JP5229767B2 - Analysis method and apparatus in laser-induced plasma spectroscopy - Google Patents

Description

  The present invention relates to a method for spectral analysis when quantitative measurement is performed in laser-induced plasma spectroscopy, which is a composition analysis method for substances (gas, liquid, solid).

Laser Induced Plasma Spectroscopy (hereinafter referred to as LIPS method) is a technique in which pulsed laser light is focused by a lens to generate plasma, and the spectrum of light emitted during the process of cooling the plasma is analyzed. Investigate the elemental composition of substances. The LIPS method is widely used as a method for examining the elemental composition of substances. Until now, the LIPS method has often been used for elemental analysis of solid samples, but in recent years, application to gas composition analysis has begun to attract attention.
The applicant's aircraft engine technology development center is conducting research to measure the fuel (air) mixing ratio (equivalent ratio) in aircraft engine combustors using the LIPS method. FIG. 7 shows the LIPS measurement system. A laser beam emitted from the laser oscillator 1 is focused on the sample by the condenser lens 4 to generate laser-induced plasma. The generated plasma is frequency-separated using a spectroscopic analyzer 8 to obtain a spectrum and imaged using an ICCD camera 9. This device comprises a device for generating laser-induced plasma and a device for light reception and spectroscopic analysis, in which 3 is a mirror (reflectance 5%), 5 is a beam damper, 6 is a lens, 7 is an optical fiber, Reference numeral 2 denotes a power meter that measures the beam energy before and after irradiation of the sample. The graph shown in FIG. 3 is an example of a spectral spectrum measured for a mixed gas having two different equivalence ratios. Although the emission wavelength varies depending on the type of element and the process of energy relaxation, this data shows that the peak value of the wavelength corresponding to the emission from the hydrogen atom increases as the equivalence ratio increases. This is due to an increase in the proportion of methane (CH ₄ ) as a fuel. Utilizing such properties, the equivalence ratio distribution inside the engine combustor is measured. This LIPS method has the advantage that the measuring device can be made compact and the optical system can be set relatively easily, so it can be used as a measurement and sensor inside a jet engine in high-temperature and high-pressure conditions where measurement is difficult. Is expected.

When performing quantitative measurement with the LIPS method, it is necessary to pay attention to the fact that the state of the generated plasma depends on the characteristics of the optical system composed of a laser and a lens. That is, it is necessary to clearly grasp the emission spectrum in which plasma state. In particular, the plasma state fluctuates greatly under the condition that the incident energy is small in the LIPS for gas. Therefore, specifying the plasma state is an important requirement for quantitative measurement. In the conventional method, a calibration experiment is performed by considering the energy used for plasma generation as a difference in laser pulse energy before and after plasma generation (see Non-Patent Document 1). However, since the plasma state also depends on an optical system such as a laser light source and a lens, it is necessary to perform a calibration experiment for each optical system to be used. In order to monitor the laser pulse energy, in the conventional apparatus, as shown in FIG. 7, the state of the generated plasma is estimated by detecting the energy difference between the incident side laser light energy and the emitted laser light after plasma generation. Therefore, it is necessary to install the power meter 2 at two places before and after the plasma. This arrangement of the power meter is a disadvantage when the device is made compact. Depending on the measurement target, there may be a case where the energy measuring device cannot be installed before and after the plasma, and there is a restriction that the measurement target is limited. In general, from the viewpoint of downsizing the apparatus, it is desirable that the power of the incident laser is small. However, since the fluctuation of the plasma state for each pulse increases as the power decreases, the conventional technique can be seen in Patent Document 1. In many cases, a relatively large laser power is used for the calibration and only one optical system is used for the calibration experiment.
US Pat. No. 6,700,660 “Method and apparatus for in-process liquid analysis by laser induced plasma spectroscopy” March 2, 2004 TX Phuoc, FP White, “Laser-Induced Spark for Measurements of the Fuel-to-Air Ratio of a Combustible Mixture” Fuel, 81 (2002) 1761-1765.

  An object of the present invention is to solve the problems of the conventional methods as described above, that is, in the LIPS measurement method, a method that can specify a plasma state from information of the plasma itself without using laser pulse energy measurement It is to provide a LIPS measuring device that is not only compact but also accurate.

