JPH0325352A - Flame tomographic measuring method - Google Patents

Abstract

PURPOSE:To image and measure the density values of plural components of an intermediate product produced in an elementary reaction process of combustion instantaneously at the same time by synthesizing laser light beams which match the absorption spectrum wavelength of a body to measured together, putting the light beams into a sheet shape and irradiating the body to be measured with the light beams. CONSTITUTION:The laser light beams from dye lasers 12 and 13 are tuned and mixed through a mirror 14, a dichroic mirror 15, a cylindrical lens system 16 and a beam splitter 17 so as to match absorption spectra of plural materials to be measured at the same time, and then the beams are changed from circular section to sheet section, and the body to be measured is irradiated with the beams. Consequently, materials in the section absorb light energy to rise in energy potential from base states to excited states and then return to the original base states in short times to emit fluorescent light. The fluorescent light is observed on a lower-wavelength side than the absorption spectrum wavelength. Those two kinds of the fluorescent light are observed by a high-sensitivity camera device 24 arranged at right angles to the tomographic surface. The fluorescent light beams are separated by an optical filter 18, so simultaneous video formation becomes possible.

Description

[Detailed description of the invention] [Industrial application field] ? The present invention relates to an optical measurement method for tomographically measuring the inside of a combustion flame.

(Conventional technology) Combustion measurement has traditionally been performed using points or lines (one-dimensional), but instead of measuring points or lines (one-dimensional).
The measurement field is expanding from two-dimensional (2-dimensional) measurement to three-dimensional measurement, and measurement methods are also changing from conventional contact methods to non-contact methods such as laser measurement. It's coming.

On the other hand, from the point of view of time resolution, instantaneous measurement is required to investigate highly unsteady turbulent combustion. Against this background and progress in technological development, it is expected that in the future it will become increasingly necessary to capture phenomena in terms of surfaces, that is, images, and to perform instantaneous high-speed measurements, and the development of elemental technologies for this will likely progress.

Conventionally, the flame cross section was measured using a sheet of laser light, and the flow and concentration were imaged using scattered light from combustion products and seed material supplied from outside.
The energy level of the electrons is excited between the energy levels of the burned or combustible materials using a laser beam, and the fluorescence and phosphorescence generated during the transition from the excited state to the ground state energy level are used to excite the raw materials. There is an image analysis of concentration and temperature.

By the way, combustion phenomena are generally difficult to investigate because many combustion parameters are complicated, but as a summation of the investigation, we first need to understand the temporal concentration of intermediate products or substances (radicals, etc.) generated during the combustion reaction process. , it is of great significance to know the spatial distribution status. It is expected that there are many types of intermediate products and that their concentration levels also vary when looking at the spatial cross section. The point of the present invention is that among these various living substances,
The advantage is that image measurements of the concentrations of multiple species of organisms can be performed simultaneously on the same cross-sectional plane. Known techniques related to the present invention include fluorescence tomographic image measurement of OH radicals performed by Kychakoff et al. (Quantit
ive visualization of c
ombustion species in a plastic
ne. Georga Kychakoff et al.
Applied OPTICS Lo Q 21NQ1
8 Sep. 1982). [Problems to be solved by the invention] Conventional tomographic measurement methods using laser light have been limited to measuring the concentration of a single substance. An object of the present invention is to provide an optical system and a measurement system that simultaneously and instantaneously image-measure the concentrations of multiple components of intermediate products generated in the elementary reaction process of combustion. [Means for solving the problem] To achieve the above purpose, laser light (
After tuning and mixing the beam (coherent light), the beam is changed from a circular cross section to a seed-like cross section using a cylindrical lens, etc., and when it is irradiated onto the object to be measured, the material on the cross section absorbs the light energy and The energy level is raised from the state to the excited state, and then within a short time (approximately 10″
During the transition to the original ground state (about 8 seconds), it emits fluorescence or phosphorescence. This fluorescence or phosphorescence is often observed on the longer wavelength side (lower frequency side) than the absorption spectrum wavelength. Now, taking the case of two substances to be measured as an example, when laser light tuned to the absorption spectrum wavelengths λL and λ2 is irradiated, fluorescence of λ1 + Δλhλ2 + Δλ2, which has a longer wavelength by ▲λl and Δλ2, is generated from the irradiated tomographic plane, respectively. do. si8l! is a high-sensitivity camera device that arranges these two types of fluorescence perpendicular to the tomographic plane! do. Wavelength separation is performed using means such as beam splitters and optical filters. [Operation] According to the above structure, by using a laser beam tuned to the wavelength of the absorption spectrum of the substance to be measured, fluorescence (multiple types) with different wavelengths generated from the substance existing on the irradiated cross section can be easily separated using an optical filter. However, since simultaneous imaging is possible, relative comparisons of the concentrations of multiple types of products are possible, and flammability diagnosis and evaluation can be made with higher accuracy.

