Atmospheric Crosswind Tests of Aspirated Jet Engine Intake Models

The ”Technische Universität Braunschweig” has commissioned a Propulsion Test Facility (PTF) for aspirated intake models of jet engines under off-design point conditions. For the commissioning of the unique facility, an aspirated intake test campaign has been carried out. Aim of the campaign was to compare the measured data in the PTF to numerical results and experimental data, which have already been measured in another test facility in the past using the same intake geometry. For the tests the Laminar Flow Reynolds Action (LARA) nacelle has been chosen. The LARA intake has been built and tested in the early 1990s at the ”ONERA F1” wind tunnel during the work on hybrid laminar flow technology. At TU Braunschweig an Aspirated-Intake-Rig (ASI-Rig) with an in-house designed fan stage was worked out, whose fan is located far enough downstream to avoid interaction with the nacelle. For the results, the static pressure distribution at the inner and outer contour of the nacelle lip and the velocity distribution in the fan face during pure crosswind conditions have been compared and analysed. As seen in the results, the PTF pressure distribution at the lip is in good agreement with the numerical and the experimental data from the ONERA. Of particular note is the deviation between the achieved peak Mach number between the two experimental setups, analysed at the 0◦/180◦ and 90◦ section, which can be explained by the Reynolds number effect. INTRODUCTION One of the most challenging design issues within the development of future civil jet engines is the balance of increasing bypass ratios without generating high penalties in cruise drag. Cruise drag is directly coupled to the engine‘s nacelle diameter which has to increase for higher bypass ratios. Nevertheless, from today‘s perspective it’s the only strategy of meeting future challenges like rising fuel costs and more strict noise emission guidelines. These guidelines are defined in the requirements for future civil aviation, which are called Flightpath 2050 [Kallas and Geoghegan-Quinn (2011)]. Within this program a comprehensive list of actions is formulated including advanced technologies with regards to lowering fuel consumption and decreasing noise emission of jet engines. In order to meet these requirements, jet engine manufacturers are challenged to further improve overall engine efficiency. Over the years, significant improvements of overall engine efficiency were mainly achieved by increasing the propulsive efficiency, which directly depends on the difference between engine exit velocity and flight velocity [Riegler and Bichlmaier (2007)]. The smaller that difference, the higher is the propulsive efficiency, which in turn corresponds to a decreased specific fuel consumption. In general, a lower nozzle exit velocity is related to a lower fan pressure ratio. Keeping the thrust at a constant level, a lower exit velocity has to be compensated by an increased total engine mass flow. This implies a larger fan diameter and thus a higher bypass ratio μ . FIGURE 1. Development of Bypass and Fan Pressure Ratio [Riegler and Bichlmaier (2007)] This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License CC-BY-NC-ND 4.0 That trend is shown in Fig.1. The most widespread technology level today is represented by Direct Drive Turbofans e.g. the V2500-A5 (1992) and the GP7000 (2007). For these engine architectures the fan pressure ratio lies between 1.45 and 1.65 and the maximum achievable bypass ratio is approximately μ = 9. Further improvements in propulsive efficiency can only be achieved by new engine concepts. A promising concept is the Geared Turbofan (GTF) [Riegler and Bichlmaier (2007)]. With that concept, bypass ratios up to approximately μ = 17−18 are in a conceivable range and fan pressure ratios will further decrease towards values of 1.3 or even lower. Without any counteraction, the fan system becomes more susceptible for aerodynamic instabilities, such as stall and flutter, so that a safe operation is no longer ensured. Accordingly, the interaction of fan performance and intake flow, especially at off-design conditions, is one of the major design criteria for such low pressure fan systems. The opportunity of testing coupled fan-intake systems on scaled models is considered to support the development process and to generate a deeper understanding of the fan-intake interaction. Classical experimental investigations on nacelle and fan aerodynamics use either wind tunnels or fan rigs. Experiments with a focus on nacelle aerodynamics were e.g. carried out by Quemard et al. (1996). These so-called ”Aspirated Intake Tests” determine the isolated intake performances of a representative nacelle model inside a classical wind tunnel. Critical conditions, such as crosswind, are achieved by adjusting the geometric angle of attack within the wind tunnel test section. The engine mass flow itself is generated by using