Effect of BLI–Type Inlet Distortion on Turbofan Engine Performance

Abstract

Boundary Layer Ingestion (BLI) is currently being researched as a potential method to improve efficiency and decrease emissions for the next generation of commercial aircraft. While reenergizing the boundary layer formed over the fuselage of an aircraft has many system level benefits, ingesting the low velocity boundary layer flow through a serpentine inlet into a turbofan engine adversely affects the performance of the engine. The available literature has only yielded studies of the effects of this specific type of inlet distortion on engine performance in the form of numerical simulations. This work seeks to provide an experimental analysis of the effects of BLI-type distortion on a turbofan engine's performance. A modified JT15D-1 turbofan engine was investigated in this study. Inlet flow distortion was created by a layered wire mesh distortion screen designed to create a total pressure distortion profile at the aerodynamic interface plane (AIP) similar to NASA's Inlet A boundary layer ingesting inlet flow profile. Results of this investigation showed a 15.5% decrease in stream thrust and a 14% increase in TSFC in the presence of BLI-type distortion.Flow measurements at the AIP and the bypass nozzle exit plane provided information about the losses throughout the fan flow path. The presence of the distortion screen resulted in a 24% increase in mass-averaged entropy production along the entire fan flow path compared to the non-distorted test. A mass-averaged fan flow path efficiency was also calculated assuming an isentropic process as ideal. The non-distorted fan flow path efficiency was computed to be 60%, while the distorted fan flow path efficiency was computed to be 50.5%, a reduction in efficiency of 9.5%. The entropy generation between ambient conditions and the AIP was compared to the entropy production along the entire fan flow path. It was found that the majority of entropy generation occurred between the AIP and bypass nozzle exit. Based on flow measurements at the bypass nozzle exit plane, it was concluded that inlet flow distortion should be located away from the tip region of the fan in order to minimize losses in a very lossy region. It was also determined that the fan and bypass duct process the different regions of the total pressure distortion in different ways. In some regions the entropy production decreased for the distorted test compared to the clean test, while in other regions the entropy production increased for the distorted test compared to the clean test. Finally, it was found that small improvements in total pressure and total temperature variation at the bypass nozzle exit plane will greatly improve the fan flow path efficiency and entropy generation, thereby decreasing performance losses.iii

Numerical study on rotor tip flow loss in a boundary layer ingesting fan under different operating conditions

“…Considering the flow loss generation in a BLI fan/compressor, Gunn and Hall 15 carried out numerical investigations on a low-speed and transonic fan stage with BLI inflow distortion and showed that about 2/3 of the additional loss is generated in the rotor. Meanwhile, the experimental results provided by Lucas et al 16 have shown that the entropy production increased substantially at the blade tip of the fan with BLI inlet distortion, indicating that the flow deterioration in the blade tip is a major contributor of flow loss. Moreover, latest studies have reported that the tip leakage flows have significant impacts on the blade tip flow fields at off-design conditions, 17–19 and deserves more attentions from designers.…”

Non-axisymmetric complete flow conditioning gauzes

“…Traditionally, stagnation pressure distortion is generated by a combination of distortion screens, grids or endwall thickeners (Place et al 1996;Lucas, et al 2014), whilst swirl is generated with inlet guide vanes (Wheeler et al 2006;Houghton and Day 2011). Gunn and Hall (2014) designed and 3D printed a precisely controlled variable porosity gauze to replicate the axial velocity profile found at the intake of a semi-buried BLI fan.…”

Experimental Quantification of Fan Rotor Effects on Inlet Swirl Using Swirl Distortion Descriptors

“…The low energy boundary layer flow that is ingested by the engines in a BLI configuration, along with vortices generated from the leading edge and body of the airframe, creates severe nonuniformities in flow direction and produces inlet swirl distortions propagating to the fan rotor. Left unabated, this nonuniform inlet flow profile can cause significant propulsive efficiency reductions, induce high aeromechanical stresses, and reduce the overall performance of the engine [7,8,[12][13][14][15][16].…”