An annular exhaust system design for being used in the bench testing of MTR390-E turboshaft engine has been performed at ITP. The exhaust system is aimed at improving the aerodynamic performance at high power compared with an existing exhaust system used in the previous version of the engine. The exhaust cone emulates to some extend the exhaust system in the helicopter and it is comprised of outer and inner cones supported by three struts.
The CFD commercial code FLUENT is used to investigate the aerodynamic performance of the baseline design and to optimise the inner and outer cone angles in the new design based on 2D axisymmetric models. Representative radial exit turbine conditions and far field conditions are imposed in the model comprising the exhaust cones plus a large external domain. Two outer and inner cone angles and two inner cone lengths are analysed at low and high power conditions. The aerodynamic performance of the exhaust shows high sensitivity to the inlet flow angle which varies up to 30°/40° between the high and low power conditions.
In all the simulated cases a large separation region is generated after the inner cone. Due to the high swirling flow the separation bubble behind the plug growths downstream hence reducing the effective flow exit area compared with the geometry area and reducing the pressure recovery downstream once the flow has been separated from the inner cone. Although all cases show similar qualitative behaviour, the best case based on the computed figures of merit (i.e., lowest total pressure loss) is chosen for the new design.
In order to further optimise the behaviour of the exhaust at high power, in the new design the three struts are aligned with the flow angle at high power conditions (struts were axially oriented in the baseline design) and the resulting geometry is analysed by 3D CFD simulations. As expected, the orientation of the struts has a dramatic impact in the aerodynamic behaviour of the exhaust. The new design shows an improvement of 29% in pressure recovery at high power compared with the baseline configuration, although it shows a degradation of 12% at low power.
Both the baseline and the new exhaust systems are tested with the real engine in the test bench. The general aerodynamic performance of the new design is compared with the CFD simulation. As a consequence of the design change an important modification in the aerodynamic behaviour of the exhaust is obtained impacting the whole engine performance. Therefore a new performance model of the exhaust system is proposed to be implemented in the whole engine performance model in order to accurately simulate the behaviour of the engine coupled with the new exhaust.