JPS6325522A - Temperature measurement by anti-stokes raman spectroscopy - Google Patents

Abstract

PURPOSE:To obtain a handy and effective optical meter, by making two first beams cross each other in a measuring area as separated in stead of overlapping them. CONSTITUTION:The temperature of a combustion flame is measured by anti- Stokes Ramans method. At this point, laser lights with two different kinds of frequencies, omega1 and omega2 (omega1>omega2) produced to irradiate non-measuring material are supplied from a YAG laser 1 and a dye laser 2. These lights are transmitted through independent optical paths to a measuring position without being mixed together while being partially cut off with beam stops 10 and 11 but the cut faces thereof are made to oppose each other with a dichroic mirror 16 while the lights are sent to a convex lens 17 as two parallel beams close to each other. The two beams are so arranged to cross each other in a combustion cylinder 18 and after checked for the generation of an anti-Stokes light, the interval thereof is adjusted by moving a prism 9. Thereafter, the two beams are removed with a dichroic mirror 20 and thus, the anti-Stokes light is converted into an electrical signal with a photodetector through a spectroscope 23.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an optical system in laser Raman spectroscopy for temperature measurement of combustion flames.

[Background of the invention]

In industrial combustion tubes such as gas turbine combustors and boilers, the maximum temperature in the combustion zone is close to 2000'C. The combustion temperature in this combustion reaction zone has a close relationship with the combustion reaction rate and the amount of combustion products such as NOx and Co produced. Regarding NOx, which is attracting attention in pollution regulations,
When the nitrogen content in the fuel is low, so-called thermal NOx generated during the combustion process becomes a problem, but since this thermal NOx is generally generated in a high temperature range, to suppress it, it is necessary to homogenize the temperature by dispersing the fuel etc. The combustion method is usually used. As described above, the temperature distribution in the combustion reaction zone is counted as one of the basic parameters that must be quantitatively understood in order to establish NOx suppression combustion technology. The conventional solution to this problem has been to insert a thermocouple to measure the temperature, but this thermocouple method has a limited measurable temperature range, and the measurement accuracy in high temperature ranges has also been affected by width correction. The problem is that accuracy compensation is difficult.

Furthermore, there was a major problem in that the insertion of the probe changed the combustion field from its original state. Therefore, a non-contact measurement method has been desired in order to measure the true temperature without disturbing the combustion field.

With the advent of high-power lasers in recent years, applied research has begun to be conducted in the field of combustion, and various methods have been proposed and discussed, but the usefulness of methods that utilize nonlinear Raman spectroscopy has received particular attention. ing. The Raman scattering phenomenon was discovered by Raman and Krishnan in India in 1927, and is a phenomenon that occurs when the frequency ω1 of a substance (molecule) is
The light is irradiated and modulated so that the frequency is ωz= (ω1−Ω) and ω
Light of s = (ω + Ω) is newly generated.

The former is called Stokes light, and the latter is called anti-Stokes light.

In later years, as shown in Figure 4, in addition to ω1, the incident light
It has been discovered that when 2 and 2 are irradiated at the same time, anti-Stokes light ω8 is abnormally enhanced and generated.

This is due to the nonlinear Raman effect, and although a detailed explanation will be omitted, the greatest feature of this method is that the generated anti-Stokes light is directionally scattered in a beam shape as coherent monochromatic light similar to laser light. It was said that Anti-Stokes light is used as a laser beam at ω1. ω2
The greatest advantage is that the light is generated from the overlapping portions of the light, so the light collection efficiency is high and the S/N ratio is good with respect to noise components caused by light emitted during combustion. This nonlinear Raman spectroscopy method is called CA RS (Coher
entAnti-8takes Raman 5pee
It is called troscopy.

FIG. 5 shows the spectral distribution of the anti-Stokes light of nitrogen molecules (N2) obtained by the CAR3 method using a spectrometer, and the temperature corresponds to about 1500'C.

By the way, it is important to briefly explain how the temperature and concentration of combustion gas are related to the spectral distribution.The basic quantification work is based on the anti-Stokes optical spectral shape and the shape of the spectrum using quantum mechanical theory. This is based on the comparison. However, since this method is time-consuming, several approximate methods have been proposed. A typical example of this is the use of the half-width of a spectrum to quantify temperature. The line width at half the height of the first spectral peak in Figure 5 is used, and this line width has a nearly linear relationship with temperature, and the line width widens as the temperature rises. is used.

Regarding the concentration, it is known that anti-Stokes light is theoretically proportional to the square of the measured molecule density, so if there is no fluctuation in the incident light, the concentration can be calculated from the height of the spectrum.

In the CAR5 method, the following two equations must be satisfied as conditions for the occurrence of an anti-Stask prior.

ua=2cv□-32...(1)kg=2kl
-kz ...... (2) → 1 to +l = □ nI = frequency ω, interaction rate C: speed of light →: vector quantity Equation (1) is the law of conservation of energy, equation (2) is conservation of momentum It represents the rules. In the case where equations (1) and (2) are satisfied and the substance to be measured is a gas, a beam arrangement as shown in FIG. 6 has already been proposed. Method (a) is called the collinear method, in which the frequencies of ωl and ω2 are mixed in advance and irradiated onto the measurement object as a single beam.The optical system is simple, but the measurement accuracy deteriorates significantly depending on the measurement environment. . The first problem is that the measurement area is not clear. Second, the intensity of anti-Stokes light is expressed as in equation (3).