As a method of laser beam tuning to the wavelength of the absorption spectrum, it is practical to use a wavelength-tunable dye laser using a high-power laser beam as a light source.

Generally, the generated fluorescence is often weak and further attenuates during the process of passing through an optical system, so an image intensifier is required, and in the present invention, an image intensifier is used as the intensifier.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. 1 to 3. FIG. 1 is a system diagram of the basic measurement system of the present invention, and here, as an example, the case where there are two substances to be measured is shown. The basic concept of optical arrangement is the same even when there are two or more objects to be measured. 1 indicates a combustion tube for burning fuel. The combustion tube shown here has the most common structure, with a combustion nozzle 3 located at the center of the upstream side of the combustion tube, an annular vent 2 on the outside of the nozzle, and a swirler 4 passing through the combustion tube. The air flows through the annular vent hole 2 while swirling, mixes with the fuel injected from the fuel nozzle, and a flame 5 is formed by a combustion reaction. Further, the combustion tube is provided with laser irradiation windows 6, 6' and observation windows 7, 7', which are large enough to allow the sheet-shaped laser beam to pass through. For the purpose of observing the products of the combustion reaction process, a window should be placed so that the area near the nozzle exit where rapid reactions can be observed can be observed. Furthermore, since there are two types of measurement cross sections: a vertical cross section and a horizontal cross section (described later), the upper observation window 8 is effective for the latter measurement.

Next, a method for producing a sheet-shaped laser beam will be described. In the case of simultaneous fluorescence measurement of two components, the wavelengths λl and λ shown in the figure, which correspond to certain absorption spectrum wavelengths, are used.
Two laser beams (coherent beams) are required,
In conventional technology, it is common to use two wavelength-tunable dye lasers using the laser source 11 as excitation light. In this optical method, the number of objects to be measured and the number of dye lasers are the same. The light of wavelength λ2 obtained from the dye lasers 1 and 12 is sent to the mirror 1.
4 changes direction by 90'. On the other hand, dye lasers 2, 13
The light with wavelength λ1 obtained at 45@dichroitsch mirror
15 to change the 90' direction. This dichroic mirror 15 is a mirror specially coded to completely transmit wavelength λ2 and completely reflect wavelength λl. Therefore, the laser light exiting the dichroic mirror 15 becomes a mixed light of wavelengths λl and λ2. At this point, although it depends on the characteristics of the laser beam, the beam shape is almost circular, but by passing it through the cylindrical lens system 16, it can be changed into a sheet-shaped beam. Since this optical system needs to pass light of a plurality of wavelengths, the cylindrical lens system 15 especially needs to include lenses with little chromatic aberration with respect to wavelength.

As shown in the figure, the sheet-shaped beam 10 enters through the left laser beam irradiation window 6, passes through the flame 5, and exits the combustion tube 1 through the right laser beam irradiation window 6'. The laser beam exiting the combustion tube is attenuated and removed using a 1-lap 9 or the like.

In the flame tomography plane irradiated with the sheet-shaped laser beam of wavelength λhλ2, a substance with absorption spectrum wavelength λn, λ2 1. Substance 2 absorbs energy from the laser beam, transitions from the stable ground state of the electronic energy level to the excited state, and in the next short period of time (
The substance emits fluorescence during the energy transition to its original ground state in about 10-δ seconds). Fluorescence has a wavelength of 582n
Although there are some cases where the absorption wavelength and fluorescence wavelength are the same (resonant radiation), such as the D line of the Na atom in m, the emission wavelength generally occurs on the longer wavelength side than the absorption wavelength. For example, OH radicals present in the ultraviolet region, which play an important role in the combustion element reaction process of hydrocarbon fuels, have an absorption band around 285 nm. 306
．． It has a fluorescent band between 4 nm and around 315 nm (
Here, regarding the energy transition of fluorescence generation, A,
G. Gaydon's book (The Identific
ation of Molecular Spectrum
a;CHAPMA}! AND }IAL), etc., and details are provided in the Ministry <). FIG. 3 schematically shows the wavelength distribution of two-component fluorescence. Now, assume that the absorption wavelengths of substance 1 and object gt2 are λ1 and λ2 (λ2>λ1, respectively).When sheet-shaped laser beams tuned to λ1 and λ2 are irradiated to substance 1 and substance 2, the sheet-shaped laser beam's cross section from the surface to the center wavelength of fluorescence, respectively, Δλ
Fluorescence shifted to the long wavelength side by 1, Δλ2 is generated.

