Hydrogen Gas Mixtures Sensing Technology Based On Acoustic Relaxation

Acoustic velocity dispersion and absorption spectrum of gas mixtures depend on the gas compositions.Based on this characteristic,acoustic gas sensing technologies can identify gas mixtures with the advantages such as simple,fast,low power consumption,no chemical reaction and no calibration,which has broad applications in industrial production.However,the existing acoustic relaxational theory for gas sensing is based on molecular vibration relaxation of gases,which cannot detect gases with rotational relaxation such as hydrogen and hydrogen gas mixtures.This deficiency constrains the development of acoustic relaxation technology in gas sensing.In this paper,we develop the rotational relaxation model of hydrogen,and combine it with the existing vibrational relaxation model to construct the mixed model for sensing hydrogen gas mixture.We focus on the following aspects:Firstly,the existing theories of acoustic relaxation are only applicable to gas molecules of vibrational relaxation,and cannot differentiate hydrogen molecules of rotational relaxation.To solve the challenge,based on the relationship between the ideal gas enthalpy and the isostatic heat capacity,we propose a molecular rotational relaxation model of hydrogen gas,and also discuss the similarities and differences between vibration relaxation and rotation relaxation.On this basis,combined the two relaxation models,a mixed relaxation model is presented in this paper.The simulation results verify the effectiveness of our model.The model provides the acoustic theoretical foundation for the detection of hydrogen gas mixtures.Secondly,the existing acoustic relaxation theory cannot explain rotational relaxation mechanism of hydrogen.To overcome the problem,we decouple the proposed relaxation model of hydrogen and extract the contribution of each decoupled relaxation process to the whole acoustic relaxation process.It also proves that the model can be simplified as a sum of some decoupled single rotational relaxation processes.On this basis,combined with the existing decoupling vibration model,a mixed decoupling model is proposed for hydrogen gas mixtures.This decoupling model reveals the influence of gas molecular relaxation on macro acoustic characteristics and calculate qualitatively and quantitatively the relationship between molecular relaxation and gas compositions.Finally,hydrogen gas mixtures are detected by our decoupling model.Thirdly,the measurements of acoustic relaxation absorption coefficient have the following problems: it needs complex device and has low accuracy;it is difficult to measure at high frequency since the signal of acoustic relaxation spectrum is submerged in large noise(the classical acoustic absorption).To overcome above challenges,we present the sensing method of locating the inflection point of acoustic velocity dispersion as follows: firstly,we capture the inflection point of the target gases by only measuring acoustic velocities at fixed frequencies;secondly,the standard detection areas of the inflection points are constructed based on our proposed decoupling model;then the measured inflection point is located into the detection areas to obtain the gas compositions qualitatively and quantitatively;finally,the measurement errors are corrected by ambient temperature since the location of the inflection points are scaled by the acoustic velocities and relaxation frequencies with the different temperatures.Finally,the existing experimental device is single-function and only measures acoustic characteristics of gas in a negative pressure.To overcome this problem,a prototype of acoustic gas sensor is developed,which not only performs measurement in a negative pressure experiment,but also in a positive pressure within 30 atm.Seven pairs of ultrasonic transducers are used to cover the frequency range of measurement.In addition,a camera with own light source inside the gas chamber is equipped,which can monitor the internal situation.The dual heating modules both inside and outside the chamber can raise the temperature to achieve constant temperature control.The fan inside the chamber can deal with gas mixtures evenly.Finally,aimed at problems such as few experimental data and small range of gas pressure,acoustic velocity data of gas mixtures are measured at the pressure range from 0.6 atm to 8 atm.Data are also processed and measurement errors are discussion.The above work provides an experimental support for our theoretical research,such as locating the inflection point of frequency-dependent velocity dispersion by acoustic relaxation to identify gas mixtures.In conclusion,we propose the model of hydrogen gas mixture based on acoustic rotation relaxation and decouple it.Using the models can analyze the internal mechanism of rotation relaxation processes of hydrogen molecules.This theoretical research makes up for the deficiency that the existing relaxation theories are not applicable to hydrogen.On this basis,the gas sensing method of the location of the inflection point of acoustic velocity dispersion is proposed,namely the inflection point is located into effective detection areas to detect hydrogen gas mixtures.The method promotes the practical application of acoustic gas sensing in industry.And the new developed equipment device provides effective experimental support for our theoretical research.