Session: 01-11 Modelling, Simulation and Validation III

Paper Number: 128701

128701 - On Hysteresis in a Variable Pitch Fan Transitioning to Reverse Thrust Mode and Back 

A Variable Pitch Fan (VPF) can be used to generate reverse thrust by changing the direction of the airflow ingested by the fan rotor. This can lead to significant mission fuel burn benefits of future aircraft as the bulky nacelle based thrust reversers are no longer required opening up the design space for additional gains with the implementation of low installation drag ‘slim-line’ nacelles. The flow transition, separation and reattachment through the engine as the VPF transitions from the nominal operation mode to the reverse thrust mode and backward is one of the key explorations required to evaluate the performance of the VPF as it recovers from a unique stall condition. The reverse thrust VPF was initially explored by NASA in turbomachinery design development programs, the Advanced Ducted Propulsor (ADP) and Quiet Clean Short-haul Efficient Engine (QCSEE), where the focus was mostly on simple isolated static configurations. Even though the evolution of flow features as the VPF transitions from the nominal operation setting to the reverse thrust mode at installed, dynamic aircraft landing conditions has been investigated by the authors, insights of the VPF recovery from reverse thrust stall conditions are necessary to ascertain the feasibility of engineering the reverse thrust VPF for future high bypass ratio engines.

In this study, an integrated airframe-engine-VPF research model is used to explore the hysteresis effect of VPF blade transitioning from nominal operation to reverse thrust mode and backward. The model consists of a modern 40000lbf geared high bypass ratio engine with complete representation of the internal bypass nozzle flow path, and a nacelle that is attached to a twin-engine airframe in landing configuration through a pylon. The dynamic flow field evolution is computed by fully transient URANS simulations while a wall motion transfer function is imposed to the fan blade aerofoils to mimic their physical rotation. An automated mesh quality probe and update routine is implemented to ensure optimal mesh quality throughout the simulation. The VPF takes 1.5 seconds to transition to the reverse thrust position and another 1.5 seconds to rotate back at the nominal forward flow setting. The timestep for the URANS analysis is 1 millisecond and the fan aerofoil rotates 0.1° at each timestep. This gradual step deformation along with the automated mesh update routine enables a high quality, near ‘real-time’ simulation of the complete transition. The core engine powering the fan is set at typical ‘Approach Idle’ conditions and the hysteresis loop is analysed for a range of typical fan rotational speeds as well as aircraft landing speeds.

As the VPF transitions to the reverse thrust mode, the conventional forward flow through the engine at the normal operation mode develops and recirculation regions are formed initially with reduction in through flow velocity until a reverse flow from the bypass nozzle to the fan passages is established. This unique condition can be considered as stall due to the fan blade rotation, and the fan recovers from this flow field, which is characterised by large separation zones and adverse pressure gradients to its initial nominal forward flow operation. The transient development of the various flow features through different stations of the engine flow path are discussed in detail in the paper. It is observed that the reverse stream is present for a longer period by 10 degrees of stagger angle approximately as the VPF transitions from the reverse thrust setting to the forward flow mode. The hysteresis loop is described in terms of ingested mass flow rates and fan power absorption. Moreover, the development of decelerating force on the airframe and the range of distorted flow conditions at the core engine inlet are explored too. The flow physics during the VPF recovery from the reverse thrust mode as described in this study is critical in understanding the ability of the VPF to fully recover as well as its performance levels during the initial phase of the landing run.

Presenting Author: Dimitrios Vitlaris Cranfield University

Presenting Author Biography: Dimitrios graduated with distinction in his Bachelor's degree in Mechanical Engineering from the University of Western Macedonia. In his final year, he utilised his dual-degree scholarship award and enrolled in the Computational Fluid Dynamics Master's course at Cranfield University. He is currently a PhD researcher within the Rolls-Royce University Technology Centre at Cranfield University working on aerodynamic investigations of a high bypass ratio turbofan engine using a variable pitch fan in reverse thrust conditions.

Authors:

Dimitrios Vitlaris Cranfield University
David John Rajendran Cranfield University
Richard Tunstall Rolls-Royce plc
John Whurr Rolls-Royce plc
Vassilios Pachidis Cranfield University