Boundary Layer Ingestion Inlet Research Articles

Behavior of flow distortion within a boundary layer ingestion inlet

One of the critical problems restricting the development of boundary layer ingestion (BLI) propulsion systems is the strong distortion and swirling flow in the inlet, which will threaten the stable operation of the fan. In this study, the effects of the Exit Mach number (MAIP), the ingested boundary layer thickness (δ) on the internal flow and flow distortion at the aerodynamic interface plane (AIP), are investigated under a freestream Mach number of 0.75. The experimental results indicate that a low total pressure region persistently located at the lower part of the AIP accompanied by a pair of vortices with a maximum swirl angle of 23° With the decrease of exit Mach number, these vortices move toward the center of the AIP, which decreases the total pressure distortion while increases the swirl intensity. However, as the ratio of the ingested boundary layer thickness to the inlet height (δ/h) increases from 18.6 % to 83.8 %, the length-scale of these vortices at AIP increases, leading to a descending trend of the total-pressure recovery coefficient from 0.942 to 0.871. The total pressure distortion gets intensified from 0.34 to 0.42, with the mean swirl intensity (SImean) dropping from 8° to 6.7° The influence of the δ/h on SImean could be attributed to the enhancement of SI in the outer ring of AIP. Meanwhile, the validated numerical results of δ/h = 83.8 % and δ/h = 43.3 % show that the vortices at AIP are originated from the coalescence of two vortices. MAIP has a substantial influence on the origin of the vortex, separation positions, and pressure distributions of the inlet. The flow separation position is associated with total pressure distortion at AIP. Further optimization design can be considered by controlling the flow separation position.

Numerical Investigations of a Non-Uniform Stator Dihedral Design Strategy for a Boundary Layer Ingestion (BLI) Fan

A distributed propulsion system has the advantage of saving 5–15% fuel burn through ingesting the fuselage boundary layer of an aircraft by fan or compressor. However, due to boundary layer ingestion (BLI), the fan stage will continuously operate under serious inlet distortion. This will lead to a circumferentially non-uniform flow separation distribution on the stator blade suction surface along the annulus, which significantly decreases the fan’s adiabatic efficiency. To solve this problem, a non-uniform stator dihedral design strategy has been developed to explore its potential of improving BLI fan performance. First, the stator full-annulus blade passages were divided into blade dihedral design regions and baseline design regions on the basis of the additional aerodynamic loss distributions caused by BLI inlet distortion. Then, to find the appropriate dihedral design parameters, the full-annulus BLI fan was discretized into several portions according to the rotor blade number and the dihedral design parameter investigations for dihedral depth and dihedral angle were conducted at the portion with the largest inflow distortion through a single-blade-passage computational model. The optimal combinational dihedral design parameter (dihedral depth 0.3, dihedral angle 6 deg) was applied to the blade passages with notable flow loss which were mainly located in the annulus positions from −120 to 60 degrees suffering from inlet distortion, while the blades in the low-loss annulus locations were unchanged. In this way, a non-uniform stator dihedral design scheme was achieved. In the end, the effectiveness of the non-uniform stator dihedral design was validated by analyzing the internal flow fields of the BLI fan. The results show that the stator dihedral design in distorted regions can increase the inlet axial velocity and reduce the aerodynamic load near the blade trailing edge, which are beneficial for suppressing the flow separations and reducing aerodynamic loss. Specifically, compared with the baseline design, the non-uniform stator dihedral design has achieved a reduction of aerodynamic loss of about 7.7%. The fan stage has presented an improvement of adiabatic efficiency of about 0.48% at the redesigned point without sacrificing the total pressure ratio. In the entire operating range, the redesigned fan has also shown a higher adiabatic efficiency than the baseline design with no reduction of the total pressure ratio, which provides a probable guideline for future BLI distortion-tolerant fan design.

Study on the aerodynamic performance of boundary-layer-ingesting inlet with various geometries

The mixing loss in S-shaped duct causes the contradiction between the total pressure recovery and circumferential distortion, which confuses the design process for boundary layer ingestion (BLI) inlet. This article is aiming to present a detailed understanding of the effect of two kinds of loss resources (mixing loss and frictional loss) on the performance for various BLI inlet geometries. To achieve the above objective, a design method including three control parameters is established. With this method, a systematic investigation of control parameters on the performance of BLI inlet is carried out, which leads to the contour map for the performance of BLI inlet. Based on the map, the flow physics of BLI inlet with two kinds of losses is then analyzed. Finally, an empirical design principle is provided. The results show that the appropriate design for BLI inlet has the potential to increase the total pressure recovery or reduce the circumferential distortion.

