Abstract

The relatively new thermometry technique “pulsed 2cLIF with MDR-enhanced energy transfer” is tested for the first time on micro-droplet streams in a heated air flow. The method allows simultaneous measurements of the droplet internal temperature-field, droplet size and droplet velocity. Plausibility of the measurement results is evaluated by comparing results of various experimental conditions among each other and by comparison with analytic modeling. Overall, measurement results are plausible. However, measured temperature-fields are biased by the integral fluorescence signal and initial conditions of the models are limited by the Rayleigh jet breakup that creates the droplet stream.Graphic abstract

Highlights

  • The following description of droplet evaporation is based on the effective thermal conductivity (ETC) model introduced by Abramzon and Sirignano (1989)

  • The averaging to a single value allows the comparison with the ETC model

  • Temperature measurement accuracy of the LIF-setup can be estimated during the temperature calibration, when droplets are expected to be isothermal

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Summary

Introduction

“Pulsed 2-color Laser-Induced Fluorescence with MDRenhanced energy transfer” (pulsed 2cLIF-EET) was recently introduced for instantaneous temperature imaging of microdroplets (Palmer et al 2016, 2017, 2018a). The pulsed laser enables simultaneous measurements of the droplet’s temperature-field, its size and velocity. In this work, pulsed 2cLIF-EET is tested for the first time in a realistic scenario, where a droplet stream is placed in a hot air flow. The comparison involves several variables that can only be deduced from the measured droplet temperature-fields including the droplet surface temperature and the surface temperature gradient These values can only be determined by pulsed 2cLIF-EET. The comparison includes the measured droplet sizes and further characteristics that allow an evaluation of the heat and mass transfer process including evaporation mass flow, Sherwood number, heat transfer coefficient and heating rate as well as the Peclet number.

Theory of pulsed 2cLIF‐EET thermometry
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Droplet evaporation modeling
Modeling the liquid phase
Modeling the gas phase of isolated droplets
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Modeling the gas phase of droplet streams
Experimental setup
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Average droplet characterization
Dynamics of the droplet stream
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Average size and temperature development
Droplet internal temperature‐fields
Influence of the integral fluorescence signal
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Comparison with evaporation modeling
Droplet size and evaporation
Droplet heating
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Droplet internal heat transfer
Convective heat transfer
Summary and outlook
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