A step of determining the plasma state based on the ratio (A1 / A2) of the peak value of one or more spectral lines in the plasma spectral analysis technique obtained in LIPS quantitative measurement of the present invention, of one material composition Calculating a ratio (A1 / B1) of the peak value A1 of the target atom A and the emission peak value of the target atom B of the other substance; and (A1 / B1) value and equivalent ratio for each (A1 / A2) value A table showing the relationship is prepared, the (A1 / A2) value obtained previously is specified with reference to the table, and the calibration value of the equivalence ratio corresponding to the (A1 / B1) value is obtained from the calibration characteristics. Steps were taken.
Furthermore, the step (A1 / C1) of calculating the ratio (A1 / C1) of the peak value A1 of the previous atom A to the emission peak value of the target atom C of the other substance was obtained previously with reference to the table (A1 / A2) A step of obtaining a calibration value of an equivalent ratio corresponding to the (A1 / C1) value from the calibration characteristics is specified .
Specifically, in the measurement of the mixed state of hydrocarbon fuel and air, hydrogen is used as the atom of interest, and the ratio of the peak values of the spectral lines of 486 nm and 656 nm is calculated and used for the determination of the plasma state.
The LIPS quantitative measurement analysis method of the present invention is prepared by calculating two sets of ratios of the peak value of the spectral line depending on the amount of one component of fuel and air and the peak value of the spectral line depending on the amount of the other component. Two calibration equivalent ratios can be obtained based on the table showing the relationship between the above two sets of ratios corresponding to the plasma state and the equivalent ratio, and measurement is performed by adopting them as appropriate values only when both equivalent ratios are within the specified values. Improved accuracy.
The analysis method for LIPS quantitative measurement according to the present invention performs analysis only when the number of plasma nuclei generated at the laser focusing point is one in order to improve the measurement accuracy in the measurement of the mixed state of fuel and air. .

The LIPS quantitative measurement apparatus according to the present invention includes a laser oscillator for irradiating a laser beam, a lens for condensing the laser beam, an optical system for receiving plasma light generated in a laser focal region, and spectroscopic analysis for analyzing the received light. In the LIPS device for measuring the mixed state of fuel and air, equipped with a detector and an ICCD camera for imaging the spectrum, the spectroscopic analyzer includes a ratio of peak values of one or more spectral lines in the obtained plasma spectrum. And a calibration curve corresponding to the plasma state is accumulated, and a calibration curve is selected based on the plasma state information obtained by the means to obtain calibration data. .
The LIPS quantitative measurement apparatus of the present invention further includes a light detection device for imaging plasma generated in a laser focal region, and the light detection device confirms the number of plasma nuclei, and the spectral analysis is performed when the number is one. The function of the vessel was enabled.

The LIPS quantitative measurement analysis method of the present invention determines the plasma state based on one or more spectral peak values in the plasma spectrum and uses it as calibration data. A measuring means becomes unnecessary and it can arrange | position to a test area | region with a compact structure. Moreover, since the plasma state is determined based on one or more spectral peak values reflecting the plasma state, not from the laser pulse energy difference, the measurement accuracy is improved. In the prior art, it was necessary to carry out a calibration experiment for each optical system. However, in the present invention, the result of a calibration experiment (calibration curve) performed in an experiment setting of a simple form that is easy to measure can be performed even in a system with a different optical system. Can be used. Furthermore, a calibration curve can be made into a database, and if a database is created, it is not necessary for each person to perform a calibration experiment.
In the measurement of the mixed state of hydrocarbon fuel and air, the hydrogen with the highest radiation level is used as the atom of interest, and the ratio of the peak value of the 486 nm and 656 nm spectral lines reflecting the plasma state is calculated and calibrated. Measurement accuracy was improved.
The analysis method for LIPS quantitative measurement of the present invention calculates the ratio of the peak value of a spectral line depending on the amount of one component and the peak value of the spectral line depending on the amount of the other component, and calculates 2 of the peak values of different spectral lines. Since the data is adopted only when the ratio shows a close value, the reliability of the data is increased and the measurement accuracy can be improved.
Since the analysis method of LIPS quantitative measurement of the present invention performs the analysis only when the number of plasma nuclei generated at the laser focusing point is one, the data when light emission information from different plasma states is mixed is excluded. Measurement accuracy can be improved.
The LIPS quantitative measurement analysis method of the present invention can improve the measurement accuracy of the temperature of the measurement target gas because the size information of the plasma generated at the laser focusing point can be integrated with the peak value information of the spectrum line. .

