Combustion Test Rig Research Articles

Delayed Formation of Thermally Grown Oxide in Environmental Barrier Coatings for Non-Oxide Ceramic Matrix Composites

The oxidation and corrosion behavior at elevated temperatures of a SiCF/SiC(N) composite with two plasma-sprayed environmental barrier coating (EBC) systems were studied. After both processes, the formation of a silica-based thermally grown oxide (TGO) layer was observed. The formation of this TGO caused two principal failure mechanisms of the EBC. Firstly, spallation of the EBC induced by stresses from volume expansion and phase transformation to crystalline SiO2 was observed. Water vapor corrosion of the TGO with gap formation in the top region of the TGO was found to be a second failure mechanism. After a burner rig test of the Al2O3-YAG EBC system, this corrosion process was observed at the TGO surface and in the volume of the Al2O3 bond coat. In the case of the second system, Si-Yb2Si2O7/SiC-Yb2SiO5, the formation of the TGO could be delayed by introducing an additional intermediate layer based on Yb2Si2O7 filled with SiC particles. The SiC particles in the intermediate layer were oxidized and served as getter to reduce the permeation of oxidants (O2, H2O) into the material. In this way, the formation of the TGO and the failure mechanisms caused by their formation and growth could be delayed.

Thermal cycling failure of the multilayer thermal barrier coatings based on LaMgAl11O19/YSZ

Thermal cycling failure of three multilayer TBCs based on LaMgAl11O19 (LaMA)/YSZ was comparatively investigated by using the burner-rig testing method in this work. Results indicate that through optimizing the weight ratio and thickness of the intermediate LaMA/YSZ composite layers, a five-layer TBC with much improved thermal cycling life of 11,749 cycles at 1372 °C surface and 1042 °C bond coat testing temperature has been realized. While, thermal cycling lifetimes of the tri- and six-layer TBCs were 7439 and 7804 cycles at surface/bond coat testing temperatures of 1378 °C/1065 °C and 1367 °C/1056 °C, respectively. Factors related to the 60 wt.% LaMA + 40 wt.% YSZ (60LaMA + 40YSZ) intermediate composite layer with the highest thermal expansion coefficient than other composite layers generating higher internal stress level to the tri- and six-layer TBCs, different bond coat temperature and TGO growth, as well as long-term stability of the LaMA coating during thermal cycling tests, were characterized and compared to understand the different thermal cycling lifetime and failure modes among such three multilayer TBCs.

Time-/space-sensitive sintering kinetics of plasma sprayed multi-modal nanostructured coating

Sintering is considered to be one of the main reasons that accelerate performance degradation and shorten lifetime of thermal barrier coatings (TBCs). The anti-sintering design and underlying sintering kinetics are of great importance for optimizing the microstructure of TBCs. In this study, the competitive sintering of a novel multi-modal nanostructured coating is systematically studied by a burner-rig test. The results suggested that, due to the coexistence of unmelted nanozone, amorphous area and recrystallized region, the sintering behavior of nanostructured coating exhibited obvious time-/space-sensitive effects. Time-sensitive feature was initially associated with the coalescence growth of nano-particles and healing of micro-pores within the unmelted nanozone. Subsequently, the sintering was accompanied by the abnormal growth of grains in the unmelted nanozone and the formation of multi-contact neck in the recrystallized region. The diffusion-driven stiffening of recrystallized region promoted the stage-progressive enhancement of elastic modulus. The space-sensitive sintering kinetics caused the spontaneous formation and evolution of multi-scale cracks at the unmelted nanozone/recrystallized region interface.

Biomass fuel identification using flame spectroscopy and tree model algorithms

ABSTRACT This paper presents an identification method for types of fuel such as biomass by combining flame spectroscopic monitoring and tree model algorithms. The features of the flame spectra are extracted, including the spectral intensity of flame radicals [OH* (310.85 nm), CN* (390.00 nm), CH* (430.57 nm) and C2* (515.23 nm, 545.59 nm)], flame radiation intensity and flame radiation energy (integration of spectral intensity). The identification models are built using four tree model algorithms, i.e., decision tree, random forest, extremely randomized trees, and gradient boost decision tree. The different type of biomass and spectra features of combustion flames are composed of sample pairs to train identification models. Experiments are carried out on a laboratory-scale biomass-air combustion test rig. Four different biomass fuels, including corncob, willow, peanut shell, and wheat straw are burnt. The results demonstrate that the identification models proposed is capable of identifying types of biomass fuels correctly with the average identification success rate of 98% in 10 trials.

