Influence of inlet distortion on fan aerodynamic performance

Abstract

Inlet distortion can lead to loss in efficiency and stability margin of the fan which in return jeopardises flight safety. These aspects driven by inlet distortion are becoming increasingly challenging for the designers of next-generation turbofan engines. With the increase of computational capability and improvements in numerical models, computational fluid dynamics (CFD) is becoming increasingly powerful and favoured by scientific researchers and industrial engineers. Time-accurate, high-fidelity CFD simulations of the compressor behaviour at extreme operational conditions (such as during stall with inlet distortion) has become possible. In the present thesis, CFD is used to determine the effects of inlet distortion on fan aerodynamic stability and stall hysteresis. NASA stage 67 is used for this study. At the very beginning an appropriate numerical strategy was developed and validated with extensive experimental data. A good match was obtained for both the flow field variables at the peak efficiency point and the stall boundary. In this research, two types of inlet distortion were examined. One is the consistent distortion which mimics the setup in experiments with slow throttling; another type is the abrupt distortion (due to sudden maneuver) whose effect is poorly understood. It will be shown that abrupt distortion can result in larger stall margin loss than consistent distortion. Therefore, experimental tests based on consistent distortion tend to be more optimistic in stall margin prediction. Thereafter the stall and recovery process of a transonic fan with both types of inlet distortions were performed. The results showed that the stall process with inlet distortion can be very different from that in uniform inflow. However, distortion has minor effect on the recovery point (corrected mass flow) of the fan and the clean flow region plays the most important role in the recovery process. In the presence of abrupt distortion, it was found that the stall margin of a fan can be influenced by the length of exit duct. This phenomenon was explained using the wave propagation theory. A shorter exit duct reduces the time lag of expansion pressure wave reflected from the nozzle, which upon arriving at fan trailing edge can prevent the fan from stalling. A critical length ratio was proposed which provides useful guidelines on test rig and engine design. A preliminary study of the behaviour of the BLI fan with serpentine intake (S intake) at near stall condition and its stall process was performed. It was found that the distortion pattern upstream of the fan is complex and can be divided into different zones radially. The stall behaviour of the fan is similar to that with a circumferential distortion, but more complex because of the coupling with the swirl distortion near the casing. Although the present work is restricted to NASA stage 67, some of the conclusions gained are general and expected to be valid for modern fan and compressor designs. Finally, during this research it has become apparent that there is a significant lack of open published measured data for fans and compressors operating under inlet distortions, which is mainly due to the difficulties and costs involved in setting up such experimental campaigns. The above indicates that validated CFD codes are going to play an even more important role in development of distortion tolerant fans. The objective of this work is to show the suitability of CFD for the modelling of fan aerodynamic performance and stability with inlet distortion, which can provide an economical alternative strategy to subscale rig tests.