Since fluorescence has a probability of transitioning to each vibrational energy level among the electronic energy levels, the wavelength of fluorescence generally has a certain width as shown in the figure. In the figure, the laser scattered light indicates that when a substance is irradiated with laser light,
Reflection, or scattering, occurs on the surface of substances (including molecules and atoms). In terms of energy, this scattering is light (photon)
This is an elastic collision between the object and the material, and does not involve a change in wavelength. Even though laser scattering is weak at the molecular level, the energy level of scattering becomes high when it comes to powders and solid particles. In particular, mu (
Mie) It is called scattering to distinguish it. Next, a method for detecting fluorescence will be described. The figure shown on the right side of FIG. 1 is a lateral sectional view taken along the plane of the laser irradiation windows 6 and 6' of the combustion pipework. 1181 at a position perpendicular to the laser irradiation windows 6, 6'! I windows 7, 7' are provided. The case where the observation window 7 or 7' is detected from either one direction will be explained based on the diagram. The present invention is characterized by the adoption of an optical imaging method that simultaneously measures two types of fluorescence distributions, which are carried out as follows.

First, the band-shaped fluorescent image radiated to the outside through the IRim window 7 is divided into two parts at right angles using the beam splitter 17. The divided images are filtered using optical filters 18 and 18' to separate the fluorescence of λl+Δλ1 and the fluorescence of λ2.
Only fluorescence light of +Δλ2 is extracted.

The optical filter used is an interference film filter that utilizes optical interference of dielectric or metal thin films. In FIG. 3, the wavelength characteristics of the optical filter for fluorescence observation are shown by broken lines. Filter characteristics depend on the wavelength band used, but the minimum half-width is about 10 nm for near ultraviolet (~300 nm) and about 40 nm for visible wavelengths (~300 nm).
(0 to 700 nm), and can manufacture products with a thickness of approximately 0.9 nm. The optical arrangement of the imaging device after the optical filters 18, 18' is the same. The fluorescence of λ1+Δλ1 extracted by the optical filter 18.18' and the fluorescence of λ2+^1. The fluorescence of 19, 19' and 20, respectively are image intensifiers for intensifying the fluorescence images using objective lenses 19 and 19'.
The image is formed on the photocathode 20'.

Photoelectric conversion is performed from light to electrons at the photocathode, and the electrons are intensified in the image intensifier and collide with the phosphor screen at the rear end of the image intensifier, thereby regenerating a new image with sufficient illuminance on the phosphor screen. do. This image is captured by a high-sensitivity camera 22.2 using means such as relay lenses 21, 21'.
After the image is formed on the image pickup tube 2' or the image pickup device and converted into a video signal, it can be displayed on various monitors and also recorded under the control of the computer 23. In addition, the captured images are processed by an image analysis device under the control of the computer 23! 124 is used to perform various types of image processing, and quantitative comparisons of concentration distributions of substances to be measured are performed. Note that, as a high-sensitivity camera, an SIP camera or the like is better if it is an image pickup tube type camera, or a cooled COD camera or the like is better if it is an image sensor type camera.

By the way, as a method of increasing the intensity of fluorescence, it is advantageous to use a high output laser beam for excitation, and in this respect, a pulsed laser with a high peak output is often used. Currently, the repetition rate of pulsed lasers is several tens to several hundred Hz. When measuring fluorescence in a flame, the S/N ratio of the fluorescence intensity decreases due to noise such as light emission from the flame and continuous spectra generated from solid soot, etc. Therefore, when using a pulsed laser, it is necessary to is being emitted, that is, approximately the time when fluorescence is observed (approximately 10
It is preferable to capture the fluorescent image only for about -a seconds).

