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DPS (distributed propulsion system) utilizing BLI (boundary-layer ingestion) has shown great potential for reducing the power consumption of sustainable AAM (advanced air mobility), such as BWB (blended-wing body) aircraft. However, the ingesting boundary layer makes it easier for flow separation to occur within the S-shaped duct, and the consequent distortion due to flow separation can dramatically reduce the aerodynamic performance of the fan, which offsets the power-saving benefit of BLI. By analyzing the source of power saving and power loss of BLI, this paper presents the LDPS (layered distributed propulsion system) concept, in which the freestream flow and boundary-layer flow are ingested separately to improve the power-saving benefit of BLI. In order to preliminarily verify the feasibility of LDPS, an existing DPS is modified. The design parameters and the system performances of LDPS are studied using a 1D engine model. The results show that there is an optimal ratio of the FPR (fan pressure ratio) for the FSE (freestream engine) to the BLE (boundary-layer engine) that maximizes the PSC (power-saving coefficient) of LDPS. This optimal ratio of FPR for the two fans can be obtained when the exit velocities of FSE and BLE are the same. Under the optimal ratio of FPR for the two fans, the PSC of LDPS is improved by 5.83% compared to conventional DPS. [ABSTRACT FROM AUTHOR]

The flow separation occurring in the S-shaped diffuser with boundary layer ingestion (BLI) has a significant effect on the performance of the embedded engine. Previous studies have found that the area ratio (AR) as well as the length-to-offset ratio (LOR) of the S-shaped diffuser are the key contributing factors that affect the flow separation features. Based on the flow phenomena observed in previous studies of an S-shaped diffuser with 100% BLI, a hypothesis that the parameter height-to-radius ratio (HRR) may also have significant effect on the flow separation features in the S-shaped diffuser is proposed. The purpose of this paper is to verify this hypothesis and to further investigate the effect of HRR on the flow separation features in the S-shaped diffuser with BLI using numerical methods. First, the hypothesis that HRR has an effect on the flow separation features in the S-shaped diffuser is verified under uniform inlet condition. Second, the effect of HRR on the flow separation features is investigated under different relative heights of inlet BLI. It is found that the flow separation features in the S-shaped diffuser are very sensitive to the change in HRR but not to the change in relative height of inlet BLI. Finally, for the fixed boundary layer height generated from the airframe, the S-shaped diffuser with a smaller design HRR can significantly suppress the flow separation and thus achieve a higher total pressure recovery and a lower distortion coefficient. The results provide improved understandings of the factor affecting the flow separation features in the S-shaped diffuser, and are useful for improving the aerodynamic performance of the embedded engine with BLI. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The boundary layer ingesting (BLI) fan is continuously working under distorted inflows. The rotor blade moves across the distorted and undistorted regions and the resultant off-design condition makes the loss present a notable non-uniform distribution feature. Accurately capturing the loss is very important for evaluations of BLI fan performances in preliminary design stage. However, unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations are extremely computational expensive even though they can well predict the loss, and the traditional empirical or physics-based loss models are usually fade when predicting loss variation trends and absolute magnitudes under complex inflow conditions. This paper develops a data-driven machine learning based region-segmentation combinational loss model to realize a fast and accurate prediction of the aerodynamic loss in a BLI fan rotor. According to the distinctions of loss sources in different spanwise fractions, a region segmentation idea is first used to create local loss model in each sub-region, and then a full-span combinational prediction model is constructed through a data-driven based neural network approach. Results show that the data-driven based region-segmentation combinational loss model presented in this work performs better than the traditional empirical or physics-based loss models in terms of predicting both the loss variation trends and absolute magnitudes. Compared with the high-fidelity URANS simulations, the averaged loss prediction errors in a BLI fan rotor are kept within 10 % under different blade load and inflow distortion conditions. [ABSTRACT FROM AUTHOR]

Boundary layer ingestion propulsion systems have attracted much attention due to the significant potential to reduce the fuel consumption of future commercial aircraft. However, the fan blade, as a key component, needs to be designed to overcome the challenge of distorted inflow due to the boundary layer ingestion. Besides the negative effects on aerodynamic performance, the aeroelastic issues such as the forced response can be caused by the low engine order due to inflow distortion. To investigate this problem, the transonic fan NASA Rotor 67 with a BLI-type inlet distortion is used as the test case. The vibration mode of a single rotor blade is analyzed and there is an intersection between the 2EO and first-order bending mode at the design rotor speed. As demonstrated by three-dimensional unsteady simulations and modal force analysis, the first-order bending vibration mode of the rotor occurs with relatively high intensity, compared to the modal force of the forced vibration induced by the effects of stator downstream. The results of the Zig-zag-shaped excitation lines in the nodal diameters versus frequency graph present the vibration mode of the full-annulus rotor blade is a 1B-2EO-2ND. Based on the unsteady flow field analysis, the periodic evolution of the flow through a passage due to the various inlet conditions could be the main reason for the propagation of 2EO. [ABSTRACT FROM AUTHOR]