K. Yakinthos

Transition on a flat plate with a semi-circular leading edge under uniform and positive shear free-stream flow

Abstract The effect of the free-stream velocity profile on the transition from laminar to turbulent flow on a flat plate was studied experimentally and numerically and it is presented in this paper. The flows investigated are based on the T3L test case of the ERCOFTAC Special Investigation Group for transition. According to this test case, the boundary layer development on a flat plate with a semi-circular leading edge is examined by means of transition due to separation, under various free-stream conditions concerning the turbulence intensity and velocity magnitude. In the present work, two free-stream velocity distributions were studied. The first was a uniform velocity one and the second, with a mean shear velocity profile with positive gradient, ∂ U / ∂ y=27.7 s −1 . Measurements using hot-wire anemometry were taken in two primary regions: far upstream of the flat plate to observe the velocity and turbulence distributions and near the flat plate to capture the boundary layer development and the transition phenomenon. The effect of the two free-stream velocity distributions was studied and it was shown that for both velocity distributions a recirculation region of the flow occurred near the flat plate wall that led to transition dominated by the boundary layer separation. For the positive velocity gradient the separation region was smaller compared to the case of uniform free-stream profile. Both cases were also studied computationally. Two widely used linear eddy-viscosity turbulence models, the k – e and the k – ω with specific low Reynolds formulations were applied and in addition, a non-linear eddy-viscosity based on the k – e model has been implemented. In general, all the k – e models gave satisfactory predictions for both flow cases regarding the predicted velocity distributions, while the k – ω model gave poor results. Concerning the longitudinal Reynolds stress distributions in the near-wall region, the non-linear k – e model gave the best predictions inside the separation zone but it over predicted the corresponding values beyond the reattachment point while beyond the separation the linear models predicted the longitudinal stresses in a more satisfactory way.

CFD Modeling and LDA Measurements for the Air-Flow in an Aero Engine Front Bearing Chamber

The continuous development of aero engines toward lighter but yet more compact designs, without decreasing their efficiency, has led to gradually increasing demands on the lubrication system, such as the bearing chambers of an aero engine. For this reason, it is of particular importance to increase the level of understanding of the flow field inside the bearing chamber in order to optimize its design and improve its performance. The flow field inside a bearing chamber is complicated since there is a strong interaction between the sealing air-flow and the flow of lubrication oil, and both of them are affected by and interacting with the geometry of the chamber and the rotating shaft. In order to understand the flow field development and, as a next step, to optimize the aero engine bearing chamber performance, in relation to the lubrication and heat transfer capabilities, the behavior of this interaction must be investigated. In this work, an investigation of the air-flow field development inside the front bearing chamber of an aero engine is attempted. The front bearing chamber is divided into two separate sections. The flow from the first section passes through the bearing and the bearing holding structure to the second one where the vent and the scavenging system are located. The investigation was performed with the combined use of experimental measurements and computational fluid dynamics (CFD) modeling. The experimental measurements were carried out using a laser Doppler anemometry system in an experimental rig, which consists of a 1:1 model of the front bearing chamber of an aero engine. Tests were carried out at real operating conditions both for the air-flow and for the lubricant oil-flow and for a range of shaft rotating speeds. The CFD modeling was performed using a commercial CFD package. Particularly, the air-flow through the bearing itself was modeled, adopting a porous medium technique, the parameters of which were developed in conjunction with the experiments. A satisfactory quantitative agreement between the experimental measurements and the CFD computations was achieved. At the same time, the effect of the important parameters such as the air and oil mass flow, together with the shaft rotational speed, and the effect of the chamber geometry were identified. The conclusions can be exploited in future attempts in combination with the CFD model developed in order to optimize the efficiency of the lubrication and cooling system. The latter forms the main target of this work, which is the development of a useful engineering tool capable of predicting the flow field inside the aero engine bearing, which can be used subsequently for optimization purposes.

