Fabio Turrini

Prediction of the Acoustic Losses of a Swirl Atomizer Nozzle Under Non-Reactive Conditions

When predicting combustion instabilities in gas turbine combustion chambers, the complex geometry and three dimensional flow configurations are often neglected. However, these may have significant influence on the overall acoustic damping behavior of the system. An important element governing the flow inside a combustion chamber is the swirl atomizer nozzle. Therein, the flow is accelerated and a swirling fluid motion is imposed. At its exit considerable high flow velocities are reached and multiple shear layers are formed which discharge into the combustion chamber. To predict damping effects in these environments, acoustic-flow interaction processes need to be taken into account. These involve scattering and refraction of incident acoustic waves in shear layers, acoustic interaction with the unstable hydrodynamic shear layers as well as acoustic wall interaction processes. Their combined effect can be studied using acoustic scattering matrices. In this paper the acoustic scattering behavior of a lean injection system developed by Avio is predicted under non-reactive conditions and compared to experiments. The numerical method is very general and works as follows: First, the fluid dynamic field is computed using a Reynolds averaged Navier-Stokes turbulence model. Then, the linearized Navier-Stokes equations are solved in frequency space around the previously computed mean flow state. The complex three dimensionality of the nozzle configuration is taken into account as well as its corresponding flow field. Results are compared against experimental measurements of a swirl atomizer nozzle at atmospheric and elevated inlet temperatures. It is shown that the scattering behavior and therefore the acoustic-flow interactions are captured accurately.Copyright

Thermoacoustic Analysis of a Full Annular Lean Burn Aero-Engine Combustor

In order to reduce NOx emissions, modern gas turbines are often equipped with lean burn combustion systems, where the engine operates near the lean blow-out limits. One of the most critical issues of lean combustion technology is the onset of combustion instabilities related to a coupling between pressure oscillations and thermal fluctuations excited by the unsteady heat release. In this work a thermoacoustic analysis of a full annular combustor developed by AVIO is discussed. The system is equipped with an advanced PERM (Partially Evaporating and Rapid Mixing) injection system based on a piloted lean burn spray flame generated by a pre-filming atomizer. Combustor walls are based on multi-perforated liners to control metal temperature: these devices are also recognized as very effective sound absorbers, thus in innovative lean combustors they could represent a good means both for wall cooling and damping combustion instabilities. The performed analysis is based on the resolution of the eigenvalue problem related to an inhomogeneous wave equation which includes a source term representing heat release fluctuations (the so called Flame Transfer Function, FTF) in the flame region using a three-dimensional FEM code. A model representing the entire combustor was assembled including all the acoustically relevant geometrical features. In particular, the acoustic effect of multi-perforated liners was introduced by modeling the corresponding surfaces with an equivalent internal impedance. Different simulations with and without the presence of the flame were performed analyzing the influence of the multi-perforated liners. Furthermore, different modeling approaches for the FTF were examined and compared with each other. Comparisons with available experimental data showed a good agreement in terms of resonant frequencies in the case of passive simulations. On the other hand, when the presence of the flame is considered, comparisons with experiments showed the inadequacy of FTFs commonly used for premixed combustion and thus the necessity of an improved FTF, more suitable for liquid fueled gas turbines where the evaporation process could play an important role in the flame heat release fluctuations.© 2013 ASME

Assessment of Aero-Thermal Design Methodology for Effusion Cooled Lean Burn Annular Combustors

