Lean-burn Injection Research Articles

Comparison of acoustic, optical, and heat release rate based flame transfer functions for a lean-burn injector under engine-like conditions

Determining the flame transfer function plays a crucial role during the development phase of lean-burn injectors to predict the overall stability of the combustion system. This study develops an enhanced acoustic post-processing strategy using acoustic network modeling and compressible large-eddy simulation for a lean-burn aero-engine injector in a realistic test rig. Results show that optical flame transfer functions experimentally obtained align with those derived from large-eddy simulation based on heat release rate. However, discrepancies, especially in gain, are observed when using the standard acoustic post-processing method. By employing an acoustic network model, it is shown that comparable flame transfer functions can be achieved through refined post-processing strategies. This study highlights the limitations of conventional acoustic post-processing and underscores the necessity of explicit acoustic modeling in complex setups to accurately obtain the flame response. A generalized formulation applicable to a broader range of setups, extending beyond simple configurations, is presented as an alternative to the conventional acoustic post-processing method.

Turbulent Flame Shape Switching at Conditions Relevant for Gas Turbines

Abstract A numerical investigation is conducted to shed light on the reasons leading to different flame configurations in gas turbine (GT) combustion chambers of aeronautical interest. Large eddy simulations (LES) with a flamelet-based combustion closure are employed for this purpose to simulate the DLR-AT big optical single sector (BOSS) rig fitted with a Rolls-Royce developmental lean burn injector. The reacting flow field downstream this injector is sensitive to the intricate turbulent–combustion interaction and exhibits two different configurations: (i) a penetrating central jet leading to an M-shape lifted flame; or (ii) a diverging jet leading to a V-shaped flame. The LES results are validated using available BOSS rig measurements, and comparisons show the numerical approach used is consistent and works well. The turbulent–combustion interaction model terms and parameters are then varied systematically to assess the flame behavior. The influences observed are discussed from physical and modeling perspectives to develop physical understanding on the flame behavior in practical combustors for both scientific and design purposes.

The effect of effusion holes inclination angle on the adiabatic film cooling effectiveness in a three-sector gas turbine combustor rig with a realistic swirling flow

The introduction of Lean Burn concept as basic Low-NOx scheme for future aero-engines is heavily affecting the aero-thermal design of combustors. A great amount of air is admitted through the injection system with relevant swirl components, producing very complex flow structures (recirculations, vortex breakdown) for flame stabilization. As a consequence a reduced quantity of air is available for liner cooling, pushing the adoption of high effectiveness cooling schemes. Effusion cooling represents one of the first choices due to its low weight and a relatively easy manufacturability. Liner metal temperature is kept low by the combined protective effect of coolant film, heat removal inside holes and an improved cold-side convection. In lean burn systems the evolution of film protection can be heavily influenced by the swirl flow interaction with combustor walls.The subject of this work is to investigate the effects of the realistic flow field of a lean burn injector on the adiabatic film cooling effectiveness on an effusion cooled combustor liner. A dedicated three-sector rig was designed with the aim of measuring film effectiveness with Pressure Sensitive Paint technique. Three effusion cooling geometries with different inclination angles were tested at various levels of pressure drops across the perforation, resulting in different blowing ratio values. It was also taken into consideration several flow rate levels of starter film realized by spent dome cooling air, injected through a dedicated plain slot. The analysis of film effectiveness measurements were supported by flow field investigation in the near wall region carried out by means of Particle Image Velocimetry.Results pointed out the relevant impact of combustor flow field on the adiabatic film cooling effectiveness as well as a significant role of the inclination angle, recommending a careful revision of standard design practices based on one dimensional flow assumption and suggesting possible holes arrangement optimization.

