Deposition in a Turbine Cascade With Combusting Flow

Abstract

This report presents the design and operation of an accelerated testing facility for the study of deposition in turbine nozzle guide vanes (NGV). The facility was designed to produce turbine deposits in a 1–2 hour test that simulates thousands of hours of turbine operation. This is accomplished by matching the net foreign particulate throughput of an actual gas turbine. The facility seeds a combusting (natural gas) flow with 10–20 micron diameter coal ash particulate. The particulate-laden combustor exhaust is accelerated through a rectangular-to-annular transition duct and expands to ambient pressure through an NGV annular sector. The cascade contains two NGV doublets (donated from industry) comprising three full passages and two half passages of flow. The inlet Mach number (0.1) and gas temperature (1000 °C) are representative of operating power turbines. The vanes are film cooled from an auxiliary air supply at nominal design operating conditions. Investigations over a range of inlet gas temperatures showed that deposition increased substantially with temperature, with a threshold for deposition occurring between 900 °C and 1000 °C. Qualitative test validation was achieved using direct comparison with deposits from service hardware. Surface topography analysis indicated that the surface structure of the generated deposits were similar to those found on actual turbine blades. Regions of heightened deposition were noted; the leading edge and pressure surface being particularly implicated. Film cooling is shown to provide substantial protection from deposition.