Study On Flame Temperature Measurement Based On CARS

Flame temperature is important for rocket engine evaluation. With the development of propulsion technology, higher demand for flame temperature measurement is put forward. This paper is to develop a Coherent Anti-Stokes Raman spectroscopy (CARS) temperature measurement system which can be used for estimating combustion efficienty of the liquid-propellant rocket engine, analyzing combustion mechanism, and providing modified design for rocket engine.Relative theory, methods and theoretical calculations of spectra are systematically studied. On this basis, the single-pulse dual-pump CARS technique combining reference optical system has been successfully applied to the measurement of temperature field for high-temperature combusting flame, with in-depth analysis of the impact of possible factors that may affect the measurement accuracy. The main researches are listed as follows:1. For the needs of this research, we have developed a single-pulse double-pumped BOXCARS measuring device include the reference optical circuit, which can perform real-time measurement of high-temperature combusting flame flow field with very high temperature gradient and great instability, and also can conduct effective suppression of non-resonant background noise. Since dual-pump methods ensure a multi-component measurement capability, with the premise of appropriate adjustments in the output frequency of the light source, we can obtain two components of the CARS spectra simultaneously. Then we can not only obtain the temperature information by fitting with the theoretical spectrum, but also can obtain the concentration information by comparing with non-resonant background signal. Finally we proposed to design CARS measurement equipment for testing the accuracy of the equipments.2. After in-depth analyzing the basic theory of CARS, we also simplified and developed the theoretical model of CARS appropriately: From the basis of nonlinear optical theory, we have derived the coupled-wave equations of CARS signal intensity, presented the expression of CARS light signal intensity, and also discussed the factors that may affect the light signal intensity such as light source linewidth, light source linetype, phase-matching conditions, coherence length and third-order nonlinear polarizability. Then we derived the mathematical model of third-order nonlinear polarizability by using density matrix. By comparison of resonant polarizability with non-resonant polarizability, the role of non-resonant polarization in the formation of optical signal is analyzed, focusing on the calculation of spectral resonance polarizability. Also we have elaborated the calculation of Raman linewidth, Raman scattering cross section, transitions between energy levels of the particle number density difference that are crucial in the third-order nonlinear susceptibility expressions.3. Based on the theoretical model of CARS, we have in-depth discussed the multi-color CARS method extended from the traditional CARS measurement method and the Electronic-Resonance-Enhanced method (ERE). The principles of dual-pump CARS and dual-broadband CARS approaches are investigated, followed by the analysis of its feasibility and advantages over traditional methods before the final theoretical model is given. In terms of the existing problems of dual-pump methods, we have designed the methods to improve the performance and extend the applicable scope, as well as the dual-pump-based measurement methods. Based on the detailed calculation of phase matching based on dual-broadband methods, it is proved that the three-beam phase matching relationship is bound to hold provided that the two-beam phase matching relationship holds. Based on the discussion of principles of ERE-CARS, we have elaborated the advantages and applications of ERE-CARS and also derived the third-order nonlinear polarizability model of nitric oxide molecules based on the ERE-CARS method, as well as designed the scheme of ERE-CARS measurement that can be applied to measuring carbon monoxide in extremely low concentration circumstances based on CARS measurements.4. For the experiment object of butane / air diffusion flame that is generated by flame spray gun PT-500, we have carried out the experiments with nitrogen as the probe molecular. The measurement results of the proposed method and that of thermocouple measurement are compared and the factors that may contribute to the difference were analyzed. The experimental results show that, the CARS measurement error of temperature is about 70K when the temperature is higher than 800K and about 40K when the temperature is between 500K and 800K.Researches in this paper are valuable for rocket flame temperature distribution determination.