the static pressure difference between the wind tunnel and ambient conditions. By using a secondary air system, a wide flow rate range can be achieved by continuously adjusting the system throttle condition. The rotating fan is not considered within such an experimental setup because of the focus on external and internal nacelle flow. Further low speed investigations were carried out by Probst et al. (2012). For this experimental setup a flow-through nacelle has been investigated in a low speed wind tunnel. Compared to realistic flight conditions for such a ”Flow-Through Setup”, the mass flow rates through the nacelle are much lower. In order to obtain a representative stall behaviour and pressure distributions the nacelle geometry was adapted [Schulze (2012)]. There exists a wide range of fan rigs for investigations with focus on fan performance. With respect to inlet conditions such compressor test facilities ensure high flow quality as a default configuration. In order to investigate new blade designs or other modifications it is not desired to have an influence triggered by the inlet conditions. Therefore, large settling chambers with turbulence screens and honeycombs ensure both: uniform velocity distributions and low turbulence levels. However, in order to evaluate distorted inlet conditions so-called distortion generators are used. These generators upstream of the rotor are intended to produce a certain local pressure drop or swirl component representing distorted inlet conditions [Wartzek et al. (2016)]. The disadvantage of such methods is, that there is no realistic interaction between the rotor and the distortion generator as it occurs in coupled fan intake systems. For example, the presence of the fan is known to increase the intake’s operating range where no separation occurs [Cao et al. (2016)]. Such behaviour can only be investigated by having realistic experimental setups that take all relevant components into account. The objective of the new propulsion test facility at TU Braunschweig is to provide a combined research setup, including both: wind tunnel capabilities and an operating fan rig. As a result, the fan system can be tested in combination with a representative intake geometry at most critical operation points that occur during a civil flight mission. The advantage of such a setup is to obtain representative intake aerodynamics, fan performances and hence interaction effects that will gain importance for future engine architectures. Since the testing strategy of the propulsion test facility in Braunschweig is an innovative concept, that has never been used before in research on fan-intake investigations, an extensive numerical and experimental validation process is taking place. In this paper the first experimental results on an aspirated-intake test are presented, showing the facility’s ability of testing intakes under pure crosswind conditions. Experimental Setup The experimental setup is divided into two parts the layout of the so-called Propulsion-Test-Facility (PTF) and the design of the Aspirated-Intake-Rig (ASI-Rig) inside the test facility, which generates the flow for aspirated intake models. Basically, the facility layout of the PTF represents an atmospheric wind tunnel (Eiffel-configuration), shown in Fig. 2. The flow is sucked in from the outside via the inlet tower and is redirected via corner vanes. Two screens and a honeycomb improve the flow quality before entering further downstream into the contraction with an area ratio of Ain/Aout = 0.20. The acceleration of the flow leads to a maximum speed of Ma = 0.20 inside the test section, where the intake model is located. The critical part of the whole facility is the diffuser. Its size is balanced between the maximum possible shaft length of the main drive, which is centred inside the diffuser, and the minimum way for the deceleration of the flow without separation. In order to prevent flow separation the diffuser is divided into three equal sub-diffusers with the same equivalent opening angles and integrated splitters. The diffusor is located between the main blowers and the test section. The blower array is located in the exit area of the diffuser and consists of eight blowers, which have a total power of around PBlower = 1 MW. In the center of the array the electric main drive and the gearbox with a total power of PMainDrive = 2 MW is located. Downstream of the drive section, turning vanes lead the flow towards the outlet tower [Krone and Friedrichs (2012)]. As a consequence of the long shaft between gearbox and fan rig, the rig itself is not pitchable. In order to generate AoA or pure crosswind conditions, a new distortion concept has been developed. As seen in Fig. 3 the test section is surrounded by a circumferential crosswind duct. At the crosswind outlet the flow is split into two parts. Half of the mass flow is guided above and the other half below the test section. Each stream is accelerated by two blowers. At the crosswi