Experimental deposition of NaCl particles from turbulent flows at gas turbine temperatures

Abstract: The deposition of micron-sized particulates from turbulent gas flow has been a subject of investigation for over 60 years. Interest from within the gas turbine community has become very significant over the last 15-20 years due to numerous threats to engine life. Ingested sand, dust, volcanic ash, salt, and ice crystals can drive various damage mechanisms which can substantially reduce component life. Studies of deposition within secondary air systems studies have shown that significant blockage of film cooling hole can occur for high pressure turbine (HPT) blade conditions. Computational studies of such geometries have tended to apply standard and widely-available numerical models for the interaction of particles with gas turbulence, including the discrete random walk. These have been shown to be inappropriate for such modelling; to address this the continuous random walk model has been applied to gas turbine flows. This model requires validation at engine-representative temperatures. This presentation features experimental data for assessment of the validity of the continuous random walk model at temperatures and Reynolds numbers representative of conditions in gas turbine secondary air systems. Horizontal pipe flow experiments are reported, using a sodium chloride (NaCl) aerosol. Tests are undertaken with both isothermal and wall-gas temperature gradient conditions in order to assess thermophoretic effects. Thermophoresis is a particle force due to temperature gradients within the gas phase, proportional to the (negative direction of) temperature gradient. When Tgas > Tmetal increasing deposition is seen ('positive' thermophoresis), for Tgas < Tmetal the reverse is observed ('negative' thermophoresis). To the authors’ knowledge very few studies have addressed negative thermophoresis experimentally.