Thermometry in dynamic and high-temperature combustion filed based on hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering

Abstract

Temperature, as an important parameter in combustion diagnostic process, will directly affect the combustion efficiency and the generation of combustion products. The accurate measuring of combustion temperature and then controlling of combustion state can not only contribute to avoiding the generation of harmful waste gas, such as carbon monoxide (CO) and oxynitride (NO_<i>x</i>), but also improve the combustion efficiency, thereby saving the energy. However, in practical applications, dynamic and high-temperature combustion field has strict requirements for measurement accuracy and response speed of the thermometry technology. As an advanced spectral thermometry technology, coherent anti-Stokes Raman scattering (CARS) has a much higher spatial resolution, and can achieve accurate temperature measurement in high-temperature environment, so CARS has the potential applications in complex combustion field. For the temperature measurement requirements in the complex dynamic and high-temperature combustion field, we demonstrate a hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering thermometry method through using the second harmonic bandwidth compression method, and achieve accurate measurements and dynamic response to temperature in dynamic and high-temperature combustion field. By using the narrow-band picosecond pulse obtained from the sum frequency process of femtosecond pulse in the BBO crystal as a probe pulse, this thermometry method can achieve single-shot, 1-kHz temperature measurement in high-temperature flame. We utilize the standard burner to simulate dynamic combustion field in a range of 1700–2200 K by changing the equivalence ratio quickly, and carry out continuous temperature measurement in 70 s by our thermometry method in this simulated dynamic and high-temperature flame. The least square method is used to fit the theoretical spectrum library to the actual single spectrum, and the fitting temperature corresponding to the actual single spectrum is obtained from the curve of fitting error. The continuous temperature measurements in 70 s exhibit superior performance in dynamic and high-temperature flame with a temperature inaccuracy less than 1.2% and a precision less than 1.8% at four different temperatures, and can track the temperature variation process within 0.2 s dynamically. These results verify the accuracy, stability and response speed in dynamic and high-temperature environment, and provide a new system scheme for thermometry in practical harsh combustion field.