Modelling Operation of System of Recuperative Heat Exchangers for Aero Engine with Combined Use of Porosity Model and Thermo-Mechanical Model

Abstract The present work describes an effort to model the operation of a system of recuperative heat exchangers of an aero engine for real engine operating conditions. The modelling was performed with the combined use of a porous medium model and a thermo mechanical model. The porous medium model was taking into account the heat transfer and pressure loss behaviour of the heat exchangers while the thermo mechanical one was used for the calculation of the wall temperature distribution of the elliptic tubes of the heat exchangers. As it is presented, the combined use of these models can provide a useful tool which can help in the prediction of the macroscopic behaviour of the system of recuperative heat exchangers of the aero engine which can be used for optimization purposes and numerical studies.

Ultra Low Emission Technology Innovations for Mid-century Aircraft Turbine Engines

Commercial transport fuel efficiency has improved dramatically since the early 1950s. In the coming decades the ubiquitous turbofan powered tube and wing aircraft configuration will be challenged by diminishing returns on investment with regards to fuel efficiency. From the engine perspective two routes to radically improved fuel efficiency are being explored; ultra-efficient low pressure systems and ultra-efficient core concepts. The first route is characterized by the development of geared and open rotor engine architectures but also configurations where potential synergies between engine and aircraft installations are exploited. For the second route, disruptive technologies such as intercooling, intercooling and recuperation, constant volume combustion as well as novel high temperature materials for ultra-high pressure ratio engines are being considered. This paper describes a recently launched European research effort to explore and develop synergistic combinations of radical technologies to TRL 2. The combinations are integrated into optimized engine concepts promising to deliver ultra-low emission engines. The paper discusses a structured technique to combine disruptive technologies and proposes a simple means to quantitatively screen engine concepts at an early stage of analysis. An evaluation platform for multidisciplinary optimization and scenario evaluation of radical engine concepts is outlined.

MODELLING OF FLOW DISTRIBUTION DURING CATALYTIC CONVERTER LIGHT-OFF

The flow field non-uniformities at the inlet of catalytic converters are considered undesirable for their performance. Computational fluid dynamics (CFD) is a powerful tool for calculating the flow field inside the catalytic converter and optimising design concepts. However, the applicability of CFD for transient simulations is limited by the high CPU demands of this technique. The present study proposes an alternative computational method for the prediction of transient flow fields in axi-symmetric converters time-efficiently. The proposed flow resistance modelling (FRM) method is validated against the results of CFD predictions during a typical warm-up case. The FRM methodology is coupled with an already available transient model for heat transfer and chemical reactions in the catalyst. The effect of flow distribution on pollutant conversion and pressure drop is examined under warm-up and steady state operation.

Derivation of an Anisotropic Model for the Pressure-Loss Through a Heat Exchanger for Aero Engine Applications

We present an effort to model the pressure loss together with the heat transfer mechanism, in a heat exchanger designed for an integrated recuperative aero engine. The operation of the heat exchanger is focusing on the exploitation of the thermal energy of the turbine exhaust gas to pre-heat the compressor outlet air before combustion and to decrease fuel consumption and pollutant emissions. Two basic parameters characterize the operation of the heat exchanger, the pressure loss and the heat transfer. The derivation of the pressure loss model is based on experimental measurements that have been carried-out on a heat exchanger model. The presence of the heat exchanger is modeled using the concept of a porous medium, in order to facilitate the computational modeling by means of CFD. As a result, inside the integrated aero engine, the operation of the heat exchanger can be sufficiently modeled as long as a generalized and accurate pressure drop and heat transfer model is developed. Hence, the porosity model formulation should be capable of properly describing the overall macroscopic hydraulic and thermal behavior of the heat exchanger. The effect of the presence of the heat exchanger on the flow field is estimated from experimental measurements. For the derivation of the porous medium pressure loss model, an anisotropic formulation of a modified Darcy-Forchheimer pressure drop law is proposed in order to take into account the effects of the three-dimensional flow development through the heat exchanger. The heat transfer effects are taken also into account with the use of a heat transfer coefficient correlation. The porosity model is adopted by the CFD solver as an additional source term. The validation of the proposed model is performed through CFD computations, by comparing the predicted pressure drop and heat transfer with available experimental measurements carried-out on the heat exchanger model.© 2009 ASME

