The configuration of an axial compressor, including the mean radius, the annulus lines, stage loading or number of stages, flow parameter, work split and stage reactions, are all of them selected in the preliminary design phase. For the success of the final design, to attain the proper selection is mandatory.
A representative geometry of the airfoils is not available at this early stage of the design process. Therefore the former parameters use to be selected based only on the designer prior experience and/or empirical correlations. Under these circumstances, the so called Smith Chart is a valuable tool that can provide simple guidelines to the designer and a preliminary assessment of the compressor efficiency. The use of this chart can be also extended to get the main features of the airfoils, like flow angles, turning, Mach and Reynolds number, diffusion factor, aspect ratio, etc. as well as to compare different design candidates.
Several authors have produced their own diagrams by analytical or semi-empirical approach. The repeating stage hypothesis, which has been usually assumed, implies no change in inlet and outlet absolute flow angles and constant axial velocity throughout the stage. The density rise through the stage is compensated by reducing the annulus height and so the annulus wall slope along the compressor is directly obtained from the continuity equation, being in most of the cases not representative of real compressors.
In order to have a more representative annulus, in the present work, the repeating stage hypothesis has not been assumed. The annulus shape (height and slope wall angle) is therefore defined by the designer and in order to close the equations of the problem, the absolute exit flow angle of every stage is required.
The optimization of the compressor by the novel proposed method is more complicate because of the higher number of variables. However the method has the advantage to reduce the design iterations due to its more reliable results.
The aim of this paper is to introduce the novel method of non-repeating stages and to show how this approach can be used in the preliminary design of an axial compressor.
