In the tunable diode laser absorption spectroscopy-based diagnostics, the absorption of the measured target species may be influenced by the interference absorption from other vapor-phase species and the extinction from particles and liquid droplets, especially at high temperatures and pressures. Here, we report the first application (to our knowledge) of a differential absorption diagnostic for interference-free, simultaneous measurement of temperature and ethylene concentration using a single distributed-feedback diode laser near 1.62 μm. According to the detailed study of the C2H4 spectra in this region, two wavelength pairs are chosen to measure the temperature based on six selection criteria. C2H4 concentration is measured by one of the selected wavelength pairs with higher differential absorption. To validate the developed system, experiments are performed in a well-controlled heated static cell at a range of temperatures (300-900 K) and pressures (1-6 atm). The measurement accuracies for temperature and ethylene concentration are 1.83% and 1.65%, respectively, over the considered ranges. The precision, stability, and detection limit are also analyzed to validate the system’s performance. This system can potentially be applied in a variety of combustion applications.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Ethylene, which is also known by its IUPAC name of ethene and its chemical formula C2H4, is a non-methane volatile organic compound that plays an important role in many fields. For example, C2H4 is widely used in the chemical industry, and its worldwide production exceeds that of any other organic compound . C2H4 is also an important natural plant hormone and is used in agriculture to accelerate the ripening of fruits . Furthermore, C2H4 is an essential intermediate product in the combustion of fossil fuels, which are most widely used as an energy source . Therefore, the time-history data of C2H4 during hydrocarbon oxidation can be used to investigate the mechanisms of fossil fuel combustion . Temperature is another fundamental parameter of combustion systems, and it determines the overall thermal efficiency of combustion . Hence, the simultaneous measurement of C2H4 concentration and temperature can provide useful information on the utilization of fuel during combustion. Recently, many practical combustion systems, such as internal combustion engines, gas turbines, and products of detonations, have been operated at high pressures for clean and highly efficient combustion . To understand the mechanisms of these combustion systems, C2H4 concentration and temperature must be determined at high temperatures and pressures.
At present, there are many methods for measurement of C2H4 concentration. Gas chromatography is one of the most sensitive methods to have been implemented for C2H4 measurement , but this method requires the operating personnel to have specific qualifications, and consumables are mandatory. Electro-catalytic or potentiometric sensors are sometimes used; however, they demonstrate saturation or wear and degradation problems without sufficient selectivity or accuracy . Moreover, it is difficult to apply these methods to in situ measurements of combustion. For optical gas sensing, tunable diode laser absorption spectroscopy (TDLAS) techniques can satisfy the necessary requirements .
TDLAS can provide a fast, sensitive, non-intrusive, and reliable method for in situ measurements of multiple flow-field parameters, such as temperature, concentration, pressure, and velocity, in various harsh environments. It can provide selectivity, accuracy, and sensitivity in gas sensing when combined with some other techniques, such as photoacoustic spectroscopy (PAS), cavity enhanced absorption spectroscopy (CEAS), cavity ring-down spectroscopy (CRDS), and direct absorption or wavelength modulation spectroscopy coupled with a multi-pass cell [10–13]. TDLAS has been widely applied to the detection of C2H4 by employing a diode laser with a wavelength from near-infrared to mid-infrared. For example, Schilt et al. detected C2H4 using quartz-enhanced photoacoustic spectroscopy and a WMS-2f detection scheme on the C2H4 absorption line near 1.62 μm . Aziz et al. developed a continuous wave cavity ring-down spectrometer for measurement of C2H4 in the air by using a DFB diode laser at a wavelength of 1.6 μm . Nguyen Ba et al. performed C2H4 detection based on quartz-enhanced photoacoustic spectroscopy using an antimonide distributed feedback quantum well diode laser operating at 3.32 μm at near room temperature in a continuous wave regime . Wang et al. developed a quartz-enhanced photoacoustic spectroscopy sensor for the detection of C2H4 using a continuous-wave distributed-feedback quantum cascade laser at 10.5 μm . Manne et al. conducted spectroscopic concentration measurements of NH3 and C2H4 by employing a pulsed distributed-feedback quantum cascade laser at 10.3 μm and an astigmatic Herriot cell with 150 m path length . McCulloch et al. developed a direct-absorption spectrometer for detection of CO2 and C2H4 based on a pulsed distributed-feedback quantum cascade laser at a wavelength of 10.26 μm and an astigmatic Herriott cell with 66 m path length .
