Small core axial compressors for high efficiency jet aircraft

[摘要] This thesis quantifies mechanisms that limit efficiency in small core axial compressors, defined here as compressor exit corrected flow between 1.5 and 3.0 lbm/s. The first part of the thesis describes why a small engine core with high overall pressure ratio (OPR) is desirable for an efficient aircraft and shows that fuel burn can be reduced by up to 17% compared to current engines. The second part examines two specific effects: Reynolds number and tip clearance. At a core size of 1.5 lbm/s, Reynolds number may be as low as 160,000, resulting in reductions in stage efficiency up to 1.9% for blades designed for high Reynolds number flow. The calculations carried out indicate that blades optimized for this Reynolds number can increase stage efficiency by up to 1.6%. For small core compressors, non-dimensional tip clearances are increased, and it is estimated that tip clearances can be up to 4.5% clearance-to-span ratio at the last stage of a 1.5 lbm/s high pressure compressor. The efficiency penalty due to tip clearance is assessed computationally and a 1.6% decrease in polytropic efficiency is found for a 1% increase in gap-to-span ratio. At the above clearance, these efficiency penalties increase aircraft mission fuel burn by 3.4%, if current design guidelines are employed. This penalty, however, may be reduced to 0.4% if optimized blades and a smaller compressor radius than implied by geometric scaling, which allows reduced non-dimensional clearance, are implemented. Based on the results, it is suggested that experiments and computations should be directed at assessing: (i) the effects of clearance at values representative of these core sizes, and (ii) the effect of size on the ability to achieve a specific blade geometry and thus the impact on loss.

[发布机构] Massachusetts Institute of Technology