Design And Optimization Of Acoustic Waveguides For Acoustic Gas Thermometry

Temperature is a measure of how hot or cold of an object is and is also one of the basic quantities in the international system of units.At present,the thermometers used by humans are all measured according to the law of properties of certain substance changes with temperature,which relatively rely on the properties of the substance.In 1848,the British physicist William Thomson(Lord Kelvin)defined the thermodynamic temperature according to the second law of thermodynamics.In contrast with traditional temperature,the thermodynamic temperature has nothing to do with the measurable substance and is one of the important parameter in thermodynamics and statistical physics.The direct measurement of thermodynamic temperature is much more difficult.Based on the thermodynamic temperature measurement results,a protocol-based international temperature scale,such as the currently used ITS-90 international temperature scale,is an approach to the thermodynamic temperature.In recent years,with the technology of acoustic temperature measurement goes deeper and deeper,it has been found that the new thermodynamic temperature measurements have deviated from the internationally defined value of ITS-90 and increase with the acceleration of temperature.Therefore,there is an urgent need for new thermodynamic temperature measurements above 600 K in the world at present.The acoustic gas thermometry is currently the method with minimum uncertainty of measuring the thermodynamic temperature.The principle of temperature measurement is to determine the relationship between the sound velocity of the ideal gas and the thermodynamic temperature.The use of acoustic gas thermometry to perform thermodynamic temperature measurements from 600 K to 1358 K is one of the hot topics in the international temperature measurement community.However,it is difficult for existing acoustic sensors to work above 400 K,and temperature-resistant acoustic waveguides are required to introduce and extract acoustic signals into and out of the acoustic resonator.The design of acoustic waveguides is a key factor affecting both the signal-to-noise ratio of acoustic signals and the energy quality factor.This paper proposes a method to reduce the energy loss and pertubation of acoustic waves using adjustable acoustic waveguides.By establishing the model of attenuation and perturbation of adjustable acoustic waveguides,this paper compares and analyzes the energy attenuation of acoustic signals in acoustic conduits of different sizes and the acoustic resonance frequencies and half-width perturbations of the cylindrical resonator and obtained the optimized size of the ducts?The major contents and achievements of the study are fllows:(1)Based on the existing theory of the acoustic field energy characteristics and the disturbance characteristics of the acoustic duct with a single inner diameter,this paper innovative establishes the model of acoustic field energy and sound field disturbance in variable diameter acoustic duct.And produces the process in designing such variable diameter acoustic duct.This paper discovers the theoretical optimal size of the second order variable diameter acoustic duct under 300 K and 101 kPa.Also,the acoustic transmission energy attenuation factor is larger than 0.3 for the inner length of the 80 mm cylindrical cavity,and the half width perturbation for sound field frequency is less than 30 ppm.(2)Based on the model for acoustic duct with second order variable diameter,this paper builds up the model for the multi-step even conical continuous energy attenuation and sound field disturbance of variable diameter acoustic duct.The calculation and analysis show that the conical duct is superior to the second order variable diameter conduit in a single axial acoustic mode.Also,Maple software is used to compile relevant calculation program,which can achieve automatic calculation of attenuation and disturbance.(3)A complete measurement system is formed with acoustic resonator,acoustic frequency measurement system,automatic control and data acquisition system with support of variable diameter acoustic duct.The acoustic resonance experimental study on the cylindrical cavity of variable diameter acoustic duct is carried out under the condition of 293 K and 101 kPa.The results of three sets of acoustic ducts and theoretical calculations are in consistent.It shows the rationality and accuracy of the established model and the optimized calculation procedure.The optimization design of the variable diameter acoustic duct and the attenuation perturbation analysis model provide the theoretical basis for the accurate measurement of the thermodynamic temperature in the high temperature region.It will also provide the idea for the revision of the international temperature scale and the Boltzmann constant measurement research.This paper provides the technical support for the development of acoustic thermometer in pyrometry.