Fluid properties measurements using wavelength modulation spectroscopy with first harmonic detection
An apparatus and method for monitoring gas velocity, temperature, and pressure in combustion systems and flow devices, in particular at inlets and isolators of scramjet engines. The invention employs wavelength modulation spectroscopy with first harmonic detection and without the need to scan the full absorption spectra.
1. A method for non-intrusively obtaining the local velocity, pressure, and temperature of a gas, the method comprising the steps of:
emitting two beams of laser light directed at two different angles across a flow of the gas;
separately detecting the two beams;
determining first harmonic line shapes from the detected beams using wavelength modulation spectroscopy and a pair of single point measurements by a computer;
calculating gas velocity and pressure from wavelength shifts of the line shapes by the computer; and
calculating gas temperature from slopes of the line shapes by the computer;
wherein wavelength of the two beams is alternated between two fixed wavelengths; and having an additional signal channel to provide reference zero-velocity and reference pressure.
2. The method of claim 1 wherein a square wave is employed.
3. The method of claim 2 wherein frequency of the square wave is at or greater than desired detection bandwidth.
4. The method of claim 1 wherein scanning full absorption line shapes is not required.
5. The method of claim 1 wherein gas velocity is determined independent of gas density changes.
6. The method of claim 1 wherein calculating gas temperature comprises employing a mole fraction of the gas.
7. The method of claim 1 wherein calculating gas temperature comprises measuring second harmonic peak signals.
8. The method of claim 1 wherein the method operates at a bandwidth of up to about 40 kHz.
9. The method of claim 1 wherein velocity is obtained at a resolution of about 10 m/s fluctuation or better.
10. The method of claim 1 wherein pressure is obtained at about plus or minus 12 torr at 2 atmospheres.
11. An apparatus for non-intrusively obtaining the local velocity, pressure, and temperature of a gas, said apparatus comprising:
two emitters of beams of laser light directed at two different angles across a flow of the gas;
detectors for the two beams;
a computer determining first harmonic line shapes from the detected beams using wavelength modulation spectroscopy and a pair of single point measurements, calculating gas velocity and pressure from wavelength shifts of the line shapes, and calculating gas temperature from slopes of the line shapes;
a laser controller alternating wavelength of the two beams between two fixed wavelengths; and
a reference cell providing reference zero-velocity and reference pressure.
12. The apparatus of claim 11 wherein a square wave is employed by said laser controller.
13. The apparatus of claim 12 wherein frequency of said square wave is at or greater than desired detection bandwidth.
14. The apparatus of claim 11 wherein scanning full absorption line shapes is not required.
15. The apparatus of claim 11 wherein gas velocity is determined independent of gas density changes.
16. The apparatus of claim 11 wherein calculating gas temperature comprises employing a mole fraction of the gas.
17. The apparatus of claim 11 wherein calculating gas temperature comprises measuring second harmonic peak signals.
18. The apparatus of claim 11 wherein said apparatus operates at a bandwidth of up to about 40 kHz.
19. The apparatus of claim 11 wherein velocity is obtained at a resolution of about 10 m/s fluctuation or better.
20. The apparatus of claim 11 wherein pressure is obtained at about plus or minus 12 torr at 2 atmospheres.