledgments This research was supported by the Office of Naval Research, with Dr. Gabriel Roy acting as technical monitor, under the ONR MURI on PDEs, and by the U.S. Air Force Office of Scientific Research, Aerospace Sciences Directorate, with Dr. Julian Tishkoff as technical monitor. The
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authors gratefully acknowledge their PDE teammates at Stanford University, D. W. Mattison, E. A. Romo, and K. M. Hinckley, for their assistance during the experiments and for many valuable discussions.
REFERENCES 1. Kailasanath, K., AIAA J. 38:1698 (2000). 2. Sanders, S. T., and Hanson, R. K., “Diode Laser-based Sensor System for Multi-parameter Measurements in Pulse Detonation Engine Flows,” paper 3592, ThirtySixth AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Huntsville, AL, 2000. 3. Sanders, S. T., Mattison, D. W., Ma, L., and Hanson, R. K., “Diode-Laser Sensors for Pulse Detonation Engines,” paper 143, Second Joint Meeting of the Combustion Institute, Oakland, CA, March 2001. 4. Brock, A., Mina-Camilde, N., and Manzanares, I. C., J. Phys. Chem., 98:4800 (1994). 5. Arroyo, M. P., and Hanson, R. K., Appl. Opt., 32:6104 (1993). 6. Sanders, S. T., Baldwin, J. A., Jenkins, T. P., Baer, D. S., and Hanson, R. K., Proc. Combust. Inst., 28:587 (2000). 7. Olson, D. B., Mallard, W. G., and Gardiner, W. C., Appl. Spectrosc. 32:489 (1978).
8. Sanders, S. T., ‘‘Diode-Laser Sensors for Harsh Environments with Applications to Pulse Detonation Engines,’’ Ph.D. thesis, Stanford University, Stanford, CA, 2001. 9. Georges, R., Bach, M., and Herman, M., Mol. Phys., 90:381 (1997). 10. Ma, L., Sanders, S. T., and Hanson, R. K., “Laserbased Fuel Diagnostics for Sensing and Control in Pulse Detonation Engines,” paper 0609, Fortieth AIAA Aerospace Sciences Meetings and Exhibit, Reno, NV, January 2002. 11. Jenkins, T. P., Sanders, S. T., Kailasanath, K., Li, C., and Hanson, R. K., “Diode Laser-based Measurements for Model Validation in Pulse Detonation Flows,” paper APS-CS-5A-3, Proceedings of the Twenty-Fifth JANNAF Airbreathing Propulsion Meeting, Monterey, CA, November 2000. 12. Li, C., Kailasanath, K., and Patnaik, G., paper 0314, Thirty-Eighth AIAA Aerospace Sciences Meetings and Exhibit, 2000. 13. Laskin, A., Wang, H., and Law, C. K., Int. J. Chem. Kinet., in press. 14. Schultz, E., and Shepherd, J., Validation of Detailed Reaction Mechanisms for Detonation Simulation, Explosions Dynamics Laboratory report FM99-54, Graduate Aeronautical Laboratories, California Institute of Technology, Pasadena, CA, 1999. 15. Schauer, F. R., Stutrud, J. S., and Bradley, R. P., AIAA paper 2001–1129.
COMMENTS Yeddidia Neumeier, Georgia Institute of Technology, USA. You have shown that with the same pulse frequency and refilling dynamics, closed loop timing was provided significant higher pressure levels than an open loop timing. Seemingly, the only differences between the two can be the ignition timing. What was the rule that set the timing for the open loop? Was it optimized? Can it be set so that the pressure will not degrade? Author’s Reply. The experiment was designed to demonstrate the feasibility of our sensor for PDE fuel control. The fuel tank was relatively small so that the fuel tank pressure declined measurably after each pulse. Thus, the fuel flow through the orifice is reduced, and a longer filling time is needed for constant fuel loading after each shot. Monitoring the fuel loading easily optimized the ignition and valve timing. Certainly a fuel tank regulator could have been used to maintain constant pressure, but these experiments were designed to demonstrate the general utility of our sensor to deal with irregular fuel flows, for example, due to valve fouling or other fuel delivery problems.
● A. Koichi Hayashi, Aoyama Gafuin University, Japan. Did you get a control (transfer) function to control the PDE system? If so, what is your control function? To get the temperature, are you using two wavelengths? How do you scan the wavelength of ⬃1.62 lm and what is the value of speed? Is it possible to decrease the L value (absorption length) of 3.8 cm? Author’s Reply. The control experiment was simply designed to demonstrate the feasibility of the sensor (see previous question/answer). The temperature is determined from a wavelength scan over the absorption feature, and the scan rate of the present laser leads to a response time of 2 milliseconds, which is sufficient for the current PDE firing rates. The direct absorption signal is linearly proportional to path length. The signal-to-noise ratio from the sensor used here is sufficient for reliable fuel control with less than 1/4 the current signal; thus, the control experiment could have performed with 1/4 the path length.