Laser Spectral Temperature Measurement Device and Method Based on Quartz Tuning Fork Detection

Abstract

The present invention discloses a laser spectral temperature measurement device and method based on quartz tuning fork detection. The device includes a semiconductor laser, a laser collimation system, a combustion field, a quartz tuning fork, a control and data acquisition processing system, and a computer, which are sequentially arranged along the propagation direction of the light beam. The semiconductor laser outputs laser light modulated by the control and data acquisition processing system. The laser light passes through the combustion field after passing through the laser collimation system. The CO gas in the combustion field absorbs part of the laser energy and then irradiates the base of the quartz tuning fork, generating an electrical signal. This electrical signal is input into the control and data acquisition processing system for signal demodulation and acquisition. The control and data acquisition processing system is connected to the computer, and temperature inversion and display are performed in the computer. The present invention solves the wavelength response limitation of the photoelectric detector in the temperature measurement device when CO is used as the detection gas in current combustion field temperature measurement.

Description

Technical Field

[0001] The present invention relates to a laser spectrum temperature measurement device and method, and in particular to a laser spectrum temperature measurement device and method based on quartz tuning fork detection. Background Art

[0002] The combustion flow field is complex and harsh, requiring real-time diagnosis of the transient environment. Optical diagnostic techniques for combustion flow fields primarily utilize laser, spectroscopy, and photoelectric detection technologies to achieve high-resolution, temporal and spatial measurements of combustion parameters such as temperature, composition, concentration, flame structure, and flow rate. Temperature is a crucial physical quantity in the combustion flow field, and its measurement is crucial for combustion dynamics, engine combustion efficiency, and pollution control.

[0003] In recent years, laser-based spectral diagnostic techniques have gradually replaced traditional temperature measurement methods and become the mainstream method for combustion testing. Among them, tunable diode laser absorption spectroscopy (TDLAS) is the most commonly used method for online monitoring of temperature and composition in combustion fields. TDLAS measurement principle is based on the Beer-Lambert law. It uses a semiconductor laser with a rapidly tunable wavelength to generate a single longitudinal mode laser. A detector measures the change in light intensity before and after the laser is absorbed by the medium.

[0004] The principle of TDLAS for measuring flow field temperature is that the distribution of molecular populations at different energy levels is related to temperature (satisfying the Boltzmann statistical distribution), while the absorption intensity of the absorption line is related to the population of the corresponding energy level. By measuring the relative intensities of different absorption lines, the temperature of the combustion field can be obtained. The specific implementation method is to select two absorption lines whose line intensities have different temperature dependencies, and use their ratio, which is a single-valued function of temperature, to invert the flow field temperature. Two diode lasers are used to scan the two absorption lines simultaneously, or a single diode laser is used to scan two adjacent absorption lines. Performing a ln(I0 / I) operation on the absorption light intensity and the baseline light intensity yields two absorption peak curves. For the wavelength scanning direct absorption method, the ratio of the areas of the two absorption peaks is equivalent to the ratio of the line intensities of the two absorption lines.

[0005] According to the Beer-Lambert law:

[0006] ln(I0 / I)=PXS(T)φ(ν)L (1)

[0007] Where I0 is the incident laser intensity; I is the outgoing laser intensity; P is the total pressure of the detection gas; X is the mole fraction of the detection component; S(T) is the spectral intensity of the absorption line; T is the temperature of the test environment; is a linear function; v is the frequency of the laser; L is the effective propagation distance of the laser in the absorbing medium.

[0008] In the wavelength scanning modulation absorption method, a sine wave is superimposed on a sawtooth wave to modulate the wavelength of the laser. The temperature is measured using the dual-line method, that is, the ratio of the second harmonic signal heights S1 and S2 (values at the center frequency) of the two absorption lines. Assuming the center frequencies of the two absorption lines are ν1 and ν2, respectively, then:

[0009]

[0010]

[0011] Where R 2f is the ratio of the second harmonic signal intensity; a is the wavelength modulation depth. The temperature measurement of the wavelength modulation absorption method is in addition to the second harmonic signal intensity R under two absorption 2f In addition to being related to , it is also related to the wavelength modulation depth a, laser intensity I, and linear function φ. The flow field temperature inversion formula of the wavelength modulation absorption second harmonic method is:

[0012]

[0013] Where E' is the lower energy level energy of the absorption line; h is Planck's constant; c is the speed of light; k is the Boltzmann constant; and T0 is the reference temperature. In existing experimental schemes, the TDLAS temperature measurement system primarily includes a light source, a modulator, a combustion field, and a photodetector. The output of a tunable semiconductor laser is collimated and then passed through the combustion field to excite the gas under test. The gas absorbs some of the laser energy, and the emitted laser illuminates the photosensitive surface of the photodetector, which is used to detect changes in light intensity.

