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Diode Laser Sensor for Scramjet Inlets

Retro-reflection is used to determine the effect of angle of attack.

The supersonic combustion ramjet (scramjet) engine is one of the more promising high-speed flight propulsion technologies. One of the reasons for this is the simplicity of the engine design, having no moving parts and requiring no external ignition source, and the fact that scramjets do not need to provide their own oxidizer. Despite this simplicity, several obstacles to the use of scramjet propulsion systems have become apparent, including the ability to produce sufficient fuel-air mixing at high speed, large total pressure losses, reduction in specific impulse with increasing flight Mach number, and the sensitivity of combustion to inlet temperature. This last problem can be very significant.

This work presents development of an oxygen-based diode laser absorption sensor designed to be used in a scramjet engine inlet. The sensor uses free-space propagation of light from a vertical-cavity surface-emitting laser (VCSEL) to determine the temperature, velocity, and angle of attack of an engine inlet in hypersonic flow.

A schematic of tunable diode laser absorption spectroscopy (TDLAS) optics for Hypersonic Inlet Measurements. The inlet configuration shows the laser passing four times through the freestream and the two shock layers.
The new sensor for the Mach number and angle of attack in a scramjet engine uses VCSEL-based absorption measurements. The method uses retro-reflection to very accurately determine the freestream velocity and the change in absorbance with line strength over the eight transitions available to the laser to determine the effect of angle of attack, determining the conditions using nonlinear least-squares fitting of the absorption spectrum against a database of computed spectra. The absorbance has been computed assuming two-dimensional analytical shock theory.

Simulation of absorption spectra with noise has been performed and has shown that the Mach number can be calculated very precisely for a given angle of attack, to less than ±0.02 from the correct value. The precision of the angle of attack measurement is lower, with an uncertainty at Mach 8 and α = 0.2 of ±0.25 degrees, for signals with no added random noise. The fits for angle of attack appear to be susceptible to convergence to a local, rather than a global, minimum using this fitting routine. This dependence can be minimized by selecting several start points for the least-squares fits, and choosing the final fit with the lowest residual. High-frequency Gaussian noise, up to 50% of the peak signal height, did not have a strong effect on the fitted velocity, but can have a significant effect on the temperature measurements. This sensitivity means that temperature measurements require a signal-to-noise ratio of 10 or greater to give a sensible value for angle of attack, because the peaks in the shock layers have approximately 10% of the signal in the freestream.

The system in its current form is capable of measuring both freestream temperature and velocity at measurement rates of 150 Hz. Sensitivity to vibration and very low absorption values prevented the system from being able to determine the angle of attack. It is possible that, with more attention to reducing the effect of vibration, some information about the angle of attack may be obtained.

Future work will be directed at reducing the susceptibility of the system to vibration, as this appears to be the largest source of uncertainty in the measurements. Vibration tests are being conducted to determine the cause of the susceptibility to vibration and reduce it by making the system move as a whole rather than having the two parts of the system move independently.

This work was done by Sean O’Byrne of the University of New South Wales, Australia, for the Asian Office of Aerospace Research and Development. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp  under the Physical Sciences category. AFRL-0164

Topics:
Combustion and combustion processes Engines Fittings Jet engines Lasers Optics Parts Physical Sciences Powertrains Research and development Scramjet engines Sensors and actuators Spectroscopy Vibration
This Brief includes a Technical Support Package (TSP).
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Diode Laser Sensor for Scramjet Inlets

(reference AFRL-0164) is currently available for download from the TSP library.

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    Defense Tech Briefs Magazine

    This article first appeared in the December, 2010 issue of Defense Tech Briefs Magazine (Vol. 4 No. 6).

    Read more articles from this issue here.

    Read more articles from the archives here.

    Overview

    The document presents a comprehensive report on the development of an oxygen-based diode laser absorption sensor designed for application in supersonic combustion ramjet (scramjet) engine inlets. Authored by Dr. Sean O’Byrne from the University of New South Wales, the report outlines the sensor's capabilities, operational principles, and experimental validation.

    The primary objective of the sensor is to measure critical parameters such as temperature, velocity, and angle of attack in hypersonic flow environments. The sensor employs free-space propagation of light from a vertical-cavity surface-emitting laser (VCSEL) to analyze the absorption spectrum of oxygen, which is crucial for determining the aforementioned parameters. The report details the calibration process conducted in a gas cell, where the sensor's performance was tested at high temperatures to ensure accuracy and reliability.

    Experimental results indicate that the sensor can effectively measure both free stream temperature and velocity at a rate of 150 Hz. However, challenges were encountered in determining the angle of attack due to sensitivity to vibrations and very low absorption values. The report suggests that with further refinement to mitigate the effects of vibration, it may be possible to extract some information regarding the angle of attack.

    The document also discusses the methodology used for data acquisition and analysis, including the use of a LabView program and a nonlinear least-squares fitting algorithm to process the spectral data. The experimental setup involved multiple measurements at various temperatures, ensuring a robust dataset for analysis. The findings demonstrate a good correlation between computed and measured spectra, validating the sensor's design and functionality.

    Additionally, the report acknowledges the funding support from the US Air Force Asian Office of Aerospace Research and Development (AOARD) and expresses gratitude to Associate Professor David Buttsworth for facilitating tests in the University of Southern Queensland’s Ludwieg tube facility.

    In summary, this report highlights the successful development and testing of a diode laser absorption sensor for scramjet applications, showcasing its potential for advancing hypersonic flow measurement technologies. The findings contribute valuable insights into the challenges and solutions associated with high-temperature gas dynamics and sensor design in aerospace applications.

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