TDLAS has also been used for the detection of C2H4 at high temperatures and pressures. Ma et al. measured the C2H4 concentration and temperature in pulse detonation engine flows based on direct absorption spectroscopy with a distributed feedback diode laser near 1.626 μm. The C2H4 sensor can provide reliable measurements from 300 to almost 900 K . Tanaka et al. developed a sensor for the detection of C2H4 in a combustion exhaust based on wavelength modulation spectroscopy with a Herriott cell and a distributed feedback interband cascade laser near 3.3 μm. In their measurements, the cell pressure and temperature were set to 3 kPa and 393 K, respectively . Parise et al. developed a two-wavelength infrared laser absorption diagnostic for measurement of C2H4 by an external cavity quantum cascade laser at 10.532 μm and demonstrated its measurement ability between 1360 and 1710 K in a shock tube .
As a result of an investigation of the related literature, it can be seen that many studies have been conducted on C2H4 measurement based on TDLAS at room temperature. However, few studies have been performed at high temperatures and pressures, especially considering the simultaneous measurement of C2H4 concentration and temperature.
C2H4 shows distinct absorption spectral features near 10.5 μm, 3.3 μm, and 1.62 μm . It is known that the absorption in mid-infrared is stronger than that in the near-infrared region. Figure 1(a) shows the measured C2H4 spectrum near 1.62 μm and a simulated one near 3.3 μm based on the HITRAN 2016 database  for the same conditions (PC2H4 = 49 torr, T = 296 K). It can be seen from Fig. 1(a) that the wavelength range near 1.62 μm shows only 5 to 8 times weaker absorption lines than at 3.3 μm, while the lasers and other elements cost less than those in the mid-infrared. Besides, the C2H4 transitions near 3.3 μm are overlapped more seriously with other primary combustion products than the transitions near 1.62 μm, as shown in Fig. 1(b). This overlap will influence measurement accuracy, especially in high temperature and pressure environments.
It also can be seen from Fig. 1(a) that the C2H4 absorption features near both 3.3 μm and 1.62 μm are dense. It is difficult to acquire the exact line shape for a C2H4 transition owing to the self-blending and overlap with other molecules at high pressure. Therefore, the concentration or temperature cannot be inferred accurately by a single line or a line pair, which is the most commonly used approach in direct absorption spectroscopy and wavelength modulation spectroscopy [25–27]. To perform interference-free absorption measurements of absolute species concentration at wavelengths that encounter self-interference and interference from other molecules with broad and unstructured absorption features, a differential absorption (or peak minus valley) scheme was developed, which takes advantage of the structural differences in the absorption spectrum. This strategy has previously been used to measure the concentrations of various hydrocarbons, including methane, acetylene, propene, and C2H4, at high temperature and pressure [22,28–30]. However, according to our investigation, it has not been used for simultaneous measurements of temperature and concentration.
In this work, we describe a TDLAS diagnostic for interference-free measurement of temperature and C2H4 concentration by employing a differential absorption strategy. A distributed feedback diode laser near 1.620 μm is employed to detect selected peak and valley wavelength pairs, which are selected based on some wavelength selection criteria. The differential absorption scheme efficiently rejects the background and the absorption of the interfering absorbers. Gas temperature is inferred from the ratio of differential absorption of the two selected wavelength pairs. After the temperature is determined, the C2H4 concentration can be determined using the wavelength pair with the larger differential absorption cross-section. The interference-free measurements of temperature and C2H4 concentration are validated in a heated cell in a temperature range of 300-900 K and a pressure range of 1-6 atm. To the best of our knowledge, this is the first report of such simultaneous measurements of temperature and C2H4 concentration employing a differential absorption scheme.