[0014] Water is a ubiquitous component in hydrocarbon fuel combustion fields, and it exhibits strong vibrational-rotational absorption bands in the visible to mid-infrared bands. In combustion field temperature measurement experiments, water is generally used for combustion field measurements. However, water vapor in the air can affect experimental measurements, resulting in reduced temperature measurement accuracy. Therefore, carbon oxides produced by combustion, such as CO and CO2, which have relatively low levels in the air, can be used as detection targets. Traditional TDLAS temperature measurement experiments use photodetectors as light intensity detection elements. However, photodetectors have wavelength response limitations. Photodetectors that respond to the mid-infrared spectral lines of CO and CO2 are relatively rare and expensive, resulting in high experimental costs. Furthermore, compared to near-infrared band elements, these photodetectors have relatively average performance, which affects temperature measurement performance. Summary of the Invention

[0015] In order to solve the problem of wavelength response limitation of photoelectric detectors in temperature measurement devices when CO is used as the detection gas in current combustion field temperature measurement, the present invention uses the principle of wavelength modulation absorption method to provide a laser spectroscopy temperature measurement device and method based on quartz tuning fork detection.

[0016] The purpose of the present invention is achieved through the following technical solutions:

[0017] A laser spectrum temperature measurement device based on quartz tuning fork detection includes a semiconductor laser, a laser collimation system, a combustion field, a quartz tuning fork, a control and data acquisition processing system, and a computer, which are sequentially arranged along the propagation direction of the light beam. The semiconductor laser outputs laser light after being modulated by the control and data acquisition processing system. The laser light passes through the combustion field after passing through the laser collimation system. The CO gas in the combustion field absorbs part of the laser energy and then irradiates the root of the quartz tuning fork. The quartz tuning fork generates an electrical signal. The electrical signal is input into the control and data acquisition processing system for signal demodulation and acquisition. The control and data acquisition processing system is connected to the computer, and temperature inversion and display are performed in the computer.

[0018] A method for laser spectroscopy temperature measurement using the above device comprises the following steps:

[0019] Step 1: Select CO as the target gas to be detected. Using the wavelength modulation absorption method, the control and data acquisition processing system controls the output characteristics of the semiconductor laser, so that the output wavelength of the semiconductor laser is scanned in the form of a sawtooth wave. A sine wave is superimposed on the sawtooth wave for modulation, and the scanning range is controlled to completely cover the two consecutive absorption lines of CO.

[0020] Step 2: The laser output by the semiconductor laser passes through the laser collimation system and is incident on the combustion field to excite the target gas CO to be measured. The CO gas medium absorbs the laser light, and the laser light intensity changes periodically. When it shines on the root of the quartz tuning fork, it stimulates the quartz tuning fork to produce periodic elastic deformation. Under the action of the laser, the quartz tuning fork resonates and swings, generating a current signal.

[0021] Step 3: The quartz tuning fork is connected to the control and data acquisition processing system for signal demodulation and acquisition, and then input into the computer for temperature inversion processing.

[0022] In the aforementioned quartz tuning fork-based temperature detection technology, CO is selected as the target gas for detection. Using the wavelength modulation absorption method, the output wavelength of the semiconductor laser is scanned in the form of a sawtooth wave. The scanning range should completely cover two consecutive absorption lines of CO. A higher-frequency sine wave is superimposed on the sawtooth wave for modulation. When the laser wavelength sweeps across the absorption line, the frequency of the laser intensity change after passing through the absorbing medium will include harmonic components in addition to the modulation frequency, because the line shape of the medium's absorption line is a Voigt function. The wavelength modulation method typically measures flow field parameters by measuring the second harmonic signal of the modulation signal after the laser passes through the absorbing medium.

[0023] The tunable semiconductor laser outputs collimated laser light, which then passes through the combustion field and irradiates the base of a quartz tuning fork. The CO gas medium in the combustion field absorbs the laser light, causing the laser's intensity to vary periodically due to the medium's absorption. When irradiated on the base of the quartz tuning fork, the photothermoelastic effect stimulates the quartz tuning fork to produce periodic elastic deformation. This means that the quartz tuning fork resonates and oscillates under the action of the laser. These oscillations generate a current signal based on the piezoelectric effect of the quartz tuning fork. The two second harmonic signals, S1 and S2, are demodulated using the lock-in amplifier unit included in the control and data acquisition processing system. The combustion field temperature can be calculated by substituting the ratio of the two second harmonic peaks into Equation (4).

