Single shot thermometry using laser induced thermal grating

摘要

With the concern of environmental protection and reducing the fossil fuel consumption, combustion processes need to be more efficient and less contaminable. Therefore, the ability to obtain important thermophysical parameters is crucial to combustion research and combustor design. Traditional surveying techniques were difficult to apply in a confined space, especially the physically intrusions of detectors can alter the combustion processes. Laser-based diagnostic techniques, like CARS, SVRS, PLIF and TDLAS, allow the in situ, non-intrusive, spatially and temporally resolved measurements of combustion parameters in hostile environments.We report here a new non-intrusive optical diagnostic technique, based on laser-induced thermal grating. Thermal gratings generated in NO_2/N_2 binary mixtures, arise from the nonlinear interaction between the medium and the light radiation from the interference of two pulsed, frequency-doubled Nd:YAG lasers (532 nm). This leads to the formation of a dynamic grating through the resonant absorption and the subsequent collisional relaxation. By the temporally resolved detection of a continuous wave, frequency-doubled Nd:YVO_4 probe laser beam (671 nm) diffracted by LITG. The temporal behavior of the signal is a function of the local temperature and other properties of gas, various parameters of the target gas can be extracted by analyzing the signal. The accurate single-shot temperature measurements were carried out at different test conditions using a stainless steel pressurized cell, data averaged on 100 laser shots were compared with simultaneously recorded thermocouple data, and the results were consistent with each other.The LITG signal is shown to grow with increasing the gas pressure and is spatially coherent, which makes the LITG thermometry technique a promising candidate in high pressure environments.