Design of a Compact Crossover Diffuser for Micro Gas Turbines Using a Mean-Line Code

Abstract

The design and validation of a Compact Crossover Diffuser (CCD) to replace the size-limited radial diffuser and axial de-swirl cascade of an existing Micro Gas Turbine (MGT) is discussed. A CCD strives to combine the performance of a channel diffuser with the operating range and efficiency of a vaneless diffuser. The development of a one-dimensional Mean-Line Code (MLC) is presented, which aids the designer in preliminary design and performance evaluation of the CCD. Design graphs indicating the performance effects of changing the primary design variables are developed and shown. The MLC is numerically validated using Computational Fluid Dynamics (CFD). Good agreement is seen between the MLC and CFD results, predicting the design point PRss(2-4) to within 1.4 %. A CFD optimized CCD was manufactured and tested. Agreement between the CFD and experimental results for PRts(0-4) is within 7.58 % at 106 kRPM. A numerically predicted increase in PRts(0-4) from 3.31, to 3.53, to 3.83 is seen for the vaneless-, MLC optimized-, and CFD optimized-design respectively. An experimental increase of 82.3 % in engine thrust and 80.0 % in total-to-static pressure recovery across the compressor stage was measured when retrofitting the BMT120KS with a new impeller and CCD.

A Large Design Space Multidisciplinary Optimization of a Mixed Flow Micro Gas Turbine Compressor Stage

“…The meridional view of a crossover diffuser is shown in Figure 2.4, including state numbering conventions used in this thesis. A vaned crossover diffuser can replace a conventional wedge diffuser with accompanying vaneless radialto-axial bend and subsequent de-swirling vanes, with a single row of curved or swept blades (Burger, 2016). Figure 2.4: Mixed flow wedge diffuser (left), and crossover diffuser (right) Figure 2.5 gives the diffuser flow angle and magnitude numbering definitions used in this thesis.…”

“…Flow entering the combustion chamber is required to meet a specified static pressure ratio, P R (01−4),DP , and swirl angle, α z4 . Additionally, a flow with low distortion of pressure and velocity in the tangential direction is desired to simplify the combustor design process (Burger, 2016). Krige (2013) and de Villiers (2014) noted significant flow irregularities in their respective radial compressors due to recirculation in the 90 • vaneless bend.…”

“…Channel diffusers, such as those used by Krige (2013), are typically used for UAV application; however, large losses are incurred in the radial-to-axial vaneless bend in such a radially constrained compressor. Successful implementation of a compact crossover type diffuser by Burger (2016), Kock (2017), and Jie and Guoping (2010) have shown a reduction in losses incurred in the radial-to-axial diffuser bend by use of a continuous vane to assist in flow turning. Jie and Guoping (2010) report a static pressure recovery coefficient, C p , of 0,65 and 11% improvement in engine thrust when compared to a standard channel diffuser.…”

“…This was introduced by a swept diffusion passage, achieving a 40% increase in static pressure recovery coefficient to a C p of 0,65. Successive optimizations and flow investigations of crossover diffusers were conducted by Burger (2016) and Kock (2017). A patent was issued for Tarnowski and Bulot (2018) which describes the crossover diffuser as mentioned above.…”

“…This necessitated a compact diffuser design of high diffusion rate to fit within the geometric constraint. The relaxed stage discharge swirl requirement further benefited the decision of using a crossover diffuser, which was found by Burger (2016) to perform well at higher discharge swirl angles due to the absence of additional de-swirling vanes.…”