High Temperature Combustion Environment Research Articles

A novel strategy to improve the corrosion resistance of SiCf/SiC composites in high-temperature combustion environments by cooperation of self-healing SiBCN and high-stability Yb2Si2O7

How to improve the resistance to water-oxygen-salt corrosion of SiCf/SiC composites is vital to their long-term service in the advanced aero-engines or gas turbines. SiCf/SiC-SiBCN-Yb2Si2O7 (SSY) self-healing composites were exposed to combustion environments at 1200–1350 °C for a long time up to 100 h. The dissolution of solid Yb2Si2O7 into low-viscosity glassy oxides derived mainly from oxidation of SiBCN leads to the formation of stable Yb2Si2O7-contianing multi-phase silicate which can inhibit the diffusion of corrosive media and alleviate corrosive damage of composites. After corrosion in H2O/O2/Na2SO4 at 1200 °C and 1350 °C, the strength retention rates of SSY are 96 % and 88 %, respectively, (6.4 ± 0.9)% higher than those of SiCf/SiC-SiBCN (SS), due to the good cooperative protective effect of SiBCN-Yb2Si2O7. However, the insufficient dissolution of Yb2Si2O7 causes the formation of plenty of solid/liquid interfaces conductive to the diffusion of corrosive media in H2O/O2 at 1200 °C and 1350 °C. As a result, the retention rates of flexural strengths of SSY are 100 % and 91 %, respectively, (2.5 ± 0.5)% lower than those of SS.

Investigation of Yb2SiO5/BSAS/Si coating on Cf/SiC composites: processing and steam corrosion behavior

Environmental Barrier Coatings (EBCs) are used to protect silicon carbide-based composites in the high-temperature steam combustion environment of the latest generation of high-efficiency gas turbine aero-engines. The Yb2SiO5/BSAS/Si coatings with gradient coefficient of thermal expansion were prepared using atmospheric plasma spraying (APS) method on the carbon fiber reinforced silicon carbide ceramic matrix composites (Cf/SiC) composites surface. The coating preparation process parameters were optimized by evaluating the influence of spraying power on the bonding strength, deposition efficiency, and porosity. The corrosion behaviors and mechanism of the optimized Yb2SiO5/BSAS/Si coating was investigated at 1200 ℃ by thermal cycle test in steam conditions. The results indicated that the decomposition of Yb2SiO5 and the conversion of SiO2 to Si(OH)4 were the main corrosion reactions of Yb2SiO5 top coating. Noticeably, the SiO2 component acted as an obvious sign of the reaction front diffusing, forming the clear SiO2-gradient layers. The gradient Yb2SiO5/BSAS/Si coating remained intact after thermal cycles for 500 h at 1200 ℃ in steam conditions which effectively enhanced the resistance of Cf/SiC composites to high temperature steam corrosion.

High Temperature Oxidation of Additively and Traditionally Manufactured Inconel 718

Abstract Additively manufactured nickel-based superalloys are now being considered as a replacement for traditionally manufactured components on commercial and military aircraft. While the potential of additive manufacturing for gas turbine engines is promising, the quality and performance of additive manufactured components still need extensive study. Coupon-sized test specimens comprised of as-printed additively and traditionally manufactured Inconel 718 with and without yttria-stabilized zirconia thermal barrier coating were tested under simulated isothermal and thermal cycling combustion conditions that were representative of gas turbine environments. Pre-test scanning electron microscopy indicated that traditionally manufactured coupons had a smooth surface finish with minor imperfections while additive manufactured coupons had a rough surface finish. Post-test scanning electron microscopy exhibited differences in oxide scale between the isothermal and thermal cycling conditions. The thermal cycling condition increased the amount of oxide scale for both additively manufactured and traditionally manufactured Inconel 718. The size of the oxide islands on traditionally manufactured coupons was significantly larger than the additively manufactured coupons. The results indicated that differences in surface roughness may affect the growth of the oxidation scale in a high-temperature combustion environment. The benefits of yttria-stabilized zirconia thermal barrier coating were characterized. The substrate of the coupons experienced little to no formation of oxide scales compared to the uncoated additive and traditionally manufactured coupons. The results further suggested that yttria-stabilized zirconia thermal barrier coating can be utilized to provide both insulation and oxidation protection when desired. Post-test energy-dispersive X-ray spectrometry results corresponded well with known elemental compositions and oxidation mechanisms.