■3αN2112I2・L2・・・・・・
・(3) N: Molecular number density (number of molecules per unit volume) engineering 1
: Laser light intensity with frequency ω1 2 : Laser light intensity with frequency ω1 L: Anti-Stokes light generation area If the temperature change is rapid within the measurement area, the anti-Stokes light is N2 as shown in the above equation. The basic problem is that the signal light from the low-temperature region becomes stronger because it is proportional to . On the other hand, Figure 6(b) is a BOXCAR that solves the above problem and increases the spatial resolution.
It is called the 5 method, and it divides ω1 into two beams and intersects the three beams within the measurement area. Anti-Stokes light is generated only from the intersection area, so it has the advantage of high spatial resolution. However, it has the disadvantage that the three beams must intersect, making it difficult to adjust the optical axis.

Next, when it comes to which optical system should be adopted for measuring actual cans, both methods (a) and (b) have problems, and some kind of improvement is required.

As an essential condition for actual can measurement (a) The optical system is simple and optical axis alignment is easy.

(b) The measurement area (spatial resolution) must be clear.

There is.

The present invention provides an optical system that satisfies the above two items and aims to realize actual can temperature measurement.

[Purpose of the invention]

An object of the present invention is to provide a simple and effective optical system when measuring the temperature inside a large combustion cylinder such as a gas turbine combustor or boiler using Raman spectroscopy.

[Summary of the invention]

Equation (3) representing the anti-Stokes light intensity shown earlier is (1
), this equation assumes that conservation of energy and conservation of momentum (also called phase matching conditions) shown in equation (2) hold true, but as a practical matter, it is difficult to strictly satisfy conservation of momentum, and some mismatch conditions may occur. It is common to think that there is The anti-Stokes light intensity in this mismatched state is expressed by the following equation.

(4) This is a quantity called the Lc coherent length, which is on the order of several meters for liquids and on the order of several tens of square meters to several meters for gases. By the way, nitrogen is 73m, and NOx is 7m.

FIG. 7 shows the intensity of anti-Stokes light in a mismatched state using the coherent length as a parameter. L
c=: oo is phase matching (phasa+matahin
g) represents a condition; It can be seen that in a small range around the anti-Stokes generation region, when Lc is larger than several 10-, there is almost no difference from the phase matching state (Lc-oo). This suggests the possibility of use even in mismatched conditions. However, in this state, there is a problem as to whether it is possible to detect the signal light intensity, which is at a low level.However, we have shown that even if the length of the measurement area is several tens of microns, by increasing the laser power (0 to 300 mJ), we can perform spectroscopic analysis. This is not an essential problem since the light intensity is sufficient for spectroscopy with the instrument.

Therefore, as a key point of the present invention, as an improved method of the collinear method shown in FIG. 6, we have proposed a method in which the beams are not superimposed at first but are made into two separate beams that intersect in the measurement area. By doing so, the optical system is relatively simple and high spatial resolution can be obtained, so that the requirements for actual can measurement described above can be met.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be described in Section 2. This will be explained with reference to FIG. FIG. 1 shows the entire optical arrangement of the CAR3 system according to the present invention. Frequency ω1. Two laser beams of ω2 are supplied from a YAG laser 1 and a dye laser (Dye laser) 2.

Now, if we were to select light with a wavelength of 532 nm as ω1 and obtain the anti-Stokes light of nitrogen molecules, the natural frequency (Raman shift) of nitrogen would be 2331an-'' in wave number expression, so ω2 would be approximately 607. It is necessary to irradiate light with a wavelength of 3 nm.Dye lasers are characterized by being able to change the wavelength of the light generated by using various dye solutions and emitting light in a wide wavelength range.This CA
In the case of the R3 method, the dye laser beam ω2 is approximately 607.3 nm.
The resulting anti-Stokes light has a center wavelength around 473.3 nm, as shown in an example in Figure 5, and has a spectral distribution that varies depending on the measurement temperature. It is detected as light with a width of .

ω1 and ω2 are transmitted through independent optical paths without being mixed to the measurement position. ω1 is prism 4. Passes through telescope 6 and prism 7.8.

ω2 is prism 3. It passes through the telescope 5 and is guided to the vicinity of the measurement position. The purpose of passing telescopes 5 and 6 on the way is
It is used to adjust the difference in beam diameter caused by the difference in the ω1 and ω2 beam spread angles. The essential part of the present invention lies in the two-beam diaphragm mechanism and its arrangement, which will be explained below.

ω1. The ω2 light can be cut into light of arbitrary width by the beam diaphragm device placed at 10.11 and transmitted to the rear. FIG. 2 is an enlarged view of the aperture device 11. As shown in FIG. The beam aperture device 1o has the same mechanism as the beam aperture device 11, and this aperture device consists of a knife 12.13 and a movable table 14.15 for moving the knife left and right.