The effect of negative shear on the transitional separated flow around a semi-circular leading edge

Abstract The effect of a negative free-stream mean-shear velocity distribution on the boundary layer development on a flat plate with a semi-circular leading edge is studied experimentally and computationally. The geometry is the same as in the T3L test case of the ERCOFTAC Special Interest Group on Transition. The existence of a negative shear is related to the transition of the boundary layer from laminar to turbulent through separation. The flow investigated here has the same general characteristics as the one presented in a recent work by the authors, Palikaras et al. [Int. J. Heat Fluid Flow 23 (2002) 455–470], where the boundary layer development has been studied under free-stream conditions of uniform and positive mean-shear velocity distributions. The negative shear flow in the core region of the wind tunnel has a value ∂ U / ∂ y=−27.7 s−1, which is the opposite to the case examined by Palikaras et al. [Int. J. Heat Fluid Flow 23 (2002) 455–470]. In the first part of the paper, a detailed description of the flow is given. The measured quantities are presented, discussed and compared with the computational analysis in order to obtain a complete picture of the investigated flow. For the computations, the non-linear k–e model of Craft et al. [Int. J. Heat Fluid Flow 17 (1996) 108–115] is used, and satisfactory predictions are reported. In the second part, a detailed comparison of the results with the cases of uniform and positive mean-shear velocity inlet distribution is carried out. In the case of negative mean velocity the separated boundary layer leads to a larger reverse flow region than the two other cases. A relation is observed between the location of the stagnation point at the leading edge and the presence or absence of shear. When mean shear is present, depending on the sign, there is a movement of the stagnation point away from the symmetry line of the flat plate and it is believed that this is the driving mechanism that affects the boundary layer development and the longitudinal size of the reverse flow region. This remark is supported by the observation that for all the three cases studied, the longitudinal RMS distribution above the reverse flow region and in the free-stream region has the same values.

Modeling an Installation of Recuperative Heat Exchangers for an Aero Engine

This work presents the complete effort to model the presence of an integrated system of heat exchangers mounted in the exhaust nozzle of an aero engine which uses an alternative but more efficient thermodynamic cycle. The heat exchangers are operating as heat recuperators exploiting part of the thermal energy of the turbine exhaust gas to preheat the compressor outlet air before combustion and to reduce pollutants and fuel consumption. The presence of the heat exchangers enforces a significant pressure drop in the exhaust gas flow which can affect the overall efficiency of the thermodynamic cycle and the potential benefit of this technology. For this reason it is important to optimize the operation of the system of heat exchangers. The main target of this optimization effort is the minimization of the pressure losses for the same amount of heat transfer achieved. The optimization is performed with the combined use of experimental measurements and CFD methods. Since the CFD modeling is taking into consideration the overall geometry of the exhaust nozzle of the aero engine where the heat exchangers are mounted, the presence of the latter is unavoidably modeled with the use of a porosity model for practical reasons, having to do with CPU and memory requirements. The porosity model is taking into account the pressure drop and heat transfer behaviour of the heat exchangers and was developed and validated with the use of detailed experimental measurements. For the validation of the CFD model, isothermal experimental measurements carried out for laboratory conditions in a 1:1 model of a quarter of the exhaust nozzle of the aero engine, including four full-scale heat exchangers, were used. The CFD results were in good agreement with the experimental measurements and the same flow structures and problematic regions were detected. Thus, a complete 3-D CFD model of the overall exhaust nozzle of the aero engine was created and validated which at the next step formed the basis for the optimization of the overall aero engine installation for real engine operating conditions. The improved design of the aero engine installation presented decreased pressure losses in relation to the initial design and a more balanced mass flow distribution, showing the applicability of the overall methodology and its advantages for producing efficient engineering solutions for similar setups.Copyright

