Temperature Dependence Of Phosphorescence Lifetime Research Articles

Simultaneous measurements of droplet size, flying velocity and transient temperature of in-flight droplets by using a molecular tagging technique

In the present study, a molecular tagging technique is introduced to achieve simultaneous measurements of droplet size, flying velocity and transient temperature of in-flight liquid droplets in a spray flow. For the molecular tagging measurements, a pulsed laser is used to “tag” phosphorescent 1-BrNp·Mβ-CD·ROH triplex molecules premixed within liquid droplets. After the same laser excitation pulse, long-lived laser-induced phosphorescence is imaged at two successive times within the phosphorescence lifetime of the tagged phosphorescent triplex molecules. While the sizes of the droplets are determined quantitatively based on the acquired droplet images with a precalibrated scale ratio between the image plane and the object plane, the displacement vectors of the in-flight droplets between the two image acquisitions are used to estimate the flying velocities of the droplets. The simultaneous measurements of the transient temperatures of the in-flight droplets are achieved by taking advantage of the temperature dependence of phosphorescence lifetime, which is estimated from the intensity ratio of the acquired phosphorescence image pair of the inflight droplets. The feasibility and implementation of the molecular tagging technique are demonstrated by conducting simultaneous measurements of droplet size, flying velocity and transient temperature of micro-sized water droplets exhausted from a piezoelectric droplet generator into ambient air at different test conditions in order to characterize the dynamic and thermodynamic behaviors of the micro-sized in-flight droplets. The unsteady heat transfer process between the in-flight droplets and the ambient air is also analyzed theoretically by using a lumped capacitance method to predict the temperature changes of the in-flight water droplets along their flight trajectories. The measured temperature data are compared with the theoretical analysis results quantitatively, and the discrepancies between measurement results and the theoretical predictions are found to be within 0.80 °C.

Experimental investigations of micro-scale flow and heat transfer phenomena by using molecular tagging techniques

Recent progress made in the development of novel molecule-based flow diagnostic techniques, including molecular tagging velocimetry (MTV) and lifetime-based molecular tagging thermometry (MTT), to achieve simultaneous measurements of multiple important flow variables for micro-flows and micro-scale heat transfer studies is reported in this study. The focus of the work described here is the particular class of molecular tagging tracers that relies on phosphorescence. Instead of using tiny particles, especially designed phosphorescent molecules, which can be turned into long-lasting glowing marks upon excitation by photons of appropriate wavelength, are used as tracers for both flow velocity and temperature measurements. A pulsed laser is used to ‘tag’ the tracer molecules in the regions of interest, and the tagged molecules are imaged at two successive times within the photoluminescence lifetime of the tracer molecules. The measured Lagrangian displacement of the tagged molecules provides the estimate of the fluid velocity. The simultaneous temperature measurement is achieved by taking advantage of the temperature dependence of phosphorescence lifetime, which is estimated from the intensity ratio of the tagged molecules in the acquired two phosphorescence images. The implementation and application of the molecular tagging approach for micro-scale thermal flow studies are demonstrated by two examples. The first example is to conduct simultaneous flow velocity and temperature measurements inside a microchannel to quantify the transient behavior of electroosmotic flow (EOF) to elucidate underlying physics associated with the effects of Joule heating on electrokinematically driven flows. The second example is to examine the time evolution of the unsteady heat transfer and phase changing process inside micro-sized, icing water droplets, which is pertinent to the ice formation and accretion processes as water droplets impinge onto cold wind turbine blades.

Simultaneous Measurements of Droplet Size and Transient Temperature Within Surface Water Droplets

A lifetime-based molecular tagging thermometry technique was developed for achieving simultaneous measurements of droplet size and transient temperature distribution within a small water droplet over a solid surface. For molecular tagging thermometry measurements, a pulsed laser is used to tag phosphorescent 1-BrNp · Mβ-CD ·ROH molecules premixed within a small water droplet Long-lived laser-induced phosphorescence is imaged at two successive times after the same laser excitation pulse. While the size of the water droplet (in terms of volume, height, contact area, and contact angle of the water droplet) can be determined instantaneously from the acquired phosphorescence images, the transient temperature measurement is achieved by taking advantage of the temperature dependence of phosphorescence lifetime, which is estimated from the intensity ratio of the acquired phosphorescence image pair. The feasibility and implementation of the lifetime-based molecular tagging thermometry technique were demonstrated by conducting simultaneous measurements of droplet size and spatially and temporally resolved temperature distribution inside a convectively-cooled microsized water droplet over an aluminum test plate to quantify the unsteady mass and heat transfer processes within the small water droplet to elucidate underlying physics to improve our understanding about microphysical phenomena associated with aircraft icing studies.

Molecular tagging velocimetry and thermometry and its application to the wake of a heated circular cylinder

We report improvements to the molecular tagging velocimetry and thermometry (MTV&T) technique for the simultaneous measurement of velocity and temperature fields in fluid flows. A phosphorescent molecule, which can be turned into a long lifetime tracer upon excitation by photons of appropriate wavelength, is used as a tracer for both velocity and temperature measurements. A pulsed laser is used to ‘tag’ the regions of interest, and those tagged regions are imaged at two successive times within the lifetime of the tracer molecules. The measured Lagrangian displacement of the tagged molecules provides the estimate of the fluid velocity vector. The simultaneous temperature measurement is achieved by taking advantage of the temperature dependence of phosphorescence lifetime, which is estimated from the intensity ratio of the tagged molecules in the two images. In relation to the original molecular tagging thermometry work of Thompson and Maynes (2001 J. Fluid Eng. 123 293–302), the improvements reported here are the use of lifetime imaging as a ratiometric method to enhance the robustness and accuracy of temperature measurements and the extension of the technique to simultaneous whole-field planar mapping of velocity and temperature fields. Compared with other simultaneous velocity and temperature measurement techniques such as combined PIV-LIF (Sakakibara et al 1997 Int. J. Heat Mass Transfer 40 3163–76, Grissino et al 1999 Proc. 3rd Int. Workshop on Particale Image Velocimetry (Santa Barbara, CA, USA, 16–18 September 1999)) and the DPIV/T technique (Park et al 2001 Exp. Fluids 30 327–38), this method accomplishes the same objectives but with a completely molecular-based approach. Because of its molecular nature, issues such as tracking of the flow by the seed particles and the thermal response of the thermal tracer particles are eliminated. In addition, the use of a single molecular tracer and a dual-frame CCD camera provides for a much reduced burden on the instrumentation and experimental set-up. The implementation and application of the new technique are demonstrated by conducting simultaneous velocity and temperature measurements in the wake region of a heated circular cylinder at a Richardson number of 0.36, a value large enough for the buoyancy effects to potentially influence the flow.

Temperature dependence of luminescence decay time of benzyl

The temperature dependence of phosphorescence lifetime in benzyl polycrystals and solid solution in polymethyl methacrylate (PMMA) is analyzed in the temperature range from 4.2 to 293 K. The temperature dependence of the phosphorescence decay probability in both crystalline benzyl and benzyl solid solution in PMMA is defined by two processes: (1) intramolecular radiationless transitions and (2) triplet state de-activation associated with the intermolecular interactions. The special features of triplet exciton migration in crystalline benzyl and energy transfer of electronic excitation from the triplet benzyl level to that of PMMA are discussed relative to the latter mechanism. The change of the benzyl molecule conformation in the excited state has been considered. The peculiarities of luminescence decay kinetics at low temperatures are found which are explained by the phenomenon of optical spin alignment in the benzyl excited triplet state.