In addition to the conventional LIPS device, the LIPS quantitative measurement device of the present invention includes a light detection device for imaging plasma generated in the laser focal region, and means for performing analysis only when the number of generated plasma nuclei is one. Since it is provided, data in the case where light emission information from different plasma states coexists can be effectively eliminated, and measurement accuracy can be improved.
In addition to the conventional LIPS device, the LIPS quantitative measurement device of the present invention includes a light detection device for imaging plasma generated in the laser focus region, and means for integrating the size information of the plasma with the peak value information of the spectral lines. Therefore, the measurement accuracy of the temperature of the measurement target gas can be improved.

The analysis method of the present invention is to determine the plasma state of the electronically excited state of one or more atoms (or ions) from the peak value (or ratio of peak values) at the corresponding wavelength on the plasma spectrum. . First, the grounds for determining the plasma state from the peak value (or peak value ratio) at the corresponding wavelength on the plasma spectrum will be described. The wavelength of the peak value on the plasma spectrum is specified by the kind of atom, and the level for each wavelength is determined in accordance with the level of excited electrons. For hydrogen, peaks of 486 nm and 656 nm are observed as can be seen from FIG. The ratio of the peak value of hydrogen at a wavelength of 486 nm to the peak value of nitrogen at a wavelength of 746 nm (H ₄₈₆ / N ₇₄₆ ) and equivalent when performing LIPS measurement for the purpose of measuring the equivalent ratio of hydrocarbon to air in an engine combustor The relationship with the ratio is shown. Hydrogen corresponds to the amount of hydrocarbons and nitrogen is an atom corresponding to the amount of air. FIG. 4 shows each data when the ratio of the peak value of hydrogen at a wavelength of 486 nm to the peak value of hydrogen at a wavelength of 656 nm (H ₄₈₆ / H ₆₅₆ ) is 0.04, 0.50, 0.60, 0.70, and 0.80, respectively. It is organized. Thus, the relationship between the (H ₄₈₆ / N ₇₄₆ ) value and the equivalence ratio is a characteristic curve that is almost linear, but it is recognized that the relationship is shifted by the (H ₄₈₆ / H ₆₅₆ ) value. That is, this (H ₄₈₆ / H ₆₅₆ ) value corresponds to the ratio of the excited level of the excited electrons of hydrogen, and it is understood that this indicates the plasma state at that time. Is done. That is, if the (H ₄₈₆ / H ₆₅₆ ) value corresponding to the plasma state at that time is grasped, the equivalent ratio can be estimated from the obtained (H ₄₈₆ / N ₇₄₆ ) value. The present invention has been made based on this finding, and a table showing the relationship between the (H ₄₈₆ / N ₇₄₆ ) value and the equivalence ratio for each (H ₄₈₆ / H ₆₅₆ ) value indicating the plasma state is used as a calibration curve. It is done.
The selection of the spectral line (wavelength) is determined by the configuration of the optical system to be used and the atom (or ion) to be examined. For example, when performing LIPS measurement for the purpose of measuring the equivalence ratio of an engine combustor, three or more independent emission peaks derived from fuel and oxidant are used in order to improve measurement accuracy. The analysis is performed only when the obtained two peak ratio values are close to each other.

