Instantaneous temperature imaging by planar laser-induced fluorescence. Effect of high laser irradiance

The temperature dependence of the fluorescence emission of certain organic dyes is utilized for measuring the temperature in liquid flows. Measurements are generally based on the assumptions that: the fluorescence signal is proportional to the intensity of the laser excitation, and the temperature sensitivity of the dye is not affected by the laser irradiance. These assumptions allow justifying the use of ratiometric methods. The influence of the laser intensity can be totally eliminated in the intensity ratio of two spectral bands of the fluorescence emission, and thus temperature measurements are not biased, when the laser propagation is disturbed in the flow. However, when pulsed lasers are utilized (mainly in planar LIF measurements), the peak irradiance usually compares or exceeds the saturation intensity of the fluorescent dyes. The present study assesses the consequences of a saturation of the dye emission on temperature measurements. Tests among fluoresceins and rhodamines reveal that the saturation can be accompanied by a significant loss of temperature sensitivity. The dyes, for which this loss of sensitivity is observed, mainly owe their temperature dependence to the fluorescence quantum yield and have a fluorescence signal decreasing with the temperature. The couple of dyes fluorescein/sulforhodamine 640 is proposed for an implementation of the ratiometric method in the saturated and partially-saturated regimes of excitation, since its high temperature dependence (+3 %/°C) is not altered at high laser irradiances. The possibility of measuring instantaneous temperature fields with this pair of dyes using a single laser shot is finally demonstrated on a turbulent heated jet injected into quiescent water.