A compact LWIR borescope sensor for 2D engine component surface temperature measurement

A compact long-wavelength infrared (LWIR) borescope imaging sensor operating in the wavelength range of 8–14 μm is designed and developed for non-contact two-dimensional (2D) surface temperature measurements in gas-turbine engines. The LWIR detection minimizes optical interferences from hot combustion gases and soot (emission within UV–mid IR region). Using simulations, the system is optimized for improving the signal collection efficiency and minimize image aberrations. The LWIR borescope probe is shielded by the custom-build compact water-cooled probe housing (outer diamter (OD) 19 mm, inner diameter (ID) 10 mm), which can sustain flame temperature up to 2400 K at a pressure of 50 bar. It facilitates long-term optical diagnostics inside the actual high-pressure combustion facilities where extreme thermal acoustic perturbation and intense heat fluxes are encountered. The design, construction, and characterization of the LWIR borescope sensor are discussed in detail. We present the 2D surface temperature measurements of a V-gutter bluff-body with simple nitrogen gas cooling in a laboratory testbed. The results indicate the sensor’s ability to accurately measure surface temperature with low background noise and to track transient cooling effect. The water-cooled probe housing was tested to verify the effectiveness of fast heat dissipation in high-temperature combustion environment to avoid unwanted thermal signals from the LWIR probe window. The developed LWIR sensor suite has promising applications in surface temperature measurements of engine components. The findings of this study may aid propulsion system engineers and researchers in designing thermal management systems and optimizing operation.

Explosion Mechanism of Lubricating Oil Droplets in High-Temperature and High-Pressure Combustion Environments

Lubricating oil-induced pre-ignition is a critical issue that requires attention in downsized gasoline engines and marine low-speed two-stroke natural gas engines. As a result, the ignition behavior of lubricating oil at high temperatures and pressures has been extensively studied. In some cases, when studying the ignition of oil droplets using a rapid compression machine, an explosion-like behavior of the oil droplets is observed, producing a soot cloud that can spread throughout the combustion chamber, especially when the ignition delay time of the ambient gas is short. To gain detailed insights into the mechanism of oil droplet explosion, the explosion process under initial pressures from 13 to 31 bar and temperatures from 700 to 1600 K was visualized using high-speed photography and microphotography on a rapid compression machine. The effects of temperature and shock waves were experimentally investigated, and droplet deformation after shock wave impact was calculated using a simple model. The results demonstrated that high temperature does not have a significant effect on droplet explosion under the conditions studied in this paper. The shock wave impact is the primary cause of the droplet’s explosion.

An environmentally friendly method to fabricate superhydrophobic cellulose fiber for oil absorption

The complex preparation process, high cost, and use of non-renewable materials limit the marketization of most synthetic superhydrophobic oil-absorbing sponges. This work provides a preparation method of superhydrophobic and nontoxic oil absorbents by surface modification of pulp cellulose fibers. A simple hydrothermal method was used to in situ grow micron-scale petal-like γ-AlOOH layers on the surface of cellulose fibers pretreated with ultrasonication and swelled with urea, and then modified with low surface energy to prepare superhydrophobic cellulose fibers. The obtained cellulose fibers have high oil absorption property, fast absorption speed and high recyclability. The fibers exhibit excellent superhydrophobic stability in boiling water, various corrosive solutions and even high-temperature combustion environments, proving that it has the ability to absorb oil in harsh environments. In addition, after being connected to a vacuum filtration device, the as-prepared fibers can continuously separate the n-hexane solution of 200 quilts of its own weight within 15 s. And in the experiment of simulating turbulent water flow, they can still effectively remove oil from oil-water mixture. More importantly, the fiber can separate micro-nano scale oil droplets from the oil (n-hexane)-in-water emulsion with a separation rate up to 93.15% (solution transparency). All these excellent properties show that the prepared fibers have high application value in the fields of marine oil spill absorption and oil-water separation.