Two-Phase Flow Pressure Drop in Corrugated Tubes Used in an Aero-engine Oil System

In modern aero-engines, the lubrication system holds a key role due to the demand for high reliability standards. An aero-engine bearing chamber contains components like bearings and gears. Oil is used for lubrication and for heat removal. In order to retain the oil in a bearing chamber, pressurized seals are used. These are pressurized using air from the compressor. In order to avoid overpressurization of the bearing chamber, air/oil passages are provided in the bearing chamber. At the top, a vent pipe discharges most of the sealing air and at the bottom, a scavenge pipe is used for discharging the oil by means of a pump (scavenge pump). The scavenge pipe is setup in most cases by tubes of circular or noncircular cross sections. When the scavenge pipe has to be routed in a way that sharp bends or elbows are unavoidable, flexible (corrugated) pipes can be used. Because of the corrugation, considerable flow resistance with high-pressure drop can result. This may cause overpressurization of the bearing compartment with oil loss into the turbomachinery with possibility of ignition, coking (carbon formation), or contamination of the aircraft’s air conditioning system. It is therefore important for the designer to be capable to predict the system’s pressure balance behavior. A real engine bearing chamber sealed by brush seals was used for generating different air/oil mixtures thus corresponding to different engine operating conditions. The mixtures were discharged through a scavenge pipe which was partly setup by corrugated tubes. Instead of a mechanical pump, an ejector was used for evacuating the bearing chamber. An extensive survey covering the existing technical literature on corrugated tube pressure drop was performed and is presented in this paper. The survey has covered both single-phase and multiphase flows. Existing methods were checked against the test results. The method which was most accurately predicting lean air test results from the rig was benchmarked and was used as the basis for extending into a two-phase flow pressure drop correlation by applying two-phase flow multiplier techniques similar to Lockhart and Martinelli. Comparisons of the new two-phase flow pressure drop correlation with an existing correlation by Shannak are presented for mixtures like air/oil, air/water, air/diesel, and air/kerosene. Finally, numerical analysis results using ansys cfx version 15 are presented.

Best Strategies for the Development of a Holistic Porosity Model of a Heat Exchanger for Aero Engine Applications

In an attempt to manage CFD computations in aero engine heat exchanger design, this work presents the best strategies and the methodology used to develop a holistic porosity model, describing the heat transfer and pressure drop behavior of a complex profiled tubular heat exchanger for aero engine applications. Due to the complexity of the profile tube heat exchanger geometry and the very large number of tubes, detailed CFD computations require very high CPU and memory resources. For this reason the complex heat exchanger geometry is replaced in the CFD computations by a simpler porous medium geometry with predefined pressure loss and heat transfer.The present work presents a strategy for developing a holistic porosity model adapted for heat exchangers, which is capable to describe their macroscopic heat transfer and pressure loss average performance. For the derivation of the appropriate pressure loss and heat transfer correlations, CFD computations and experimental measurements are combined. The developed porosity model is taking into consideration both streams of the heat exchanger (hot and cold side) in order to accurately calculate the inner and outer pressure losses, in relation to the achieved heat transfer and in conjunction with the selected heat exchanger geometry, weight and operational parameters. For the same heat exchanger, RAM and CPU requirement reductions were demonstrated for a characteristic flow passage of the heat exchanger, as the porosity model required more than 80 times less computational points than the detailed CFD model. The proposed porosity model can be adapted for recuperation systems with varying heat exchanger designs having different core arrangements and tubes sizes and configurations, providing an efficient tool for the optimization of the heat exchangers design and leading to an increase of the overall aero engine performance.Copyright