RESUMO
In this paper, three ultrashort-pulse coherent anti-Stokes Raman scattering (CARS) thermometry approaches are summarized with a theoretical time-domain model. The difference between the approaches can be attributed to variations in the input field characteristics of the time-domain model. That is, all three approaches of ultrashort-pulse (CARS) thermometry can be simulated with the unified model by only changing the input fields features. As a specific example, the hybrid femtosecond/picosecond CARS is assessed for its use in combustion flow diagnostics; thus, the examination of the input field has an impact on thermometry focuses on vibrational hybrid femtosecond/picosecond CARS. Beginning with the general model of ultrashort-pulse CARS, the spectra with different input field parameters are simulated. To analyze the temperature measurement error brought by the input field impacts, the spectra are fitted and compared to fits, with the model neglecting the influence introduced by the input fields. The results demonstrate that, however the input pulses are depicted, temperature errors still would be introduced during an experiment. With proper field characterization, however, the significance of the error can be reduced.
RESUMO
The physics of combusting flows consists of a complex interaction between chemical reactions, fluid mechanics and radiation. Temperature is one of the most important parameters for the processes. Laser-based imaging techniques are frequently used to assess temperature information from reactive systems without perturbing the system under study. To verify the feasibility of the temperature measurement of UV tunable absorption spectroscopy technology the methane/air premix flat flame was selected as the target of test because of the combustion stability of this kind of flame. Before the temperature measurement the distribution of OH radical in the premix flat flame under different operating conditions were obtained by using planar laser induced fluorescence (PLIF). At the low equivalence ratio the OH radicals distribute uniformly in the flame for the adequate oxygen in the premix gas. The condition with uniform distribution of OH in the flame was selected for the UV tunable absorption spectroscopy measurement. For the selection of absorption lines of the measurement the spectrum of OH A-X(0,0) band have been simulated by LIFBASE. Considering the slope sensitivity and SNR of the test the transitions P1(2) and Q1(8) were suitable for the temperature measurement of the flame. A dye laser pumped by a frequency doubled Nd:YAG laser was used to generated the UV laser. The dye laser was operated with the mixed dye of DCM and PM580 for high conversion efficiency at 310 nm. To investigate the transitions of Q1(8) and P1(2) of OH A-X(0,0) the laser was tuned from 309.225~309.255 and 308.625~308.655 nm separately with the step of 0.4 pm, 30 pulses were recorded at each step. The laser pulses reflected by the beam splitter were collected by detector A, and the pulses passed the flame were collected by detector B. The signal of these two detectors were recorded by the oscilloscope and acquired by the computer automatically. The line shape of these transitions can be obtained after fitting the experimental data with the Voigt function. The integral ratio between the fitting results of these two lines was calculated. Then temperature of the flame could be deduced by the integral ratio. The temperatures of different positions above the surface of burner and varied heights of the flame center were obtained by measuring the integrated absorption ratio of these two transitions. The test results of this method are compared with the report in reference, in which temperature of the burner with the same structure was measured by other ways. The results of these two tests agree well. It shows that this method has the potential to be a calibration for the two-dimension thermometry in flame such as two-line PLIF.
