Aerothermodynamic Benefits of Mixed Exhaust Turbofans

Abstract

The preference for mixed exhaust turbofan installations in corporate, regional, and in some commercial airliners is primarily because of the claimed improvement in thrust and Specific Fuel Consumption (SFC) with mixing. This improvement (2 to 3%) has been demonstrated in prior component tests of mixed exhaust configurations in powered wind tunnels. This paper presents back-to-back whole engine cycle studies of both mixed exhaust (Figure 1) and separate exhaust turbofans (Figure 2) with design By-pass Ratios (BPR) of 4, 6, 7, and 8 toward predicting the improvement for the whole engine system. For all configurations modeled the top of climb (TOC,35000’/0.80) net thrust (FN) was kept constant (9000 lb) resulting in different core sizes and fan diameters for the clean sheet designs. The model calculations included propulsive efficiency, core efficiency, thermal efficiency, and transmission efficiency. Comparison between mixed exhaust and separate exhaust configurations with the same design BPR showed an altitude SFC advantage with mixed exhaust designs up to a design BPR of 7 (2 to 3% for BPR of 4-6), primarily resulting from a higher transmission efficiency. However, for design BPR of 8 the mixed exhaust design showed worse SFC than the separate exhaust turbofan. Also, a comparison of gross thrust (FG) was made at the off-design condition of Sea Level Take-Off (SLTO) when both models were run to the same fuel flow. Again the mixed exhaust design showed a higher gross thrust up to a design BPR of 7, and a loss in thrust for a design BPR of 8. The improvement with mixing (1 to 2% in FG) has been attributed to higher mixed temperature upstream of the common nozzle as explained in the paper. In addition to the back-to-back cycle study, a standalone MATLAB model of the mixer/nozzle was constructed requiring as input the entering separate stream properties available from cycle data. Calculations of the stand alone model showed a similar trend as the cycle model results. The reason for the decrease in performance with mixing for BPR of 8 has been attributed to both reduced mixed gas temperature upstream of the common nozzle and increased mixer pressure loss from increased mixer Mach number. Literature on experimental and numerical investigations of mixer designs toward improving mixer pressure loss while achieving the required mixing effectiveness is listed.