  FIG. 1 shows a schematic diagram of a measurement system according to the present invention. In this system, a laser beam emitted from a laser oscillator 1 is focused on a sample by a condenser lens 4 to generate laser-induced plasma, and the generated plasma is frequency-separated using a spectroanalyzer 8 to obtain a spectrum. The point of imaging using the ICCD camera 9 is the same as that of the conventional LIPS apparatus shown in FIG. 7, and the above configuration is adopted. However, since this system does not estimate the plasma state from the difference in laser pulse energy before and after plasma generation, this apparatus does not require two mirrors and two energy meters in the conventional apparatus, but the obtained spectral information A different process for grasping the plasma state will be performed. The lens 6 in the figure captures the plasma emission, and the plasma light is not directional, so it is not limited to the arrangement perpendicular to the laser beam as shown in the figure. The beam damper 5 is not essential and may be omitted in some cases. Further, as a preferred embodiment of the system according to the present invention, an optical detector 10 for observing a plasma phenomenon such as an ICCD camera having a function described later is disposed.

FIG. 2 shows a flowchart assuming analysis in the present invention, here, measurement of equivalent ratios of different substances. First, in step 1, a laser pulse is emitted from the laser oscillator 1 to bring the sample in the focal region into a plasma state. In step 2, spectral information about the plasma generated for each pulse is acquired via an optical system. In step 3, the ratio (A ₁ / A ₂ ) of emission peak values of different frequencies of the atom A of interest is calculated in order to grasp the plasma state. In step 4 calculates the ratio of the emission peak value of a target atom B of the peak value A ₁ and the other materials of interest atoms A of one material (A ₁ / B _1). In step 5, a table indicating the relationship between the (A ₁ / B ₁ ) value and the equivalence ratio for each (A ₁ / A ₂ ) value is prepared, and the table (A ₁ / A ₂ ) value is specified, and a calibration value of an equivalent ratio corresponding to the (A ₁ / B ₁ ) value is obtained from its calibration characteristics. In step 6, the ratio (A ₁ / C ₁ ) between the peak value A ₁ of the previous atom A and the emission peak value of the target atom C of the other substance is calculated. In step 7, the previously obtained (A ₁ / A ₂ ) value is specified with reference to the table, and the calibration value of the equivalent ratio corresponding to the (A ₁ / C ₁ ) value is obtained from the calibration characteristics. Steps 6 and 7 may be performed in parallel with steps 4 and 5. In step 8, it is confirmed that the equivalence ratio calculated based on the emission peak values of different atoms obtained is within the specified limit (for example, 2.5%). If it is not within the limits, the measurement is terminated as non-adoption. B ₁ and C ₁ need only be spectral lines with different atoms contained only in other substances, and are not limited to spectral lines with different atoms, but may be spectral lines with the same atom having different wavelengths.

Steps 9 to 11 in the portion surrounded by the broken line on the right side in FIG. 2 are not indispensable components, but are optional recommended methods. In this step, it is necessary to arrange a photodetector 10 (see FIG. 1) such as a diode array or a CCD element array with an image enhancement function for monitoring the plasma state. In step 9, the plasma size and the number of generated plasma nuclei are imaged by the plasma observation photodetector 10. In step 10, it is determined how many plasma nuclei are present from the captured image information. In step 11, only when the number of plasma nuclei is one, the plasma state is specified and LIPS analysis is performed. In step 3, the “ratio of emission peak values of different frequencies of atom A (A ₁ / A ₂ )” is performed. The calculation is executed, and when there is one or more, the calculation is stopped. That is, it is used as an ON / OFF condition for determining whether or not to perform measurement depending on the number of plasma nuclei. The fact that there are a plurality of plasma nuclei means that the spectra of a plurality of plasma states are mixed, and correct measurement cannot be performed. By adding this step, the reliability of the measured values obtained is increased.
In step 9, the plasma observation optical detector 10 can detect the size of the plasma in addition to the number of plasma nuclei. The detection value can be determined in combination with the spectrum level information, thereby further improving the temperature measurement accuracy of the measurement gas.

