Feasibility of Mixed Flow Compressors in Aero Engines

The term, “mixed flow compressor”, refers to a type of compressor that combines axial and radial flow paths. This phenomenon produces a fluid outflow angle somewhere between 0 and 90 degrees with respect to the inlet path.  Referred to as the meridional exit angle, the angled outflow of this mixed flow configuration possesses the advantages of both axial and centrifugal compressors.  Axial compressors can produce higher order efficiencies for gas engines, but they have relatively low-pressure ratios unless compounded into several stages.  Centrifugal compressors can produce high-pressure ratios in a single stage, but they suffer from a drop in efficiency.  The geometrical distinction of mixed flow compressors allows for higher efficiencies while maintaining a limited cross-sectional area.  The trade-off for a mixed flow compressor when introduced to aero gas turbines is that there is an associated weight increase due to the longer impellers needed to cover this diagonal surface.  However, when related to smaller gas turbines, the weight increase becomes less significant to the overall performance of the engine.

Figure 1 - Mixed Flow Compressor Arrangement in AxSTREAM
Figure 1 – Mixed Flow Compressor Arrangement in AxSTREAM

Since the advent of more advanced Unmanned Air Vehicles (UAVs) in the 1990’s, successful integration of gas turbines into these aircrafts required high performance and lower cross-sectional areas. These requirements facilitated the introduction of mixed flow compressors as a strategic alternative. In order to analyze the feasibility of these types of compressors for aero engines, several tactics must be put in place to ensure the design is both effective and reliable. With the use of a structured database and various analysis methods, the designer can ensure an accurate study of this proposed alternative for smaller gas turbines. Design of Experiment (DoE) methods study the effect that multiple variables have on the outcome of the system simultaneously. Multiple parameters must be considered before considering this mixed flow arrangement as a feasible design. The engineer must look at the variation of the pressure ratio and flow coefficient with the meridional exit flow angle. As well, studies on the effects that different pressure ratios, meridional exit flow angles, and power variations have on the mass flow rate of the system are crucial to the design. All of these simultaneous parameters and objectives must be analyzed within a proper database to guarantee an optimized design. To learn more about the DoE optimization methods seen on SoftInWay’s AxSTREAM platform please follow this link: http://www.softinway.com/software-functions/optimization-doe/

 

References:

http://www.nal.res.in/cfdupload/Udocument/15.PDF

https://etd.lib.metu.edu.tr/upload/12611105/index.pdf

 

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