Combustor Geometry Research Articles

Perspectives on oxy-fuel combustion for supercritical CO2 direct-fired power cycle

This paper explores the potential of oxy-fuel direct-fired supercritical carbon dioxide (sCO2) power cycles, proposing them as a promising strategy towards achieving near-total carbon capture while utilizing existing fossil fuels. It offers insights into the future of CO2- and sCO2-diluted combustion science and combustor design, supported by a review of the current state of the art. The paper is divided into four sections: chemical kinetics and the development of chemical mechanisms, numerical simulations tools, combustion and laser ignition experimental efforts, and the current state of the art and perspectives of combustor design efforts. The paper underscores the need for additional experimental measurements to validate chemical mechanisms, numerical simulations, and combustor design to advance understanding of CO2 and sCO2-diluted combustion science. The authors advocate for increased collaboration within the scientific community and the development of standardized lab-scale burners and combustor geometries to facilitate comparison and validation as well as reduce development costs. The paper emphasizes that significant research and development efforts are crucial to ensuring the safety, reliability, and efficiency of CO2 and sCO2-diluted combustion processes and combustor design. The knowledge and strategies applicable to conventional gas turbines may not directly transfer to sCO2 cycles, necessitating dedicated research efforts to advance this promising technology towards widespread adoption.

Experimental analysis of hemispherical combustion geometry of diesel engine (neem oil) biodiesel with nano additives

Experimental analysis of hemispherical combustion geometry of diesel engine (neem oil) biodiesel with nano additives

Scale effects on rotating detonation rocket engine operation

Rotating detonation rocket engines are propulsive devices employing detonation waves moving circumferentially around an annular channel that consume axially fed propellants. Theoretically, this provides benefits with respect to combustion pressure gain and thermodynamic efficiency when compared to deflagration-based combustors. To facilitate size scaling of these devices, the relationships between geometric parameters, performance, and wave dynamics have been investigated with gaseous methane-oxygen propellant. Empirical relations were derived between combustor geometry, fueling conditions, and engine operation, as well as correlation to thermodynamic parameters calculated with chemical kinetics codes. The radius of curvature effects were explored in annular combustors having outer diameters of 25 mm, 51 mm, and 76 mm with a fixed gap width of 5 mm. The injectors were scaled to have same oxidizer-to-fuel injector port area ratio, impingement distance, and injector-to-gap area ratio. Larger combustors had higher wave counts during operation at a given mass flux and equivalence ratio. Combustor axial pressures were found to be more dependent on propellant mass flux and equivalence ratio than geometry. Mass flux and the inner-to-outer radius ratio, the latter of which was related to other geometric ratios, dictated the operating mode transition thresholds and the number of resulting waves, respectively.

Numerical Analysis of High Frequency Transverse Instabilities in a Can-Type Combustor

Abstract Dry low emissions (DLE) systems are well-known to be susceptible to thermoacoustic instabilities. In particular, transverse, spinning modes of high frequency may appear, and lead to severe damage in a matter of seconds. The thermoacoustic response of an engine is usually specific to the combustor geometry, operating conditions and difficult to reproduce at the lab-scale. In this work, details of high frequency dynamics (HFD) observed during the early development phase of a new DLE system are provided, where a multipeaked spectrum was noticed during testing. Beginning with an analysis of the measured pressure spectra from three different concepts, an analytical model of the clockwise and anticlockwise transverse waves was fitted to the experimental data using a nonlinear curve fitting approach to produce a simple yet useful understanding of the phenomena. A flamelet-based large eddy simulation (LES) of the entire combustion system was used to complement this analysis and confirm the mode shapes using dynamic mode decomposition (DMD). Both approaches independently identified a spinning second-order mode as the dominant one in the high frequency regime. The LES indicates the coupling of a distortion of swirl profile with a precessing vortex core as a possible cause for the onset of instability. With regard to modeling sensitivities, it is shown that subgrid scale combustion modeling has a strong impact on predicted amplitudes. Ultimately, a thickened-flame model with a modified efficiency function provided consistent results.

