David J. Arend

Aircraft System Study of Boundary Layer Ingesting Propulsion

A trade-factor-based system study has been carried out to identify fuel burn benefits associated with boundary layer ingestion (BLI) for generation-after-next (N+2) aircraft and propulsion system concepts. The analysis includes detailed propulsion system engine cycle modeling for a next-generation, Ultra-High-Bypass (UHB) propulsion system with BLI using the Numerical Propulsion System Simulation (NPSS) computational model. Cycle modeling was supplemented with one-dimensional theory to identify limiting theoretical BLI benefits associated with the blended wing body reference vehicle used in the study. The system study employed low-order models of engine extractions associated with inlet flow control; nacelle weight and drag; fan performance; and inlet pressure losses. Aircraft trade factors were used to estimate block fuel burn reduction for a long-range commercial transport mission. Results of the study showed that a 3-5% BLI fuel burn benefit can be achieved for N+2 aircraft relative to a baseline high-performance, pylon-mounted, UHB propulsion system. High-performance, distortion-tolerant turbomachinery, and low-loss, low-drag inlet systems, were identified as key enabling technologies. Larger benefits were estimated for N+3 configurations for which larger fractions of aircraft boundary layer can be ingested.

Preliminary Design for Embedded Engine Systems

This paper presents a collaborative research program to advance the analysis and design capabilities for embedded engines in Blended Wing Body Boundary Layer Ingesting (BWB- BLI aircraft. Embedded engine propulsion systems for hybrid wing/body aircraft face several key technical issues stemming from the ingestion of the low-momentum boundary layer to the close coupling of the airframe and propulsion systems in ways that challenge current design procedures and require a system design perspective. To address these challenges, a new high performance flow-control-enabled embedded engine inlet was designed. The inlet bleed flow control actuation was integrated with the aircraft Environmental Control System (ECS), thus providing a viable robust inlet design with improved system performance while supplying the flow required by the ECS. Without flow control, a thick ingested boundary layer leads to high levels of distortion at the AIP plane and impacts the overall fan performance and operability. The impact of inlet distortion on fan performance was estimated through numerical simulations based on unsteady flow simulations. Of particular interest, is the increased flow unsteadiness in the fan passages and its associated detrimental impact on blade aeromechanics and acoustic responses. The proposed flow control solution is predicted to have significant reduction in the harmonic content associated with blade strain. In addition, reductions in total radiated acoustic power are predicted to be achievable, especially at well cut-on modes, significantly reducing aircraft noise at approach conditions. These results suggest that inlet flow control technology has the potential to address the challenges of BWB-BLI aircraft.

Aerodynamic Analysis of a Boundary-Layer-Ingesting Distortion-Tolerant Fan

The paper describes the aerodynamic CFD analysis that was conducted to address the integration of an embedded-engine (EE) inlet with the fan stage. A highly airframe-integrated, distortion-tolerant propulsion preliminary design study was carried out to quantify fuel burn benefits associated with boundary layer ingestion (BLI) for “N+2” blended wing body (BWB) concepts. The study indicated that low-loss inlets and high-performance, distortion-tolerant turbomachines are key technologies required to achieve a 3–5% BLI fuel burn benefit relative to a baseline high-performance, pylon-mounted, propulsion system. A hierarchical, multi-objective, computational fluid dynamics-based aerodynamic design optimization that combined global and local shaping was carried out to design a high-performance embedded-engine inlet and an associated fan stage. The scaled-down design will be manufactured and tested in NASA’s 8′×6′ Transonic Wind Tunnel. Unsteady calculations were performed for the coupled inlet and fan rotor and inlet, fan rotor and exit guide vanes. The calculations show that the BLI distortion propagates through the fan largely un-attenuated. The impact of distortion on the unsteady blade loading, fan rotor and fan stage efficiency and pressure ratio is analyzed. The fan stage pressure ratio is provided as a time-averaged and full-wheel circumferential-averaged value. Computational analyses were performed to validate the system study and design-phase predictions in terms of fan stage performance and operability. For example, fan stage efficiency losses are less than 0.5–1.5% when compared to a fan stage in clean flow. In addition, these calculations will be used to provide pretest predictions and guidance for risk mitigation for the wind tunnel test.Copyright

Active Control of Flow in Serpentine Inlets for Blended Wing-Body Aircraft

The Blended Wing-Body commercial aircraft concept promises performance improvements in noise, emissions, and fuel consumption. Highly integrated airframepropulsion systems featuring embedded engines offer further improvements. Embedded engine systems are envisioned which require Boundary Layer Ingesting (BLI) serpentine inlets to provide the needed airflow to the engine. Due to the ingestion of a large boundary layer as well as the geometry of the serpentine inlet, significant flow distortions are developed that will affect engine performance and the stability of the fan. A bleed flow control system was tested that utilized no more than 2% of the total inlet flow. Two bleed slots were employed, one near the entrance of the BLI inlet and one near its aft exit. The bleed system successfully reduced inlet distortions by as much as 30%, implying improvements in stall margin and engine performance.

