Fluorescence Lifetime Imaging Applied To The Mixing Between Two Non-Isothermal Sprays

Abstract

Droplets temperature is a key parameter for the study of heat and mass transfers in many spray applications (spray cooling, spray combustion, spray drying, spray scrubber for cleaning air and other gases of various pollutants and dust particles…). In this study, we develop a new method based on the measurement of the fluorescence lifetime to characterize the droplet temperature in sprays. The method is tested in the mixing region of two water sprays, which are injected with a significantly temperature difference. Time Correlated Single Photons (TCPSC) is applied to characterize the fluorescence decay in the time domain which is normally mono exponential for a fluorescent dye at a given temperature. For some well-chosen fluorescent dyes, like rhodamine B (RhB), the fluorescence lifetime strongly varies with the temperature. Hence, in the mixing region of two non-isothermal sprays, the fluorescence is expected to follow a biexponential decay. In this study, we discuss the possibility of using the fluorescence decay to determine the temperature of the two sprays as well as their respective volume fraction. In a first step, a simplified configuration is considered where the two sprays are seeded with eosin Y and rhodamine 6G separately. Since these dyes have very different lifetimes and no temperature dependence, the fluorescence decay changes with the mixing fraction alone. A calibration is necessary to evaluate the amount of liquid originating from each of the two sprays. The femtosecond laser used for the ultrashort excitation of the fluorescent molecules is focused inside the spray using a long-distance microscope lens to obtain two-photon absorption within a small volume of a few tens of microns. The out-of-field fluorescence usually affecting the measurements in dense sprays when one-photon absorption is used, is suppressed using this approach. In a second step, both the temperature and the mixing fraction are measured seeding the two sprays by rhodamine B only. The fluorescence decay is further analysed to determine the lifetime and the signal contribution of both sprays. Results show that the volume fraction of a given spray must exceed about 10% to make it possible to determine its temperature and its volume fraction with a reasonable accuracy (typically a few percent for the volume fraction and 2°C for the temperature). Simultaneous measurements of the two sprays temperatures and volume fractions provide a means to calculate the mixing temperature (the average between the temperatures of the two sprays weighted by their volume fraction). One of the benefits of the newly developed technique is to remove the problems inherent to intensity-based measurements. Given that rhodamine B has a more intense emission of fluorescence at low temperatures, intensity-based measurements are known to be deficient for evaluating the mixing temperature, the coldest spray always has a larger signal and therefore a greater importance in the measured temperature.