Deposition Control Using Transpiration

An experimental and theoretical study of deposition of small particles is presented to evaluate the concept of transpiration as a deposition control strategy. The application of this work is the control of deposition of small particles (1–5μm) contributing most of the mass of the solid carryover entering turbines burning fuels derived from coal. The experimental study is carried out in a wind tunnel facility containing a flat porous transpired section. The Reynolds and Stokes numbers are chosen to be representative of the conditions found in industrial gas turbines. Measurements of the velocity profiles were conducted for high injection rates (1.5%<F<3%). A theory developed for the transpired turbulent boundary layer, which is described by an “outer boundary layer” entraining the injected flow for large injection rates, agrees well with the experimental data. Concentration profiles of glass particles of both very narrow and wide size distributions were conducted for different injection rates under the isothermal conditions. The measurements clearly indicate the conditions under which transpiration can avoid the deposition of particles and they show the effect of particle size on the concentration profiles and consequently on the arrival rates. The study also provides a clear insight into the turbulent diffusion of particles for a Stokes number of between 1 and 3.5. The Schmidt numbers obtained from the measurements are in agreement with the theoretical prediction of Tchen.