Polarization (ellipsometric) measurements of liquid condensate deposition and evaporation rates and dew points in flowing salt/ash-containing combustion gases

Abstract

An optical polarization (“ellipsometric”) technique is developed for measuring rapidly growing and evaporating transparent liquid condensate films (e.g., boric oxide) on solid surface exposed to flowing combustion product gases at film thicknesses well below the onset of complications due to run-off. To demonstrate the validity of optical techniques both ellipsometric and optical interference techniques were first used to measure, under similar geometric, flow, and thermal conditions, the growth rate of solid molybdenum trioxide films on molybdenum, with excellent agreement being obtained. Moreover, our new polarization results for the B 2O 3( l) deposition rate from BCl 3-seeded propane/air flames agree well with the results of our earlier interference measurements, and also with theoretical chemical vapor deposition predictions. While previously unable to measure interferometrically B 2O 3( l) evaporation rates, these rates (from platinum ribbons into seeded/propane air flames) are here estimated using the polarization technique. It appears that, compared with the interference method, the polarization technique places less stringent requirements on surface quality, which may justify the added optical components needed for such measurements. We conclude that the complementary real-time optical methods of polarization (ellipsometry) and interference hold considerable promise for application to rapid remote measurement of condensation and evaporation rates in high temperature (e.g., combustion product) flow environments at film thicknesses below the onset of complications in interpretation due to liquid film run-off.