Ex situ calibration technique for simultaneous velocity and temperature measurements inside water droplets using temperature-sensitive particles

Phosphorescence-based temperature measurements usually employ in situ calibration in macro-scale flow domains. However, the application of conventional in situ calibration in millimetre scales such as in 2 mm droplets is difficult because temperature probing using an intrusive technique such as thermocouple-based measurement can deform a droplet interface and even its integrity. Therefore, in this paper we propose an ex situ calibration technique for combined 2D fluid temperature and flow velocity measurements inside pendant droplets using thermographic phosphorescence and particle image velocimetry. This calibration technique has the potential to perform measurements not only inside droplets but also in other small-scale fluid domains that are sensitive to intrusive temperature probing. To develop this technique, the effect of local phosphorescence intensity on the phosphorescence decay constant (initial intensity effect) was studied and quantified. We observed that the phosphorescence light intensity of the first image among the decay image series affects the local decay constants as the coefficient of a half sigmoid function. This novel ex situ calibration technique was used to investigate high-spatial-resolution temperature and flow velocity distributions inside a pendant water droplet located in an air stream, and an anti-distortion algorithm was used to correct the optical distortion induced by the curved surface of the droplet. The measured temperature and velocity fields were found to be reasonable and consistent with those obtained in previous studies. The proposed technique is expected to be useful in conducting further investigations on the mechanisms of heat and mass transfers between droplets and carrier gases. Furthermore, it can potentially improve our understanding of the heat and mass exchange mechanisms within a droplet's internal flow structures and temperature gradients. (paper)