Air-breathing rotating detonation engine supplied with liquid kerosene: propulsive performance and combustion stability

Experimental results are presented for a rotating detonation engine supplied with liquid kerosene and preheated air without liquid or gaseous additions to the propellant mixture. Various combustion modes for the generic combustor geometry design were observed—from deflagration, through pulsed combustion and high-frequency instabilities, to stable detonation propagation. Attention was paid to detonation stability (if present), its characteristics, and the propulsive performance of the combustor with a focus on specific thrust and pressure gain through thrust and outlet total pressure measurement. These parameters measured for the observed modes were compared. The stability of the detonation combustion proved not to be critical to achieve high performance of the combustion chamber. For example, high performance was achieved for combustion modes with high-frequency instabilities.

Continuous Injector Geometry Variation to Augment Rotating Detonation Combustor Operation and Performance

Sensitivity of rotating detonation combustor operation and performance to oxidizer injector pressure drop was characterized using continuous variation of the injector area during combustor operation. As the oxidizer injector area was both increased and decreased, the sensitivity of the combustion process to varying injector pressure drop was characterized using high-frequency measurements of pressure and chemiluminescence intensity. Detonation wave strength and coherence were characterized using peak-to-peak intensity and power fraction calculated from point-chemiluminescence measurements. Propulsive performance of the combustor was evaluated using thrust and equivalent available pressure, relating them back to reactant supply pressures for assessment of combustor pressure gain. Pressure gain increased during a test as the oxidizer injector area was increased and the corresponding manifold pressure was decreased. At larger injector areas, pressure gain decreased as the operating mode of the combustor transitioned from detonation to deflagration, concomitant with a reduction of gross thrust. Modeling of injector recovery time revealed that the injector operated in both choked and unchoked regimes, which was used to explain detonation wave number transitions in the experiment. A broadened range of detonative operability enabled by active variation of combustor geometry resulted in higher performance with a lower injector pressure drop.

Control of Intrinsic ThermoAcoustic instabilities for methane and hydrogen air flames: DNS and network analysis

Replacing methane by hydrogen in premixed burners is a typical decarbonation scenario. This is usually done by ensuring a constant flame speed, i.e. by using ultra-lean H2 mixtures. This change can modify the thermoacoustic properties of burners. This study focuses on one class of thermoacoustic instabilities, Intrinsic ThermoAcoustic (ITA) modes, for flames stabilized on a diaphragm. An acoustic network approach is built to construct an ITA stability criterion (called ITAS criteria) for a three-duct configuration and a fully analytical expression is obtained in the limit of thin diaphragms. It shows that instability will grow when the Flame Transfer Function (FTF) gain n−π, located at the frequency at which u′-q′ phase ϕ=−π, is greater than a critical gain nc expressed by the ITAS criteria. Two-dimensional DNS, with anechoic terminations, of methane–air and hydrogen–air flames stabilized on a diaphragm are simulated with similar laminar flame speed sL0 and theoretical flame length, with an equivalence ratio of 0.73 and 0.4 respectively. The theory developed allows to analyze the differences with these two strong unstable ITA modes. For ultra-lean hydrogen flame, preferential diffusion near the diaphragm lips and tip opening reduces the flame time delay and shifts the ITA frequency towards higher values compared to methane. The assessment of two new control strategies is possible with the help of the analytical network model by increasing nc. A first strategy involves the use of preheated leaner mixtures: increasing Tu and reducing ϕ while keeping the flame speed sL0 constant. A second strategy consists of modifying the combustor geometry by changing the section ratio S2/S0, between plenum and combustion chamber. Both strategies successfully mitigate the hydrogen–air ITA modes and seems promising strategies for methane cases despite the fact that instabilities are not fully mitigated for this fuel at the conditions of interest in this study.

Numerical study of inlet air conditions on methane flameless combustion characteristics

The current study investigates the influence of inlet air momentum and temperature for flameless characteristics. Three-dimensional computations were implemented by ANSYS FLUENT. A small-scale experimental combustor geometry was used for validation study. Turbulence modelling was performed by RNG k- ε model and the GRI-Eddy Dissipation Concept (GRI-EDC) model was used for the combustion modelling. The influence of inlet air velocity was investigated through using different air jet diameters at constant flow rate. Five different air jet diameters (4,6,8,10 and 12 mm) were studied. It was found that decreasing air jet diameter leads to an increase in the exhaust gas recirculation which reserve in mixture dilution (low O2 content mixture) subsequently, the reaction rate decreases which support the flameless mode creation. Also, this recirculated gas participates in heating up of fresh reactants which make the mixture tends to complete combustion. The influence of combustion air preheating temperature on NO and CO emissions were studied. CO and NO concentration were decreased with reducing the temperature of inlet preheated combustion air.