The knife 12.13 has a plate shape with its tip cut at an acute angle, and the beam shape of the light passing through the diaphragm 10.11 before the knife is almost circular, as shown by the diagonal lines in Figure 2. In this case, the cross section of the light after passing through the knife will take on the shape of a semicircular arc, for example, as shown in FIG. 2, depending on the cutting ratio of the beam. The beam shape cut in this way will be referred to as a partial arc beam. The beam shaped by 10.11 becomes two parallel beams close to each other by a dichroic mirror 16 arranged at 45° and is sent to a convex lens 17. The back surface of the dichroic mirror 16 is coated with a special coating that has a reflection efficiency of nearly 90% for ω1 and almost transmits ω2.

Figure 3 shows the frequency ω1゜ω2 formed as above.
It shows the arrangement of the partial arc beams and the generation of anti-Stokes light generated from the intersection of the two beams.

The reason for shaping the beam into a partial arc is that the non-measurement area near the lens is usually at a temperature close to room temperature, so the first condition is to determine the beam spacing so that there is no contact between the two beams in this area. However, if the interval is too large, the area where both beams intersect will become smaller, resulting in a problem of weakening the intensity of the anti-Stokes light and an increase in the angle that the anti-Stokes radiation direction makes with the lens center axis.

The problem arises that it becomes difficult to collect the light on the receiving side, so there is inevitably a limit to the beam spacing, but by making the opposing surfaces of the beams straight lines, it is possible to It has the advantage that the beam spacing can be made smaller compared to the conventional method. In addition, in the colinear method, the optical axis adjustment was extremely easy because a single beam entered the lens surface, but with this method as well, the first step is to set up a colinear optical system and check the generation of signal light. The spacing between the beams can be adjusted from First, the dichroic mirror 17 shown in Figure 1
Behind ω1. The arrangement is such that the ω2 light is superimposed, and after confirming the generation of anti-Stokes light, the ω1 light is transferred to the dichroic mirror 16 by moving the prism 9.
By moving left and right on the top, the distance from the ω2 beam can be changed to a predetermined position. Since this work can be done while monitoring the generation of signal light, the setting of the optical system can be completed in a relatively short time.

Anti-Stokes light is detected by the following procedure.

The anti-Stokes light forms a beam from the intersection of the two beams and is emitted in a direction substantially close to the two beam traveling directions ω1 and ω2. This anti-Stokes light enters a convex lens 19 disposed outside the combustion tube 18 and is focused at the focal point of the lens. At this time, since the light of ω1°ω2 is not needed, it is reflected by the diquick mirror 20 provided at the rear of the lens and thrown away to the outside. An optical fiber receiver 21 is placed at the focal point of the lens 19, and the signal light is guided into an optical fiber cable 22 and transmitted to a spectrometer 23. The spectrometer 23 has a focal length of 1
m, a double diffraction grating of 1800 lines/I is used. The separated spectrum is converted into an electrical signal by a photodetector 24 having 512 channels of photodiodes, and then digitally processed and displayed on a monitor as a spectrum.

As explained with the diagram above, when measuring the flame temperature of industrial combustion equipment, etc., which has a relatively wide flame cross section, by applying the CAR3 method, which is a type of nonlinear Raman scattering, the collinear method that has been commonly used When measuring objects with steep temperature gradients, since the signal light generation region is unclear, the influence of the low temperature range is reflected in the spectrum, which has been seen as a problem in terms of accuracy.The present invention solves this drawback. for 2
Because the beams are placed close together and intersect only at the measurement section, information from only the measurement section is reflected in the spectrum, making it possible to achieve highly accurate measurements6.Also, because it is an optical system very similar to the collinear method, The practical advantage is that the optical system can be adjusted in a short time.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the entire optical system of the CAR8 method according to an embodiment of the present invention, Fig. 2 is a perspective view of a beam diaphragm device that cuts the beam, and Fig. 3 is a beam arrangement using a partially circular arc beam. Figure 4 is an explanatory diagram showing the principle of signal light generation using the CAR8 method, Figure S is a diagram showing the spectral distribution of anti-Stokes light, and Figure 6 is a previously proposed beam arrangement method. FIG. 7 is a diagram showing the relationship between the anti-Stokes generation region length and the light intensity using the coherent length as a parameter.

Claims (1)

[Claims] 1. A method of measuring the temperature of combustion flame using anti-Stox Raman spectroscopy (CARS), which is a type of Raman spectroscopy.
Two types of frequencies ω_1 and ω_2 (
The final convex lens surface on the light emission side is used to partially cut the laser beams of ω_1>ω_2) in the middle of the optical system and focus these lights on the substance to be measured.
A temperature measurement method using anti-Stox Raman spectroscopy, characterized in that the spatial resolution of the measurement area into which the beams are focused can be controlled by arranging the beams of _1 and ω_2 with their cut surfaces facing each other and with a slight distance between the two beams.