Nature of Transonic Compressor Flow and Its 3D Design Implications

Abstract A key problem in transonic compressor and fan design is that although a 3D description of the flow is necessary to correctly capture the shock, accounting for it during the sectional design is difficult because the key driving design parameters are unknown. In this paper, it is shown that for inlet relative Mach numbers between 0.85 and 1.10, the pre-shock Mach number is a function of the 3D streamtube area at the throat At over the inlet area A1. This key finding is based on three key transonic flow features, discussed in detail within this paper, being present together across a wide range of 10,000 representative transonic compressor and fan designs published online.1 This unique wide-ranging web-interactive dataset reveals that the effect of changes in the blade geometry, or the 3D streamtube height, on the transonic flow field is one of the same and can be explained simply by keeping track of the associated changes in At/A1. Surprisingly, the pre-shock Mach number at a given At/A1 is shown to be insensitive to the details of the blade surface geometry. Only geometric design choices made in the preliminary design phase, such as the maximum thickness and inlet relative flow angle, are shown to have a second-order effect. These findings suggest that the sectional design phase should focus solely on achieving the desired spanwise 3D At/A1 distribution. The second half of the paper addresses the level of fidelity necessary when calculating the spanwise 3D At/A1, for it to positively influence design; especially when approaching a Mach number of unity. A key conclusion is that failing to resolve the subtle 3D radial flow changes within the blade passage at the appropriate level of fidelity during the early throughflow multistage compressor design stage could mislead the transonic design process. As a result, for the rapid exploration of future compressor designs, this paper advocates utilizing the more than 10,000 transonic design databse to generate an initial 3D blade, which is then assessed early in the design process using At/A1 extracted from 3D CFD.