Flow Field and Performance Characteristics of Combustor Diffusers: A Basic Study

Results of a detailed study concerning the influence of geometrical as well as of fluid mechanic parameters on the performance of a plane model combustor diffuser in cold flow are presented. For a qualitative insight into the complex flow field inside the pre-diffuser, the sudden expansion region and into the flow field around the flame tube dome, results of a flow visualization study with the hydrogen bubble method as well as with the ink jet method are presented for different opening angles of the pre-diffuser and for different flame tube distances. Also, quantitative data from detailed measurements with LDV and conventional pressure probes in a geometrically similar air driven setup are presented. These data clearly demonstrate the effect of boundary layer thickness as well as the influence of different turbulence levels at the entry of the pre-diffuser on the performance characteristics of combustor diffusers. The possibility of getting an unseparated flow field inside the pre-diffuser even at large opening angles by appropriately matching the diffuser’s opening angle and the flame tube distance is demonstrated. Also, for flows with an increased turbulence level at the entrance — all other conditions held constant — an increased opening angle can be realised without experiencing flow separation. The comparison of the experimental data with predictions utilizing a finite-volume-code based on a body-fitted coordinate system for diffusers with an included total opening angle less than 18° demonstrates the capability of describing the flow field in combustor diffusers with reasonable accuracy.