2. Theory of the differential absorption
The theoretical details of the differential absorption scheme for concentration measurement have been previously developed [22,28,30]. Here, we will present the first development (to the best of our knowledge) of the theory on temperature measurement based on the differential absorption scheme. A brief review of the method for measurements of concentration and temperature is presented here in order to introduce the necessary notations and guide the discussion.
For a monochromatic laser beam through a uniform gas medium, the absorption of the light can be described by the Beer-Lambert relation
When the laser at a selected frequency can also be absorbed by other species or there is extinction/scattering caused by particles and liquid droplets, additional terms should be added to the above equation to describe these effects:
To eliminate the influences of αint and τext on the measurement of the target species, the differential absorption technique can be combined with direct absorption spectroscopy. In the differential absorption technique, the absorbance of the interfering species should be nearly the same at the selected peak and valley wavelengths. Then, the influence of αint can be eliminated by the differential absorbance measured at the selected peak and valley of the absorption feature of the investigated molecule. The total extinction (absorption and scattering) from particles and liquid droplets τext varies with wavelength. However, variations in τext are very small over the 2 cm−1 tuning range of the laser, and can, therefore, be neglected. Hence, the influence of τext can also be neglected in the differential absorption technique. Then, the fractional transmissions measured at the selected peak (-ln(I/I0)ν, p) and valley (-ln(I/I0)ν, v) wavelengths can be subtracted as
When the differential cross-section σν, p-v is measured at a certain temperature and pressure, the mole fraction of the target species can be obtained from the fractional transmissions measured at the selected peak and valley according to
To measure the gas temperature, another wavelength pair must be used. The gas temperature can be inferred from the ratio of the differential fractional transmissions simultaneously measured for the two selected wavelength pairsν1 and ν2:
Before the measurement of the temperature and species concentration, the cross-section at the selected peak and valley wavelengths can be measured over the range of temperatures and pressures considered in the experiment. The temperature and pressure dependencies of the differential absorption cross-sections of the target species can be formulated using the measured cross-section data, and the relations can be described as
Then, we substitute the two relations into the ratio f(T, P), and Eq. (5) can be expressed as
Hence, if the ratio f(T, P) and the total pressure are measured, the gas temperature can be inferred using this equation.
3. Experimental detail
The differential absorption-based setup for the measurements of temperature and C2H4 concentration is schematically illustrated in Fig. 2. The measurements are taken in a heated static cell to validate the system at high temperature and pressure. This is an effective way to ensure the reliability and accuracy of the system owing to the well-controlled environment provided by the static cell. The heated static cell is made from stainless steel with a total length of 50 cm and an inner diameter of 1.2 cm. The optical window used is a fused quartz window which has a shape similar to a nail (JGS3 rod in Fig. 2). The head part has a thickness of 1 cm and a diameter of 2.5 cm and the rod part has a length of 14.5 cm and a diameter of 1.15 cm. It allows a low-temperature vacuum seal outside the heating zone while spanning the thermal gradient region to access the high temperature test section. The 1.5° wedge angle at the end of the rod is for eliminating the optical interference. To minimize the deviation of the laser beam the end surfaces of both windows are adjusted as parallel as possible. When a mid-infrared laser source is employed in the measurement, CaF2 or sapphire can be used as the window material. The 21 cm test section is filled with the sample gas and heated by a furnace with a length of 38 cm and can supply a temperature as high as 1000 K. The 5-cm long water cooling tubes wrapped near the flanges maintain both ends of the cell at low enough temperature, so that the test section can be sealed with Viton O-rings. The temperature uniformity of the test section is better than 1% at 1000 K. Three K-type thermocouples with accuracy of ± 1% and precision of 0.1 K are equally spaced along the test section of the cell. The temperature of the gas in the cell is determined by the average of the three thermocouple readings. The vacuum of the static cell is provided by a mechanical pump and a turbo molecular pump (Leybold, TW300). After the middle section is evacuated to the order of 10 −4 Torr, the sample gas is delivered to the test section from a mixing tank via stainless steel tubes. Two pressure transducers provide measurements of pressure inside the gas cell: one for low pressure (0-1 atm) and another for high pressure (1-10 atm). The leakage rate of the static cell is ~7.2 Torr/h at 10 atm and 1000 K, which demonstrates the high gas impermeability of the cell. In the experiment, the gas sample is allowed to thermally stabilize for a sufficient time before each measurement.