Experimental Investigations of the Flow Field Structure and Interactions between Sectors of a Double-Swirl Low-Emission Combustor

In this paper, the flow field characteristics of a double-swirl low-emission combustor were analyzed by using Particle Imaging Velocimetry (PIV) technology in an optical three-sector combustor test rig. The interactions between sectors and the flow field structure were explained. The results illustrated that there was a big difference between the flow field structures of the middle sector and the side sector under the same pressure drop, which was mainly induced by the interactions between sectors. The interactions made the swirl intensity of the middle sector weaker than that of the side sector, which made the recirculation zone of the middle sector be smaller than that of the side sector. With the increase of swirler pressure drop, the jet velocity at the exit of the swirler, the jet expansion angle, the width of the recirculation zone and the recirculating speed of the central axis became larger, enhancing the interactions between air streams from middle sector and side sector. The flow velocity in the central plane between sectors was small, especially the radial velocity, mainly because of the loss of the swirl intensity by the interactions between flow field of adjacent sectors. The expansion angle determined the position of the vortex in the primary recirculation zone; the axial and radial position of the vortex move downstream and radial outward with the increase of the jet expansion angle. The results of the mechanism of flow field organization in this study can be used to support the design of new low-emission combustor.

Experimental investigations of the spray structure and interactions between sectors of a double-swirl low-emission combustor

In this paper, the spray characteristics of a double-swirl low-emission combustor are analyzed by using Particle Imaging Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) technologies in an optical three-sector combustor test rig. Interactions between sectors and the influence of main stage swirl intensity on spray structure are explained. The results illustrate that the swirl intensity has great effect on the flow field and spray structure. The spray cone angle is bigger when the swirl number is 0.7, 0.9 than that when the swirl number is 0.5. The fuel distribution zone is larger and the distribution is more uniform when the swirl number is 0.5. The fuel concentration in the center area of the center plane of side sector (Plane 5) is larger than that of the center plane of middle sector (Plane 1). The spray cone angle in Plane 5 is larger than that in Plane 1. The width of spray cone becomes larger with the increase of Fuel–Air Ratio (FAR), whereas the spray cone angle under different fuel–air ratios are absolutely the same. The results of the mechanism of spray organization in this study can be used to support the design of new low-emission combustor.

YAlO3—A Novel Environmental Barrier Coating for Al2O3/Al2O3–Ceramic Matrix Composites

Ceramic matrix composites (CMCs) are promising materials for high-temperature applications. Environmental barrier coatings (EBCs) are needed to protect the components against water vapor attack. A new potential EBC material, YAlO3, was studied in this paper. Different plasma-spraying techniques were used for the production of coatings on an alumina-based CMC, such as atmospheric plasma spraying (APS) and very low pressure plasma spraying (VLPPS). No bond coats or surface treatments were applied. The performance was tested by pull–adhesion tests, burner rig tests, and calcium-magnesium-aluminum-silicate (CMAS) corrosion tests. The samples were subsequently analyzed by means of X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Special attention was paid to the interaction at the interface between coating and substrate. The results show that fully crystalline and good adherent YAlO3 coatings can be produced without further substrate preparation such as surface pretreatment or bond coat application. The formation of a thin reaction layer between coating and substrate seems to promote adhesion.

Determination of Acoustic Scattering Matrices from Linearized Compressible Flow Equations with Application to Thermoacoustic Stability Analysis

The acoustic transmissions and reflections of plane waves at duct singularities can be represented with so-called scattering matrices. This paper shows how to extract scattering matrices utilizing linearized compressible flow equations and provides a comparative study of different governing equations, namely the Helmholtz, linearized Euler and linearized Navier–Stokes equations. A discontinuous Galerkin finite element method together with a two-source forcing is employed. With this method, the scattering matrix for a radial swirler of a combustion test-rig is computed and validated against the results of a fully compressible Large-Eddy-Simulation. Analogously, the scattering behavior of an axial swirler is investigated. The influence of acoustic-hydrodynamic interactions, viscous effects as well as unsteady boundary layers on the results is investigated for both configurations. A thermoacoustic stability analysis of the combustion test-rig housing the axial swirler is carried out, utilizing the scattering matrix of the swirler. Major influence of the reflections coming from the swirler on the thermoacoustic eigenfrequencies is found.

