Abstract: A method for the parameterization of an S-shaped transition duct is presented. The midway offset ratio (MWOR), a parameter closely related to the curvature of the duct, is introduced, to enable the duct geometry to be expressed by only two independent parameters: MWOR and control-point area ratio (CPAR) in a selected axial position. Approaches of controlling the wall pressure gradient through MWOR and the area ratio is then investigated respectively. Finally, the method is applied to the design of a compressor transition duct to explore the design principle. The results show that larger values of MWOR tend to move the loading forward, while they decrease the adverse pressure gradient in the rear half of the inner wall where separation is likely to occur, thus reducing the loss. In addition, the area variation in an increase-decrease sequence along the duct, which can be controlled by CPAR, generates a mean pressure gradient counteracting that of the inner wall, and the integral pressure gradient can be further diminished. For the transition duct with equal area at its inlet and outlet, the loss is significantly decreased with the MWOR between 0.55 and 0.65, and CPAR around 1.1.

Key words: transition duct, S-shaped, fan/compressor, pressure gradient, aerodynamic design