The continuous wave distributed-feedback tunable diode laser used in the experiment can emit a typical power of ~10 mW near 1.62 μm. The laser is placed in a commercial laser mount (ILX Lightwave LDM-4980) and driven by a low noise controller (ILX Lightwave LDC-3724C). The laser frequency is tuned by scanning the current (∼0.005 nm/mA) using a 1 kHz triangle ramp generated from a function generator (RIGOL, DG1032Z). The emitted laser beam is split into two parts using a 1 × 2 fiber splitter with a 50/50 split ratio. The first part is collimated using a fiber collimator, propagated through the heated static cell, and then focused by a lens onto an InGaAs detector (New Focus Model 2011). However, owing to the relatively low power of the laser used in the experiment, the blackbody radiation of the heated static cell might affect laser absorption measurements. To eliminate this potential problem, an aperture (AP) and a narrow bandpass filter (BPF, 1.62 ± 0.013 μm) are added downstream of the heated static cell. The second part is introduced into a wavemeter (Bristol-671B-MIR) to monitor the frequency of the laser with a repeatability of ± 1 ppm. The detector signal is acquired via a DAQ card (NI, USB6361) using a computer. The data acquisition and post-processing analysis are performed by versatile software based on the LabVIEW program.
4. System development
4.1. Wavelength pair selection
An important step in the system development is the selection of optimum wavelength pairs. Wavelength pair selection rules for concentration measurements of species with a resolved, structured absorption spectrum have been discussed in . In this work, we will reconstruct these selection criteria of optimum wavelength pairs for simultaneous measurements of temperature and concentration. Here, six selection rules listed in Table 1 are used.
Usually, the optimum lines can be selected based on databases such as HITRAN/HITEMP or PNNL. Because the C2H4 transitions near 1.62 μm are not included in HITRAN/HITEMP and because we are not able to access PNNL database, we measure the spectra of C2H4 in the experimental ranges of temperature and pressure. Figure 3 shows a part of the C2H4 spectra measured in the heated static cell at different temperatures. We investigate the measured spectra according to the six selection criteria, and two wavelength pairs are selected for the differential absorption scheme for the measurements of temperature and concentration of C2H4. The two selected wavelength pairs are indicated on the measured C2H4 absorption profile shown in Fig. 4, and their properties are summarized in Table 2. The wavelength pair B is still used for the measurement of C2H4 concentration owing to the higher differential absorption at high pressure and temperature. The simulated spectra of some primary species in combustion are also shown in Fig. 4. The graph and table can help validate the selection of the wavelength pair for this experiment.
It can also be seen from Fig. 4 that the absorbance of CO2 is slightly different between the peak and valley for the selected wavelength pair B, which will lead to some error in the measurement results if the selected wavelength pairs are used for an experiment in an actual furnace. But the aim of this work is to show the ability of a differential absorption diagnostic for simultaneous measurement of temperature and C2H4 concentration at high temperature and pressure. Therefore, it should be emphasized that the selected wavelength pair is optimum for the conditions in this experiment. If it is applied to other experimental systems (shock tubes, rapid compression machines, flames of different types, and flow reactors, etc.), the optimum wavelength pairs should be re-selected according to the corresponding conditions in those experiments.
