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Research Papers: Evaporation, Boiling, and Condensation

Enhanced Evaporation of Microscale Droplets With an Infrared Laser

[+] Author and Article Information
Luis A. Ferraz-Albani, Alberto Baldelli, Reinhard Vehring, David S. Nobes, Larry W. Kostiuk

Department of Mechanical Engineering,
University of Alberta,
Edmonton, AB T6G 2G8, Canada

Chrissy J. Knapp, Wolfgang Jäger

Department of Chemistry,
University of Alberta,
Edmonton, AB T6G 2G2, Canada

Jason S. Olfert

Department of Mechanical Engineering,
University of Alberta,
Edmonton, AB T6G 2G8, Canada
e-mail: jolfert@ualberta.ca

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received February 1, 2016; final manuscript received August 11, 2016; published online September 20, 2016. Assoc. Editor: Milind A. Jog.

J. Heat Transfer 139(1), 011503 (Sep 20, 2016) (8 pages) Paper No: HT-16-1048; doi: 10.1115/1.4034486 History: Received February 01, 2016; Revised August 11, 2016

Enhancement of water droplet evaporation by added infrared radiation was modeled and studied experimentally in a vertical laminar flow channel. Experiments were conducted on droplets with nominal initial diameters of 50 μm in air with relative humidities ranging from 0% to 90% RH. A 2800 nm laser was used with radiant flux densities as high as 4 × 105 W/m2. Droplet size as a function of time was measured by a shadowgraph technique. The model assumed quasi-steady behavior, a low Biot number liquid phase, and constant gas–vapor phase material properties, while the experimental results were required for model validation and calibration. For radiant flux densities less than 104 W/m2, droplet evaporation rates remained essentially constant over their full evaporation, but at rates up to 10% higher than for the no radiation case. At higher radiant flux density, the surface-area change with time became progressively more nonlinear, indicating that the radiation had diminished effects on evaporation as the size of the droplets decreased. The drying time for a 50 μm water droplet was an order of magnitude faster when comparing the 106 W/m2 case to the no radiation case. The model was used to estimate the droplet temperature. Between 104 and 5 × 105 W/m2, the droplet temperature changed from being below to above the environment temperature. Thus, the direction of conduction between the droplet and the environment also changed. The proposed model was able to predict the changing evaporation rates for droplets exposed to radiation for ambient conditions varying from dry air to 90% relative humidity.

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Figures

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Fig. 1

Schematic of the experimental setup

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Fig. 2

Comparison between the experimental and model results for different conditions of air temperature (T∞ = 19.8 °C and T∞ = 60 °C) and 0% relative humidity. Dashed lines represent a two standard deviation band in the measured initial diameters.

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Fig. 3

Evolution of squared diameter (micrometers squared) with respect to time (seconds) for pure water droplets with conditions of RH = 0%, T∞  = 20 °C, initial diameter of 48.12 μm, and varying infrared radiation

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Fig. 4

Variation of the surface temperature of water droplets, Ts, with respect to droplet diameter for various infrared radiant flux densities, RH = 0%, and T∞  = 293.15 K

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Fig. 5

Comparison between the experimental and numerical model results for water droplets subject to infrared radiation and different conditions of relative humidity. Dashed lines represent two standard deviations in the initial droplet diameter. (a) T∞ = 24.0 °C, RH = 0%, power = 2.33 W, and standard deviation of initial droplet diameter (Sd,0) = 0.81 μm; (b) T∞ = 24.6 °C, RH = 30%, power = 2.33 W, and Sd,0 = 0.81 μm; (c) T∞ = 23.7 °C, RH = 60%, power = 1.91 W, and Sd,0 = 0.67 μm; and (d) T∞ = 24.3 °C, RH = 90%, power = 2.37 W, and Sd,0 = 0.65 μm.

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