Effects of secondary air velocity on NO emission with coal preheating technology

This study investigated experimentally the effects of the injection velocity of the secondary air on combustion and NO emissions of Shenmu semi-coke in a 30 kW coal preheating combustion test rig. Results showed that the self-preheating burner could provide with the high temperature preheated fuel (coal gas and coal char) stably and continuously without extra heat input. Most of volatile-N was released following with the release of volatiles during the preheating process, which was conductive to inhibit NO formation. The combustion temperatures were uniform in the down-fire-combustor and flame images were all dark red with no obvious flame fronts. The coal gas was rapidly consumed in the upper part while the coal char was burnt out in the lower part of the down-fire-chamber. The presence of two combustion zones would contribute to leading to more uniform temperatures. Char-N was the main source of NO in the research, and NO was reduced by the coal gas (CO, CHi, NHi and other species) and char simultaneously above 1000 mm. While below 1000 mm, char was dominating for NO reduction. In the paper, the relationship of injection velocity of secondary air and exit NOx emissions was not clearly revealed. It was believed that the effects of secondary air velocity were multifactorial. Mean temperature and root mean square temperature were associated with the exit NOx emissions. When mean temperature and root mean square temperature are low, the exit NOx emissions are low correspondingly.

Oxide Dispersion Strengthened Bond Coats with Higher Alumina Content: Oxidation Resistance and Influence on Thermal Barrier Coating Lifetime

The oxidation resistance of the bond coat in thermal barrier coating systems has significant influence on thermal cycling performance of the protective coating. In this study, the influence of varying the alumina content of plasma-sprayed oxide dispersion strengthened bond coats with CoNiCrAlY matrix material on the oxidation resistance was analysed by thermogravimetric analysis, SEM and TEM. Yttrium ions at the alumina scale grain boundaries and the grain size in the scale appear as major factors influencing oxidation properties. The ODS material with 2, 10 and 30 wt% alumina content was applied in TBC systems as an additional thin bond coat. The thermal cycling performance of those advanced TBC systems, in burner rig tests, was evaluated with respect to the ODS material properties. Thermal cycling behaviour is in good correlation with the isothermal oxidation resistance. All results indicate that TBC systems with 10 wt% alumina content in the ODS bond coat have a superior thermal cycling performance, as compared to ODS bond coats with lower or higher alumina content.

Durability of Gadolinium Zirconate/YSZ Double-Layered Thermal Barrier Coatings under Different Thermal Cyclic Test Conditions.

Higher durability in thermal barrier coatings (TBCs) is constantly sought to enhance the service life of gas turbine engine components such as blades and vanes. In this study, three double layered gadolinium zirconate (GZ)-on-yttria stabilized zirconia (YSZ) TBC variants with varying individual layer thickness but identical total thickness produced by suspension plasma spray (SPS) process were evaluated. The objective was to investigate the role of YSZ layer thickness on the durability of GZ/YSZ double-layered TBCs under different thermal cyclic test conditions i.e., thermal cyclic fatigue (TCF) at 1100 °C and a burner rig test (BRT) at a surface temperature of 1400 °C, respectively. Microstructural characterization was performed using SEM (Scanning Electron Microscopy) and porosity content was measured using image analysis technique. Results reveal that the durability of double-layered TBCs decreased with YSZ thickness under both TCF and BRT test conditions. The TBCs were analyzed by SEM to investigate microstructural evolution as well as failure modes during TCF and BRT test conditions. It was observed that the failure modes varied with test conditions, with all the three double-layered TBC variants showing failure in the TGO (thermally grown oxide) during the TCF test and in the ceramic GZ top coat close to the GZ/YSZ interface during BRT. Furthermore, porosity analysis of the as-sprayed and TCF failed TBCs revealed differences in sintering behavior for GZ and YSZ. The findings from this work provide new insights into the mechanisms responsible for failure of SPS processed double-layered TBCs under different thermal cyclic test conditions.