4.2. Measurements of absorption cross-sections
In order to develop correlations for differential absorption cross-sections of the selected two wavelength pairs as a function of temperature and pressure, absorption cross-sections at the selected peak and valley wavelengths are measured. These measurements are taken in the heated static cell in the temperature range of 300-900 K with increments of 100 K and the pressure range of 1-6 atm with increments of 1 atm. At each set point of temperature and pressure, the spectra of six C2H4-N2 mixtures with different C2H4 concentrations are recorded. According to Eq. (1), the absorption cross-section can be determined by plotting the natural log of the fractional transmission versus the C2H4 concentration and then fitting them by a straight line. The slope of the line divided by the product of the total molarity and the laser path length directly gives the absorption cross-section at the respective wavelength. The linear fit can eliminate any systematic error in the zero of the pressure gauge. Figure 5 illustrates the absorption cross-section obtained at the peak wavelength of wavelength pair A at the pressure of 1 atm, where different symbols indicate measurements at different temperatures.
Figure 6 presents the peak, valley, and differential cross-sections of the two selected wavelength pairs at different temperatures for measurements taken at 1 atm. Similar measurements are also taken at pressures of 2, 3, 4, 5, and 6 atm. These measurements can be used to analyze the temperature- and pressure-dependence of the differential cross-sections. The measured differential absorption cross-sections of the two selected wavelength pairs as a function of temperature and pressure are shown in Fig. 7. It can be seen from Fig. 7 that both differential cross-sections show significant pressure dependencies at 1 atm. However, as the pressure increases, the pressure dependence weakens. This is because the transitions merge into each other as the pressure increases, and the profile of the spectrum becomes relatively smooth at high pressure.
The correlations for differential absorption cross-sections of the two selected wavelength pairs as a function of temperature and pressure can be formulated by these measurements. The best fit for the measured differential cross-sections as a function of temperature and pressure over the range of 300-900 K and 1-6 atm measured in this work is as follows:
- • Wavelength pair A: , where T0 = 304 K, P0 = 1 atm, and σ0 = 0.285 m2 mol−1.
- • Wavelength pair B: , where T0 = 304 K, P0 = 1 atm, and σ0 = 0.749 m2 mol−1.
The ratios of differential absorption cross-sections of the two selected wavelength pairs as a function of temperature at each pressure are shown in Fig. 8. As shown in Fig. 8, the differential absorption cross-section ratio is single-valued with temperature over the expected range. This means that the relation between the ratio of differential fractional transmissions and the gas temperature is monotonic over the temperature and pressure ranges measured in this work. The temperature can only be determined from the ratio of the measured differential fractional transmissions of the two selected wavelength pairs. Here, the temperature sensitivity of the two selected wavelength pairs can be defined as the unit change in the normalized differential absorption cross-section ratio, ΔR/R, for a unit change in the normalized temperature, ΔT/T. According to the definition, the temperature sensitivity decreases from ~1.8 to ~1.1 as the pressure increases from 1 atm to 6 atm. Therefore, the sensitivity for the target temperature range (300-900 K) does not fall below 1 over the 1-6 atm pressure range, which is the minimum value for temperature measurements with small uncertainty.
It should be emphasized that all the measured cross-sections and the formulated relations are valid over the temperature and pressure ranges measured in this work. If the system is used for high temperature and pressure applications, the relations should be formulated using the cross-sections measured over the corresponding ranges of temperature and pressure or new wavelength pairs should be selected.
