A Generalized Approximation for the Thermophoretic Force on a Free-Molecular Particle

A general, approximate expression is described that can be used to predict the thermophoretic force on a free-molecular, motionless, spherical particle suspended in a quiescent gas with a temperature gradient. The thermophoretic force is equal to the product of an order-unity coefficient, the gas-phase translational heat flux, the particle cross-sectional area, and the inverse of the mean molecular speed. Numerical simulations are used to test the accuracy of this expression for monatomic gases, polyatomic gases, and mixtures thereof. Both continuum and noncontinuum conditions are examined; in particular, the effects of low pressure, wall proximity, and high heat flux are investigated. The direct simulation Monte Carlo (DSMC) method is used to calculate the local molecular velocity distribution, and the force-Green's-function method is used to calculate the thermophoretic force. The approximate expression is found to predict the calculated thermophoretic force to within 10% for all cases examined. The source file mentioned in this article is available online at http://taylorandfrancis.metapress.com/openurl.asp?genre=article&id=doi:10.1080/02786820490490001 . To access this file, click on the issue link for 38(7), then select this article. In order to access the full article online, you must either have an institutional subscription or a member subscription accessed through www.aaar.org.