The Research Of Concentration Dectection For Gas Mixtures Based On Acoustic Relaxation Theory

In the modern world and industrial production, gas concentration detection has widely demands and application prospects. The real-time concentration measurement of multi-component gas mixture has been extensively developed by many researchers. The concentration detection for gas mixtures based on acoustic relaxation theory, which both has solid theoretical background and brightly developing orientation, is being studied vigorously by the researchers, because it has the numerous advantages, such as lower cost, longer service duration, smaller power loss, faster response speed, considerably wide concentration scope for detecting, as well as simultaneously gaining results of multiple gases with different characteristics. The concentration detection for gas mixtures based on acoustic relaxation theory is a novel theory and technology. It utilizes the characteristics of acoustic attenuation and sound speed along with the changes of concentrations of gaseous components, and founds models of acoustic attenuation and sound speed depending on acoustic frequencies and concentrations of gases components. The detection is conducted by using sound speed, acoustic attenuation coefficients, effective relaxation frequency and these models. It is an interdisciplinary study based on mathematics, physics, electronics, signal processing.In this thesis, according to the natures of gases relaxation acoustics, we studied the classical acoustic velocity theory, the classical acoustic attenuation theory, the acoustic relaxation attenuation theory, and the gas molecular collision and energy transfer theory. Through researching, expanding and innovating of these theories, we have solved the key technologies of the concentration detection algorithms for three-component and four-component gas mixtures.For the gas molecular collision and energy transfer model and the algorithm of effective relaxation frequency, this thesis demonstrated it is reasonable that effective relaxation frequency can be considered as a third gas acoustics parameter. By ignoring the near resonance modes in relaxation energy transitions of gas molecular collision process under normal temperature, we improve the gas molecular collision and energy transfer model. For the first time, we gain the relaxation matrix algorithm to calculate the effective relaxation frequency for four-component gas mixtures. The simulation results verify that two results of the effective relaxation frequency, which come from the relaxation matrix algorithm based on the developed acoustic multi-relaxation attenuation theory and the algorithm based on the maximal dimensionless relaxation attenuation coefficient per wavelength respectively, become consistent basically. Therefore, the effective relaxation frequency can be introduced to the concentration detection algorithms of four-component gas mixtures based on acoustic relaxation theory.For the concentration detection algorithms of four-component gas mixtures based on acoustic relaxation theory, in this thesis, the effective relaxation frequency is first applied to acoustic gas concentration detection for four-component gas mixtures. We establish several multidimensional models for the concentrations of the constituents versus the effective relaxation frequency, relaxation attenuation, and acoustic velocity, respectively. Based on these models, we can use the measured parameters: effective relaxation frequency, relaxation attenuation coefficient and acoustic velocity, to predict the concentration of each component in the mixture. Testing the simulation results of sample gas mixtures demonstrates that the algorithm has high accuracy, strong stability and robustness for a wide range of acoustic frequencies.In the concentration detection algorithm of three-component gas mixtures discussed here, we boldly remove the classical acoustic attenuation from the total attenuation due to the certain relativity between the classical acoustic velocity and the classical acoustic attenuation. Then we provided dependences between relaxation attenuation coefficients and other acoustic parameters when ultrasound propagates in gas mixtures, such as concentrations of constituents and acoustic frequencies, and established a three dimensional model between concentrations of mixture constituents and relaxation absorption, acoustic velocity, separately. Further more, we gave out the two dimension relationship between relaxation absorption and acoustic frequencies. We propose a simplified algorithm to calculate the carbon monoxide concentration by measuring relaxation absorption and acoustic velocity. By analyzing the models of the energy transfer in molecular collision process and the acoustic relaxation attenuation in gas, we find the reason why ruggedness appears when the gas constituents have weak concentrations. We use the smooth window, a typical method in signal processing, to address acoustic relaxation attenuation data, and develop the algorithm for weak gas concentration detection of three-component gas mixtures. The measuring accuracy of this algorithm achieves 0.001%. Simulation results not only prove the feasibility of this method, but also indicate the appropriate range of acoustic frequencies, which is one 10 octave under the effective relaxation frequency. The reasons of application errors can be reduced through mean tested value, because of linear acoustic frequency displacement characteristic of the algorithm.For the acoustic relaxation attenuation theory, in this thesis, we introduced the fundamental definition and theory of gases relaxation acoustics, and the acoustic relaxation attenuation theory resulted from the gas molecular collision and energy transfer theory. Then we completed the foundation and solution of the acoustic relaxation attenuation theory according to the existing gas molecular collision and energy transfer models, and confirmed the correctness of the acoustic relaxation attenuation model by using other researcher's theoretical calculation result and experiment metrical data.For the gas acoustic experimental equipment, in this thesis, we summarized the structure merits and design proposals of overseas gas acoustic experimentation equipments, then designed and manufactured a prototype of gas acoustic experimentation equipment independently. Although the equipment is not finished, we still believe it has a high probability to be actualized. The design concepts of the prototype can be used for reference for other researchers.The achievements of the research work in this thesis are expansion and the innovation not only to theory of gas acoustic relaxation attenuation, but also to technologies of the concentration detection for gas mixtures based on acoustic relaxation theory. The algorithm of the concentration detection for gas mixtures based on acoustic relaxation theory had been considered to be applied in experiment, and it has a strong feasibility.