The impact of swirling flow strength on lean-dome LDI pilot mixers’ operability and emissions

In an effort to reduce the environmental impact of aviation, lean-dome combustion concepts such as lean direct injection (LDI) are being pursued for their potential to achieve very low emissions. However, low-emissions potentials may be accompanied by operability challenges. In the present effort – part of a series of fundamental investigations into LDI operability challenges – systematic experiments of lean-dome-relevant pilot combustor devices are conducted in a single-element methane-fueled combustor test rig with two different LDI configurations by using a suite of optical diagnostics. While maintaining all other design features constant, the clockwise vane angle of the outer air swirler is altered between 60° (LDI-60-CW configuration) and 45° (LDI-45-CW configuration), representing strong and weak swirling flows, respectively. Measurements of the non-reacting/reacting flow fields and flame characteristics are performed as a function of overall equivalence ratio via time-resolved particle image velocimetry, OH*/CH*/NO2* chemiluminescence, OH planar laser-induced fluorescence, and broadband flame imaging. It is found that a large center recirculation zone exists only with the LDI-60-CW configuration, while the LDI-45-CW flow is dominated by a swirling jet flow. In addition, it is shown that the center recirculation zone in the LDI-60-CW configuration results in noticeably lower lean blowout limits than the LDI-45-CW configuration, but at the cost of much higher NOx production for vigorously-burning equivalence ratios. At lower equivalence ratios near lean blowout conditions, both configurations result in similar NOx production. These results are discussed in terms of identifying possible research directions for optimizing LDI designs for reduced NOx emissions while maintaining or improving operability relative to current rich-dome combustion technology.

Thermal Shock and Oxidation Stability Tests to Grade Plasma Sprayed Functionally Gradient Thermal Barrier Coatings

Functionally graded layers in thermal barrier coatings reduce the stress gradient between the overlaid ceramic coatings and the underlying metallic component. Introduced to alleviate early onset of spallation of the coating due to thermal expansion mismatch, this facilitates improvement in the life of the component. Conventional thermal barrier coatings typically comprise of duplex layers of plasma sprayed 8% yttria stabilized zirconia (ceramic) coatings on bond coated (NiCrAlY) components/substrates (Inconel 718 for example). This work highlights the superiority of plasma sprayed coatings synthesized from blends of the intermetallic bond coat and ceramic plasma spray powders on Inconel 718 substrates in three-layer configuration over the duplex layered configuration. Assessed through (a) thermal shock cyclic tests (at 1200oC and 1400oC) in laboratory scale basic burner rig test facility and (b) oxidation stability test in high temperature furnace (at 800oC and 1000oC) the functionally graded coatings of certain configurations exhibited more than double the life of the conventional 8% yttria stabilized zirconia duplex (double layer) coatings. Micro- and crystal structure analysis support the findings and results are detailed and discussed.

Microstructure and self-healing properties of multi-layered NiCoCrAlY/TAZ/YSZ thermal barrier coatings fabricated by atmospheric plasma spraying

Abstract In this paper, the multi-layered self-healing thermal barrier coatings (TBCs) have been fabricated by atmospheric plasma spraying (APS). The phase composition of the sprayed feedstock and the as-sprayed coating have been characterized. The thickness of each layer for the self-healing TBCs have been optimized based on previous finite element simulation results. The self-healing effect of the TBCs under burner rig test (BRT) has been characterized via in-situ acoustic emission technique. The self-healing ability of the TBCs has been also characterized via static high temperature oxidation test. The investigation results indicate that the thickness of each layer for the as-sprayed self-healing TBCs is nearly in our optimization range based on our previous work. The AE signals of the TBCs before and after self-healing are distinctly different. When the self-healing effect has appeared, some key AE signals for the crack propagation has disappeared. The AE signals which are located at the higher frequency have been inhibited at a certain degree. The static high temperature oxidation results indicate that the TBCs with self-healing effect has lower oxidation rate compared with that of the TBCs without self-healing. The effective thickness of the TGO layer for the TBCs with self-healing treatment is lower compared with that of the TBCs without self-healing treatment.

