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Technical Briefs

Heating and Ignition of Metal Particles in the Transition Heat Transfer Regime

[+] Author and Article Information
Salil Mohan, Mikhaylo A. Trunov, Edward L. Dreizin

Department of Mechanical Engineering,  New Jersey Institute of Technology, Newark, NJ 07102

J. Heat Transfer 130(10), 104505 (Aug 07, 2008) (5 pages) doi:10.1115/1.2945881 History: Received April 27, 2007; Revised December 27, 2007; Published August 07, 2008

This paper considers the heating and ignition of small metallic particles in hot gases for a range of Knudsen numbers, for which the continuum description of heat transfer is not valid. Modified Fuchs’ model for the transition heat transfer analysis was adapted to treat diatomic gas with properties changing as a function of temperature. The dimensionless heat transfer coefficient, Nusselt number, was calculated as a function of the particle diameter for the transition heat transfer regime. Heat transfer rates in the transition regime are somewhat different from one another for the cases of particle heating and cooling while the absolute values of the particle-gas temperature difference are the same. This effect does not exist for the continuum heat transfer model. It is observed that the applicability of the continuum heat transfer model for particles of different sizes depends on pressure and particle-air temperature difference. For example, for particles at 300K heated in air at 2000K, the continuum heat transfer model can be used for particle diameters greater than 10μm and 1μm at the pressures of 1bar and 10bars, respectively. Transition heat transfer model must be used for the analysis of heat transfer for nanosized particles. For calculating the ignition delay, the continuum model remains useful for particle diameters greater than 18μm and 2μm for 1bar and 10bars, respectively. The sensitivity of the transition heat transfer model to the accommodation coefficient is evaluated. It is found that for metallic particles, the accommodation coefficient has a relatively weak effect on the heat transfer rate.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 1

Constant Knudsen number lines as a function of particle temperature and gas temperature

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Figure 2

Nusselt number as a function of Knudsen number (and particle diameter) calculated for the transition regime heat transfer using Fuchs’ model for monoatomic gas (16) and for air considering air properties as a function of temperature

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Figure 3

Nusselt number calculated as function of the particle diameter and pressure for the transition heat transfer regime at pressures of 1bar and 10bars. Particle heating: Tg=2000K and Tp=300K. Particle cooling: Tg=300K and Tp=2000K.

Grahic Jump Location
Figure 4

Temperature histories for Mg particle inserted in air at the air temperatures just above (solid line) and just below (dashed line) of the ignition threshold. The ignition delay is measured from time t=0, the moment when the particle is exposed to hot air.

Grahic Jump Location
Figure 5

Calculated ignition delay of Mg particle at 300K inserted in air at 2000K as a function of particle diameter and pressure

Grahic Jump Location
Figure 6

Calculated ignition delay of Mg particle at 300K inserted in air at 2000K as a function of particle diameter and accommodation coefficient

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