Two-photon fluorescence lifetime imaging applied to the measurement of the droplet temperature in sprays, Measurement of instantaneous fully 3D scalar dissipation rate in a turbulent swirling flow

Abstract

Droplets temperature is a key parameter for the study of heat and mass transfers in many spray applications. Time correlated single photons counting (TCSPC) is applied to monitor the fluorescence decay and determine the droplet temperature in the mixing zone of two sprays which are injected with significantly different temperatures. For some well-chosen fluorescent dye, like rhodamine B (RhB), the fluorescence lifetime strongly varies with the temperature. Provided sufficiently different fluorescence lifetimes for the droplets of the two sprays, the fluorescence decay is expected to follow a multiple exponential decay. In this study, different approaches are tested for measuring the temperature of the two sprays as well as their mixing fraction based on the analysis of the fluorescence decay. Firstly, the measurement of the mixture fraction alone is tested by considering a configuration where one spray is seeded with eosin Y (EY) and the other with rhodamine 6G (Rh6G). Given the very different lifetimes of these dyes, which are not temperature dependent, the fluorescence decay is function of the volume fraction of liquid from each spray in these tests. A calibration is necessary to evaluate the mixing fraction. Both sprays are mounted on an automated platform allowing 3D scanning and motions which allows obtaining maps of the fluorescence decay. The out-of-field fluorescence, observed in dense sprays when fluorescence is induced by one-photon absorption, is suppressed by using a two-photon fluorescence excitation. This approach significantly improves the spatial resolution of the measurements. Finally, both the droplet temperature and the mixing fraction are measured simultaneously using a single dye, namely RhB, whose fluorescence lifetime is temperature dependent. Special care must be paid to the fact that RhB does not have a purely monoexponential decay at a given temperature. The fluorescence decay in the mixing zone of the two sprays is considered as a combination of two biexponentials. Results show that the volume fraction of a spray must exceed about 10% to make it possible to determine its temperature with an accuracy of about 2–3 °C. Simultaneous measurements of the 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 fractions). This paper describes the measurement methodology for quantifying the instantaneous full 3D scalar dissipation rate (SDR) in order to characterize the rate of mixing. Measurements are performed in a near field of a jet-in-swirling-coflow configuration. All three components of SDR are measured using a dual-plane acetone planar laser-induced fluorescence technique. To minimize noise, a Wiener filtering approach is used. The out-of-plane SDR component is validated by assuming isotropy between axial and azimuthal components of SDR. An optimum laser-sheet separation distance is identified by comparing the SDR components on the basis of instantaneous, mean, and probability density function data. The in-plane resolution needs to match the Batchelor scale for the central difference scheme-based SDR deduction. However, the out-of-plane resolution requirement is different owing to the use of two-point difference based SDR and systematic biases. The optimum out-of-plane resolution is found to be 2.5xBatchelor scale. Finally, measurement guidelines are provided to assess the accuracy of 3D SDR measurements.