Journal
JOURNAL OF QUANTITATIVE SPECTROSCOPY & RADIATIVE TRANSFER
Volume 214, Issue -, Pages 146-157Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.jqsrt.2018.04.034
Keywords
Phase rainbow refractometry; Droplet evaporation rate; Temperature; Size change; Refractive index
Categories
Funding
- German Research Foundation (DFG) [SPP 1934, MA 3333/12-1]
- UK's Engineering and Physical Science Research Council [EP/K020528/1, EP/M009424/1]
- Engineering and Physical Sciences Research Council [EP/K020528/1, EP/M009424/1] Funding Source: researchfish
- EPSRC [EP/K020528/1, EP/M009424/1] Funding Source: UKRI
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The accurate measurements of droplet temperature, size and evaporation rate are of great importance to characterize the heat and mass transfer during evaporation/condensation processes. The nanoscale size change of a micron-sized droplet exactly describes its transient mass transfer, but is difficult to measure because it is smaller than the resolutions of current size measurement techniques. The Phase Rainbow Refractometry (PRR) technique is developed and applied to measure droplet temperature, size and transient size changes and thereafter evaporation rate simultaneously. The measurement principle of PRR is theoretically derived, and it reveals that the phase shift of the time-resolved ripple structures linearly depends on, and can directly yield, nano-scale size changes of droplets. The PRR technique is first verified through the simulation of rainbows of droplets with changing size, and results show that PRR can precisely measure droplet refractive index, absolute size, as well as size change with absolute and relative errors within several nanometers and 0.6%, respectively, and thus PRR permits accurate measurements of transient droplet evaporation rates. The evaporations of flowing single n-heptane droplet and monodispersed n-heptane droplet stream are investigated by two PRR systems with a high speed linear CCD and a low speed array CCD, respectively. Their transient evaporation rates are experimentally determined and quantitatively agree well with the theoretical values predicted by classical Maxwell and Stefan-Fuchs models. With the demonstration of evaporation rate measurement of monocomponent droplet in this work, PRR is an ideal tool for measurements of transient droplet evaporation/condensation processes, and can be extended to multicomponent droplets in a wide range of industrially-relevant applications. (C) 2018 Elsevier Ltd. All rights reserved.
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