• Energy Research

Stricter legislation limits concerning NOx emissions are leading main aero-engine manufacturers to update the architecture of the combustors towards the implementation of lean burn combustion concept. Cooling air availability for the thermal management of combustor liners is significantly reduced, demanding even more effective liner cooling schemes. The state-of-the-art of liner cooling technology is represented by effusion cooling, consisting in a very efficient cooling strategy based on multi-perforated liners, where metal temperature is lowered by the combined protective effect of coolant film and heat removal inside the holes. The present research study aims at deepening the knowledge of effusion systems, exploiting the results of a thorough experimental campaign carried out in two different planar test rigs, equipped with a complete liner cooling scheme composed by slot injection and effusion array. The film cooling protection was analysed using PSP (Pressure Sensitive Paint) technique, while the effect of cooling injection and extraction from the annulus on heat transfer distribution were studied by means of TLC (Thermochromic Liquid Crystals) thermography. Thermal measurements were supported by flow field investigation with standard 2D PIV (Particle Image Velocimetry) in order to highlight the typical velocity distributions generated by a realistic lean injector. These detailed experimental data were exploited in a 1D thermal flow-network solver that allows to better assess the main cooling mechanisms characterising the proposed cooling system. Moreover, an optimized cooling configuration with enhanced back-side convective cooling was proposed and compared with the standard configuration in terms of metal temperature and cooling consumption.

This work presents the results of an experimental campaign investigating the behavior of an industrial burner operated with simulated Exhaust Gas Recirculation (EGR). EGR is recreated by diluting standard air with CO2, and tests are performed at ambient pressure using natural gas as fuel. Burner characterization has been performed in terms of emission measurements both in standard conditions and with CO2 vitiated air. Flame topology has been studied with OH* chemiluminescence, evaluating the effect of fuel split and CO2 addition. CO2 addition has been found to trigger thermoacoustic instabilities up to a certain threshold, therefore limiting the EGR operability window.

In modern lean-burn aero-engine combustors, highly swirling flow structures are adopted to control the fuel-air mixing and to provide the correct flame stabilization mechanisms. Aggressive swirl fields and high turbulence intensities are hence expected in the combustor-turbine interface. Moreover, to maximize the engine cycle efficiency, an accurate design of the high-pressure nozzle cooling system must be pursued: in a film-cooled nozzle, the air taken from last compressor stages is ejected through discrete holes drilled on vane surfaces to provide a cold layer between hot gases and turbine components. In this context, the interactions between the swirling combustor outflow and the vane film cooling flows play a major role in the definition of a well-performing cooling scheme, demanding for experimental campaigns at representative flow conditions. An annular three-sector combustor simulator with fully cooled high-pressure vanes has been designed and installed at THT Lab of University of Florence. The test rig is equipped with three axial swirlers, effusion-cooled liners, and six film-cooled high-pressure vanes passages, for a vortex-to-vane count ratio of 1:2. The relative clocking position between swirlers and vanes has been chosen in order to have the leading edge of the central airfoil aligned with the central swirler. In this experimental work, adiabatic film effectiveness measurements have been carried out in the central sector vanes, in order to characterize the film-cooling performance under swirling inflow conditions. The pressure-sensitive paint (PSP) technique, based on heat and mass transfer analogy, has been exploited to catch highly detailed 2D distributions. Carbon dioxide has been used as coolant in order to reach a coolant-to-mainstream density ratio of 1.5. Turbulence and five-hole probe measurements at inlet/outlet of the cascade have been carried out as well, in order to highlight the characteristics of the flow field passing through the cascade and to provide precise boundary conditions. Results have shown a relevant effect of the swirling mainflow on the film cooling behavior. Differences have been found between the central airfoil and the adjacent ones, both in terms of leading edge stagnation point position and of pressure and suction side film coverage characteristics.

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.

Abstract The introduction of Lean Burn concept as basic Low- N O x 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.