Evaluation of Nox Emissions Associated to Non-Premixed Combustion of H2/Natural Gas Blends in a 40 MW Heavy-Duty Gas Turbine

Abstract The tightening in the international regulations is leading the energy production sector toward the usage of hydrogen, which is a zero-carbon energy carrier. In the field of gas turbine lifetime extension, a nonpremixed approach including hydrogen blending with conventional fuels is the most promising. However, high-temperature spots might occur, thus increasing thermo-mechanical stresses and NOx emissions. Therefore, a reliable evaluation of the impact of hydrogen blends in combustors characterized by nonpremixed flames is necessary. In the present work, a 40 MW heavy-duty multican combustor belonging to EthosEnergy is investigated by means of steady, reactive simulations by using the ANSYS® FLUENT® solver. The combustor geometry is simplified by removing the casing volume, being the flow split among the holes already available. Such simplification allows for paying major attention to the chemical kinetics thanks to the use of the extended reaction mechanism for natural gas developed by National University of Ireland Galway. Simulations include the assessment of the natural gas base load configuration together with hydrogen blends up to 50% in volume, while maintaining unaltered the turbine inlet temperature (TIT). The obtained data provide some retrofitting guidelines in the field of hydrogen usage in nonpremixed combustion and prove for a modified temperature field in the combustor core and close to the basket. A linear increase in NOx emission is also associated with hydrogen addition, thus suggesting the need for NOx abatement technologies (e.g., water injection).

Combustion organization strategies for a variable geometry RBCC combustor under low total temperature conditions

A rocket-based combined cycle (RBCC) engine experiences a low Mach number phase during flight operations. Through combustor geometry adjustment technology, the engine can combust more efficiently under low-temperature inflow conditions during this phase, thereby improving the engine efficiency. In this paper, we investigate the fuel blending and combustion processes in rocket ramjet and pure ramjet modes under Ma 2.5 low-temperature inflow conditions, analyze the fuel combustion and flow and engine performance changes under different operating modes, and propose a new method for centralized heat release combustion organization under low-temperature inflow conditions. The method is based on the RBCC engine combustor with a geometry throat and relies on both direct-connect experiments and numerical calculations. The results show that (1) when the primary rocket operates, its jet reacts with the inflow in the shear layer and ignites the kerosene fuel injected by the pylons, forming a high-temperature zone downstream of the fuel pylons. When the primary rocket stops working, this flame is stabilized in the low-speed reflux region at the exit of the rocket by injecting a small equivalent ratio of fuel in the isolator to mix with the air. This approach can be a good substitute for the rocket jet to stabilize the flame. (2) When the primary rocket operates, the secondary fuel is mainly concentrated in the area from the fuel pylons to the geometric throat. After the rocket is turned off, the interval of heat release from combustion is also concentrated in the combustor from the fuel pylons to the geometric throat. (3) After the primary rocket stops working, the performance of the combustor improves significantly.

Experimental and numerical comparisons of geometric scaling criteria for lean premixed swirl combustor

This study experimentally and numerically investigates the applicability of the DaI and Re criteria for scaling the geometry of a lean premixed swirl combustor during a reaction and in the absence of it. We first set up an experimental system to test the loss of pressure, the flow field, and NOx emissions in a prototype combustor and two models of it scaled to 3/5 of its size. The results showed that the friction in the flow in the prototype decreased with an increase in its intensity, and the corresponding constant DaI model (M-D) exhibited a similar trend, while the constant Re model (M-R) exhibited an adverse trend to that of the prototype. The results of particle image velocimetry (PIV) of the flow field in the non-reactive state showed that regardless of the criterion used and the state of the reaction, the flow fields of the prototype and the models were similar under flows of different strengths. However, a quantitative comparison of their distributions of velocity showed that the peak velocity of the rotating jet of M-R was significantly lower than that of the prototype. PIV results of the flow field in the reactive state exhibited similar phenomena. Moreover, the NOx emissions of M-D were consistent with those of the prototype, while emissions from M-R were significantly higher. The numerical results also showed that the shape of the flame and the pattern of flow of M-R were significantly different from those of the prototype.

