Utilize este identificador para referenciar este registo: http://hdl.handle.net/10400.6/3692
Título: A comparison of different gas turbine engines ground vortex flows
Autor: Manquinho, Pedro Alexandre Rodrigues
Orientador: Barata, Jorge M. M.
Silva, André R. R.
Palavras-chave: Motor -- Vórtices de solo
Motor -- Fluxo
Motor -- Admissão
Motor -- Vento cruzado
Data de Defesa: 2010
Resumo: The design of an engine intake is very important because, it is one of the engine components that directly interface with the flow around the engine and the internal airflow. The inlet is designed to give the appropriate amount of airflow required from the free-stream conditions to the conditions required at the entrance of the compressor with minimal pressure loss by the engine1. When the pressure losses and the flow distortions are very low, the performance of the engine is optimal, and that is the reason why the airflow has to be as uniform as possible when entering into the compressor. This airflow condition is necessary in all flight configurations including when the aircraft is maneuvering on ground tasks. The intake performance depends on the mass-flow delivered to the compressor. The internal mass-flow stays constant from the captured stream tube to the compressor face and assuming that the flow is incompressible due to low speed velocities, it will be given by . Since the mass-flow is constant and the area ratio is related to the stream tube contraction ratio, the area ratio can be expressed as . The capture ratio is controlled by the engine, the engine mass-flow, the inlet diameter and the free-stream velocity. The area defined by the boundary between the air that enters in the engine and the air that does not is called the intake captured area. The flow ratio and the stream-tube shape vary with the operation conditions of the aircraft engine. In near static configuration since the ambient air is at rest, the engine must accelerate the air using maximum thrust. The extreme local acceleration of the flow at the inlet lip can lead to airflow separation in this region2,3. In cross-wind configuration, the shape of the stream tube is modified near the lip. The cross-flow leads to an increase in the flow velocity near the lip depending on the strength of the cross-flow, high velocity origins in flow separation leading to a total pressure loss at the engine fan. The formation of ground vortices depends on engine power, wind velocity and engine inlet height and size. Previous published work show that the phenomenon can only occur with the presence of a stagnation streamline between the ground and the intake which is dependent on the velocity ratio and the non-dimensional height of the engine axis above the ground, h/Di (Fig. 1). In static conditions, the inlet airflow demand increases, and the inlet capture surface increases in diameter and starts including the ground to bring the necessary airflow to the fan4,5. Typically the formation of ground vortices is characterized by low h/Di and high that corresponds to an engine operating close to the ground at a high inlet mass flow6. Hence, the mechanism of intake formation is strongly dependant of the height of the engine axis above the ground, the velocity ratio and the presence of an upstream velocity. Four different types of conditions leading to the formation of inlet ground vortices have been identified. A vortex can be generated without ambient wind and with a low ratio h/Di (typically less than one). Due to the ground proximity, high levels of suction beneath the engine inlet leads into a strong flow underneath the inlet upstream towards the intake lip7. In these conditions, it is possible to visualize at the engine intake and at the ground two upward spiraling vortices. Under no-wind condition, it appears that the two vortices are counter-rotating, and the vorticity is induced by the boundary layer. Is a head-wind flow, when the air is sucked into the engine inlet, the flow field underneath the intake starts to roll up into two upright counter-rotating vortices and a fast flow into the opposite direction of the wind appears between them8,9. For high velocity ratios (>20) the sense of rotation of the two vortices switches to the same as in the no-wind mechanism. With a 90° cross-wind two different kinds of vortices appear around the intake: an inlet vortex and a trailing vortex (Fig.3). When the engine intake is oriented at a 90° yaw angle and with the presence of cross-wind with far upstream vertical vorticity, there is the formation of a single vortex inside the intake. In this case the sense of rotation of the vortex is opposite to the ambient vorticity. Other mechanisms of formation also exist and can be considered as combinations of the previous ones that lead to a large number of possible combinations that are responsible for the need of more studies in order to understand all the physics involved. CFD tools have been applied recently to the understanding of the ground vortex with relative success. Nakayama & Jones10 used panel methods to simulate the inlet and ground interaction, noting that the wind speed needed to blow the vortex away was lower than the measured experimentally. Barata et al.11 report Navier-Stokes calculations and predicted successfully the ground vortex phenomena using real operational conditions for the case of the engine Trent 900. The ground vortex formation in irrotational crosswind flow is analyzed in detail for this configuration, and the formation of the trailing vortex was associated to a very complex flow. In the present paper the previous work of Barata et al.11 is extended to include several engines that are being used in the present. The ground vortex flows produced by different engines are compared and discussed for each operational condition.
URI: http://hdl.handle.net/10400.6/3692
Designação: Dissertação apresentada à Universidade da Beira Interior para a obtenção do grau de mestre em Engenharia Aeronáutica
Aparece nas colecções:FE - DCA | Dissertações de Mestrado e Teses de Doutoramento

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