AN EXPERIMENTAL, AERODYNAMIC EVALUATION OF DESIGN CHOICES FOR A LOW-PRESSURE COMPRESSOR TRANSITION DUCT

The S-shaped duct which transfers flow from the low-pressure fan to the engine core of modern large civil turbofans presents a challenging design problem. Aerodynamically it must accommodate a spatially and temporarily non-uniform inlet in conjunction with a complex flow field which will develop under the combined influence of pressure gradients and streamline curvature. It must also allow for the transfer of structural loads and services across the main gas path. This necessitates the use of structural vanes which can compromise aerodynamics, introduce unwanted component interactions and erode performance. Furthermore, this must all be achieved with minimum length/weight and without flow separation. This paper presents a comprehensive aerodynamic evaluation of three options for a low-pressure compressor transition duct containing (i) a long-chord, structural compressor outlet guide vane, (ii) a more aerodynamically optimal but non-structural outlet guide vane in conjunction with a small number of discrete radial load bearing struts and (iii) a fully integrated outlet guide vane and strut design. Evaluation was performed using a fully annular, low-speed test facility incorporating a 1 1/2 stage axial compressor and transition duct representative of an engine design. Aerodynamic data were produced from miniature five-hole probe area traverses conducted at several locations with compressor/duct. The data suggest that all the options were viable. However, the aerodynamic vane and discrete struts produced the lowest system loss with the other two options being comparable. The performance of the structural vane was seen to be sensitive to off-design conditions producing a notably increased loss at a low flow coefficient. The more optimized aerodynamic vanes were much less sensitive to off-design conditions whilst the fully integrated design showed only very small changes in loss.