Aircraft propulsion integration
Engine installation performance optimization
Improvement of engine installation performance
– Reduction of fuel consumption and polluting emissions
– Increased efficiency of the intake and exhaust nozzle
– Reduction of inlet flow distortions
– Increase in compressor stall margin
Designing efficient airframe aerodynamic shapes is extremely important to increase aircraft propulsive efficiency and thus reduce fuel consumption, with particular reference to the components which affect engine performance.
Optimized inlet and exhaust geometries can be obtained by coupling CFD codes with advanced optimization algorithms, automatically searching for optimal solutions among a prescribed search space.
When considering engine installation components, such as air intakes, the involved shapes can be very complex: the design process has usually to deal with many variables and a complex system of structural and architectural constraints, making the traditional trial and error design approach very tedious, time-consuming and scarcely effective. In such context, the development of automatic design methodologies, involving the application of advanced optimization algorithms coupled with CFD solvers, provides an alternative and very powerful tool for parametric analysis and optimization of general airframe components.
This must be done for different flight conditions and engine loads. Due to this requirement, an engine off design performance prediction software should be employed to validate the installation design, which means in practice to find a way to predict the fuel consumption reduction attained.
We applied this automated methodology in the ERICA tilt-rotor research project case (Clean Sky 1 European funded project), in which we have been involved in the past.
The main goal of the proposed methodology is based on the coupling between CFD simulation results and engine simulation code outputs to assess engine performance related to different installation configurations.
Once a CFD model related to a single configuration is built, a first simulation run allows the estimation of the installation performance parameters. However, in order to run a CFD simulation correctly, the air mass flows entering and exiting the engine and the turbine exit temperature must be known.
These values are only available after running an engine performance simulation model; to this aim we employed TSHAFT, an in-house validated gas turbine simulation tool. Since the installation losses affect the air mass flow inside the engine, an iterative procedure is needed to obtain reliable results, otherwise there is a real risk of optimizing the intake and exhaust shapes at the wrong operating point. The procedure is as follows:
1. A first simulation is run using TSHAFT where unitary values for intake and exhaust nozzle efficiencies (initial values) are entered. The values of engine mass flow and turbine exit temperature are used to determine the boundary conditions of the air flows in the CFD model.
2. A CFD simulation of the flow entering and exiting the engine is then carried out using the mass flow and temperature values previously calculated. The values of the intake and exhaust pressure losses are thus obtained.
3. A new TSHAFT simulation is performed using the new value for the pressure losses. If the new mass flow differs from the previously calculated one outside a predefined tolerance, the new values of mass flow and temperature are inserted into a new CFD simulation.
4. The above procedure is repeated until convergence. When convergence on the air mass flow is obtained, the turbine exit temperature should also present a negligible discrepancy between CFD and the TSHAFT model.
Since a little modification of the installation performance parameters produce a slight variation in the engine mass flow (but not negligible for performance improvement calculations), usually two or three iterations are sufficient to achieve an acceptable tolerance.
An increase in total pressure at the engine inlet, along with reduced flow distortion, was verified in all flight conditions with CFD analyses. But the most important engine performance parameter is fuel consumption: the percentage difference was calculated between baseline and optimized configurations for different flight conditions.
The engine performance increase calculated using the validated engine performance simulator TSHAFT was found to be always positive. The maximum reduction encountered in fuel consumption is more than 2% in cruise, and more than 1% in hover. These are extremely encouraging values, since a few percentage points of fuel consumption may determine the feasibility of a particular concept design. In addition, these have been obtained also decreasing the compressor stall margin.
The redesign optimization procedure applied to the intake/exhaust installation for the ERICA rotorcraft was a success, later confirmed also by wind tunnel testing. In fact, even if the optimization was meant to be focused on the cruise condition, it also improved hover performance, for every power loading condition analyzed. The flow reversal encountered in the baseline geometry was completely solved by means of the new optimized geometry, with very high pressure recovery factor in the intake and improved main exhaust nozzle efficiency.