A Blast Furnace Burden Surface Deeplearning Detection System Based on Radar Spectrum Restructured by Entropy Weight

Accurately capturing the burden surface information of a blast furnace (BF) is helpful to adjust the burden distribution matrix and improve the gas flow distribution, which is essential in the steel smelting industry. However, it is difficult to detect the burden surface because of the high temperature, high pressure, high dust and flame combustion environment condition inside the BF, in addition to the fluidization characteristics of the burden surface. With the fusion of high-temperature metallurgy, radar detection and image processing, a new BF surface deep-learning detection system based on energy weight was developed in this study to visualize the smelting status of a BF and digitize the information of the burden surface. First, the original burden surface echo spectrogram was captured using a frequency-modulated continuous wave radar with a high-temperature-resistant sensor. Second, the point cloud distribution map was constructed based on the principle of synthetic aperture radar (SAR) imaging. The coordinate transformation and matrix compensation were completed by the entropy weight method and Gamma correction to convert point cloud map to the burden surface grayscale image, constructing datasets for the network. Third, with a 3rd-order hourglass feature extraction network, a BF surface detection network based on the principle of YOLOv3 was designed and trained to predict the position of the burden surface in the grayscale image and track the changing trend of the burden line. The proposed method, which is tested on two steel plant datasets, achieved an average prediction precision of 99.74%. Finally, compared with the traditional burden line extraction algorithm, burden line oscillations and local outliers can be avoided by the proposed method. The continuous detection of the BF surface information was realized.

Ytterbium Silicate Environmental Barrier Coatings Deposited Using the Solution-Based Precursor Plasma Spray

Gas turbine efficiency can be improved by increasing the operating temperature and by reducing the need for cooling air, both of which are enabled by higher temperature materials. Silicon carbide based ceramic matrix composites (CMCs) have shown promise for achieving higher temperatures. However, to survive in the high-temperature high-humidity combustion environment, environmental barrier coatings (EBCs) are essential to limit silicon loss of CMCs and its associated damage. Atmospheric plasma spray (APS) is a popular process for making such coatings. In this work, an alternative deposition process is explored, specifically the solution precursor plasma spray (SPPS), where it will be shown for ytterbium monosilicate EBCs that such process directly deposits thin (< 50 μm), dense and crystalline EBC topcoats without annealing of amorphous phases as is needed in the APS process. In addition, the SPPS process allows for quick compensation of system-specific element loss observed in both SPPS and APS processes. Finally, the secondary phase present in EBCs are much smaller in size for the SPPS process and hence less likely to produce cracking compared to APS EBCs.

Y5Si3C and Y3Si2C2: Theoretically predicted MAX phase like damage tolerant ceramics and promising interphase materials for SiCf/SiC composites

Researching for interphase materials that can protect SiC fibers from oxygen and water vapor attacks has become one of the most important issues for the applications of SiCf/SiC composites in high-temperature combustion environment. However, such kinds of interphase materials are not available yet. Herein, we report theoretically predicted properties of two promising interphase materials Y5Si3C and Y3Si2C2. Although crystallizing in different structures, they share the common features of layered structure, anisotropic chemical bonding, anisotropic electrical and mechanical properties, and low shear deformation resistance. The bulk moduli for Y5Si3C and Y3Si2C2 are 78 and 93 GPa, respectively; while their shear moduli are 52 and 50 GPa, respectively. The maximum to minimum Young’s modulus ratios are 1.44 for Y5Si3C and 3.27 for Y3Si2C2. Based on the low shear deformation resistance and low Pugh's ratios (G/B = 0.666 for Y5Si3C and 0.537 for Y3Si2C2; G: shear modulus; B: bulk modulus), they are predicted as damage tolerant and soft ceramics with predicted Vickers hardness of 9.6 and 6.9 GPa, respectively. The cleavage plane and possible slip systems are (000l) and (0001)[112¯0] and (101¯0)[0001] for Y5Si3C, and those for Y3Si2C2 are {h00} and (010)[101]. Since the oxidation products are water-vapor resistant Y2Si2O7, Y2SiO5 and/or Y2O3 upon oxidation, and the volume expansions are ca 140% and ca 26% for Y5Si3C and Y3Si2C2, they are expected to seal the interfacial cracks in SiCf/SiC composites. The unique combination of easy cleavage, low shear deformation resistance, volume expansions upon oxidation, and the resistance of the oxidation products to water vapor attack warrant them promising as interphase materials of SiCf/SiC composites for water-vapor laden environment applications.

