Results and Discussions The measured absorption spectra of the used target acetylene line pair at different content and different temperatures are shown in the paper (Fig. 5). The average bias between the measured temperatures and thermocouple readings is about 23.1 K. The measured acetylene content for each level is averaged and plotted against the known content [Fig. 6(a)]. The measured values from the tunable diode laser absorption spectroscopy sensor agree well with the known values. The linear relationship between the counterparts is about 0.995, and the average relative standard deviation of the acetylene measurement is 2.65%. Additionally, we compare the temperatures obtained depending on the thermometry of the sensor and the averages of the three thermocouple readings [Fig. 6(b)]. The temperatures determined by the sensor agree well with the thermocouple readings in the tested temperature range of 500-1000 K. The correlation of those measured points R² is about 0.998, and the average relative standard deviation is 3.05%.

In the past 40 years, tunable diode laser absorption spectroscopy has developed into a remarkable optical diagnostic technique because it can provide a reliable, fast, time-resolved, sensitive, non-intrusive in situ measurement for various gas parameters, such as concentration, temperature, pressure, velocity, mass flux, and density. Hence, it has been widely used in many fields including environmental monitoring, combustion diagnosis, industrial process control, and biomedical sensing. In some special applications, there is not only gas but also some particles and liquid droplets in the measurement area, such as the soot in the flow field of combustion and fine particles (PM_2.5 and PM₁₀) in the atmospheric air. Then the variation in the intensity of the tunable diode laser is induced by both the absorption of the target gas molecule and the extinction/scattering caused by particles and liquid droplets. Thus, how to extract the absorption spectrum of the target absorption line of the measured gas molecule from the mixed signal is significant for the development of gas sensors based on tunable diode laser absorption spectroscopy. Here we design a gas-solid two-phase optical sample cell and apply it to simulate a gas detection environment in the presence of both particulate matter and gas. Temperature and acetylene content are measured in the gas-solid two-phase optical sample cell, which can demonstrate how to detect gas in a particulate environment.

The gas-solid two-phase optical sample cell used in the experiment is made of stainless steel and located in a ceramic fiber furnace whose temperature is controlled by a custom-made temperature control system. The heated static cell has a height of 290 mm and a diameter of 120 mm. The JGS3 quartz rods with a length of 90 mm and a diameter of 52 mm are inserted into the cell, and each quartz rod has an inclination of 1.5^o against the vertical direction to further reduce interference fringe noise. The temperature in the sample cell is measured by three K-type thermocouples with accuracies of ±1% and precision of 0.1 K. In the experiment, the C₂H₂-N₂ mixtures which are controlled by a mass flowmeter carry the quartz sand particles with a diameter of 125 μm into the gas-solid two-phase optical sample cell. A distributed feedback (DFB) laser with a central wavelength of 1540 nm is used to detect the absorption spectra of the target acetylene lines at 6489.07 cm^-1 and 6490.02 cm^-1. A multi-pass absorption technique is combined in the system to enhance performance. The temperature is inferred from the integrated absorption ratio of the two lines, and the acetylene content is determined from the integrated absorption of the line at 6490.02 cm^-1 due to its high absorption intensity.

In this work, temperature and acetylene content are simultaneously measured by means of a DFB laser with a central wavelength of 1540 nm in a particulate environment which is simulated by a gas-solid two-phase optical sample cell. Two target acetylene lines at 6489.07 cm^-1 and 6490.02 cm^-1 are chosen as the optimum line pair for the tunable diode laser absorption spectroscopy sensor to detect temperatures in the target range of 500-1000 K at atmospheric pressure. The comparison of the values measured by the sensor with the well-controlled gas temperature and acetylene content confirm the accuracy and reliability of the sensor. At present, the optical path length is only 0.36 m. The short path length limits the detection sensitivity of the sensor. In the future, the detection sensitivity will be decreased due to the increase in the reflection time of multi-pass absorption. Then the system can be used to detect the gas volume fraction and temperature in the flow fields of real flames or internal combustion engines, where both gas and particles coexist.