High precision temperature measurements for reacting flows

低排放燃气轮机的有效和快速设计关键取决于工程师对燃烧室内气体温度做出准确和精确预测的能力。该项目的目标是在模型和工业规模的燃烧室中产生有史以来最高精度和精确度的瞬时温度测量。这些精密测量的目的不仅是为工业和学术用户验证模型提供基础，也是为了开发一种低成本、高精度的测温技术，以便在现实的燃烧室中部署。控制连续流燃烧器设计的两个关键因素是保持低排放--特别是氮氧化物--以及使系统远离热声不稳定性。燃烧气体温度的空间分布和统计分布是控制一氧化氮(NO)生成的最重要因素：在典型燃烧温度下，50K的局部变化可以导致热NO生成率变化70%。由于缺乏关于温度的统计和空间信息，排放预测模型的有效性受到限制。热声不稳定性是由一种反馈效应造成的，在这种反馈效应中，在受限环境中燃烧过程中流动的非定常加速产生的声波导致热释放的进一步不稳定。与火焰相关的两个因素是重要的：火焰对声学扰动的响应和温度不均匀(称为熵斑)的产生：前者直接导致密度涨落和声波，后者耦合边界条件以压力波的形式反射。确定燃烧不稳定性的原因是复杂的，因为几个因素可以引起波动，但通常只有压力信息可用，有时借助化学发光的相对总放热波动。然而，无论是模型还是工业规模的燃气轮机火焰，温度的统计测量都相对少见，因为依赖于信号幅度校准的物理探头或光学方法存在困难。建议的测量不依赖于幅度，而是依赖于信号频率的测量，与同类技术相比，信号频率的测量可以显著更精确(误差低至0.2%)。此外，目前的测量将能够用单一激光直接同时测量NO和温度，从而为模型验证创建一个独特的统计数据库。最后，这项技术将能够通过喷嘴以非常高的精度测量温度波动，这是以前从未做过的。高精度的测量将直接影响对NO和不稳定性的模型预测的质量，并在转化为设计规范时，转化为更清洁、更稳定的动力和推进系统的设计。

The effective and fast design of low emission gas turbines depends critically on the ability of engineers to make accurate and precise predictions of gas temperatures within the combustion chamber. This project aims to produce instantaneous temperature measurements of the highest accuracy and precision ever in model and industrial scale combustors. These precision measurements aim not only provide the basis for validation of models by industrial and academic users, but also to create a path for development of a lower cost, high precision thermometry technique for deployment in realistic combustors. The two key factors governing the design of continuous flow combustors are maintaining low emissions - particularly nitric oxides - and keeping the system away from thermoacoustic instabilities. The spatial and statistical distribution of burned gas temperatures is the single most important factor governing the formation of nitric oxide (NO): a local change of 50 K can lead to a change of 70% in thermal NO formation rates at typical combustion temperatures. Validation of emission prediction models is hemmed by the lack of availability of statistical and spatial information on temperatures. Thermoacoustic instabilities are created by a feedback effect in which acoustic waves generated by the unsteady acceleration of the flow during combustion in a confined environment lead to further unsteadiness in heat release. Two factors associated with the flame are important: the response of the flame to acoustic perturbation, and the generation of temperature non-uniformities (called entropy spots): the former leads directly to density fluctuations and acoustic waves, and the latter couple the boundary conditions to reflect as pressure waves. The identification of the origin of combustion instabilities is complex, as several factors can contribute fluctuations, yet usually only pressure information is available, sometimes aided by relative total heat release fluctuations via chemiluminescence. Nevertheless, statistical measurements of temperatures in either model or industrial scale gas turbine flames are relatively uncommon, because of difficulties with physical probes or optical methods relying on calibration of signal amplitudes. The proposed measurements do not rely on amplitudes, but on the measurement of signal frequency, which can be made significantly more precisely (down to errors of 0.2%) than comparable techniques. Furthermore, the present measurements will enable the direct simultaneous measurements of NO and temperature with a single laser, thus creating a unique statistical database for model validation. Finally, the technique will enable for the measurement of temperature fluctuations through a nozzle at very high precision, which has not been done previously. The high precision measurements will have a direct impact on assessing the quality of model predictions for NO and instabilities, and when translated into design codes, into the design of cleaner and more stable power and propulsion systems.