Next, examples of experimental results under two different experimental conditions are shown. The first condition was a room temperature condition of a relatively small methane-air premixed burner having a heat generation rate of 10 kw. An Nd: YAG laser oscillator having a specification of 120 mJ-8 ns was used as the light source, and equivalent measurement was performed by the method of the present invention. The graph of FIG. 3 actually shows the average plasma spectrum under these two equivalence ratio conditions. The emission value of each element was determined by removing the background level at each wavelength (average value in a range where no peak appears) and then taking an average of ± 3 nm across the peak.
In another experiment, a methane-air premixed combustor preheated to 700 K with a heat generation rate of 100 kW class was targeted. As a light source, an Nd: YAG laser having a specification of 400 mJ-7 ns was used, and measurement by the method of the present invention was performed. The graph of FIG. 5 shows the equivalence ratio measurement result under this condition together with the result under the first experimental condition. Since equivalence ratio measurement results in different experimental settings show good agreement, it can be confirmed that general calibration is performed by the analysis method proposed by the present invention.

FIG. 6 shows the difference in accuracy between the case where the calibration is performed by the conventional method and the case where the calibration is performed by the proposed method. It is the graph which took equivalence ratio on the horizontal axis | shaft, took uncertainty of the measured value on the vertical axis | shaft, and showed the uncertainty of the measured value for every equivalence ratio in%. (H ₄₈₆ / H ₆₅₆ ) Although the reliability of the measurement data obtained by the analysis method of the present invention when the values are 0.50, 0.60, and 0.70 is slightly different from each other, it is much higher than the reliability of the conventional method. You can see what is going on. In addition, the measured value obtained by confirming that the equivalence ratio calculated based on two different emission peak values is within the specified limit (2.5%) must be more reliable. I understand. It was confirmed that when the calibration by the proposed method was performed, the accuracy was remarkably improved in a wide equivalence ratio range.

  Industrial applicability is diverse, such as measuring equivalence ratios in automobile engines and gas turbine engines, and measuring harmful metal concentrations at the outlet of burners. Improvement of measurement accuracy can be expected by incorporating this analysis method in all cases where LIPS measurement is performed. In particular, it can be said that the effect is great in the measurement in which the fluctuation of the plasma state becomes a problem.

It is a figure which shows the basic composition of the LIPS quantitative measurement apparatus of this invention. It is a flowchart of the analysis method in the LIPS quantitative measurement of this invention. The average plasma spectrum in two equivalence ratio conditions is shown. It is the graph which showed the influence of the hydrogen excitation state in the relationship between the peak ratio of _H486 and _N746 , and an equivalence ratio. 6 is a graph showing peak ratios of _H486 and _N746 under two different experimental conditions according to the technique of the present invention. It is a graph which shows the improvement of the measurement precision by the method of this invention. It is a figure which shows the basic composition of the conventional LIPS quantitative measurement apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser oscillator (Nd-YAG laser) 2 Power meter 3 Mirror (5% reflection) 4 Condensing lens 5 Beam damper 6 Lens 7 Optical fiber 8 Spectrometer 9 ICCD camera 10 Photodetector for plasma observation

Claims (3)

A step of determining a plasma state based on a ratio (A1 / A2) of peak values of spectral lines of one or more identical atoms in the obtained plasma spectrum, and a peak value A1 of the target atom A of one substance of the composition And calculating the ratio (A1 / B1) of the emission peak value of the target atom B of the other substance and the relationship between the (A1 / B1) value for each (A1 / A2) value and the equivalent ratio of both substances leave preparing a table, the step of obtaining a calibration value of the equivalent ratio corresponding to (A1 / B1) values from the calibration characteristics of the table identifies the by referring to the table obtained in advance (A1 / A2) value And step on
A step of calculating the ratio (A1 / C1) of the peak value A1 of the previous atom A and the emission peak value of the target atom C of the other substance, and specifying the previously obtained (A1 / A2) value with reference to the table In addition, a step of obtaining a calibration value of the equivalent ratio corresponding to the (A1 / C1) value from the calibration characteristics is added, and the measurement accuracy is improved by adopting it as an appropriate calibration value only when both equivalent ratios are within the specified value. An analysis method for LIPS quantitative measurement, characterized by   2. The measurement of a mixed state of a hydrocarbon fuel and air, which uses hydrogen as a target atom and calculates a ratio of peak values of spectral lines of 486 nm and 656 nm to be used for determination of a plasma state. Analytical method. The analysis method according to claim 1 or 2 , wherein in the measurement of the mixed state of fuel and air, the analysis is performed only when the number of plasma nuclei generated at the laser focusing point is one.

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