Increasingly stringent limitations imposed on aircraft engine emissions have led many manufacturers toward lean combustion technology, which involves a relevant increase in mass flow rate dedicated to primary combustion, leading as a consequence to a reduction of air dedicated to cooling of liners. One of the most promising cooling techniques in such conditions is represented by effusion cooling, which consists of an array of closely spaced discrete film cooling holes. This cooling method is based on a protective layer of cooling flow on the hot side of the liner, enhancing at the same time the heat removal within the holes. In the latest years many aero engine manufacturers have increased the research and technology investment on this combustion technology. Working in partnership with the University of Florence, specific component design tools and experimental techniques have been improved by Avio Aero for combustor gas turbine investigation.From a design perspective, CFD analysis has become a key tool up to the early stages of novel combustor design process, producing affordable direct 3D optimization of combustor aerodynamics. Nevertheless, a RANS simulation of even only a single sector of an annular combustor still presents a challenge when the cooling system is taken into account. This issue becomes more critical in case of modern effusion cooled combustors, which may contain up to two thousand holes for the single sector. For this reason, many efforts have been devoted to develop methodologies based on film cooling modeling. Among the approaches published in the literature, models based on local sources represent a good compromise between simplicity and accuracy, with the capability to automatically perform a Conjugate Heat Transfer analysis. This type of methodology has been already defined and validated by the authors, with comparison on effusion cooled plates in terms of experimental overall effectiveness measurements as well as the application on a tubular combustor test case. In the context of this work, the proposed approach has been applied to the analysis of a lean annular combustor with the purpose of investigating pressure losses, flow split and metal temperature field. The results obtained have been compared to experimental data and different numerical tools exploited during the preliminary design of these devices.© 2014 ASME

On Swirl Stabilized Flame Characteristics Near the Weak Extinction Limit

One of the most promising methods for reducing NOx emissions of jet engines is the lean combustion process. In order to realize this concept the percentage of air flowing through the combustor dome has to be drastically increased. This requirement leads to nozzles with high effective area and to high mean velocities in the primary zone of the combustor chamber. The investigation of the lean blow out limit for those nozzles is of main interest for the design of lean combustor technology. It is reported on investigation of a kerosene-fueled, swirl stabilized flame at atmospheric conditions. Two lean operation conditions are investigated, one in stable regime and the other very close to the weak extinction limit. It has been determined, that the flame shape changes when shifted from the stable regime to the other one close to the weak extinction limit (also referred to further as LBO — lean blowout). Since all field measurement schemes are similar, the gained data can be associated and conclusions regarding the flame stabilization at lean conditions can be drawn. The velocity data yields information about the topology of both isothermal and reacting flow fields in the combustion chamber. The internal recirculation and the corner recirculation zones can be well distinguished, because it can be measured directly in the nozzle exit plane. The comparison of the experimental data at stable and near LBO conditions shows the importance of inner and outer recirculation zones for the stabilization process. Furthermore, a comparison with a gaseous fuel nozzle will exhibit the differences between liquid and gaseous fuel combustion.Copyright

Characteristics of an Ultra-Lean Swirl Combustor Flow by LES and Comparison to Measurements

The lean combustion process is one of the most promising methods for reducing NOx emissions of jet engines. Since the risk of flash back is major for premixed concepts a diffusion flame concept is applied. In order to realize the lean condition in this concept the percentage of air flowing through the injection system and combustor dome has to be drastically increased. This leads to nozzles with a high effective area and to high volume flux in the primary zone of the combustor chamber. Such an injection system demands a particular focus towards flame stability at low load. Hence, it is essential to gain information on characteristics such as vortex breakdown, turbulent mixing and coherent structures (e.g. PVC) of the flow by means of numerical simulations. In the paper it is reported on the flow characteristics of the PERM injection system, which equips the AVIO annular combustor designed and developed within NEWAC, an integrated project co-funded by the European Commission. For this injections system RANS and LES simulations have been performed to investigate the isothermal flow field. The results are compared to detailed field measurements of velocity components and Reynolds stresses.Copyright

Adiabatic Effectiveness and Flow Field Measurements in a Realistic Effusion Cooled Lean Burn Combustor

