Aerodynamic design framework for low-pressure compression systems

Aircraft engine manufacturers strive to improve current state of the art designs through continuous development efforts. By improving existing designs and exploring new alternatives, the goal is to reduce the fuel consumption - a topic of high relevance due to the remarkable growth rate of air traffc. To achieve a low fuel consumption, turbofan engines should operate at a high overall pressure ratio which is commonly achieved by\ua0an axial compressor. An axial compressor consists of a set of consecutive stages, each consisting of a rotating and stationary blade row. While a compressor should operate\ua0with a high pressure ratio, it should not operate too close to its stability limit where surge\ua0can occur. If surge occur in the compressor, the compressor blades will be subject to large\ua0transient forces and the ow may even reverse direction. The main focus of this thesis is the further development of an aerodynamic design framework for low-pressure systems,\ua0where an appropriate level of modeling is selected and compressor stages are optimized with respect to effciency and stability. Different approaches are used to evaluate stability\ua0of a compressor stage and it is concluded that the static pressure rise capability of the stage is an appropriate measure to use for ranking designs in an optimization. As a part\ua0of this thesis, all three stages of a three-stage compressor are optimized using steady state\ua0RANS calculations, and the performance of the three-stage compressor is evaluated as an assembly. The possibility of replacing blade geometries to improve part- or design\ua0speed stability of the three-stage compressor is shown. Other aspects which may penalize efficiency are investigated, namely the in uence of surface roughness and manufacturing\ua0variations on performance. The in uence of surface roughness on optimal stage designs\ua0is assessed by optimizing compressor blades with and without taking surface roughness\ua0into account. The impact of manufacturing variations on performance at a design point\ua0is investigated by utilizing measurements of a manufactured compressor blisk

Feasibility Study of a Radical Vane-Integrated Heat Exchanger for Turbofan Engine Applications

The density of liquid hydrogen (LH2), at the normal boiling point, is two times higher than that of highly compressed hydrogen. This makes LH2 the prime candidate for hydrogen storage in aviation. However, LH2 is stored at cryogenic temperatures that require adequate insulation, as well as the integration of heat exchangers to warm up the hydrogen on its way to the combustion chamber. Ideally, the required heat exchangers are strategically placed in the engine core to produce optimum heat management, thus improving the engine efficiency, increase its durability as well as to reduce emissions. Moreover, the combination of hydrogen high specific heat with cryogenic temperatures results in formidable cooling capacity, that can be explored by more compact HEX solutions. The present work numerically investigates a novel concept of a compact compressor vane-integrated heat exchanger, for application in cryogenically fuelled gas turbine engines. The baseline engine used for establishing the HEX requirements is a large geared turbofan, operating on liquid hydrogen. The HEX aero-thermal performance is first estimated using zero-dimensional models and Chalmers in-house gas turbine performance tool GESTPAN. After, the conceptual design of an outlet guide vane-HEX is developed and integrated into a three-stage low-pressure compressor. The baseline compressor geometry is a lightly loaded high-speed booster with a design pressure ratio of 2.8. The multi-stage compressor with the integrated HEX is evaluated using steady-state computational fluid dynamics. Results allow to estimate the heat exchanger performance in terms of total pressure loss, heat transfer effectiveness, and the potential enhanced radial flow turning capability. Further, the impact of the new developed OGV-HEX on the compressor characteristics is also reported and discussed

Multipoint aerodynamic design of a nacelle for an electric fan

Attention to aircraft electrification has been growing quickly since such technology carries the potential of drastically reducing the environmental impact of aviation. This paper describes the re-design of a nacelle for an electric fan, which is developed as part of the EleFanT (Electric Fan Thruster) project. A multipoint nacelle design approach was carried out. Initially the nacelle shapes were optimized for a cruise condition by employing an evolutionary genetic algorithm (GA). The flow field around the nacelles was calculated by conducting 2D axisymmetric computational fluid dynamics (CFD) simulations, and the objective functions were computed by means of thrust and drag bookkeeping. It was found that the optimizer favored two types of nacelle shapes that differed substantially in geometry. The designs were referred to as low spillage and high spillage types. The optimum low spillage and high spillage cases were selected and investigated further by the means of 3D CFD simulations at cruise and at an end of runway takeoff condition, where the nacelle is subjected to high angle of attack. Whilst the low spillage case provided a slightly better performance at cruise, it presented high levels of distortion and boundary layer separation at takeoff, requiring a substantial shape modification. The high spillage case performed well at takeoff; however, supersonic velocities could be observed at the cowling when it was subjected to incoming flow at an angle of attack. Nonetheless, such problem was easily corrected by drooping the inlet. Due to its superior performance at takeoff, the drooped high spillage design type was recommended

The heat transfer potential of compressor vanes on a hydrogen fueled turbofan engine

