The engine bay is an essential component of the AgustaWestland AW609 tilt rotor nacelle, since it determines the performance of the engine and assures safe maneuvers for every flight condition. The correct design of the nacelle and of the exhaust are critical as they help in achieving these operative purposes, especially in terms of engine bay cooling.

Several attempts have been made in the past to design an efficient secondary exhaust, but thermal problems, such as panels scorching and hot impingement on the ground, forced AW Engineering to evaluate a new shape. A CFD analysis on an accurate simulation model was required to compare with experimental data collected on the aircraft by telemetry and to monitor the benefits introduced by the new configuration. AW Engineering used CD-adapco's STAR-CCM+ for the CFD analysis largely due to the code’s ability to address all of AW’s simulation requirements.

The project begins

A 3D model was designed in CATIA by taking into account the entire nacelle, the air intake system, and the engine bay. All components of the nacelle were represented except for the three-blade rotor, whose downwash effect has been demonstrated to have no influence on the amount of fresh air entering the engine bay.

The air intake system carries air to the compressor inlet and it contains a blower, which releases any undesired foreign objects like sand, asphalt dust, glass pieces, and rain to the outside environment. To ensure an easier treatment of the engine bay cooling in the simulation, engine accessories were omitted. This decision allowed for a quick turnaround time without undermining the simulation accuracy.

The inclusion of the primary exhaust enabled the study of the development of the flow along the duct, which determines the intensity of the fresh air pumping effect and generates the mixing of hot gases and fresh air with significant repercussions on the temperature, pressure, and velocity at the main outlet.

The main purpose of this analysis was to assess the efficiency of the cooling generated by the pumping effect of this new secondary exhaust configuration. The engine can reach wall temperatures of the order of 750 K, and the introduction of fresh air into the engine bay is a necessity to meet the thermal operating constraints imposed by the engine provider. Exploiting the high kinetic energy of the hot exhaust gases to suck fresh air in is a novel way to guarantee the safe performance of the aircraft under any flight condition.

Moreover, the temperature, velocity, and direction of the exhaust flows have a direct effect on the exhaust configuration since they can cause serious impacts on the nacelle cowling heating and result in asphalt deterioration, especially during the maneuvers on ground.

At the same time, this discharge system has to limit the exhaust back pressure loss, which influences the engine performance in terms of power production and fuel consumption. Therefore, the right duct shape can reduce flow detachments and wake turbulence. The performance of the engine is also subject to the air intake system. The air mass flow, in fact, has to be sucked in and carried to the compressor inlet with a small pressure drop and in the most uniform way possible to avoid undermining the engine efficiency. The CFD study contributed to evaluating both the benefits and deficiencies of various configurations and has enabled visualizing the streamlines of intake flows.

Flight simulations

The analysis involved four main flight operations, with the nacelle assuming different inclination angles depending on the aircraft altitude and velocity. The simulation of these four phases helped to observe both the engine and the nacelle behavior during a complete flight, from the hovering operation in helicopter mode to the cruise flight condition in airplane mode. Comparison with experimental data was used to determine the quality and accuracy of the model. The four flight conditions and the corresponding tilting angles of the nacelle were as follows:

• Hover flight condition with a 90° tilting angle of the nacelle with respect to the ground (helicopter mode),

• Climb flight condition with a 75° tilting angle of the nacelle,

• Level flight condition with a 50° tilting angle of the nacelle, and

• Maximum cruise power climb flight condition with a 0° tilting angle of the nacelle (airplane mode).

The hover flight with a 90° tilting angle and the maximum cruise power climb flight with a 0° tilting angle represent the two extreme cases for the aircraft configurations. Hovering is the maneuver following takeoff, where the engines reach their maximum powers and temperatures. The maximum cruise climb flight, on the other hand, represents the flight condition at high altitude where the aircraft achieves the maximum speed while in airplane mode. Therefore, these two main operating conditions are very important in a first flight test with a new exhaust configuration, and they have to be analyzed properly to ensure the correct working of the engines.

The other two flight cases cover the two most important intermediate steps between one configuration and the other. During these, the aircraft lifts to a higher altitude and gains velocity. It is fundamental to analyze the influence of these maneuvers on the amounts of air intake for the compressor and of fresh air for the engine bay cooling.

The nacelle was positioned at the center of an external cubic domain (50 x 5 0 x 50 m³) in STAR-CCM+ and it was possible to simulate the four nacelle tilting angles in this domain. A velocity inlet and pressure outlet boundary conditions were set to the entrance and exit of the cubic domain respectively.

Simulations were run in steady state with Menter’s SST k-ω turbulence model, which was validated internally at AW for this application. A volume mesh consisting of approximately 1.5 million polyhedral cells, 9.5 million faces, and 7.6 million vertices was generated and used. The segregated solver in STAR-CCM+ was employed for the treatment of the Navier-Stokes equations, since it allowed for an accurate calculation of the velocity profile for slightly compressible flows while ensuring fast convergence.

The results from the STAR-CCM+ simulations compared agreeably with experimental data for each flight condition. The efficiency of cooling is strictly connected to the mass flow of fresh air coming into the engine bay and the maximum relative error of temperature from simulations was about 6%. The numerical simulations showed that the thermal operating constraints were respected by the new design and that the cooling margin of every engine component stayed within the regulations imposed by the provider.

Six press probes installed at the primary outlet in the experiments provided a means for a fundamental validation of the simulation model. The pressure values from the simulations at the probe locations were within 2.9% of the experimental data, and the exhaust back pressure losses from STAR-CCM+ compared well with the experimental measurements.

The nacelle panels scorching problem was also solved with the new design, as the simulations showed that the exhaust flows stay clear of the structure, which was the cause of the scorching. The risk of impingement on the ground was also reduced, since the mixing of hot gases with fresh air ensures that the exhaust temperatures stay below the temperature at which the ground asphalt melts and that the aircraft taking off does not present any risks for the crew on the ground.

Sebastiano Felice, AgustaWestland S.p.A., and Alessandro Introini, Politecnico di Milano, wrote this article for Aerospace Engineering.