Stability of split flow fans

Abstract

The performance requirements of turbofan engines demands a stability and transient capability beyond that associated with the past generations of gas turbine engines. The axial flow fan unit is most vulnerable to loading limitations due to the primary problems associated with the compression process, its sensitivity to inlet distortion and the difficulty to design for an overall optimum blade duty in a machine of wide radial blade loading distribution. The development of mathematical models with some capability of predicting the stable operating range of an axial flow fan has to overcome the difficulties associated with the modelling of the radially distinct flow regions and their dynamic interaction. ' The current investigation combined the available knowledge of one-dimensional models (based on the principles of conservation of mass, linear momentum and energy) with the assumptions of the parallel compressor theory, in order to develop a linearized system of equations for stability analysis (surge prediction). The stability conditions which emerged from this approach were applied on the experimentally derived characteristics of a low hub to tip ratio split flow fan in a manner which involved the modelling of the dynamic interaction of the inner and outer flow region of the fan. The development of the governing equations was achieved by applying one-dimensional flow analysis to the inner and outer section of the fan. Their interaction was modelled on the experimentally obtained radial movement of the splitter streamline and the discharge ,static pressure 'radial

distribution. The inner and outer region were treated as a lumped volume element search operating on a local masflow averaged total pressure rise characteristic and alternatively acting in conjunction with a common nozzle and separate nozzles. The experimental investigation was carried out on a low hub totipratio two-stage split flow fan(with the facility of independent bypass and core throttles)in order to examine the localised and overall performance of such a fan(and the staling processes involved)and to enable the application of the stability analysis. The influence of reducing the distance between the fan flow spliter and the last bladerowasal so investigated, «The mathematical mode1s predicted the point of dynamic instability within 4.52 of the experimental observed mas flow rate and pressure is value.