The combustion chamber of aircraft engines plays an important role in achieving the optimum performance during an engine overhaul. For long decades, it has been common understanding in the MRO business that a well overhauled compressor and turbine are required to get an engine with low SFC and high EGT margin. In recent work at Lufthansa Technik AG, a comprehensive CFD analysis of the combustion chamber showed that, in contrast to this, small geometrical features influence the mixing process in the combustion chamber and can have an effect on the exit temperature profile. This in turn can reduce the accuracy of the EGT measurement significantly and create measurement errors and misinterpretations of the real engine performance.
In order to get insight into the flow topology, a very detailed digital model has been created using scans of the hardware available in the shop. Important geometrical features such as the cooling provisions and swirl creating components have been included in a very detailed manner with an efficient hexahedral mesh. The model includes the HPT vanes and the cooling flow extraction from the secondary cold flow. CFD results have been generated using the flow solver Ansys CFX 17.1, which is able to predict all relevant physical effects such as injection of liquid fuel, evaporation, and combustion of Jet A1 fuel using the Burning-Velocity combustion model. The flow in the combustion chamber shows large natural fluctuations. Subsequently, for each case a transient calculation has been carried out in order to allow an evaluation of the time-averaged flow field. Different geometrical features are investigated to predict the effect of geometry deviations on the exit temperature profile, e.g. the shape and size of the dilution holes.
Finally along the example of two CFM56 engines it will be shown how the data obtained by the detailed CFD model is used to optimize work-scoping and maintenance procedures. On the two cases put forward the combination of extended test-cell instrumentation and detailed modeling enabled not only the identification but also the rectification of combustion chamber deviations. This in turn minimized the necessary work, whereas in the past combustion chamber issues often went unnoticed and consequently resulted in extensive additional work.