Non-uniform stator loss reduction design strategy in a transonic axial-flow compressor stage under inflow distortion

In a boundary layer ingesting (BLI) propulsion system, the fans/compressors continuously operate at a highly distorted inflow condition due to ingesting the boundary layers on the aircraft fuselage. The BLI inlet distortion induces non-uniform whirl and axial velocities at the rotor outlet, leading to a circumferential and radial non-uniform incidence distribution for the downstream stator. One of the main challenges for the compressor performance is a bad match between the rotor outflow and the stator inlet geometry, which would cause flow separations in blade passages. This paper has employed a non-uniform stator loss reduction design strategy to make a redistribution of stator incidence at the compressor off-design working point so as to reduce the potential separation loss and enhance the stage aerodynamic performance. Firstly, three-dimensional full-annulus unsteady numerical simulations are carried out to reveal the specific flow features of the non-uniform incidence distribution at the stator inlet. Then, according to the circumferential and radial non-uniform incidence distribution, a spatial non-uniform stator loss reduction design strategy based on adjusting the local inlet metal angles of the blade airfoil sections is utilized to make it a good match between the rotor outflow and the stator inlet geometry. Finally, the inner flow fields are analyzed to test the effectiveness of the non-uniform loss reduction design strategy. The computational results indicate that the high-incidence regions at the stator inlet are notably reduced and a more uniform incidence distribution has been achieved along both the circumferential and radial directions. Moreover, the non-uniform loss reduction design has successfully suppressed the flow separations at both the hub and tip regions. As a result, the stator has achieved an 18.2% reduction of the aerodynamic loss at the re-designed point and the stage efficiency has been improved by 0.83%. In the meanwhile, the re-designed stator has decreased the aerodynamic losses over the whole compressor operating range, and the re-designed stage has also achieved a higher efficiency than the baseline one while maintaining the total pressure ratio.

Flow simulation and optimal shape design of N3-X hybrid wing body configuration using a body force method

System studies show that NASA's N3-X hybrid wing-body aircraft with a turboelectric distributed propulsion system using a mail-slot inlet/nozzle nacelle can meet the environmental and performance goals of N+3 generation transports (three generations beyond the current air transport technology level). In this paper, we present flow calculations and shape optimization of the integrated N3-X configuration, including the mail-slot propulsor, based on an unstructured-mesh Navier–Stokes solver. To simulate the propulsor fans flow, a novel body force model is suggested for a realistic and efficient representation of flow turning, pressure rise and losses resulting from the fan blades and the inlet-fan interactions for distorted inflow conditions. This work is the first of its kind analysis and design study on boundary layer ingestion inlet and fan interaction problems using the body force method. A mail-slot geometry was generated and installed on the N3-X configuration with an efficient CAD-free method. An optimal shape design of the mail-slot nacelle surface with the adjoint method was successfully conducted to reduce the strength of shock waves and flow separations on the cowl surface.

Analysis of Fan Stage Conceptual Design Attributes for Boundary Layer Ingestion

This paper describes a new conceptual framework for three-dimensional turbomachinery flow analysis and its use to assess fan stage attributes for mitigating adverse effects of inlet distortion due to boundary layer ingestion (BLI). A nonaxisymmetric throughflow analysis has been developed to define fan flow with inlet distortion. The turbomachinery is modeled using momentum and energy source distributions that are determined as a function of local flow conditions and specified blade camber surface geometry. Comparison with higher-fidelity computational and experimental results shows the analysis captures the principal flow redistribution and distortion transfer effects associated with BLI. Distortion response is assessed for a range of (i) design flow and stagnation enthalpy rise coefficients, (ii) rotor spanwise work profiles, (iii) rotor–stator spacings, and (iv) nonaxisymmetric stator geometries. Of the approaches examined, nonaxisymmetric stator geometry and increased stage flow and stagnation enthalpy rise coefficients provide the greatest reductions in rotor flow nonuniformity, and may offer the most potential for mitigating performance loss due to BLI inlet distortion.

Jet Engine Fuel Burn Reduction Through Boundary Layer Ingestion

This article explains various technical aspects of the boundary layer ingestion (BLI) concept. Using BLI, airliner designs featuring close-coupled, rear-mounted turbofans are being considered, with a fuselage sculpted to sweep a large part of the fuselage boundary layer into engine inlets for reduced fuel consumption. With an engine array fuselage-centered, rather than splayed out on wings, reduced rudder control is needed in the event of a single engine outage. This reduces the size of a BLI tail assembly, saving weight and reducing drag. A near-future goal of the BLI studies is to determine if modern engine front-mounted fans can be designed to operate efficiently and stably under BLI inlet conditions. The D8 design is aimed at the huge single-aisle, narrow-body market, now dominated by the Boeing 737 and Airbus 320 families. Airframe and engine designers strive to achieve 'clean' inlet flow conditions for jet engines.