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Research Papers: Evaporation, Boiling, and Condensation

Modeling of Thermophysical Processes in Liquid Ceramic Precursor Droplets Heated by Monochromatic Irradiation

[+] Author and Article Information
Saptarshi Basu

Mechanical Engineering Department, University of Connecticut, Storrs, CT 06269-3139

Baki M. Cetegen1

Mechanical Engineering Department, University of Connecticut, Storrs, CT 06269-3139cetegen@engr.uconn.edu

1

Corresponding author.

J. Heat Transfer 130(7), 071501 (May 16, 2008) (8 pages) doi:10.1115/1.2908426 History: Received March 06, 2007; Revised July 26, 2007; Published May 16, 2008

A transient heat and mass transfer model is formulated to describe radiative heating of ceramic precursor droplets in a nonconvective environment. Heating causes vaporization of solvent from the droplet and concentration of the solute within the droplet leading to precipitation of the solute. It is found that the temperatures within the droplets are fairly uniform, but show different spatial profiles depending on the characteristics of solute absorptivity and duration of radiative heating. Incident laser irradiance and wavelength were found to play a significant role in the temperature profiles within droplets due to the absorption characteristics of the solute and the solvent. Lower levels of incident laser irradiation allows longer times for mass diffusion within a droplet leading to a gradual increase of the solute concentration from its center to its surface. Based on an equilibrium homogeneous precipitation hypothesis, it is found that the droplets heated with low laser irradiance tend to form thick precipitate shells as compared to those exposed to higher irradiances and consequently faster rates of vaporization. Large droplets form thin shells through surface precipitation, while small droplets may precipitate into shells of varying thickness depending on the magnitude of irradiance. Comparisons with convective heating in a high temperature plasma indicate that, with proper tuning of the laser irradiance, similar internal temperatures and solute concentration distributions are achievable. These modeling results suggest that different particle morphologies can be obtained from processing of liquid ceramic precursor containing droplets by proper tailoring of radiation parameters (wavelength and irradiance level).

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 6

Effect of irradiance on the radial distribution of temperature and solute concentration of a 10μm droplet after (a) 0.5ms of heating and (b) prior to precipitation at 10.6μm wavelength. τprecipitation=0.5ms(I*=0.938) and 1.2ms(I*=0.469).

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Figure 7

Comparison of radial distributions of temperature and species concentration of 10μm droplets prior to precipitation plasma heating and radiative heating at 1.2ms

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Figure 8

Radial distribution of temperature and species concentration of 5μm droplets prior to precipitation for variable absorption coefficient for two different irradiances of I*=0.469 and 0.938. τprecipitation=0.56ms(I*=0.938) and 1.23ms(I*=0.469).

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Figure 9

Radial distribution of temperature and species concentration of 20μm droplets after (a) 0.4ms and (b) prior to precipitation for variable absorption coefficient for irradiances of I*=0.469 and 0.938. τprecipitation=0.43ms(I*=0.938) and 1.1ms(I*=0.469).

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Figure 10

Radial distributions of temperature and species concentration of 10μm diameter droplet for different initial solute levels at irradiance of I*=0.469 after 0.5ms

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Figure 11

Radial distributions of temperature and species concentration of 20μm diameter droplet for different initial solute levels at an irradiance of I*=0.469 after 0.5ms

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Figure 12

Radial distributions of temperature and species concentration of 100μm diameter droplet after 0.125ms

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Figure 13

Variation of shell thicknesses for different droplet diameters and initial solute concentrations for an irradiance of I*=0.938

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Figure 14

Variation of shell thicknesses for different droplet diameters for two values of irradiance, for χZr,i=0.2

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Figure 4

Effect of source function on the radial distribution of temperature and solute concentration of a 10μm diameter droplet after 0.5ms of heating at an irradiance of I*=0.469 at 10.6μm wavelength

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Figure 5

Comparison of radial distributions of temperature and species concentration of 10μm diameter droplets at wavelengths of 10.6μm and 6.9μm after (a) 0.5ms and (b) prior to precipitation for I*=0.469. τprecipitation=1.2ms (I*=0.469, 10.6μm) and 0.95ms (I*=0.469, 6.9μm).

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Figure 1

Solute containing droplet vaporization and precipitation routes: (A) Uniform concentration of solute and volume precipitation leading to solid particles; (B) supersaturation near the surface followed by (I) fragmented shell formation (low permeability through the shell, (II) unfragmented shell formation (high permeability), (III) impermeable shell formation, internal heating, pressurization, and subsequent shell breakup and secondary atomization from the internal liquid; (C) elastic shell formation, inflation, and deflation by solid consolidation.

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Figure 2

Comparison of directional laser heating model of Park and Armstrong (17) with the spherically developed symmetric heating model with Q(r)=3.5×1011W∕m3 for a 20μm water droplet

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Figure 3

Variation of absorption coefficient of zirconium acetate solution at 942cm−1(10.6μm) and at 1446cm−1(6.9μm) as a function of the mole fraction of zirconium acetate

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