Surge Margin Tracking for Active Control of Quick Windmill Relighting

Abstract

Surge margin tracking logic is developed for use in the control of quick windmill relighting (QWR) at sub-idle. Using existing high pressure compressor (HPC) characteristics (but without any gas turbine engine model), the surge margin can be calculated and used to approximate the air flow which is currently not measured in flight. During the QWR flight test, only limited measurements excluding the airflow measurement are available. Based on the fact that a beta value is equivalent to the position of the throttle valve in a compressor test rig, the role of the beta value is here to interrelate between the PRC (pressure ratio of compressor) and NDMF (non-dimensional mass flow) values for the measured CNH (corrected high pressure spool speed) and PRC values. Using the proposed scaling factors (SFs), the HPC map in terms of PRC is adaptively scaled with the engine parameters to cover the operating pressure ratio of the HPC. These account qualitatively for the effects of heat soakage and stability aids such as bleed and VSV (variable stator vane) on the compressor map. The simulation results show that the variable SF approach is more realistic in estimation of the surge margin, compared to the fixed SF approach. As a result of this proposed surge margin tracking logic, an active control for QWR may be possible using an estimated surge margin to adjust the fuel flow. This improves the pull-away time to reach idle power without danger of stall or surge during QWR.