Further studies on effects of thermophoresis on seeding particles in LDV measurements of strained flames

The axial velocity profiles for counterflow premixed and diffusion flames were experimentally measured by laser-doppler velocimetry (LDV) and computationally simulated with detailed reaction mechanism and transport properties. The LDV measurements were found to agree well with the computed values in the cooler, decelerating part of the flow upstream of the flame, but to significantly deviate from the calculated values in the rapidly- accelerating preheat region of the flame in which substantial thermal expansion occurs over a very short distance. An analysis of the motion of the LDV seeding particles under the influence of viscous drag and thermophoresis in these well-characterized counterflow flame environments demonstrates that such deviations are consequences of thermophoresis. Furthermore, since the thermophoretic force is in the direction opposite to that of the temperature gradient, and its influence on the motion of the particle depends upon the local flow velocity, a rich variety of LDV velocity profiles were observed for flames with different temperature profiles and distances to the stagnation surface. The stoichiometric mixture fraction was found to be a useful parameter to characterize the velocity profile variation. The study emphasizes the importance of accounting for the effects of thermophoresis in interpreting LDV as well as PIV (particle image velocimetry) data in flames, both laminar and turbulent. An approach to closely simulate experimental counterflow flames is also presented.