Hydrogen is a promising fuel for future aviation due to its CO2-free combustion. In addition, its excellent cooling properties as it is heated from cryogenic conditions to the appropriate combustion temperatures provides a multitude of opportunities. This paper investigates the heat transfer potential of stator surfaces in a modern high-speed low-pressure compressor by incorporating cooling channels within the stator vane surfaces, where hydrogen is allowed to flow and cool the engine core air. Computational Fluid Dynamics simulations were carried out to assess the aerothermal performance of this cooled compressor and were compared to heat transfer correlations. A core air temperature drop of 9.5\ua0K was observed for this cooling channel design while being relatively insensitive to the thermal conductivity of the vane and cooling channel wall thickness. The thermal resistance was dominated by the air-side convective heat transfer, and more surface area on the air-side would therefore be required in order to increase overall heat flow. While good agreement with established heat transfer correlations was found for both turbulent and transitional flow, the correlation for the transitional case yielded decent accuracy only as long as the flow remains attached, and while transition was dominated by the bypass mode. A system level analysis, indicated a limited but favorable impact at engine performance level, amounting to a specific fuel consumption improvement of up to 0.8\ua0% in cruise and an estimated reduction of 3.6\ua0% in cruise NOx. The results clearly show that, although it is possible to achieve high heat transfer rate per unit area in compressor vanes, the impact on cycle performance is constrained by the limited available wetted area in the low-pressure compressor

Multidisciplinary assessment of a year 2035 turbofan propulsion system

A conceptual design of a year 2035 turbofan is developed and integrated onto a year 2035 aircraft model. The mission performance is evaluated for CO2, noise and NOx and is compared with a notional XWB/A350-model. An OGV heat exchanger is then studied rejecting heat from an electric generator, and its top-level performance is evaluated. The fan, the booster and the low-pressure turbine of the propulsion system are subject to more detailed aero design based on using commercial design tools and CFD-optimization. Booster aerodynamic modelling output is introduced back into the performance model to study the integrated performance of the component. The top-level performance aircraft improvements are compared to top-level-trends and ICAO estimates of technology progress potential, attempting to evaluate whether there is some additional margin for efficiency improvement beyond the ICAO technology predictions for the same time frame

Aerodynamic Investigation of Air Inlets on Aircrafts with Application of Computational Fluid Dynamics

Air inlets in some form are used on all commercial airliners today. The type of air inlet investigated in this report is a NACA inlet submerged into a surface. This surface is within this thesis a test section wall of a wind tunnel. The considered wind tunnel is TWG in Göttingen (Germany) that operates in transonic speeds. Submerged inlets have the main advantage of low aeroynamic drag from the inlet itself. The importance of reducing drag, and the attention given to this subject is increasing as fuel prices rise as well as public awareness of environmental impact by all of us. The outcome of this thesis contributes to the government-funded project ECOCENTS which deals with the design of innovative new aircraft cooling systems and the detailed flow analysis of these systems. This thesis was carried out at the company Airbus in Bremen, Germany. The main objective of this report was the evaluation of the ram pressure efficiency of four different ramp angles of a NACA inlet and the estimation of the drag caused by these geometries with the use of Computational Fluid Dynamics (CFD). The flow solver used was TAU, a Reynolds-Averaged Navier-Stokes (RANS) solver developed by the German Aerospace Center (DLR). The inlet consisted of one ramp section where the ramp angle was fixed at 7 degrees, and a second variable ramp section. The following different angles were investigated: 4, 7, 10 and 15 degrees. These configurations were evaluated at a velocity of Mach 0.8 and a Reynolds number of 10*10^6. The ramp angle of 7 degrees was evaluated at two additional velocities (Mach 0.73 and Mach 0.87) and at two additional Reynolds numbers (5*10^6 and 15*10^6) at Mach 0.8. The inlet efficiency outcome of this study was located between two other investigations. The results of this RANS computation predicted a higher total pressure at the inlet throat plane compared to a previous CFD investigation where a different RANS solver at the same geometry was used. In comparison to an estimation method mainly based on experimental data (ESDU method), the recent study showed a lower total pressure at the inlet throat plane. The aerodynamic drag that arised by the presence of the inlet system was calculated within this thesis to be higher than the outcome of the experimental data based (ESDU) method. The advantage of using a NACA type inlet was observed to be highly related to the ramp angle. Vortices are originated and develop along the edges of the intake ramp walls. These two vortices help to transport higher energy flow from the free stream into the inlet and therefore reduce the boundary layer thickness in the inlet region. For lower mass flows (0.10 - 0.20 kg/s) a ramp angle of 7 degrees was seen to be prefered in view of ram pressure efficiency. At a higher mass flow (0.25 kg/s) the 10 degrees ramp angle was prefered, followed by the 15 degrees ramp angle at the highest investigated mass flows (0.30 - 0.35 kg/s). In view of drag, the lowest ramp angle possible for a given mass flow was seen to be most advantagous. Future work on this subject will include simulation of an inlet in combination with a heat exchanger and a ram air outlet. This arrengement will be the same as in the investigation at the TWG test campaign and therefore comparable. The difference in outcome of the separate CFD analysis was discussed within this investigation but could not be completely cleared