In the continuous demand of increasing cooling efficiency for novel combustor liners, it is necessary to have a comprehensive understanding of the interaction of hot gases with coolant flows. The aim of the present study is the experimental characterization of the flow field and the measurement of liner heat transfer coefficient in a combustion chamber model equipped with an axial swirler and a liner slot cooling scheme. The test rig geometry consists in a linear three sector chamber fed by an open loop blower. The system is operated at isothermal conditions. A highly swirled main stream flow is achieved by considering an injector geometries that produce flow structures which interact with film cooling flow delivered by a simplified slot at the inner wall of the liner. To study the effects of this mutual interaction, the flow field and the liner heat transfer are investigated at different slot cooling and injector flow rates. A 2D PIV (Particle Image Velocimetry) technique is employed to investigate the test section flow field on two different planes. An experimental campaign focused on liner heat transfer measurement is carried out using a TLC (Thermochromic Liquid Crystals) steady state technique with a thin Inconel heating foil fed by two copper bus bars. Results obtained indicate an appreciable role of film cooling flow on both swirler aerodynamics and the liner heat transfer coefficient. When the slot cooling flow rate is increased, the observed peak of heat transfer coefficient, due to liner-swirl flow interaction, gradually reduces. This is a consequence of a reduction in swirling jet expansion when slot cooling is increased, which also affects the amount of flow recirculation due to vortex breakdown.

Abstract While the employment of Exhaust Gas Recirculation (EGR) is a well-established technique in Internal Combustion Engines to limit NOx emissions, its adoption in Gas Turbine engines hasn’t yet found a practical application due to its expensive and complex installation that doesn’t justify the emissions reduction when compared to already established DLN combustion technologies. EGR becomes an interesting option in GT engines considering the possibility of increasing the CO2 content of the exhaust gases to improve the efficiency of Carbon Capture and Storage (CCS) units. However, the decrease in oxygen content of the combustion air is extremely challenging in terms of combustion stability and therefore of engine operability. In the present work, a low NOx burner was studied at ambient pressure in a reactive single burner test rig. The burner was fed with methane and characterised in terms of emissions and stability limits at different operating conditions. In addition, the flame position and shape were studied through OH* chemiluminescence imaging together with the flow field thanks to PIV measurements. The effects of CO2 addition on the flame were then investigated at different EGR increasing levels, highlighting the impact of the oxygen content on the combustion reaction intensity. Variations in emissions and burner stability limits in terms of maximum sustainable CO2 content were also studied, to detail the burner operating window. Data have been thoroughly analysed to gather information on the burner behaviour to support the design of new technical solutions capable of ensuring both proper flame stability and low CO and NOx emissions.

Abstract The development and the optimization of a novel dry low NOx burner may require several steps of improvement. The first step of the overall development process has been documented by authors in a previous paper and included an exhaustive experimental characterization of a set of novel geometries. The in-depth results analysis allowed to correlate the investigated design parameters to burner performances, discovering possible two-fold optimization paths. Recurrent verifications of the assumptions made to define prototypes design are considered a mandatory step to avoid significant deviation from the correct optimization path, which strongly depends on both objective function definition and selection of design variables. Concerning the objective function, a proper mathematical formulation was proposed in the previous work, which represented a balance between two apparently conflicting aspect like flame stability and low emissions. Concerning design variables, outcomes of the first test campaign have been used in the present work to define new burner geometries. Starting from a new baseline who has showed the widest low NOx operating window, additional geometrical features have been considered in this survey as potentially affecting flame stabilization. Thanks to the degree of freedom offered by DMLM technology, rapid prototyping of alternative geometries allowed to easily setup a new experimental plan for the second optimization step. Exploiting the same approach used in the first test campaign, new geometries have been tested in a single-cup test rig at gas turbine relevant operating conditions, showing Stable low-NOx operating windows have been evaluated throughout dedicated objective functions for all geometries and results showed lower NOx and CO emissions as a consequence of the newly introduced geometrical modifications. Moreover, the comparison with the estimates of the previous campaign proved the existence of the identified optimization path. Indeed, it furnished valid elements for further using of the proposed methodology for the improvement of emission and blow-out characteristics of novel burners and, more in general, for the development of a novel dry low NOx technology.