Radiation heat transfer and soot thermophoresis in laminar tube flow

The transport of soot particles suspended in laminar hot gas, flowing in a tube, has been modeled numerically and studied parametrically. The objective was to assess the coupled effects of radiation heat transfer and thermophoresis on the transport of monodisperse, as well as polydisperse soot particles. The wall material was assumed to he transparent to radiation for wavelengths shorter than a threshold wavelength at high temperatures, an essential property of high temperature-resistive materials. The results indicated that, as a consequence of strongly coupled thermal radiation transport and thermophoresis, a radially nonuniform temperature profile develops as the distance from the tube inlet is increased, leading to the development of sharp, nonuniform radial soot concentration profiles. The particles tend to accumulate in the vicinity of the point of inflection of the radial temperature profiles. The pace of the development of the nonuniform radial temperature and soot concentration profiles is sensitive to several parameters. Higher average soot concentration and higher tube wall temperature both promote the development of sharp radial profiles. The development of temperature and concentration profiles is also sensitive to particle size distribution. In comparison with model predictions obtained with the more realistic log-normally distributed particles, and for the range of parameters covered in our simulations, the assumption of monodisperse particles leads to significant over-prediction of the overall effect of thermophoresis, and of the pace of development of nonuniformity in the radial distributions of soot concentration and gas temperature.