Generation After Next Propulsor Research: Robust Design for Embedded Engine Systems

The National Aeronautics and Space Administration, United Technologies Research Center and Virginia Polytechnic and State University have contracted to pursue multi-disciplinary research into boundary layer ingesting (BLI) propulsors for generation after next environmentally responsible subsonic fixed wing aircraft. This Robust Design for Embedded Engine Systems project first conducted a high-level vehicle system study based on a large commercial transport class hybrid wing body aircraft, which determined that a 3 to 5 percent reduction in fuel burn could be achieved over a 7,500 nanometer mission. Both pylon-mounted baseline and BLI propulsion systems were based on a low-pressure-ratio fan (1.35) in an ultra-high-bypass ratio engine (16), consistent with the next generation of advanced commercial turbofans. An optimized, coupled BLI inlet and fan system was subsequently designed to achieve performance targets identified in the system study. The resulting system possesses an inlet with total pressure losses less than 0.5%, and a fan stage with an efficiency debit of less than 1.5 percent relative to the pylon-mounted, clean-inflow baseline. The subject research project has identified tools and methodologies necessary for the design of next-generation, highly-airframe-integrated propulsion systems. These tools will be validated in future large-scale testing of the BLI inlet / fan system in NASAs 8 foot x 6 foot transonic wind tunnel. In addition, fan unsteady response to screen-generated total pressure distortion is being characterized experimentally in a JT15D engine test rig. These data will document engine sensitivities to distortion magnitude and spatial distribution, providing early insight into key physical processes that will control BLI propulsor design.

Development of a Flow Field for Testing a Boundary-Layer-Ingesting Propulsor

The test section of the 8by 6-Foot Supersonic Wind Tunnel at NASA Glenn Research Center was modified to produce the test conditions for a boundary-layer-ingesting propulsor. A test was conducted to measure the flow properties in the modified test section before the propulsor was installed. Measured boundary layer and freestream conditions were compared to results from computational fluid dynamics simulations of the external surface for the reference vehicle. Testing showed that the desired freestream conditions and boundary layer thickness could be achieved; however, some non-uniformity of the freestream conditions, particularly the total temperature, were observed.

Development of a Rotating Rake Array for Boundary-Layer-Ingesting Fan-Stage Measurements

The recent Boundary-Layer-Ingesting Inlet/Distortion Tolerant Fan wind tunnel experiment at NASA Glenn Research Center’s 8by 6-foot Supersonic Wind Tunnel (SWT) examined the performance of a novel inlet and fan stage that was designed to ingest the vehicle boundary layer in order to take advantage of a predicted overall propulsive efficiency benefit. A key piece of the experiment’s instrumentation was a pair of rotating rake arrays located upstream and downstream of the fan stage. This paper examines the development of these rake arrays. Pre-test numerical solutions were sampled to determine placement and spacing for rake pressure and temperature probes. The effects of probe spacing and survey density on the repeatability of survey measurements was examined. These data were then used to estimate measurement uncertainty for the adiabatic efficiency.

Data Analysis Techniques for Fan Performance in Highly-Distorted Flows from Boundary Layer Ingesting Inlets

The design of a unique distortion-tolerant fan for a high-bypass ratio boundary-layer ingesting propulsion system has been completed and a rig constructed and tested in the NASA Glenn 8’x6’ wind tunnel. Processing the data from the experiment presented some interesting challenges because of the complexity of the experimental setup and the flow through the test rig. The experiment was run in three phases, each of which employed a unique complement of inlet throat and fan face instrumentation to avoid the blockage that would have resulted from simultaneously installing all of the rakes. The measurement from the individual test points were subsequently combined to compute the overall stage performance. A CFD model of the experiment was used to gain understanding of the flow field and to test some of the techniques proposed for interpolating and extrapolating the measurements into regions where measurements were not made. This capability became extremely useful when it was discovered that there was an unexpected total temperature distortion in the tunnel. The CFD model was modified by inserting a total temperature profile at the upstream boundary that mimicked the measured distortion where measurements were available and that CFD solution was used to investigate methods to infer the complete total temperature field at the fan face.

Performance Calculations for a Boundary-Layer-Ingesting Fan Stage from Sparse Measurements

A test of the Boundary Layer Ingesting Inlet/Distortion Tolerant Fan was completed in NASA Glenn’s 8by 6-Foot Supersonic Wind Tunnel. Inlet and fan performance were measured by surveys using a set of rotating rake arrays upstream and downstream of the fan stage. Surveys were conducted along the 100% speed line as well as a constant exit corrected flow line passing through the aerodynamic design point. These surveys represented only a small fraction of the data collected during the test. For other operating points, data was recorded as “snapshots” without rotating the rakes which resulted in a sparser set of recorded data. This paper will discuss an approach to the analysis of these additional, lower-measurement-density data points to expand our coverage of the fan map. Several techniques will be used to enhance snapshot data and compare with survey data to assess​ ​the​ ​quality​ ​of​ ​the​ ​approach.