Preliminary Desing and Optimization of a Power Gearbox for the Ultrafan TM Geared Engine

Abstract

The need for a more efficient and environmentally-friendly aviation has led to the development of low-emission, low-noise engines. Increasing the bypass ratio is known to be an effective solution in order to reduce the specific fuel consumption. This has motivated the development of high-diameter fans. However the increasing difference between the fan and low-pressure turbine diameters poses a great challenge when finding an optimum rotational speed to suit both components. While an excessive rotational speed results in aerodynamic instabilities in the fan blades, a reduced rotational speed increases the required number of low-pressure turbine stages. The high bypass ratio geared turbofan permits to de-couple the fan and low-pressure turbine rotational speed. This allows the development of high bypass ratio engines while precluding the mentioned disadvantages. The main drawback of this technology is the additional weight associated with the gearbox and the heat exchanger required for cooling purposes. The mechanical losses and the often prohibitive dimensions of this arrangement are other of the challenges associated with this design solution. This work undertakes the preliminary design and optimisation of a power transmission gearbox to be used on a geared turbofan. For this purpose a computer-based tool has been developed in order to design the gearbox depending on the power conditions, the gear ratio and the lubricant properties. This approach permits to estimate the gearbox weight and total heat rejection. Performing a multi-objective optimisation to assess the trade-off between weight and efficiency, the most suitable gearbox configurations in terms of these two parameters are presented. The suitability of the designs is finally checked via a stress analysis. The method has been validated against existing gearboxes and the maximum discrepancies in terms of weight, power loss and stress limits do not exceed 10%.The method implemented is to be employed within the Advanced UltraFanTM Engines research activity, performed in Cranfield University Technology Centre (UTC) in performance engineering..