Evolution of porosity, crack density, and CMAS penetration in thermal barrier coatings subjected to burner rig testing

AbstractDegradation of thermal barrier coatings (TBCs) in gas‐turbine engines due to calcium–magnesium–aluminosilicate (CMAS) glassy deposits from various sources has been a persistent issue since many years. In this study, state of the art electron microscopy was correlated with X‐ray refraction techniques to elucidate the intrusion of CMAS into the porous structure of atmospheric plasma sprayed (APS) TBCs and the formation and growth of cracks under thermal cycling in a burner rig. Results indicate that the sparse nature of the infiltration as well as kinetics in the burner rig are majorly influenced by the wetting behavior of the CMAS. Despite the obvious attack of CMAS on grain boundaries, the interaction of yttria‐stabilized zirconia (YSZ) with intruded CMAS has no immediate impact on structure and density of internal surfaces. At a later stage the formation of horizontal cracks is observed in a wider zone of the TBC layer.

Prediction of critical rupture of plasma-sprayed yttria stabilized zirconia thermal barrier coatings under burner rig test via finite element simulation and in-situ acoustic emission technique

In the current work, the yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) have been fabricated by atmospheric plasma spraying. The failure mechanism of the TBCs during burner rig test (BRT) has been investigated via finite element modeling (FEM) and in-situ acoustic emission (AE) technique systematically. During the BRT, the oxygen-propane flame was heated onto the surface of the coating samples with the dwell time 3 min, then the surface and the backside of the coating samples were cooled by the compressed air simultaneously for 3 min. This can be viewed as one thermal cycle. Then repeat the same heating and cooling process. The history and distribution of the temperature along the through-thickness direction of the coating samples has been obtained via FEM, and the residual stress at different stages has been also calculated. The FEM results have indicated that large tensile stress and compressive stress existed at the heating stage and cooling stage of each thermal cycle, respectively. The crack propagation tends to occur at the initial period of heating stage in the BRT process. The in-situ acquisition of the AE signals during the BRT has also been adopted. The crack propagation patterns have been obtained based on the analysis of AE signals. The filtering technique has been used to exclude the disturbance of the noise in the process of BRT in order to capture the actual and effective AE signals which represent the crack propagation and the deformation of the coating systems. Based on the characteristic waveform of effective AE signals, the Fast Fourier Transformation (FFT) and wavelet transformation has been adopted to analyze the key distribution range of the amplitude and frequency. The investigation results indicate that the acquired AE signals during BRT mainly include signals which came from the plastic deformation of the substrate and creep of each layers, propagation of the vertical crack and propagation of the interfacial cracks (horizontal crack). The detailed failure mechanisms of the TBCs during BRT have been clarified in the current work.

Failure of Gadolinium Zirconate and Yttria Stabilized Zirconia Thermal Barrier Coatings Subjected to High Temperature Calcia-Magnesia-Alumino-Silicate Attack

Nowadays, the contribution of rare-earth oxide compounds is extensively investigated with the aim to improve the service life of gas turbine engine components protected by thermal barrier coatings (TBCs) against the environmental Calcia-Magnesia-Alumino-Silicate (CMAS) attack. Therefore, the TBCs consisting of a NiCrAlY bond coat, Yttria Stabilized Zirconia (YSZ) and/or mixture of YSZ and Gadolinium Zirconate (YSZ+GZ) interlayers, and a GZ top coat, which were all deposited by atmospheric plasma spraying onto the nickel-based superalloy substrates, are introduced in this contribution. The CMAS-attack resistance was evaluated using an indirect method. Firstly, the thin layer of CMAS prepared from colloidal solutions was deposited onto the top coat surface and, after drying, the samples were heat treated with the aim to glassified the CMAS. Secondly, the coated samples containing CMAS glass debris were subjected to rapid heating (up to 1200 °C) and enforced cooling cycles at the burner-rig test device and the failure of TBC was investigated. In all cases, the failure mechanism due to the CMAS attack was the top coat spallation. The functional graded TBC was found to be the most resistant system.