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] 1. The quartz tuning fork with a wide spectral response range, high sensitivity and low cost is selected as the detection unit of the light intensity change, which solves the problem of limited coverage and high price of existing mid-infrared detection band detectors;

[0026] 2. The quartz tuning fork has the advantage of a high quality factor Q>10000. Its detection bandwidth (a few hertz or even lower) is several orders of magnitude narrower than that of optical infrared detectors, and can effectively isolate the interference caused by other spectra in the combustion field and environmental noise. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 The figure is a schematic diagram of the structure of the laser spectrum temperature measurement device based on quartz tuning fork detection;

[0028] Figure 2 This is a diagram of the experimental setup for the laser spectroscopic temperature measurement device based on quartz tuning fork detection;

[0029] Figure 3 is the second harmonic signal measured in a plane flame burner environment. DETAILED DESCRIPTION

[0030] The technical solution of the present invention is further described below with reference to the accompanying drawings, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention that does not depart from the spirit and scope of the technical solution of the present invention should be included in the scope of protection of the present invention.

[0031] In order to solve the problem of wavelength response limitation of the photoelectric detector in the temperature measurement device when CO is used as the detection gas in the current combustion field temperature measurement, the present invention designs a temperature detection method and device based on quartz tuning fork detection by using the principle of wavelength modulation absorption method. The design uses a quartz tuning fork instead of a photoelectric detector as the light intensity detection element. The laser is emitted by a semiconductor laser. The collimated laser passes through the combustion field and then irradiates the root of the quartz tuning fork. The laser wavelength scan includes two CO absorption lines. When the laser modulation frequency is the resonant frequency of the quartz tuning fork, the light intensity excitation causes the quartz tuning fork to resonate and enhance. The mechanical vibration signal is converted into an electrical signal through the piezoelectric effect of the quartz tuning fork. The harmonic signal of the electrical signal is demodulated by the phase-locked amplifier unit in the control and data acquisition and processing system. Finally, the signal is processed to obtain the second harmonic signals S1 and S2. Substituting them into formula (6) can calculate the combustion field temperature.

[0032] like Figure 1 and Figure 2 As shown, the device includes a semiconductor laser 1, a laser collimation system 2, a combustion field 3, a quartz tuning fork 4, a control and data acquisition and processing system 5, and a computer 6, which are sequentially arranged along the propagation direction of the light beam. The semiconductor laser 1 modulated by the control and data acquisition and processing system 5 outputs laser light, which is incident on the combustion field 3 after passing through the laser collimation system 2. The CO gas in the combustion field 3 absorbs part of the laser energy and then irradiates the root of the quartz tuning fork 4. Due to the thermoelastic effect and the piezoelectric effect, the quartz tuning fork 4 will generate an electrical signal, which is input into the control and data acquisition and processing system 5 for signal demodulation and acquisition. The control and data acquisition and processing system 5 is connected to the computer 6, and temperature inversion and display are performed in the computer 6. The specific implementation process is as follows:

[0033] Step 1: The control and data acquisition processing system 5 controls the output characteristics (power, wavelength, modulation rate, etc.) of the semiconductor laser 1;

[0034] Step 2: The laser output by the semiconductor laser 1 passes through the laser collimation system 2 and is incident on the combustion field 3 to excite the target gas CO to be measured. After CO absorbs the laser, the emitted laser irradiates the root of the quartz tuning fork 4.

[0035] Step 3: The quartz tuning fork 4 is connected to the control and data acquisition processing system 5 to demodulate and collect the signal, and then input it into the computer 6 for inversion processing.

[0036] In the present invention, the semiconductor laser 1 is a distributed feedback single longitudinal mode semiconductor laser with near-infrared continuous wave output, and the line width should not be greater than 10 MHz.

[0037] In the present invention, in order to cover two absorption lines, the tuning range of the semiconductor laser 1 should be greater than 0.1 nm.

[0038] In the present invention, in order to increase the thermoelastic energy amplitude of the quartz tuning fork 4, the laser power of the semiconductor laser 1 should be greater than 10 mW.

[0039] In the present invention, wavelength modulation technology is used to reduce system noise. The control and data acquisition processing system 5 modulates the output wavelength of the semiconductor laser 1. The modulation frequency of the semiconductor laser 1 should be equal to half the resonance frequency of the quartz tuning fork.

[0040] In the present invention, the control and data acquisition processing system 5 includes a lock-in amplifier, a data acquisition card and a signal generator. In order to realize the demodulation of the high-frequency modulated signal, the demodulation bandwidth of the lock-in amplifier should be greater than 10kHz.

[0041] In the present invention, in order to improve the temperature measurement speed and temperature resolution, the resonance frequency of the quartz tuning fork 4 should be greater than 10 kHz.

[0042] In the present invention, in order to reduce the influence of the combustion field 3 on the thermal noise of the quartz tuning fork 4 , the quartz tuning fork 4 should be at least 5 cm away from the combustion field 3 .

[0043] In the present invention, in order to ensure the accuracy of temperature measurement by the quartz tuning fork 4, the relative sensitivity of the selected absorption line pair should be greater than 1.

