Research Papers: Radiative Heat Transfer

Temperature Effect on Phase-Transition Radiation of Water

[+] Author and Article Information
M. Q. Brewster

Fellow ASME
Department of Mechanical
Science and Engineering,
University of Illinois at Urbana-Champaign, Urbana, IL 61801
e-mail: brewster@illinois.edu

K.-T. Wang, W.-H. Wu, M. G. Khan

Department of Mechanical
Science and Engineering,
University of Illinois at Urbana-Champaign,
Urbana, IL 61801

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received March 21, 2013; final manuscript received January 18, 2014; published online March 10, 2014. Assoc. Editor: Zhuomin Zhang.

J. Heat Transfer 136(6), 062704 (Mar 10, 2014) (9 pages) Paper No: HT-13-1156; doi: 10.1115/1.4026556 History: Received March 21, 2013; Revised January 18, 2014

Infrared radiation associated with vapor-liquid phase transition of water is investigated using a suspension of cloud droplets and mid-infrared (IR) (3–5 μm) radiation absorption measurements. Recent measurements and Monte Carlo (MC) modeling performed at 60 °C and 1 atm resulted in an interfacial radiative phase-transition probability of 5 × 10−8 and a corresponding surface absorption efficiency of 3–4%, depending on wavelength. In this paper, the measurements and modeling have been extended to 75 °C in order to examine the effect of temperature on water's liquid-vapor phase-change radiation. It was found that the temperature dependence of the previously proposed phase-change absorption theoretical framework by itself was insufficient to account for observed changes in radiation absorption without a change in cloud droplet number density. Therefore, the results suggest a strong temperature dependence of cloud condensation nuclei (CCN) concentration, i.e., CCN increasing approximately a factor of two from 60 °C to 75 °C at near saturation conditions. The new radiative phase-transition probability is decreased slightly to 3 × 10−8. Theoretical results were also calculated at 50 °C in an effort to understand behavior at conditions closer to atmospheric. The results suggest that accounting for multiple interface interactions within a single droplet at wavelengths in atmospheric windows (where anomalous IR radiation is often reported) will be important. Modeling also suggests that phase-change radiation will be most important at wavelengths of low volumetric absorption, i.e., atmospheric windows such as 3–5 μm and 8–10 μm, and for water droplets smaller than stable cloud droplet sizes (<20 μm diameter), where surface effects become relatively more important. This could include unactivated, hygroscopic aerosol particles (not CCN) that have absorbed water and are undergoing dynamic evaporation and condensation. This mechanism may be partly responsible for water vapor's IR continuum absorption in these atmospheric windows.

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Fig. 1

Schematic of experimental system for measurement of water cloud transmissivity

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Fig. 2

Ray tracing (geometric optics) model of multiple chances for surface absorption

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Fig. 3

Transmissivity versus liquid water volume fraction at 75 °C, experimental results and MC simulations for fixed probability constant (a21) and varying d* (μm)

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Fig. 4

MC simulation and experiment results comparison at 75 °C in the absence of surface absorption for pure absorbing (nonscattering), pure scattering (nonabsorbing), and mixed absorbing and scattering droplets

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Fig. 5

MC simulation results for different surface absorption coefficients (i.e., varying, a21) at 75 °C

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Fig. 6

MC simulation results for different d* with no surface absorption contribution (a21 = 0) at 75 °C

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Fig. 7

Comparison of Qa,v and Qa,s for d = 6.75 μm (fv = 0.1 × 10−4) versus wavelength

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Fig. 8

Comparison of Qa,v and Qa,s for d = 18.3 μm (fv = 2 × 10−4) versus wavelength

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Fig. 9

Comparison of experimental transmissivity measurement for 60 °C and 75 °C

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Fig. 10

Best simulation fit for 60 °C experiment with the presence of surface absorption

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Fig. 11

MC estimation of transmissivity at 50 °C, based on extrapolation of simulations at 60 °C and 75 °C




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