4.3. Measurement of the incident laser intensity I0
In direct absorption spectroscopy, the fractional transmission (I/I0) is calculated by the transmitted and incident laser intensities I and I0. Usually, the transmitted laser intensity I is recorded directly by a detector, and the incident laser intensity I0 is determined by mathematically modeling the part of the transmitted signal with no absorption by a polynomial when the total scan includes the far wings on both sides of the probed feature. However, in this work, it is difficult to find the section with no absorption due to the density of the lines and the high pressure. Here, we substitute the fitted incident laser intensity with a measured one. In the experiment, we measure the transmitted laser intensity at each pressure and temperature set point while the static cell is filled with pure nitrogen. These measured signals can be regarded as the incident laser intensity I0 for the corresponding temperatures and pressures. To verify this method, the transmitted intensity I is recorded while the test section of the cell is filled with ~1 kPa of C2H4 and I0 is recorded when it is filled with ~1 kPa of N2. Figure 9 compares the measured I0 with the fitted one. An enlarged view of the area covered by the green circle is shown in this graph for clarity. The average deviation between the two I0 values is ~0.3%. The calculated spectra from the two incident laser intensity I0 are also shown in the inset. As shown in Fig. 9, the spectra can be calculated accurately by the measured I0. This validates the method performed here to acquire the I0.
4.4. Improvement of signal-to-noise ratio
There are many intrinsic noise sources in a TDLAS system, such as dark current noise, photon shot noise, thermal noise (Johnson noise), and flicker noise. The digital filtering technique of the wavelet transform is an efficient tool to reduce noise from a polluted signal without additional hardware. It has been widely used in many spectroscopic techniques, such as Raman spectroscopy , laser-induced breakdown spectroscopy , Fourier transform infrared spectroscopy , and TDLAS [34–36]. To improve the sensitivity of the system, a 5th order wavelet denoising method is employed to reduce the noise of the direct absorption signal measured in the experiment. A comparison between the raw signal (gray line) and the denoised signal (red line) is shown in Fig. 10. It can be seen from Fig. 10 that the signal-to-noise ratio is improved significantly by employing the wavelet denoising method.
5. Results and discussion
5.1. Accuracy of measurements
In order to investigate the performance of the system for simultaneous measurements of temperature and C2H4 concentration using the differential absorption scheme, a set of static heated cell experiments with well controlled C2H4-N2 mixtures are performed over the considered temperature range (300-900 K) and pressure range (1-6 atm).
Figure 11 displays the measured temperature and C2H4 concentration using the differential absorption scheme versus the known values. A comparison between the measured temperatures and the thermocouple readings is shown in Fig. 11(a). It can be seen from Fig. 11(a) that the measured temperatures from the system are in good agreement with the thermocouple readings over the entire temperature range. The correlation of those measured points has an R2 value of 0.991, which indicates good linearity of the measurements. The accuracy of the temperature measurements is 1.83% according to the linear fitted slopes of the scatter plot of 0.9817 ± 0.0138. The average temperature bias between the measured values and the thermocouple readings (δT = \ Tmeasured - Treading \) is ~12 K over the tested temperature range. Figure 11(b) shows a comparison between the measured C2H4 concentrations (xmeasured) and the concentrations recorded when the mixtures were prepared (xreference). The C2H4 concentrations are measured using wavelength pair B due to the higher differential absorption. The measured C2H4 concentrations are also in good agreement with known mixture values. The standard deviation between the measured and reference values is 2.65% for the measurements of CO2 concentrations. Uncertainties in the temperature and C2H4 concentration measurements primarily come from uncertainty in the measurements of cross-sections, thermocouple and pressure readings, laser path length, C2H4 concentrations recorded during the preparation of the C2H4-N2 mixtures, the analysis of the measured spectroscopic data, etc.
5.2. Precision and stability
To further evaluate the measurement precision and stability of the system, time series measurements of ~0.275% and ~0.543% C2H4-N2 mixtures are performed for 1000 s at 900 K and 1 atm and 900 K and 6 atm, respectively. In the measurements, each point is obtained in 1 s, so one thousand C2H4 concentration points are obtained. Histogram plots of measured concentration points are shown in Fig. 12 to evaluate the measurement precision of C2H4 concentration. Each histogram demonstrates a good Gaussian distribution, and the measurement precision is determined by the half width at half maximum (HWHM) of the Gaussian profile. It is found that the measurement precisions for C2H4 concentration are 19 ppm and 25 ppm at 900 K and 1 atm and 900 K and 6 atm, respectively.
