Swirl Cup Modeling - Part VI: Dilution Jet Modeling

Abstract

CFD simulations of gas turb ine combustors, typically, do not include the inner and outer passages of the diffuser in the models for design analyses. This necessitates the specification of boundary conditions at the exit of the dilution holes into the combustor domain. This paper e xplores an approach for specifying this boundary condition that is applicable for combustor operating conditions at different power levels. First, the predictions for discharge coefficient are validated against the NACA benchmark data for different circul ar hole sizes and wall thickness. The agreement with data is within 5%. For this configuration, simulations are also performed to understand the effects of cross -flow on discharge coefficient and jet angle. Results indicate that these parameters are not very sensitive to the crossflow momentum ratio. Simulations are also performed for a CFM -56 swirl cup package in a 3 - inch square flame tube. To understand the flow characteristics at dilution hole, the configuration included an outer flow passage. Resu lts for this case indicate that distributions of flow variables at the dilution holes are two -dimensional and complicated due to existence of reverse flow regions. The swirling flow in the flame tube also has an impact on the dilution jet as the flow is s ubsonic. Since the flow variables distributions do not follow a pattern amenable for developing scaling rules, accurate modeling of dilution flow requires integrated diffuser -combustor analysis.