The Influence of Thermal Transient Rates on Coated Turbine Parts' Life Expectancy

During rapid engine throttling operations, turbine airfoils can experience very rapid heating and cooling, particularly at take-off conditions. These rapid transient events lead to the generation of high thermal gradients and nonuniform stress distributions through the thermal barrier coating (TBC), environmental barrier/bond coating, and substrate. This, in turn, can lead to coating delamination, overheat of the substrate materials, creep, and thermo-mechanical fatigue of the part. We present the process and computer modeling methodology for a physics-based prediction of deformation, damage, crack propagation and local failure modes in coated turbine airfoils and other parts operating at hot section turbine environment conditions as a function of engine operational regimes, with a particular emphasis on rapid transient events. The overall goal is to predict the effects and severity of the cooling and heating thermal rates on transient thermal mechanical fatigue life of coated hot parts (turbine airfoils, blade outer air seals, and combustor liners). The computational analysis incorporates time-accurate, coupled aerothermodynamics with nonlinear deformation thermal-structural finite element modeling, and fracture mechanics modeling for high-rate thermal transient events. TBC thermal failure and spallation are introduced by the use of interface fracture toughness and interface property evolution as well as dissipated energy rate. The spallation model allows estimations of the part remaining life as a function of the heating/cooling rates. Applicability of the developed model is verified using experimental coupons and calibrated against burner rig test data for high-flux thermal cycles. Our results show a decrease in TBC spall life due to high-rate transient events.

Assessment of SLW-1 model in the presence of gray and non-gray particles

In this study, predictive accuracy of Gray Gas and SLW-1 approximations is benchmarked against Spectral Line-Based Weighted Sum of Grey Gases Model (SLW) in multidimensional enclosures involving gray/non-gray absorbing, emitting and scattering particles. Input data required for the radiation code and its validation are provided from two combustion tests previously carried out in a 300 kWt Atmospheric Bubbling Fluidized Bed Combustor (ABFBC) test rig burning low calorific value Turkish lignite with high volatile matter/fixed carbon (VM/FC) ratio in its own ash. Comparisons reveal that SLW-1 approximation leads to one order of magnitude higher accuracy in heat flux and source term predictions compared to that of Gray Gas approximation in the presence of gray particles while maintaining a similar computational efficiency. In the presence of non-gray particles, SLW-1 approximation is again found to improve the predictive accuracy compared to that of Gray Gas approximation. However, it is seen that errors in heat flux and source term predictions with SLW-1 approximation are noticeably higher in the presence of non-gray particles compared to those with gray particles.

Development of a wide range-operable, rich-lean low-NOx combustor for NH3 fuel gas-turbine power generation

Low-NOx NH3-air combustion power generation technology was developed by using a 50-kWe class micro gas-turbine system at the National Institute of Advanced Industrial Science and Technology (AIST), Japan, for the first time. Based on the global demand for carbon-free power generation as well as recent advances involving gas-turbine technologies, such as heat-regenerative cycles, rapid fuel mixing using strong swirling flows, and two-stage combustion with equivalence ratio control, we developed a low-NOx NH3-air non-premixed combustor for the gas-turbine system. Considering a previously performed numerical analysis, which proved that the NO reduction level depends on the equivalence ratio of the primary combustion zone in a NH3-air swirl burner, an experimental study using a combustor test rig was carried out. Results showed that eliminating air flow through primary dilution holes moves the point of the lowest NO emissions to the lesser fuel flow rate. Based on findings derived by using a test rig, a rich-lean low NOx combustor was newly manufactured for actual gas-turbine operations. As a result, the NH3 single fueled low-NOx combustion gas-turbine power generation using the rich-lean combustion concept succeeded over a wide range of power and rotational speeds, i.e., below 10–40 kWe and 75,000–80,000 rpm, respectively. The NO emissions were reduced to 337 ppm (16% O2), which was about one-third of that of the base system. Simultaneously, unburnt NH3 was reduced significantly, especially at the low electrical power output, which was indicative of the wider operating range with high combustion efficiency. In addition, N2O emissions, which have a large Global Warming Potential (GWP) of 298, were reduced significantly, thus demonstrating the potential of NH3 gas-turbine power generation with low environmental impacts.