AEROSPACE
AIRCRAFT PROPULSION
INTEGRATION
Engine installation performance
optimization for the aeronautical industry
CLIENT
Aeronautical industry
CHALLENGE
Improvement of engine installation performance
SUCCESS
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
01
CHALLENGE
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 approach very tedious, time-consuming and scarcely effective.
In such context, the development of automatic design methodologies — advanced optimization algorithms coupled with CFD solvers — provides an alternative and very powerful tool for parametric analysis and optimization of general airframe components.
This methodology can be applied to airplanes, helicopters and tiltrotors, for different flight conditions and engine loads. An engine off-design performance prediction software is needed to validate the installation design, which in practice means finding a way to predict the fuel consumption reduction attained.
Nablawave founders applied this automated methodology in an EU funded research project, in collaboration with prof. Ernesto Benini of the University of Padova.
02
SOLUTION
The main goal of the proposed methodology is the coupling between CFD simulation results and engine simulation code outputs, to assess engine performance across different installation configurations.
Once a CFD model for a single configuration is built, a first simulation run allows the estimation of the installation performance parameters. But to run a CFD simulation correctly the air mass flows entering and exiting the engine and the turbine exit temperature must be known — and those are only available after running an engine performance simulation model. To this aim we employed TSHAFT, a gas turbine simulation tool validated in collaboration with the University of Padova.
Since the installation losses affect the air mass flow inside the engine, an iterative procedure is needed; otherwise there is a real risk of optimizing the intake and exhaust shapes at the wrong operating point. The procedure is as follows:
01
A first TSHAFT simulation is run with unitary values for intake and exhaust nozzle efficiencies. The resulting engine mass flow and turbine exit temperature set the boundary conditions of the air flows in the CFD model.
02
A CFD simulation of the flow entering and exiting the engine is carried out using those mass flow and temperature values. The intake and exhaust pressure losses are thus obtained.
03
A new TSHAFT simulation is performed with the new pressure losses. If the new mass flow differs from the previous one beyond a predefined tolerance, the new mass flow and temperature values feed a further CFD simulation.
04
The procedure repeats until convergence. When the air mass flow converges, the turbine exit temperature shows a negligible discrepancy between the CFD and TSHAFT models.
Since a small modification of the installation performance parameters produces only a slight variation in engine mass flow — though not a negligible one for performance calculations — two or three iterations are usually sufficient to reach an acceptable tolerance.
03
RESULTS
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 using the validated engine performance simulator TSHAFT, and the improvement 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 — extremely encouraging values, since a few percentage points of fuel consumption may determine the feasibility of a particular concept design. These were obtained while also increasing the compressor stall margin.
The redesign optimization procedure applied to the intake/exhaust installation of a tiltrotor aircraft was a success, later confirmed by wind tunnel testing. Even though the optimization was 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 the new optimized geometry, with a very high pressure recovery factor in the intake and improved main exhaust nozzle efficiency.
