声学法直接测量高温热力学温度的探索

Practical thermometers are based on the thermophyiscal properties of the material, which means the temperature value is strongly influenced by the chosen material. The thermodynamic temperature, which is not related to the material of the thermometers, guarantees the physical truths of absolute temperature. The international temperature scale, which is established on the measurement of the thermodynamic temperature, is the best approximation to the thermodynamic temperature. However, new results from the acoustic thermometry shows that there is an obvious deviation between present accurate measurements and the international temperature scale at 600 K, which means the present temperature scale does not represent the true temperature. There is no available data at higher temperature. This project focuses on the fundamental scientific issues and techniques in high temperature acoustic thermometry with cylindrical resonator method. The research includes the influence of the temperature gradient in acoustic ducts, the perturbation from the imperfection of the cavity caused by thermal expansion at diffident temperatures, the heat transfer between the resonator and the complex boundaries, the design and optimization of the acoustic and microwave transducers and the design of the high temperature acoustic and microwave cavity. Then speed of sound in argon gas at different pressures will be measured along different isotherms at temperature up to 900 K. The thermodynamic temperature will be determined from the speed of sound data. The measurement of the thermodynamic temperature will contribute to the improvement of the international temperature scale. The understanding and new techniques will also help to explore practice acoustic gas thermometers in extreme environment.

通用温度计依据某一特定物质的性质进行测量，使得温度数值有赖于选定的物质。热力学温度与测温物质完全无关，本质上保证了温度的基本性和绝对性。基于热力学温度测量结果而建立的实用协议性国际温标，是对热力学温度的无限逼近。然而已有研究表明，国际温标与新近更高水平的声学法结果在600 K产生了明显偏差，意味着现有温标偏离了物理真实，而更高温区国际上尚无研究结果。本项目针对圆柱声学法测量高温热力学温度的科学问题和技术关键，在理论上深入分析导管内温度梯度对共振频率的影响，探索高温区热膨胀导致的腔体非理想性对声场和微波场的扰动机理，研究共鸣腔在复杂边界条件下的传热特性，实验中优化高温声学和微波传感器设计，自行研制高温共鸣腔，测量常温至900 K不同压力Ar声速，获得热力学温度。本项目将实现高温热力学温度的直接测量，研究成果用于国际温标的赋值，并为极端环境实用化声学温度计研制提供解决方案。

从2019年5月20日起，温度单位开尔文（K）将采用玻尔兹曼常数重新定义，这是温度计量史上最重大的变革之一。温度单位的重新定义，将开启温度测量的新篇章。未来的温度量值溯源体系，一方面基于国际温标（当前为ITS-90）进行传递，并行的另一个途径是，通过测量系统的平均能量，结合玻尔兹曼常数，直接复现热力学温度。本项目前瞻性的开展声学法直接测量热力学温度的研究，建立了基于圆柱谐振腔的高温热力学温度测量装置，包括精密声学和微波谐振腔、耐压压力舱、基于热管的精密恒温系统、声学和微波谐振频率测量系统、高纯气路系统、压力测量系统以及数据自动采集和控制系统等。发展了圆柱腔内声学和微波谐振频率的高信噪比测量方法，通过变径声波导管提高测量信噪比、降低导管对声学谐振频率的扰动，提出了声波导管的优化设计方法。深入研究了微波谐振法测量圆柱腔内长及热膨胀系数的方法；在903 K，对单一模式声学共振频率测量分辨率优于6E-6，对微波谐振频率测量分辨率优于5E-8。建立了超声共振谱法测量材料等温压缩系数的实验系统，测量了不同材料的等温压缩系数等弹性参数，分析了影响测量准确性的因素，测量的共振峰匹配误差（RMS）可小于0.04%。测量了不同压力和温度下氩气的折射率，获得了热力学温度，初步获得797 K热力学温度的相对标准不确定度为2.84E-5。同时，开展了实用声学温度计的探究，采用声波导管声学传感器，测量了圆柱腔内He从常温至1100 K的声学共振频率，重复性优于0.1%，结果显示，将声学温度计应用于我国第四代核反应堆高温气冷堆的温度测量极具前景。