A model of cool simulation

The engine bay is one of the essential components of the AgustaWestland AW609 TiltRotor nacelle, since its correct working determines a good performance of the engine and assures safe manoeuvres in every flight conditions. The correct designs of the nacelle and the exhaust are two fundamental elements, which enable these operative purposes to be achieved, especially in terms of engine bay cooling.

Several attempts have been made in the past to design an efficient secondary exhaust, but thermal problems, like panels scorching and too hot impingement on to the ground, have forced AgustaWestland Engineering to evaluate a new shape. A computational fluid dynamics (CFD) analysis has been requested to create an accurate model so that it has been possible to interface with experimental data collected on the aircraft by telemetry and to monitor any benefits introduced by the new configuration. AgustaWestland Engineering decided to invest in STAR-CCM+ due to its ability to address all of its requirements.

The 3D model has been realised thanks the design software CATIA by taking in account the entire nacelle, the air intake system and the engine bay. The nacelle has been represented with its whole architecture without the three-blade rotor, whose downwash effect has been demonstrated not to influence the fresh air amount entering the engine bay. The air intake system carries air to the compressor inlet and it contains a blower, which has the task to release outside any undesired foreign objects, which correspond to sand, asphalt dust, glass pieces and rain.

To ensure an easier treatment of the engine bay cooling, every engine accessory has been omitted. This choice has enabled a reduction of the realisation time without undermining the simulation accuracy. The inclusion of the primary exhaust has enabled study of the flow development along the duct, which determines the fresh air pumping effect intensity and generates the mixing among hot gasses and fresh air with consequences on the temperature, pressure and velocity at the main outlet.

Meeting the aims

The main purpose of this analysis has been to assess the cooling efficiency generated by the pumping effect of this new secondary exhaust configuration. The engine can reach wall temperatures of the order of 750K and the necessity to introduce a mass flow of fresh air into the engine bay has become a necessity to assure the respect of the thermal operating constraints imposed by the engine provider. Exploiting the high kinetic energy of hot exhaust gasses to suck fresh air is a novel way to guarantee a safe performance of the aircraft in all flight conditions. Moreover, the temperature, the velocity and the direction of the exhaust flows are direct consequences of the exhaust model, since they can reveal serious impacts on the nacelle cowling heating and on the asphalt deterioration especially during the manoeuvres on the ground.

At the same time this discharge system has to limit the exhaust back pressure loss, which influences consistently the engine performance in terms of power production and fuel consumption. Therefore, a correct duct shape can reduce vein detachments and rate turbulence. The correct performance of the engine is also subject to the air intake system. The air mass flow, in fact, has to be sucked and carried to the compressor inlet with small pressure drops and in the most uniform way possible not to undermine the engine efficiency. CFD study has contributed to evaluate both benefits and deficiencies of any kinds of configuration and to visualise the streamlines of intake flows with relative performance impacts.

The analysis has involved four main flight operations, where the nacelle assumes different inclination angles with respect to altitude and velocity attained by the aircraft. The simulation of these four phases has enabled observation of both the engine and the nacelle behaviours during a whole flight, starting from the hovering operation in helicopter mode and arriving at the cruise flight disposition in airplane mode. Then, the experimental comparison has determined the quality and the accuracy of the model. The four flight conditions and the correspondent tilting angles of the nacelle are described as follows: hover flight condition with a 90° tilting angle of nacelle with respect to ground (helicopter mode); climb flight condition with a 75° tilting angle of nacelle; level flight condition with a 50° tilting angle of nacelle; maximum cruise power climb flight condition with 0° tilting angle of nacelle (airplane mode).

The hover flight with 90° tilting angle of nacelle and the maximum cruise power climb flight with 0° tilting angle of nacelle represent the two extreme cases for what concerns the aircraft configurations. Hovering is the manoeuvre following take-off, where the engines reach their maximum power and temperatures. The maximum cruise climb flight, instead, represents the flight condition at high altitudes where the aircraft achieves the maximum speed 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 taken under control and monitored in a detailed way to check the correct working of the engines. The other two flight cases are the two main intermediate steps, which define the conversion from one configuration to the other. During these two nacelle rotations, the aircraft lifts at a higher altitude and gains velocity. It has been fundamental to analyse the incidence of these manoeuvres on the amounts of the air intake for the compressor and of the fresh air for the engine bay cooling.

Simulate to innovate

The nacelle has been positioned at the centre of the external cubic domain (50 x 50 x 50m3) and in this way it has been possible to simulate the four nacelle tilting angles. The imposition of velocity inlet (magnitude and direction) and pressure outlet, as boundary conditions applied to the external domain, has assured the Navier-Stokes equations set resolution with the achievement of the final results. Simulations have been run in steady state with Menter's SST k-ω turbulence model, which have shown important agreement in internal validation studies.

In order to meet strict deadlines, a volume mesh consisting of approximately 1.5 million of polyhedral cells, 9.5 millions of faces and 7.6 millions of vertices has been generated and used. The Segregated Solver has been employed for the treatment of the Navier-Stokes equations set, because it has shown a good calculation velocity for slightly compressible flows cutting the necessary time for convergence down.

Analyses have given accurate results in each flight condition and the correspondences with experimental data have been very good. The efficiency cooling is strictly connected to the mass flow of fresh air coming into the engine bay and the maximum relative error of temperature has been about 6%. The thermal operating constraints have been respected by simulations and the cooling margin of every engine component has followed the regulations imposed by the provider. The installation of the six pressure probes at the primary outlet has revealed a fundamental validation means of the model, because the maximum gap with the results recorded in simulations has been 2.9% and the exhaust back pressure losses have corresponded to the relative experimental measurements.

The nacelle panels scorching issue has been solved, since the exhaust flows have shown to stay far away from the structure without licking it. As a result, the impingement on ground has been reduced, because the mixing of hot gasses with fresh air has determined exhaust temperatures beneath the asphalt melting point and the aircraft take-off does not present any risks for crew on the ground.

AgustaWestland has at its disposal a reliable model thanks to the help of STAR-CCM+, which can be used to simulate any further flight conditions. The addition of new technical changes will be dealt with faster with time and cost reductions for mesh generation. Post-processing activities will be more efficient to find any performance improvements and to address future developments for the best way forward.

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