Femtosecond Chirped-Probe-Pulse Coherent Anti-Stokes Raman Scattering for Thermometry in Dynamic Combustion Fields
Objective As China enters the middle and late stages of industrialization,there is an urgent need to advance energy system reform. Given the various challenges,transitioning to new clean energy sources as a primary energy supply in the short term is difficult. Therefore,enhancing the combustion efficiency of traditional energy sources and reducing combustion pollutants is crucial for advancing ecological civilization. Accurately understanding the chemical reaction kinetics and basic physics of combustion is key to designing and developing efficient combustion systems. Temperature plays a critical role in combustion efficiency and the formation of combustion products. Precise temperature measurement and regulation of the combustion state help minimize harmful exhaust gases such as carbon monoxide (CO) and oxides of nitrogen (NOx),while also improving combustion efficiency and saving energy. However,practical applications involve dynamic high-temperature combustion fields,which present challenges for temperature measurement methods in terms of time resolution,spatial resolution,accuracy,stability,and response speed. Traditional contact temperature measurement methods struggle to meet these requirements.Methods Coherent anti-Stokes Raman scattering (CARS) is a significant laser diagnostic technique for high-temperature gas or flame measurements. Nanosecond CARS thermometry is well-established,but its limitations,such as inelastic molecular collisions and low frequency,make it unsuitable for rapidly changing combustion fields like high-temperature turbulent flames. With the advent of femtosecond (fs) laser,femtosecond CARS has increasingly applied. This technique offers high spatiotemporal resolution,accuracy,and spectral acquisition efficiency,making it an efficient tool for measuring temperatures in complex dynamic high-temperature combustion fields. The coupled wave equation,incorporating material equations and medium susceptibility,is solved to derive the electric field of the CARS spectrum and establish the theoretical model. By constructing a model involving three beams and the molecular response,the fs CARS spectrum model is fully developed. In addition,the influence of key parameters on the fs CARS spectrum model for femtosecond CARS is also studied. Due to the sensitivity of femtosecond CARS in the time domain,a time-domain nonlinear optical model was also studied. In the experiment,a precise CARS optical platform with a phase-matching configuration is set up to demonstrate the subsequent progress.Results and Discussions In the experiments,the temperature of a hot fan chassis is measured first. With the spectrometer exposure time set to 0.01 s,1000 data sets at the same temperature (e.g.,679 K) are recorded and fitted (Fig. 7). Subsequently,the temperature of an unsteady combustion field,specifically the flame of a butane Bunsen lamp,is measured. The spectrometer exposure time is set to 0.1 s,collecting experimental spectra as superpositions of 100 pulse-generated signals. The temperature information from three points on the central axis of the butane Bunsen lamp flame is measured and fitted (Fig. 8). The accuracy and precision of the experimental temperature measurements are high (Table 1). To verify the high time resolution of the CARS temperature measurement system,measurements with a time resolution of 0.001 s are conducted,and data fitting is carried out (Fig. 9). Results show that the accuracy and precision of the temperature measurements at this resolution are still high (Table 2).Conclusions For temperature measurement in complex dynamic high-temperature combustion fields,femtosecond chirped-probe-pulse coherent anti-Stokes Raman scattering (fs CPP) is used with a time resolution of 1 ms. The measurement results are consistent with those obtained from thermocouples and previous literature,achieving an accuracy of 1.5％ and a precision of 4％. This technique enables millisecond-level temperature monitoring of dynamic high-temperature combustion fields. The simultaneous arrival of the pump and Stokes pulses at the probe volume and excites numerous Raman transitions,which initially oscillate in phase but then shift to their natural frequencies,leading to interference and decay of the Raman coherence. The CARS signal is generated as the chirped probe pulse interacts with this coherence. By introducing a glass rod into the optical system to chirp the probe pulse and broadening its duration to the picosecond level,the temporal decay of the Raman coherence is mapped to the CARS signal pulse frequency. Data from the fs CARS spectrum at different temperatures are collected and fitted to the fs CARS spectrum model to determine the flame temperature. The study verifies the accuracy and precision of fs CARS spectroscopy in high-temperature unsteady flame temperature measurement and provides a viable method for high-speed temperature measurement of large aero engines,supersonic engines,and small-scale high-temperature combustion devices.
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