Laser absorption spectroscopy (LAS) technology, as a non-contact measurement method with high measurement sensitivity and fast response, combined with tomography technology, can simultaneously achieve visual measurement of flame temperature and components concentrations distribution, and further analyze combustion reaction rules. In this paper, we developed a compact LAS tomography system for the distribution images of temperature and species concentrations. The system mainly consists of two parts, a 32-path sensor and a self-made compact case. In the sensor, 32 pair of collimators with optical fibers are used. The collimator-to- collimator design can help reduce background interference from infrared radiation due to fuel combustion. In the compact case, two distributed feedback lasers, two laser controller units, a 2/1 fiber combiner, a 1/32 fiber splitter and a 32-channels data acquisition unit are integrated. The output lights of lasers are divided into 32 beams by the splitter. The sampling rate of each channel of data acquisition unit can reach up to 1MHz. The measured data is transmitted to the computer for processing and can realize uninterrupted long-term monitoring. In the experiment, a flat flame burner is chosen as the target and the wavenumbers of lasers are scanning around 7185.6cm-1 and 7444.3cm-1 respectively. The temperature and water vapor concentration distribution images are reconstructed by the system. The reconstructed image trends of symmetrically distributed. Compared with the temperature testing result of thermocouple thermometer, the temperature distribution trends of the two methods are consistent.
Hydrogen has a wide range of applications in various industries, including fuel cells, chemical processing, and electronics. However, hydrogen is highly flammable and explosive at a wide concentration range (4%~75%), which poses a significant safety concern. Hydrogen sensors with higher sensitivity, better selectivity, faster response, and wider dynamic range are of increasing importance in connection with the development. In this paper, a scheme of four-channel hydrogen sensor is proposed which is based on stimulated Raman spectroscopy and nanofiber. The tightly confined evanescent field associated with the nanofiber enhances the Raman gain per unit length by a factor of more than 104 over free-space beams. Time division multiplexing technology is used to implement multichannel detection. The whole sensing system consists of commercial optical modules such as DFB laser, EDFA and optical switch, and several homemade circuits instead of the commercial instruments for signal processing and controlling channel switching. Every channel can connect one probe which is based on nanofiber and packaged by special metal structure and method. The switching time can be millisecond order so that the signal period would not be influenced. The four-channel hydrogen sensor is realized in the experiment that demonstrates hydrogen detection from hundreds parts per million to 100% with each channel. The reported sensor could be used in the field of new energy, electric power and aerospace for detection of hydrogen leakage or monitoring of transformer health conditions with advantages of low cost, compact size and high resolution.
Methane, as the main component of gas and natural gas, its flammable and explosive characteristics to industrial production, urban safety and daily life has brought great potential hazards. Accurately measuring the concentration of methane gas is of great significance for urban development and environmental protection. At present, tunable semiconductor laser absorption spectroscopy (TDLAS) has become one of the important technologies for methane sensor concentration measurement. However, this technology is easily affected by environmental temperature, and the relative error of concentration measurement caused by environmental temperature changes is greater than 50%. Therefore, this article proposes a temperature compensation calibration method for methane sensors based on TDLAS technology. This article first analyzes the effect of temperature on the absorption intensity of methane gas, and studies the relationship between its concentration value and temperature. Then, based on theoretical analysis, the negative temperature coefficient variation between the concentration value and temperature is achieved through power normalization processing algorithm. At the same time, a temperature compensation system is established to compensate for the concentration value and suppress the interference caused by environmental temperature to the detection. Finally, the feasibility of the temperature compensation calibration method was verified through experiments. The experimental results show that under the same testing conditions, the relative error of the measured concentration after temperature compensation can be reduced to less than 5%, significantly improving the detection accuracy of the methane sensor.
Hydrogen is an important source used as an energy carrier and a chemical reactant or industrial material. However, if not handled properly, hydrogen content as low as 4% can lead to a life-threatening catastrophe. Hydrogen sensors with higher sensitivity, better selectivity, faster response, and wider dynamic range are of increasing importance in connection with the development. In this paper, a scheme of hydrogen sensor that satisfies these requirements with a single sensing element is proposed which is centered on a nanofiber. The sensor is based on stimulated Raman scattering spectroscopy but the tightly confined evanescent field associated with the nanofiber enhances the Raman gain per unitlength by a factor of more than 104 over free-space beams. In addition, the homemade signal processing circuit plays an important role in the whole sensing system instead of the commercial instruments, which makes it possible to develop a principle prototype. The circuit intergrates tne DFB laser driver circuit, the photoelectric detection circuit and the main control circuit which outputs modulation signal and acts as a digital lock-in amplifier. Several silica nanofibers operating in the telecom wavelength band has been manufactured and measured in an experiment that demonstrates hydrogen detection from hundreds parts per million to 100%. The reported sensor could be used in the field of new energy, electric power and aerospace for detection of hydrogen leakage or monitoring of transformer health conditions with advantages of low cost, small size and outstanding performance.
Natural gas and biogas, as commonly used combustible gases, are widely used in urban residents and industrial enterprises. Gas leakage accidents occur frequently. In order to more accurately distinguish whether it is a natural gas leak or a biogas leak, this paper proposes a method for simultaneous detection of methane and ethane based on TDLAS, and develops a set of methane and ethane dual gas detection devices. Firstly, this paper studied the gas absorption lines of methane and ethane. According to the absorption lines, a 1680nm DFB laser beam was used to scan methane and ethane at the same time. Secondly, the phase-locked amplifier module of TDLAS technology is used to obtain the second harmonic signals of methane and ethane concentration detection at the same time, and identify the components of methane and ethane based on the peak position information and FWHM information of the second harmonic signal. Finally, the concentration calculation function is obtained by fitting the peak and valley difference of the second harmonic. Experimental results show that the detection method and device proposed in this paper can achieve simultaneous detection of methane and ethane
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