Flow Field Characterization at the Outlet of a Lean Burn Single-Sector Combustor by Laser-Optical Methods

High overall pressure ratio (OPR) engine cycles for reduced NOx emissions will generate new aggravated requirements and boundary conditions by implementing low emission combustion technologies into advanced engine architectures. Lean burn combustion systems will have a significant impact on the temperature and velocity traverse at the combustor exit. Lean burn fuel injectors dominate the combustor exit conditions. This is due to the fact that they pass a majority of the total combustor flow, and to the lack of mixing jets like in a conventional combustor. With the transition to high-pressure engines, it is essential to fully understand and determine the high energetic interface between combustor and turbine to avoid excessive cooling. Velocity distributions and their fluctuations at the combustor exit for lean burn are of special interest as they can influence the efficiency and capacity of the turbine. A lean burn single-sector combustor was designed and built at DLR, providing optical access to its rectangular exit section. The sector was operated with a fuel-staged lean burn injector. Measurements were performed under idle and cruise operating conditions. Two velocity measurement techniques were used in the demanding environment of highly luminous flames under elevated pressures: particle image velocimetry (PIV) and filtered Rayleigh scattering (FRS). The latter was used for the first time in an aero-engine combustor environment. In addition to a conventional signal detection arrangement, FRS was also applied with an endoscope for signal collection, to assess its practicality for a potential future application in a full annular combustor with restricted optical access.

Unsteady Computational Fluid Dynamics Investigation of Effusion Cooling Process in a Lean Burn Aero-Engine Combustor

This work describes the main findings of a computational fluid dynamics (CFD) analysis intended to accurately investigate the flow field and wall heat transfer as a result of the mutual interaction between a swirling flow generated by a lean burn injection system and a slot–effusion liner cooling system. In order to overcome some limitations of Reynolds-averaged Navier–Stokes (RANS) approach, the simulations were performed with shear stress transport (SST)–scale-adaptive simulation (SAS), a hybrid RANS–large eddy simulation (LES) model. Moreover, the significant computational effort due to the presence of more than 600 effusion holes was limited exploiting two different modeling strategies: a homogeneous model based on the application of uniform boundary conditions on both aspiration and injection sides, and another solution that provides a coolant injection through point mass sources within a single cell. CFD findings were compared to experimental results coming from an investigation carried out on a three-sector linear rig. The comparison pointed out that advanced modeling strategies, i.e., based on discrete mass sources, are able to reproduce the effects of mainstream–coolant interactions on convective heat loads. By validating the approach through a benchmark against time-averaged quantities, the transient data acquired were examined in order to better understand the unsteady behavior of the thermal load through a statistical analysis, providing useful information with a design perspective.

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.

Experimental Investigation of the Flow Field and the Heat Transfer on a Scaled Cooled Combustor Liner With Realistic Swirling Flow Generated by a Lean-Burn Injection System

Lean-burn swirl stabilized combustors represent the key technology to reduce NOx emissions in modern aircraft engines. 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 processing vortex core, which may deeply interact in the near wall region of the combustor liner. This interaction and its effects on the local cooling performance make the design of the cooling systems very challenging, accounting for the design and commission of new test rigs for detailed analysis. The main purpose of the present work is the characterization of the flow field and the wall heat transfer due to the interaction of a swirling flow coming out from real geometry injectors and a slot cooling system which generates film cooling in the first part of the combustor liner. The experimental setup consists of a nonreactive three sector planar rig in an open loop wind tunnel; the rig, developed within the EU project Low Emissions Core-Engine Technologies (LEMCOTEC), includes three swirlers, whose scaled geometry reproduces the real geometry of an Avio Aero partially evaporated and rapid mixing (PERM) injector technology, and a simple cooling scheme made up of a slot injection, reproducing the exhaust dome cooling mass flow. Test were carried out imposing realistic combustor operating conditions, especially in terms of reduced mass flow rate and pressure drop across the swirlers. The flow field is investigated by means of particle image velocimetry (PIV), while the measurement of the heat transfer coefficient is performed through thermochromic liquid crystals (TLCs) steady state technique. PIV results show the behavior of flow field generated by the injectors, their mutual interaction, and the impact of the swirled main flow on the stability of the slot film cooling. TLC measurements, reported in terms of detailed 2D heat transfer coefficient maps, highlight the impact of the swirled flow and slot film cooling on wall heat transfer.