Over the last ten years, there have been significant technological advances toward the reduction of NOx emissions from civil aircraft engines, strongly aimed at meeting stricter and stricter legislation requirements. Nowadays, the most prominent way to meet the target of reducing NOx emissions in modern combustors is represented by lean burn swirl stabilized technology. The high amount of air admitted through a lean burn injection system is characterized by very complex flow structures such as recirculations, vortex breakdown, and precessing vortex core (PVC) that may deeply interact in the near wall region of the combustor liner. This interaction makes challenging the estimation of film cooling distribution, commonly generated by slot and effusion systems. The main purpose of the present work is the characterization of the flow field and the adiabatic effectiveness due to the interaction of swirling flow, generated by real geometry injectors, and a liner cooling scheme made up of a slot injection and an effusion array. The experimental apparatus has been developed within EU project LEMCOTEC (low emissions core-engine technologies) and consists of a nonreactive three-sectors planar rig; the test model is characterized by a complete cooling system and three swirlers, replicating the geometry of a GE Avio PERM (partially evaporated and rapid mixing) injector technology. Flow field measurements have been performed by means of a standard 2D PIV (particle image velocimetry) technique, while adiabatic effectiveness maps have been obtained using PSP (pressure sensitive paint) technique. PIV results show the effect of coolant injection in the corner vortex region, while the PSP measurements highlight the impact of swirled flow on the liner film protection separating the contribution of slot and effusion flows. Furthermore, an additional analysis, exploiting experimental results in terms of heat transfer coefficient, has been performed to estimate the net heat flux reduction (NHFR) on the cooled test plate.

Effect of Slot Injection and Effusion Array on the Liner Heat Transfer Coefficient of a Scaled Lean-Burn Combustor With Representative Swirling Flow

International standards regarding polluting emissions from civil aircraft engines are becoming gradually even more stringent. Nowadays, the most prominent way to meet the target of reducing NOx emissions in modern aero-engine combustors is represented by lean burn technology. Swirl injectors are usually employed to provide the dominant flame stabilization mechanism coupled to high efficiency fuel atomization solutions. These systems generate very complex flow structures such as recirculations, vortex breakdown and processing vortex core, that affect the distribution and therefore the estimation of heat loads on the gas side of the liner as well as the interaction with the cooling system flows.The main purpose of the present work is to provide detailed measurements of Heat Transfer Coefficient (HTC) on the gas side of a scaled combustor liner highlighting the impact of the cooling flows injected through a slot system and an effusion array. Furthermore, for a deeper understanding of the interaction phenomena between gas and cooling flows, a standard 2D PIV (Particle Image Velocimetry) technique has been employed to characterize the combustor flow field.The experimental arrangement has been developed within EU project LEMCOTEC and consists of a non-reactive three sectors planar rig installed in an open loop wind tunnel. Three swirlers, replicating the real geometry of a GE Avio PERM (Partially Evaporated and Rapid Mixing) injector technology, are used to achieve representative swirled flow conditions in the test section. The effusion geometry is composed by a staggered array of 1236 circular holes with an inclination of 30deg, while the slot exit has a constant height of 5mm. The experimental campaign has been carried out using a TLC (Thermochromic Liquid Crystals) steady state technique with a thin Inconel heating foil and imposing several cooling flow conditions in terms of slot coolant consumption and effusion pressure drop. A data reduction procedure has been developed to take into account the non-uniform heat generation and the heat loss across the liner plate.Results, in terms of 2D maps and averaged distributions of HTC have been supported by flow field measurements with 2D PIV technique focussed on the corner recirculation region.Copyright

Multi-Coupled Numerical Analysis of Advanced Lean Burn Injection Systems

Lean burn aero-engine combustors usually exploit advanced prefilming airblast injection systems in order to promote the formation of highly homogeneous air-fuel mixtures with the main aim of reducing NOx emissions. The combustion process is strongly influenced by the liquid fuel preparation and a reliable prediction of pollutant emissions requires proper tools able to consider the most important aspects characterizing liquid film evolution and primary breakup. This paper presents the numerical analysis of an advanced lean burn injection system using a multi-coupled two-phase flow three-dimensional solver developed on the basis of OpenFOAM modelling and numerics. The solver allows the coupled solution of gas-phase, droplets and liquid film exploiting correlation-based and theoretical models for liquid film primary atomization. A detailed analysis of the liquid film evolution is presented together with an investigation of the influence of film modelling and primary breakup on the combustion process at different operating conditions. The combustion field is strongly influenced by the characteristics of droplet population generated by the liquid film and this study proposes a computational setup, suitable for industrial calculations, able to account for all the main physical processes that characterize advanced prefilming airblast injection systems.Copyright

