Nature of Transonic Compressor Flow: Importance of 3D Athroat/Ainlet Part I: Subsonic Mach Numbers

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 detailed 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 to 1.30, the pressure rise across the shock is primarily 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 more than 3000 representative transonic compressor and fan designs published online: (https://whittle.digital/). Only the subsonic regime approaching a Mach number of unity is analysed in this paper, whilst the supersonic regime is analysed separately in Part II. Moreover, it is shown 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 the inlet relative flow angles, are shown to have a second-order effect. These findings suggest, that the purpose of the sectional detailed design phase should be solely to make the desired changes in the real spanwise 3D At/A1. The final part of the paper concerns itself with the level of fidelity necessary when calculating the spanwise 3D At/A1, for it to positively influence design; especially while approaching a Mach number of unity when small changes in At/A1 become increasingly important. A key conclusion is that not resolving the subtle changes in the 3D radial flow within the blade passage at the appropriate level of fidelity, especially at the early through/low multistage compressor design stage, could potentially mislead the transonic design process. As a result, this paper advocates the use of 3D CFD, even at this early design stage, for the rapid exploration of future compressor designs.