A Modelling Approach of Variable Geometry for Low Pressure Ratio Fans

Abstract

This paper presents the development and application of a modelling approach of variable geometry concepts for low pressure ratio fans; namely Variable Area Nozzle and Variable Pitch Fan. An enhanced approach for Outlet Guide Vane pressure loss predictions and an aerothermodynamic analysis of variable pitch concept are developed and integrated into a multi-disciplinary conceptual engine design framework. A streamline curvature algorithm is deployed for the derivation of the off-design fan performance map, alleviating scaling issues from higher pressure ratio fan designs. Correction deltas are derived through the variable pitch analysis for calculating the re-shaped off-design fan performance map. The aforementioned variable geometry concepts are evaluated in terms of surge margin at engine and aircraft level for a low pressure ratio aft-fan of a hybrid-electric configuration. Performance assessments carried out suggest that a +8° closing of fan blade cascades leads to a 33% surge margin improvement (with reference being the surge margin without variable geometry) compared to a 25% improvement achieved by +20% opening the nozzle area at EOR take-off conditions. Although weight and complexity implications of variable geometry are not considered, the integrated modelling approach is shown to be able to assess and compare such novel engine technologies for low pressure ratio fans in terms of operability