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Research Papers: Radiative Heat Transfer

Apparent Emissivity of Combustion Soot Aggregate Coating at High Temperature

[+] Author and Article Information
Tai Ran Fu

Key Laboratory for Thermal Science
and Power Engineering of Ministry of Education,
Beijing Key Laboratory of CO2 Utilization and
Reduction Technology,
Department of Thermal Engineering,
Tsinghua University,
Beijing 100084, China
e-mail: trfu@mail.tsinghua.edu.cn

Ji Bin Tian

Key Laboratory for Thermal Science and
Power Engineering of Ministry of Education,
Beijing Key Laboratory of CO2 Utilization and
Reduction Technology,
Department of Thermal Engineering,
Tsinghua University,
Beijing 100084, China

Hua Sheng Wang

School of Engineering and Materials Science,
Queen Mary University of London,
London E1 4NS, UK

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received June 19, 2016; final manuscript received October 23, 2016; published online January 10, 2017. Assoc. Editor: Laurent Pilon.

J. Heat Transfer 139(4), 042701 (Jan 10, 2017) (7 pages) Paper No: HT-16-1405; doi: 10.1115/1.4035102 History: Received June 19, 2016; Revised October 23, 2016

Soot aggregates frequently occur during combustion or pyrolysis of fuels. The radiative properties of soot aggregates at high temperature are important for understanding soot characteristics and evaluating heat transfer in combustion systems. However, few data for soot radiative properties at high temperature were available. This work experimentally investigated the apparent emissivity of the soot aggregate coating at high temperature using spectral and total hemispherical measurements. The soot aggregate coatings were formed on nickel substrates by a paraffin flame. The surface and inner morphology of the coatings were characterized by scanning electron microscope (SEM). The thickness of the coating was 30.16 μm so the contribution of the smooth nickel substrate to the apparent radiation from the coating could be neglected. The total hemispherical emissivity of the coating on the nickel substrate was measured using the steady-state calorimetric method at different temperatures. The spectral directional emissivity of the coating was measured for the wavelength of 0.38–16.0 μm at the room temperature. The measurements show that the total hemispherical emissivity decreases from 0.895 to 0.746 as the temperature increases from 438 K to 1052 K. The total hemispherical emissivity of the coating deposited on the nickel substrate is much larger than those of the nickel substrate and a nickel oxidization film. The measured spectral emissivity of the coating at the room temperature was used to theoretically calculate the total hemispherical emissivity at different temperatures by integration with respect to wavelength. The measured and calculated total hemispherical emissivities were similar, but their changes relative to temperature were completely opposite. This difference is due to the fact that the spectral emissivity of the coating is a function of temperature. The present results provide useful reference data for analyzing radiative heat transfer at high temperature of soot aggregates in combustion processes.

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Figures

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Fig. 1

The test sample: (a) photo of the sample and (b) SEM image of the coating surface morphology

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Fig. 2

SEM images of a cross section of the coating at different magnifications: (a)×3000, (b) ×30,000, and (c) ×50,000

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Fig. 3

Schematic of the test sample: (a) front view and (b) top view

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Fig. 4

Total hemispherical emissivity as a function of temperature for three samples: S1, soot aggregates sample; S2, oxidized nickel sample [37]; and S3, smooth nickel sample [34]

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Fig. 5

Spectral normal emissivity of the coating against wavelength at room temperature

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Fig. 6

Total hemispherical emissivity calculated based on the spectral emissivity data at room temperatures

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