Eulerian based population balance modeling of agglomeration in a bubbling fluidized bed combustor

Renewable energy sources are contributing to the displacement of fossil fuel use for power generation and biomass combustion is a viable alternative for baseload power. Fluidized bed combustion (FBC) technology has become a favorable option for biomass (including industrial wastes) fired boilers for its benefits including fuel flexibility, high combustion efficiency, lower emissions (NO x , SO x ) in comparison to other solid fuel fired boilers. However, the variation of inorganic species and corrosive deposits lead to bed agglomeration resulting in unscheduled outages. In the present work, biomass combustion and bed agglomeration in a pilot scale atmospheric-fluidized bed combustor is simulated and analyzed. The Eulerian–Eulerian simulation is validated with the experimental data available in literature using a two-dimensional simplified geometry of a pilot scale fluidized bed combustor. Two each homogeneous and heterogeneous reactions are considered for devolatilization and char combustion processes during combustion with the species transport model. Population Balance Model (PBM) is implemented to calculate agglomerate hydrodynamics with air fed as fluidizing medium from the bottom of the combustor. The model results compared well and found consistent with available experimental data. Prediction of defluidization time of the bed showed a good agreement with the results of experiments.

Pressure gradient effect on flame–vortex interaction in lean premixed bluff body stabilized flames

This investigation considers the effect of axial pressure gradient on the dynamics of flame–vortex interaction for a lean premixed bluff body stabilized flame. Large eddy simulations (LESs) of four different combustor geometries generated through combustor wall adjustments that resulted in mild to strong pressure gradients are studied. A bluff body stabilized combustor for a propane/air flame is analyzed first. The results are compared with all available experimental data with the purpose of validating the LES methodology used in OpenFOAM and obtaining a base solution for the study of the pressure gradient effect on flame–vortex interaction. The role of the pressure gradient on flame structure, emission characteristics, vortex dynamics, and flame stability is presented. The mild favorable pressure gradient due to the decelerated flow in diffuser configurations influences flame–vortex dynamics by suppressing flame-induced vorticity sources, baroclinic torque and dilatation, and hence resulting in augmented hydrodynamic instabilities. The sustained hydrodynamic instabilities maintain the large flame wrinkles and sinusoidal flame mode in the wake region. The nourished near-lean blowoff dynamics also affect the emission characteristics, and the emission of species increases. However, the accelerated flow in the nozzle configuration amplifies the flame-induced vorticity sources that preserve the flame core, resulting in a more organized, symmetric, and stable flame. Ultimately, the combustion performance and operation envelope in the lean premixed flames can be increased by maintaining the flame stability and suppressing the limiting lean blowoff dynamics and emissions with the help of a strong favorable pressure gradient generated through adjusting the combustor geometry.

Experimental Assessment on the Coupling Effect of Mixing Length and Methane-Ammonia Blends on Flame Stability and Emissions

Lean premixed combustion mode has become attractive for utilization in industrial gas turbines due to its ability to meet strict emissions regulations without compromising engine efficiency. In this combustion mode, the mixing process is the key player that affect the flame structure and stability, as well as the generated emissions. Many studies have investigated the aspects that influence premixed flames, including the effects of turbulence, combustor geometry, and level of partial premixing, while mostly using conventional natural gas fuel represented by methane. Recently, ammonia, a sustainable energy source, has been considered in gas turbines due to its carbon-free fuel producing no CO2. Utilizing 100% ammonia or a blend of methane and ammonia alters the combustion performance of a premixed flame due to the variation associated with the physical and chemical properties of ammonia. Thus, investigating the coupling between blend ratios and mixing length of methane-ammonia on flame stability and emissions is an essential step toward implementing ammonia in industrial gas turbines. In this study, the influence of various methane-ammonia blends, from 0 (pure methane) to XNH3 = 75%, and mixing lengths on the flame performance were studied. The mixing length was altered by delaying the injection (i.e., partially premixing) of the ammonia while using a fixed injection location for the reference methane-air mixture. This was done by using three fuel ports located at three different heights upstream of the combustion chamber. The results showed that the flame stability is negatively influenced by increasing (decreasing) ammonia fraction (mixing length ratio) and is more sensitive to the ammonia fraction than to the mixing length. At a constant equivalence ratio, the CO and NOx performances improved positively by increasing the ammonia volume fractions (especially at XNH3 = 75% compared to XNH3 = 25% and 50%) and the mixing length.

