Experimental study of temperature gradient inside an IR heated oil droplet by the use of Planar 2c LIF

Fluorescence-based methods (Laser Induced Fluorescence, Planar-LIF, 2 Colours LIF…) represent attractive means for studying liquids (flows or droplets) in a non-intrusive way. For example, the dependency of fluorescence on temperature makes possible the visualization of the entire temperature field within a droplet or a flow. The utilization of ratiometric methods based on the ratio of the fluorescence measured at two different wavelengths (also called 2 colours LIF), allows one to disregard the variations of some experimental parameters, such as incident laser power, dye concentration, measurement volume. That is particularly relevant in the case of evaporating droplets where, for example, significant variations in the concentration of the dye and of the measurement volume can be expected (here droplets are of millimetric size, with a typical D0 around 2 mm). The present study focuses on the application of 2 colours LIF for temperature field measurements within n-tetradecane droplet radiatively heated. To this aim, a fluorescent tracer miscible with hydrocarbons (BOD-Me) was completely characterized in terms of spectral bands of absorption and fluorescence, dependency of fluorescence signal with temperature in the range [20°C- 200°C] prior to its use for temperature measurements within n-tetradecane droplets. In the experimental conditions studied, the temperature inside droplets appear quite homogenous, with a maximal temperature gradient of 15°C in a droplet at vaporization temperature. Precision on temperature measurements is estimated with a +/- 7°C uncertainty. Internal Rayleigh number is estimated during the heating phase, with Ra around 10 000, where the typical critical value in this type of configuration is Racrit = 1700.