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Technical Briefs

Soot Optical Properties in the Terahertz Spectra Domain

[+] Author and Article Information
Fei Wang1

State Key Laboratory of Clean Energy Utilization,  Zhejiang University, Hangzhou 310027, Chinawangfei@zju.edu.cn

Ting Xu, Zhishen Qiang, Qunxing Huang, Dong Liu, Jianhua Yan, Kefa Cen

State Key Laboratory of Clean Energy Utilization,  Zhejiang University, Hangzhou 310027, Chinawangfei@zju.edu.cn

1

Corresponding author.

J. Heat Transfer 134(7), 074501 (May 18, 2012) (4 pages) doi:10.1115/1.4006034 History: Received March 31, 2011; Revised December 12, 2011; Published May 17, 2012; Online May 18, 2012

For understanding and accurately modeling combustion, the important questions are what species are present in the flame, and the spatial distribution and temperature of these species. Traditional optical methods used only the electromagnetic waves in the wavelength region from the ultraviolet region up to the infrared. Terahertz time-domain spectroscopy technique can be used for the combustion research as a novel tool. However, for some sooty combustion environments, the strong absorption, spectral interference from soot scattering, and fluorescence from large molecules must be considered. The optical properties of soot in the terahertz domain are the main basic data for terahertz application. In this paper, the terahertz time-domain spectroscopy technique was used to study the optical properties of flame soot within 0.2–1.6 THz. The complex refractive indices of the soot were deduced by the fixed-point iteration method. In order to validate the results, the complex refractive indices of the soot from the four different fuel flames were deduced. It was found that the complex refractive indices in the terahertz domain of the soot from the different fuel flames are very close to each other. The comparisons of complex refractive indices between the visible–IR domain and the terahertz domain indicate that the value of absorption index in terahertz domain is smaller than in IR domain, which implies that the terahertz wave will penetrate the sooty flame with smaller absorption than the IR rays. The results can provide the basic optical data of flame soot for the application of terahertz time-domain spectroscopy technique in the optical combustion diagnostics and extend the optical combustion diagnostics application area.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

The electromagnetic spectrum. The terahertz frequency lies between microwave frequencies and the infrared portion of optical spectrum. It is loosely defined by frequencies lying between 100 GHz and 10 THz (wavelengths 3 mm to 30 μm)

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Figure 2

The schematic setup of the terahertz time-domain spectroscopy

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Figure 3

The time-domain spectra of references and samples: (a) methane flame soot, (b) ethylene flame soot, (c) propane flame soot, and (d) candle flame soot.

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Figure 4

The complex refractive index of different soot samples: 1, ethylene flame soot; 2, methane flame soot; 3, propane flame soot; 4, candle flame soot.

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Figure 5

Comparisons of the complex refractive index of soot in the terahertz domain and visible–IR domain [21].

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