Study of the droplet size effect coupled with the laser light scattering in sprays for two-color LIF thermometry measurements

The present paper deals with a new optical diagnostic that aims to perform droplet temperature measurements based on the two-color laser-induced fluorescence (2cLIF). The 2cLIF requires a single tracer, and the fluorescence intensity is collected on two spectral bands. The ratio of both intensities measured in the case of single monodisperse droplets depends in principle only on the temperature. However, the application in the case of polydispere sprays failed. Indeed, a dependence of the droplet size correlated to the depth of field of the optics, used for fluorescence collection, induces a bias in the ratio value. This work analyses these coupled effects by using combined LIF and Phase Doppler Analyser measurements. This device allows achieving fluorescence ratio measurements per droplet size class. The study is conducted with two different sprays in terms of particles size and density. The use of a third spectral band of detection (3cLIF) and a long distance microscope leads to correct the size effect and reduce the depth of field effect, respectively. These investigations are then demonstrated by measuring temperatures in an overheated water spray.