Dynamic Similarity in Turbine Deposition Testing and the Role of Pressure

Abstract

The role of absolute pressure in deposition testing is reviewed from first principles. Relevant dimensionless parameters for deposition testing are developed and dynamic similarity conditions are assessed in detail. Criteria for establishing appropriate conditions for laboratory studies of deposition are established pursuant to the similarity variables. The role of pressure is particularly singled out for consideration relative to other variables such as temperature, particle size, and test article geometry/scaling. A case study is presented for deposition in a generic array of impinging jets. A fixed quantity (2 g) of 0–10 μ Arizona road dust (ARD) is delivered to the impingement array at three different temperatures (290, 500, and 725 K) and at fixed pressure ratio. Deposition results are presented for operating pressures from 1 to 15 atm. Surface scans show that the height of deposit cones at the impingement sites decreases with increasing pressure at constant temperature and pressure ratio. This reduction is explained by the lower “effective” Stokes number that occurs at high particle Reynolds numbers, yielding fewer particle impacts at high pressure. A companion computational fluid dynamics (CFD) study identifies the additional role of Reynolds number in both the impingement hole losses and the shear layer thickness.