Fabrication and Thermal Structural Characteristics of Ultra-high Temperature Ceramic Struts in Scramjets

ZrB2-SiC based ultra-high temperature ceramic (UHTC) struts were firstly proposed and fabricated with the potential application in the combustor of scramjets for fuel injection and flame-holding for their machinability and excellent oxidation/ablation resistance in the extreme harsh environment. The struts were machined with electrospark wire-electrode cutting techniques to form UHTC into the desired shape, and with laser drilling to drill tiny holes providing the channels for fuel injection. The integrated thermal-structural characteristic of the struts was evaluated in high-temperature combustion environment by the propane-oxygen free jet facility, subject to the heat flux of 1.5 MW/m2 lasting for 300 seconds, and the struts maintained integrity during and after the first experiment. The experiments were repeated for verifying the reusability of the struts. Fracture occurred during the second repeated experiment with the crack propagating through the hole. Finite element analysis (FEA) was carried out to study the thermal stress distribution in the UHTC strut. The simulation results show a high thermal stress concentration occurs at the hole which is the crack initiation position. The phenomenon is in good agreement with the experimental results. The study shows that the thermal stress concentration is a practical key issue in the applications of the reusable UHTC strut for fuel injection structure in scramjets.

Transmitting and Friction Behavior of a Carbon/silicon Carbide Composite Hinge in High Temperature Combustion Environment

An all-ceramic hinge based on the two dimensional carbon fiber reinforced silicon carbide matrix (C/SiC) composites was prepared by chemical vapor infiltration.The high temperature combus- tion wind tunnel realized simulation of the transmitting and friction behavior of the hinge at 1800℃in an oxidizing atmosphere.Based on the coupling stress equivalent simulation system,a characterization method with the change of torque was proposed to evaluate the friction behavior.The friction and wear behavior as well as wear mechanism were studied both at high temperatures in combustion environment and at room temperature in air.The results indicate that the friction torque is stable and insensitive to the sliding time at high temperatures,which demonstrates stable and reliable friction property and thermal load-carrying ability of the hinge.The tribochemical reaction products on contact surface may moderate the stress distribution by providing a reaction layer for wear protection and lubrication.

Transmitting and Friction Behavior of a Carbon/silicon Carbide Composite Hinge in High Temperature Combustion Environment

An all-ceramic hinge based on the two dimensional carbon fiber reinforced silicon carbide matrix (C/SiC) composites was prepared by chemical vapor infiltration.The high temperature combus- tion wind tunnel realized simulation of the transmitting and friction behavior of the hinge at 1800℃in an oxidizing atmosphere.Based on the coupling stress equivalent simulation system,a characterization method with the change of torque was proposed to evaluate the friction behavior.The friction and wear behavior as well as wear mechanism were studied both at high temperatures in combustion environment and at room temperature in air.The results indicate that the friction torque is stable and insensitive to the sliding time at high temperatures,which demonstrates stable and reliable friction property and thermal load-carrying ability of the hinge.The tribochemical reaction products on contact surface may moderate the stress distribution by providing a reaction layer for wear protection and lubrication.

Stress-Oxidation Behavior of a Carbon/Silicon Carbide Composite in a High-Temperature Combustion Environment

A carbon/silicon carbide composite with a silicon carbide coating was prepared by chemical vapor infiltration. Stressed oxidation testing was performed on the composites in a self-built high-temperature combustion environment. The gas in this environment contained oxygen, steam, carbon dioxide, and some nitrogen. Test conditions were controlled at temperatures of 1300°, 1500°, and 1800°C, and the stress was sustained at 40, 80, 120, 160, and 200 MPa. The effect of combustion environment and applied load on stress-oxidation behavior was discussed by analyzing the residual strength and weight loss. The morphology of the fracture surface of the tested specimens was observed by scanning electron microscopy. The high-temperature combustion environment and the high sustained stress above 80 MPa enhanced the material failure and led to strength reduction by determining crack openings and thus oxidation of fibers. However, sustained stress below 80 MPa resulted in no strength degradation after exposure for 10 min at 1500°C.

