Fluorescence lifetime measurements applied to the characterization of the droplet temperature in sprays

Obtaining accurate droplet temperature is essential to study heat and mass transfers in a wide range of spray applications such as spray cooling and spray combustion. A novel measurement technique based on the fluorescence lifetime is developed to tackle the challenge of measuring the droplet temperature in sprays. Similarly, to the intensity of the fluorescence signal, the fluorescence lifetime can vary with the temperature for some specific organic dyes, such as rhodamine B and kiton red. In the past, applications of laser-induced fluorescence (LIF) to the measurement of the temperature in sprays have been based exclusively on the fluorescence intensity by using intensity ratios of the signal detected by two or three photodetectors (PMTs, Cameras) operating at different spectral regions. Nonetheless, this approach is not straightforward, and corrections are usually required in dense sprays to mitigate some biases, which are arising in particular from the out-of-field fluorescence and multiple light scattering by the droplets. In contrast, measuring the fluorescence lifetime makes use of only a single detection spectral band and provides an absolute measurement unlike intensity-based measurements, which are always relative. In the present study, the time-correlated single-photon counting (TCSPC) is applied for the first time to measure the lifetime of the fluorescence emitted by droplets in a spray. The measurement technique provides the mean temperature of the liquid phase. No evidence of measurement biases could be pointed out during the tests performed at different injection conditions. Besides, a very high signal-to-noise ratio could be achieved even at a relatively far distance from the injection nozzle, resulting in an absolute error on the measured temperature that typically does not exceed ± 1 °C. Graphical Abstract