Optical Methods for Studies of Self-Excited Oscillations and the Effect of Dampers in a High Pressure Single Sector Combustor

Self-excited periodic instabilities in a staged lean burn injector could be forced by operating the combustor at off-design conditions. These pressure oscillations were studied in a high pressure single sector combustor with optical access. Two damper configurations were installed and tested with respect to their damping efficiency in relation to the configuration without dampers. For a variety of test conditions, derived from a part load case, time traces of pressure in the combustor were measured, and amplitudes were derived from their Fourier transformation. These measurements were performed for several combinations of the operating parameters, i.e., injector pressure drop, air/fuel ratio (AFR), pilot/main fuel split, and preheat temperature. These tests “ranked” the respective damper configurations and their individual efficiency with respect to the configuration without dampers. Although a general trend could be observed, the ranking was not strictly consistent for all operating conditions. For several test cases, preferably with pronounced self-excited pressure oscillations, phase-resolved planar optical measurement techniques were applied to investigate the change of spatial structures of fuel, reaction zones, and temperature distributions over a period of an oscillation. A pulsating motion was detected for both pilot and main flame, driven by a pulsating transport of the liquid fuel. This pulsation, in turn, is caused by a fluctuating air velocity, in connection with a prefilming airblast type atomizer. A phase shift between pilot and main injector heat release was observed, corresponding to a shift of fuel penetration. Local Rayleigh indices were calculated qualitatively, based on phase-resolved OH chemiluminescence used as marker for heat release, and corresponding pressure values. This identified regions, where a local amplification of pressure oscillations occurred. These regions were largely identical to the reaction regions of pilot and main injector, whereas the recirculation zone between the injector flows was found to exhibit a damping effect.

Three-dimensional organization of the flow structure in a non-reactive model aero engine lean burn injection system

An investigation of the three-dimensional flow field of a turbulent swirling jet at Re=50×103 generated by a non-reactive model aero engine lean burn injector is carried out in a water facility with tomographic Particle Image Velocimetry. This work is focused on the organization of the coherent structures arising within the near field of the swirling jet both in free and confined configurations. The confinement causes an increase of the Swirl number: the measured values are equal to 0.90 and 1.27, respectively for free and confined swirling jets. The effects of the confinement induce a larger spreading of the swirling jet promoting the enhancement of turbulence at the nozzle exit, but the expected upstream displacement of the reverse flow region is not observed. The instantaneous flow field is characterized by the presence of the precessing vortex core (PVC), of the outer helical vortex and of smaller turbulent structures perturbing the structure of the PVC. A three dimensional modal analysis of the velocity field using the Proper Orthogonal Decomposition (POD) highlights that the flow is dominated by the precessing vortex core. Using the first two POD modes a low order reconstruction of the velocity field is calculated. It is found that the small-scale structures shown in the instantaneous velocity field are not captured in the low order reconstruction due to smoothing effects, but the precessing vortex core and the outer helical vortex are properly represented.

Rapid Liquid Fuel Mixing for Lean-Burning Combustors: Low-Power Performance

Designers of advanced gas turbine combustors are considering lean direct injection strategies to achieve low NOx emission levels. In the present study, the performance of a multipoint radial airblast fuel injector Lean Burn injector (LBI) is explored for various conditions that target low-power gas turbine engine operation. Reacting tests were conducted in a model can combustor at 4 and 6.6 atm, and at a dome air preheat temperature of 533 K, using Jet-A as the liquid fuel. Emissions measurements were made at equivalence ratios between 0.37 and 0.65. The pressure drop across the airblast injector holes was maintained at 3 and 7–8 percent. The results indicate that the LBI performance for the conditions considered is not sufficiently predicted by existing emissions correlations. In addition, NOx performance is impacted by atomizing air flows, suggesting that droplet size is critical even at the expense of penetration to the wall opposite the injector. The results provide a baseline from which to optimize the performance of the LBI for low-power operation.