Stressed oxidation behaviors of SiC matrix composites in combustion environments

Performance of three kinds of continuous fiber reinforced SiC matrix composites prepared by chemical vapor infiltration (CVI) method, i.e. 2D C/SiC, 3D C/SiC and 3D SiC/SiC composites, have been investigated in the high temperature combustion environment gas with various creep stresses. The relationship between the life time of composite and the normalized peak strength, defined by the ratio of the test stress to the material strength, was studied. The life time of composites decreased with increasing the normalized peak strength following an exponential relationship. The oxidation resistance of the SiC/SiC composite was the best and that of the 2D C/SiC composite was the worst in the high temperature combustion environment with an applied stress. The experimental results suggested that there was a critical normalized peak strength which controls the oxidation mechanism of C/SiC. Below the critical normalized peak strength, the degradation of C/SiC in the combustion environment was controlled by the diffusion of oxygen and water vapor through the cracks in the composite. Above the critical normalized peak strength, the degradation was controlled by the oxidation of C fibers.

High temperature corrosion of coatings and boiler steels in reducing chlorine-containing atmosphere

Unacceptably high corrosion rates are often experienced, when chlorine-containing fuels are combusted. Reducing conditions that may occur in various boilers accelerate corrosion even further. Protective oxide scales are not formed on low-alloy steels if partial pressure of oxygen is too low. Materials rich in oxide formers, such as chromium and aluminum, are needed to resist corrosion in reducing combustion atmospheres, but processibility of such bulk alloys is very limited. Various coating technologies are considered as potential solution for corrosion problems in high temperature combustion environments with low partial pressure of oxygen. High temperature corrosion tests were performed on one ferritic boiler steel, one austenitic boiler steel, five high velocity oxy-fuel (HVOF) coatings, one laser-melted HVOF coating, and one diffusion chromized steel. Synthetic atmosphere simulating reducing conditions in combustion of chlorine-containing fuels was created for the tests. The test atmosphere contained 500 ppm HCl, 600 ppm H 2S, 20% H 2O, 5% CO, and Ar as a balance. The test temperature was 550 °C and the test duration was 1000 h. Corrosion resistance of steels and homogeneous coatings was mainly determined by chromium content. Homogeneous and dense coatings with high chromium content performed well and were able to protect the substrate. Some of the HVOF coatings were attacked by corrosive species through interconnected network of voids and oxides at splat boundaries.

Accelerated oxidation of SiC CMC's by water vapor and protection via environmental barrier coating approach

Abstract Silicon carbide fiber reinforced silicon carbide matrix composites (SiC/SiC CMC's) are attractive materials for use in gas turbine hot sections due to the potential for high temperature mechanical properties and overall lower density than metals. Potential SiC/SiC CMC gas turbine components include combustion liners, and turbine shrouds, vanes, and blades. Engine design with SiC/SiC CMC's will allow optimization for performance, efficiency, and/or emissions. However, SiC/SiC CMC's are silica formers under oxidizing conditions and have been shown experimentally to undergo accelerated oxidation due to exposure to steam in high temperature combustion environments such as found in the gas turbine hot section. Oxidation by steam in a flowing gas stream has been shown to exhibit paralinear behavior and result in unacceptable recession of the surface. Thus, prior to the successful introduction of SiC/SiC CMC's for long life use in gas turbines, the problem of accelerated oxidation needs to be addressed and resolved. To this end, one approach has been the development of the environmental barrier coating (EBC) to prevent accelerated oxidation by limiting oxidant access to the surface of the silica former. This paper will review the accelerated oxidation of silica formers such as silicon carbide, the experimental testing confirming the problem, and EBC approaches resolving the problem.

Corrosion of thermal barrier coatings by vanadium and sulfur compounds

Hot corrosion studies of two plasma-sprayed coatings, yttria-stabilized zirconia and calcium silicate, were undertaken in order to compare the performance of these materials for use as thermal barrier coatings in high-temperature combustion environments. The coatings were tested in contact with vanadium pentoxide at 1,000°C and, also, under conditions in which they were exposed to sulfur-containing compounds at 900°C or 1,000°C. The samples were subsequently characterized by scanning electron microscopy and X-ray diffraction analysis to identify the effects of these tests on the microstructure and composition. The results indicate that reactions with V2O5 lead to a disruptive phase transformation in zirconia that rapidly degrades the coating. For calcium silicate, the reactions with V2O5 appear to be more limited and less disruptive so that the coating is much more slowly degraded by the vanadium compounds. Exposure to SOx and sulfate salts at high temperature caused rapid degradation of the calcium silicate coatings through a reaction involving the formation of CaSO4. Under similar conditions, the yttria-stabilized zirconia coatings experienced much less attack.