A gating unit 25 is used for this purpose. This is a device that emits a gate signal in synchronization with the pulse of the laser source (in the case of a pulsed laser) 11, and this gate signal is transmitted to the image intensifier 20.20'
On/off control of the high voltage applied to the image intensifier is performed in synchronization with the gate signal sent to the image intensifier.

In other words, when the gate signal is being sent, the gate width (
A voltage is applied corresponding to the time), and if the gate signal does not come, it is turned off. As a result, high-sensitivity camera 2
lI at 2.22'! The measured fluorescence image will be observed in synchronization with the laser pulse. In Figure 1, we explained the case where there are two substances to be measured, but the basic configuration is the same even when there are three or more substances, and a laser beam generator (dye laser in this text) matching the absorption spectrum wavelength is Only the number of substances is required. The problem when the number of substances to be measured increases is that a high-output laser source is required. It also depends on the optical performance of the dichroic mirror 15 when mixing laser beams matching the absorption spectrum. Since the wavelength characteristics of dichroic mirrors also have production limits, it is necessary to select a wavelength that takes into account the absorption spectrum wavelength of the substance to be measured and the characteristics of the dichroic mirror.

Although Fig. 1 shows the case where the sheet-shaped laser beam is arranged in the longitudinal direction of the combustion tube, it is naturally possible to arrange it in the cross section of the combustion tube. Figure 2 shows the case where a laser beam is placed on this cross section.
In this case, w48I! in the direction perpendicular to the laser sheet surface! It is necessary to provide one part, and in Fig. 2, the upper Il! It shows how light is brought in through the measuring window 8. The image detection unit is the same as that shown in Figure 1, so its description is omitted. The above has described the fluorescence distribution measurement of two measuring substances, but as a modification of the present invention, it is also possible to perform image measurement based on fluorescence and laser scattered light. As shown in FIG. 3, for the laser beam wavelengths λ2 and λ2, light having the same wavelength as the laser beam wavelength is reflected from the substance. As mentioned earlier, this is called laser scattering.

The intensity of laser scattering is related to the material's intrinsic scattering cross section. Since the scattering cross section of substances generated in a flame is on the same level as the scattering cross section of air, it is normally difficult to detect scattered light in a flame. Therefore, by introducing fine particles such as TiOz into the fuel and air lines, the particle density distribution at each cross section of the flame can be determined from Tilt laser scattering. Information on fuel diffusion and flow can be obtained from this particle density distribution. To select the wavelength of the laser scattered light, the same type of optical filter as explained for fluorescence measurement can be used, and the arrangement of other imaging equipment can be the same as in the previous method. [Effects of the Invention] Since the present invention is configured as described below, it produces the effects described below. That is, by combining laser beams that match the absorption spectrum wavelength of the object to be measured and irradiating it in the form of a sheet, information on the concentration of the object to be measured within the flame tomography can be obtained as a fluorescence intensity distribution image. Although the number of objects to be measured is limited by the optical system and the output of the laser source, simultaneous measurement of two or three objects can be done without any problem with the current technology level. Using a pulsed laser is advantageous in efficiently operating the present invention, and by performing on-off control of the video in synchronization with the pulsed laser, it is possible to create a high S/N ratio video and instantaneous imaging.

Furthermore, in one modification of the present invention, taking as an example the case where there are two substances (items) to be measured, the fluorescence from the substance and the laser (mu) scattered light distribution are simultaneously imaged as shown in Figure 3. It is also possible to obtain information on the concentration of reaction products from the former fluorescence, and information on diffusion and flow from the latter laser (mu) scattering.

[Brief explanation of drawings]

Claims (1)

[Claims] 1. In an optical method for two-dimensional planar tomographic measurement of the combustion state within a combustion flame, as a method for simultaneously measuring image concentration distributions of several types of combustion products, a method that matches the wavelength of the absorption spectrum of the products The wavelength of the laser beam is tuned so that the wavelength of the laser beam is tuned, and an optical means is provided to mix the light.Furthermore, the mixed light is changed into a sheet-shaped beam with a minute width using a cylindrical lens, and the sheet-shaped beam is applied to one cross-section of the flame. It is characterized by comprising an optical means that penetrates the sheet-like beam surface and simultaneously observes fluorescence or laser scattered light generated from the sheet-like beam surface from a direction perpendicular to the beam surface, and performs image analysis to extract the relative concentration distribution status, etc. Flame tomography method.