Experimental and Theoretical Investigation of Thermal Effectiveness in Multi-Perforated Plates for Combustor Liner Effusion Cooling

State-of-the-art liner cooling technology for modern combustors is represented by effusion cooling (or full-coverage film cooling). Effusion is a very efficient cooling strategy based on the use of multi-perforated liners, where metal temperature is lowered by the combined protective effect of coolant film and heat removal through forced convection inside each hole. The aim of this experimental campaign is the evaluation of the thermal performance of multi-perforated liners with geometrical and fluid-dynamic parameters ranging among typical combustor engine values. Results were obtained as adiabatic film effectiveness following the mass transfer analogy by the use of Pressure Sensitive Paint, while local values of overall effectiveness were obtained by eight thermocouples housed in as many dead holes about 2 mm below the investigated surface. Concerning the tested geometries, different porosity levels were considered: such values were obtained both increasing the hole diameter and pattern spacing. Then the effect of hole inclination and aspect ratio pattern shape were tested to assess the impact of typical cooling system features. Seven multi perforated planar plates, reproducing the effusion arrays of real combustor liners, were tested imposing 6 blowing ratios in the range 0.5–5. Test samples were made of stainless steel (AISI304) in order to achieve Biot number similitude for overall effectiveness tests.To extend the validity of the survey a correlative analysis was performed to point out, in an indirect way, the augmentation of hot side heat transfer coefficient due to effusion jets. Finally, to address the thermal behaviour of the different geometries in presence of gas side radiation, additional simulations were performed considering different levels of radiative heat flux.Copyright

Evaluation of a Piloted Lean Injection System in Terms of Emission Performance and Flame Structure at Elevated Pressure

A new compact injection system design for piloted lean combustion has been developed to reduce the pollutant emissions in aero engines. The system includes an integrated premixing zone to achieve a homogenous fuel distribution, so that peak temperatures in the combustor are avoided. This leads to low NOx emissions at lean conditions. The risks of flame flashback and auto ignition have been considered in the design and neither of them has been detected by the performed tests. To avoid the formation of a recirculation zone within the mixing zone an axial air jet has been introduced. This axial jet also works as an air assisted pilot fuel atomizer, which is a major innovation as compared to other lean injection systems using pressure-swirl atomizers for the pilot fuel like e.g. the PERM (Partial Evaporation and Rapid Mixing) concept developed in a previous research program [1], [2]. The main fuel injection of the current configuration is performed by four circumferentially arranged pressure swirl atomizers, which is also an alternative approach compared to previous concepts. The emission performance of the injection system using kerosene Jet A-1 has been investigated in a tubular combustor with air inlet temperatures up to 733 K and combustor pressures up to 10 bar. The dependencies of pilot fuel split, air to fuel ratio, combustor pressure and air inlet temperature on emissions have been determined. Over a wide range of operating conditions a low amount of pollutant emissions are achieved and the stability range is broadened by the pilot fuel injection. The flame structure has been analyzed by OH* chemiluminescence measurements. The Abel transformation technique has been applied to the images to generate the radial distribution. The main flame is lifted and its shape remains similar for different combustor pressures. The lift off height with only pilot fuel injection decreases with increasing combustor pressure and the flame shape is changing. This behavior is explained based on the effects of combustor pressure on fuel atomization, droplet traces and the distribution of evaporated fuel. The development and testing have been conducted in cooperation of AVIO and Karlsruhe Institute of Technology in the frame of the European Commission co-financed research project TECC-AE (Technology Enhancement for Clean Combustion in Aero Engines).Copyright