Diode-Laser Sensors for Real-Time Control of Pulsed Combustion Systems

A diode-laser based, closed-loop control system has been developed to nonintrusively optimize a pulsed, 50-kW dump combustor. The adaptive control system used temperature and water mole fraction measurements obtained at 10-kHz rates from the peak absorbance values of H 2 O features near 1.4 μm. In addition, measurements of CO, C 2 H 2 , and C 2 H 4 concentrations in the exhaust, determined from diode-laser absorption spectra recorded using a fast-sampling probe and a multipass absorption cell (nominal 33-m-long path), were used to evaluate the effectiveness of the control strategies. A correlation was established between the magnitude of the observed temperature oscillations and the measured CO concentration in the exhaust. Adaptive control strategies were then applied to maximize the coherence of the burning vortices in the combustion region and thus optimize the combustor performance. The closed-loop control system was able to adaptively tune the phase and amplitude of the applied forcing within 100 ms and the forcing frequency within 10 s. These results demonstrate the applicability of multiplexed diode-laser absorption sensors for rapid, continuous measurements and control of multiple flowfield parameters, including trace species concentrations, in high-temperature combustion environments.

Advanced diode laser absorption sensor for in situ combustion measurements of CO 2, H 2O, and gas temperature

A diode-laser sensor system based on absorption spectroscopy techniques has been developed to measure CO2, H2O, and temperature nonintrusively in high-temperature combustion environments. An external-cavity diode laser operating near 2.0 μm was used to scan over selected CO2 [(12°1)–(00°0) band] and H2O transitions [(011)–(000), (021)–(010) bands] near 1.996 and 1.992 μm for measurements of CO2 and H2O concentration and gas temperature. Gas temperature was determined from the ratio of integrated line intensities. Species concentration was determined from the integrated line intensity and the measured temperature. The system was applied to measure temperature and species concentrations in the combustion region of a premixed C2H4-air flat-flame burner operating at fuel-lean conditions. The laser-based temperature measurements were in agreement with values determined using a (type S) thermocouple to within 3%. In addition, the measured CO2 and H2O concentrations agreed to within 6% and 3%, respectively, with calculated equilibrium values at measured temperatures. The minimum CO2 detectivity was 200 ppm (for =0.51, 1470 K, a l-m path length, 200-Hz detection bandwidth). These results represent the first in situ combustion measurements of CO2 concentration using room-temperature near-IR diode lasers. Furthermore, the results demonstrate the utility of diode-laser absorption sensors, operating near 2.0 μm, as attractive diagnostic tools for in situ combustion measurements of temperature and the concentrations of CO2 and H2O.

Real-time adaptive combustion control using diode-laser absorption sensors

A wavelength-multiplexed, diode-laser absorption sensor system, comprised of two distributed feedback (DFB) In GaAsP diode lasers and fiber-optic components, has been developed to nonintrusively measure gas temperature and H2O concentration over a single path in the combustion region of a 5-kW, atmospheric-pressure, non-premixed, annular dump combustor. The wavelengths of the DFB lasers were independently current-tuned at 10-kHz rates across H2O transitions near 1343 nm (v1+v3 band) and 1392 nm (2v1, v1+v3 bands). Temperature was determined from the ratio of measured peak absorbances. Water-vapor mole fraction was determined from the measured absorbance and gas temperature values. The diode-laser sensors were applied for closed-loop control of the combustor, which utilized the concepts of acoustic forcing to improve the fuel-air mixing and thus the combustion efficiency. Adaptive control strategies were investigated that used the measured mean H2O concentration and the instantaneous temperature as sensed parameters. The relative phase between the acoustic forcing of the central air and the fuel (C2H4) flow, as well as the fuel-forcing amplitude, was adjusted to maximize the coherence and the extent of reaction in the combustion region and thus optimize the combustor performance. The closedloop control system was able to adaptively optimize the combustor performance within 100 ms, which corresponded to seven characteristic time constants of the actuator. The results obtained demonstrate the applicability of multiplexed diode-laser absorption sensors for rapid, continuous measurements and control of multiple flowfield parameters in high-temperature combustion environments.