Temperature measurement in combustion flow field with femtosecond laser-induced filament

Abstract

Laser-based diagnostic techniques are critical nonintrusive methods of measuring the in-situ temperature in combustion flow fields. Developing temperature measurement techniques with high accuracy and precision is of great significance for studying the combustion. At present, nanosecond (ns) lasers are commonly used in these methods. However, the researches based on femtosecond (fs) lasers are relatively few. Here, we develop a thermometry technique for combustion fields based on fs laser-induced filament. When the fs laser propagates in an optical medium, a long uniformly distributed plasma channel (also named filament) will be generated. The clamped intensity inside the filament is high enough to generate excited atoms/molecules through fs laser-induced photochemical reactions. Subsequently, the excited atoms/molecules release fluorescence signals. The length of the filament can be measured by imaging the fluorescence signal with an ICCD camera, which is evaluated by the full width at half maximum (FWHM) of the spatial distribution of the filament emission signal. Based on theoretical analysis, the experimental data of the filament length are fitted with a power function, and the result is satisfactory compared with the <i>R</i>-squared measure of goodness (<i>R</i>²) of 0.984. This indicates that the filament length is correlated well with the temperature of the combustion field. A monotonic quantitative relationship between the filament length and the temperature can be established by a calibration process, and then the temperature of the combustion field can be measured. When the temperature changes from 1630 to 2007 K, the length of the filament shortens by 38%. This indicates that the filament length is sensitive to the temperature of the flow field. When the temperature is 2007 K, the absolute uncertainty of the measurement is ±25 K, and the relative uncertainly is about 1.2%. The spatial resolution of the measurement system is 50 μm, which was determined by a USAF 1951 Target. Based on the spatial resolution, the measurement precision can arrive at 17 K. Although, at present, this temperature measurement technique based on femtosecond laser-induced filament is used only in laminar premixed flames, it has potential applications in temperature measurements ranging from room temperature to combustion temperatures.