abstract

An accelerated deposition test facility was operated with four different coal ash species to study the effect of ash composition on deposition rate and spatial distribution. The facility seeds a combusting (natural gas) flow with 10–20 micron mass mean diameter coal ash particulate. The particulate-laden combustor exhaust is accelerated through a rectangular-to-annular transition duct and expands to ambient pressure through a nozzle guide vane annular sector. For the present study, the annular cascade consisted of two CFM56 aero-engine vane doublets, comprising three full passages and two half passages of flow. The inlet Mach number (0.1) and gas temperature (1100 °C) are representative of operating turbines. Ash samples were tested from the three major coal ranks: lignite, subbituminous, and bituminous. Investigations over a range of inlet gas temperatures from 900 °C to 1120 °C showed that deposition increased with temperature, though the threshold for deposition varied with ash type. Deposition levels varied with coal rank, with lignite producing the largest deposits at the lowest temperature. Regions of heightened deposition were noted; the leading edge and pressure surface being particularly implicated. Scanning electron microscopy was used to identify deposit structure. For a limited subset of tests, film cooling was employed at nominal design operating conditions but provided minimal protection in cases of severe deposition.

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