Wall Condensation Modelling in Convective Flow

Modelling condensation of water vapour is important in a number of engineering applications, such as nuclear reactor containment, rocket engine nozzles and heat exchangers. The current study investigates the possibilities of modelling condensation induced by a cold surface in a flow at high velocity and temperature. A number of non-condensable gases are present in the flow. The possibilities of condensation modelling are investigated in ANSYS CFX and ANSYS Fluent, with focus on ANSYS CFX. A case study is done of a 2D flat plate, with water vapour and non-condensable gases at varying temperatures and velocities. The condensation models in ANSYS CFX are investigated for a few basic flow setups and the model deemed most appropriate for wall condensation is investigated in greater detail. The wall condensation model in CFX is investigated to a greater extend, and compares well with an analytical solution for laminar flow. The complexity of the flow is gradually increased to determine limitations and best practise settings for flows at high velocity and temperature. For isothermal walls and for a conjugated analysis, using a solid with a specified adiabatic wall temperature and heat flux coefficient to induce condensation, the wall condensation model works well for grids with a y+ above 1. For finer grids, convergence is found to be difficult to achieve. The choice of material properties for water vapour was found to play an important role in terms of stability. Real gas properties to define the water vapour material properties are deemed important to avoid unphysical results in terms of the temperature. The wall condensation model in ANSYS CFX is deemed to be an appropriate choice for future work with respect to validity and reduced complexity. If the wall condensation model in ANSYS CFX does not prove adequate, it is recommended to investigate an Euler-Euler multiphase model in ANSYS Fluent with condensation and the Eulerian wall film model enabled

Simulation and Optimization of an Axial Compressor Considering Tip Clearance Flow

With ever-increasing demands for high efficiency in axial compressors, it has become important to consider more geometrical features of the manufactured component in the design phase. Many possibilities open if the level of fidelity of the computational model can be increased. A higher level of detail leads to enhanced performance of components, as the need to be conservative in the design phase is reduced. Better performance is important for reducing fuel consumption and the weight of the component and, consequently, decreasing the environmental impact.An axial compressor consists of rotating and stationary blade rows, where a distance between the rotating blades and the inner casing (shroud) is called the tip gap or tip clearance and is required to avoid contact of the blades with the shroud during engine operation. Large tip gaps in relation to the blade height can typically be found in the rear stages of transonic compressors. If the size of the tip gap is large in relation to the blade height, it can affect the flow in the rotor passage significantly. Including a tip gap in the optimization process of a compressor stage can therefore be of importance even in the early design phase to find geometries that will reach the design point at the design rotational speed.In this thesis, different turbulence models and wall modeling approaches are used tocalculate the flow in a transonic compressor stage with a large tip clearance. The benefitsof including the tip clearance in the optimization process of a transonic compressor areshown and discussed. It is shown that considering the tip clearance in the optimizationprocess is important to be able to reach the specified design point. Furthermore, fromthe optimization results it is shown that redistribution of the flow as a result of blockage in the tip region impacts the design variables over the entire span

Simulation and Optimization of an Axial Compressor Considering Tip Clearance Flow

With ever-increasing demands for high efficiency in axial compressors, it has become important to consider more geometrical features of the manufactured component in the design phase. Many possibilities open if the level of fidelity of the computational model can be increased. A higher level of detail leads to enhanced performance of components, as the need to be conservative in the design phase is reduced. Better performance is important for reducing fuel consumption and the weight of the component and, consequently, decreasing the environmental impact.An axial compressor consists of rotating and stationary blade rows, where a distance between the rotating blades and the inner casing (shroud) is called the tip gap or tip clearance and is required to avoid contact of the blades with the shroud during engine operation. Large tip gaps in relation to the blade height can typically be found in the rear stages of transonic compressors. If the size of the tip gap is large in relation to the blade height, it can affect the flow in the rotor passage significantly. Including a tip gap in the optimization process of a compressor stage can therefore be of importance even in the early design phase to find geometries that will reach the design point at the design rotational speed.In this thesis, different turbulence models and wall modeling approaches are used tocalculate the flow in a transonic compressor stage with a large tip clearance. The benefitsof including the tip clearance in the optimization process of a transonic compressor areshown and discussed. It is shown that considering the tip clearance in the optimizationprocess is important to be able to reach the specified design point. Furthermore, fromthe optimization results it is shown that redistribution of the flow as a result of blockage in the tip region impacts the design variables over the entire span

Less Religion, Better Society? On Religion, Secularity and Prosperity in Scandinavia.

Phil Zuckerman argues in his book Society without God that Scandinavian secularity is strongly correlated to Scandinavian prosperity. In this article, we argue that such usage is premature. First, there are methodological issues that are not properly dealt with. Second, providing a causal narrative in addition to mere correlation is needed. Third, we argue that the causes of Scandinavian prosperity are found in close connection to Scandinavian Lutheranism