The stability of the system based on the differential absorption scheme is evaluated by Allan analysis. Figure 13 shows the Allan variance obtained from the same continuous time series measurements used in the histogram plot for measurements of C2H4 concentration at 900 K and 1 atm and 900 K and 6 atm, respectively. The Allan analysis in Fig. 13 shows that a sensitivity of ~10 ppm can be achieved with an averaging time of 300 s for C2H4 concentration at 900 K and 1 atm. As shown in Fig. 13, it allows an averaging time of 200 s and a corresponding sensitivity of ~20 ppm at 900 K and 6 atm.
5.3. Detection limit of the system
The C2H4 concentration detection limits of the system are calculated under different pressure and temperature conditions based on these measurements. The minimum detectivity is calculated for a C2H4-N2 mixture with a path length of 21 cm, at laser power of 10 mW, and 1 kHz bandwidth. Based on the correlations for differential absorption cross-sections of wavelength pair B as a function of temperature and pressure given in Section 4.2, the C2H4 concentration detection limit (SNR = 1) is estimated under different temperature and pressure conditions, as presented in Fig. 14. As shown in Fig. 14, the detection limit increases along with temperature, ranging from 18 ppm at 300 K to 90 ppm at 900 K (at 1 atm) owing to the small differential cross-section σν, p-v at high temperature. It can also be seen from Fig. 14 that the detection limit decreases along with pressure, ranging from 90 ppm at 1 atm to 28 ppm at 6 atm (at 900 K). According to Eq. (3), the minimum detectivity is inversely proportional to the differential cross-section σν, p-v, the total gas pressure P, and the laser path length L. For wavelength pair B used in our experiment, it can be seen from Fig. 7(b) that its σν, p-v at 1 atm is only four times larger than its σν, p-v at 6 atm over the temperature range of 300-900 K. Hence, the detection limit decreases with increasing pressure. In our measurements based on the differential absorption scheme, the minimum detectivity is limited by the short path length. Because the detection limit is inversely proportional to the effective path length, cavity-based techniques can be introduced into the system to decrease the minimum detectivity.
6. Summary and future work
In this work, a TDLAS based system was developed for simultaneous measurements of temperature and C2H4 concentration at high temperatures and pressures. To eliminate the interferences from absorption by other vapor-phase species and the effect of extinction from particles and liquid droplets, the differential absorption scheme (peak minus valley) was employed in this system. The spectra of C2H4 were measured in the ranges of temperature and pressure required to select the optimum wavelength pairs for the system. Based on these measurements, two wavelength pairs were selected for the differential absorption scheme for the measurements of temperature and concentration of C2H4 according to six selection criteria. The cross-sections at the selected peaks and valleys were measured in the temperature range of 300-900 K and the pressure range of 1-6 atm. The performance of the system was validated in a well-controlled static heated cell. The analyses of accuracy, precision, stability, and detection limit confirmed the potential of the system. The system has the potential to measure temperature and C2H4 concentration in a variety of high temperature and pressure environments.
In the future, we will improve the time resolution and detection limit of the system to extend its applicability to many other fields. To increase the time resolution of the system, a quasi-fixed-wavelength scheme, which is similar to that reported in our earlier work , can be employed in the system. Besides that, a vertical-cavity surface-emitting laser can be used to replace the DFB diode laser owing to its much higher scan rate and hence higher temporal resolution. The minimum detectivity can be decreased by introducing a sensitive technique such as multi-pass absorption spectroscopy or cavity enhanced absorption spectroscopy. Then, the system can be used to study the combustion kinetics in shock tubes, internal combustion engines, or flames of different types. In those environments, the influence of the interference absorption by other vapor-phase species and the effect of extinction from particles and liquid droplets can be effectively eliminated by the differential absorption technique.
National Natural Science Foundation of China (NSFC) (61875079, 61805110, 61475068, 11104237).
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