[0044] In the present invention, in order to reduce the interference of flame jitter on the laser light intensity signal, the wavelength scanning frequency of the semiconductor laser 1 should be greater than 10 Hz.

[0045] In the present invention, the wavelength modulation depth of the semiconductor laser 1 will affect the modulation absorption spectrum and should be optimized first to maximize the signal value.

[0046] In the present invention, the laser light after passing through the combustion field 3 should be irradiated on the root of the quartz tuning fork, and the specific position should be optimized to maximize the signal value.

[0047] In the present invention, the computer 6 is connected to the control and data acquisition and processing system 5, and performs real-time control and signal acquisition and processing through serial port instructions.

[0048] Example:

[0049] This embodiment uses a Mckenna standard flat flame burner as a test object and verifies the technical solution of the present invention under the condition of an equivalence ratio of 1.0. The light intensity signal detected by the quartz tuning fork is demodulated and the result obtained is as follows: Figure 3As shown in Figure 2. The secondary signals corresponding to the two absorption lines are S1 and S2. According to formula (4), the temperature of the area to be measured is 2100K, and the test result is close to the calibration value of the burner. Therefore, this method can be used for temperature measurement.

Claims

1. A laser spectrum temperature measurement device based on quartz tuning fork detection, characterized in that The device includes a semiconductor laser, a laser collimation system, a combustion field, a quartz tuning fork, a control and data acquisition processing system, and a computer, which are sequentially arranged along the propagation direction of the light beam. The semiconductor laser outputs laser light after being modulated by the control and data acquisition processing system. The laser light passes through the combustion field after passing through the laser collimation system. The CO gas in the combustion field absorbs part of the laser energy and then irradiates the root of the quartz tuning fork. The quartz tuning fork generates an electrical signal. The electrical signal is input into the control and data acquisition processing system for signal demodulation and acquisition. The control and data acquisition processing system is connected to the computer, and temperature inversion and display are performed in the computer.

2. The laser spectrum temperature measurement device based on quartz tuning fork detection according to claim 1 is characterized in that The semiconductor laser is a distributed feedback single longitudinal mode semiconductor laser with near-infrared continuous wave output, and the line width is not greater than 10 MHz.

3. The laser spectrum temperature measurement device based on quartz tuning fork detection according to claim 1 is characterized in that The tuning range of the semiconductor laser is greater than 0.1 nm.

4. The laser spectrum temperature measurement device based on quartz tuning fork detection according to claim 1 is characterized in that The laser power of the semiconductor laser is greater than 10 mW.

5. The laser spectrum temperature measurement device based on quartz tuning fork detection according to claim 1 is characterized in that The modulation frequency of the semiconductor laser is equal to half of the resonance frequency of the quartz tuning fork, and the resonance frequency of the quartz tuning fork is greater than 10 kHz.

6. The laser spectrum temperature measurement device based on quartz tuning fork detection according to claim 1 is characterized in that The demodulation bandwidth of the lock-in amplifier in the control and data acquisition and processing system is greater than 10 kHz.

7. The laser spectrum temperature measurement device based on quartz tuning fork detection according to claim 1 or 5, characterized in that The quartz tuning fork should be at least 5 cm away from the combustion field.

8. The laser spectrum temperature measurement device based on quartz tuning fork detection according to claim 1 is characterized in that The wavelength scanning frequency of the semiconductor laser is greater than 10 Hz.

9. A method for laser spectroscopy temperature measurement using the device according to any one of claims 1 to 8, characterized in that The method comprises the following steps: Step 1: Select CO as the target gas to be detected. Using the wavelength modulation absorption method, the control and data acquisition processing system controls the output characteristics of the semiconductor laser, so that the output wavelength of the semiconductor laser is scanned in the form of a sawtooth wave. A sine wave is superimposed on the sawtooth wave for modulation, and the scanning range is controlled to completely cover the two consecutive absorption lines of CO. Step 2: The laser output by the semiconductor laser passes through the laser collimation system and is incident on the combustion field to excite the target gas CO to be measured. The CO gas medium absorbs the laser light, and the laser light intensity changes periodically. When it shines on the root of the quartz tuning fork, it stimulates the quartz tuning fork to produce periodic elastic deformation. Under the action of the laser, the quartz tuning fork resonates and swings, generating a current signal. Step 3: The quartz tuning fork is connected to the control and data acquisition processing system for signal demodulation and acquisition, and then input into the computer for temperature inversion processing.

10. The laser spectrum temperature measurement method according to claim 9, characterized in that In step 3, the temperature inversion formula is: Where E' is the lower energy level of the absorption line; h is Planck's constant; c is the speed of light; k is the Boltzmann constant; T0 is the reference temperature; R 2f is the ratio of the second harmonic signal intensity; I is the output laser intensity; T is the temperature of the test environment; v is the frequency of the laser; S1 and S2 are the second harmonic signal heights of the two absorption lines.

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