Adaptive operating mode switching process in rotating detonation engines

In this study, the adaptive mode switching process of rotating detonation waves (RDWs) in response to the change of inlet conditions is numerically explored and analytically explained. Once a quasi-steady rotating detonation flow field is obtained, the inlet total pressure of the reservoir is decreased and maintained at different levels for a certain period of time in order to observe the feedback on the RDWs. It shows that instead of being quenched immediately, the RDWs can adapt to the change of inlet conditions by a mode switching adjustment; it decreases the number of detonation waves and begins to run in a new operating mode, a process that is found to occur swiftly and is corroborated by experimental observations; moreover, an analytical analysis indicates that for a stable RDW flow field, the characteristic parameters such as the number and height of detonation waves are related to the combustor geometry and inlet conditions, which determine whether the RDWs are required to switch into a new operating mode as a means to ensure stability. Additionally, the results indicate that the rotating detonation engine (RDE) can work in different operating modes at the same mass flow rate, or conversely, the RDE can remain in the same operating mode even if the mass flow rate varies.

The Role of Hydrodynamic Instabilities on Near-Lean Blowout Flame Shapes in a Swirl-Stabilized Spray Combustor

Abstract This study investigates the role of hydrodynamic instabilities on near-lean blowout (LBO) flame shapes in a swirl-stabilized spray combustor. Hydrodynamic instabilities often manifest themselves in swirling flows as a helical vortex that winds around the vortex breakdown bubble. However, the heat released from combustion tends to suppress coherent vortex structures, which can limit the helical vortex to certain combustor geometries and operating conditions. Flame shape changes often accompany changes in hydrodynamic stability because they reposition the heat release and consequently modify the degree of coherent vortex suppression. In this study, laser diagnostics measurements were used to characterize the flow fields and spray patterns corresponding to different flame shapes that were observed in the Air Force Research Laboratory (AFRL) referee combustor. In particular, the flame fluctuated between its original shape, FS1, and a new flame shape, FS2, when the combustor operated on the threshold of LBO. Proper orthogonal decomposition (POD) was used to analyze the measurements. POD showed that the appearance of FS2 coincided with coherent vortex structures that resembled those in the hydrodynamically unstable nonreacting flow field. Furthermore, fuel Mie scattering measurements and phase-averages of the velocity field provided evidence that the FS2 spray was periodically disturbed by a helical vortex. Near the swirler exit, this helical vortex structure involved both outer and inner shear layer vortices that appeared to be synchronized with each other. However, the inner shear layer vortices decayed as the flow progressed downstream and only the outer shear layer vortices remained throughout the measurements' field of view. In contrast, there was no indication of a helical vortex structure in either the flow field or fuel spray measurements corresponding to FS1.

A Preliminary Research on Combustion Characteristics of a Novel-Type Scramjet Combustor

In this work, a new configuration of strut-based scramjet is proposed, and a series of simulations are conducted to investigate its possibility of practical application. The simulation results are verified via the classical DLR ramjet and an experiment conducted on the connected pipe facility. The inlet area ( A in ) and air intake height ( H ) of the combustor are varied independently to investigate their performance. The results indicate that the flow field and shock wave structure of such engine reveal similar characteristics as the classical DLR engine, and the variation in engine geometry can significantly affect its combustion characteristics. Moreover, the combustion efficiency could be enhanced by 2% as the A in varied from 900π mm2 to 1100π mm2; increasing the air intake path ( H ) to 12 mm can increase the combustion efficiency by 25%. In general, the present work proposes a new geometry of the scramjet combustor; this combustor has possibility of practical application, but a further and detailed investigation is still needed.

Effects of scaling laws on flow and combustion characteristics of lean premixed swirl burners

Modern heavy gas turbine combustors are always huge, so it is difficult and costly to do experiment. Thus, geometry scaling method has come into sight. In this paper, based on a single lean premixed swirl burner, validated computational fluid dynamic (CFD) model was used to study the effects of different scaling laws on various scalling models from 1/2 to 1/10. Experimental study on prototype combustor and the 3/5 scale model under full operating condition is also carried out to verify the NOx emission under different laws. Results showed that DaI scaling law was able to maintain good similarity under combustion state when scaling factor = 1/2–1/5, while Re scaling law would bring significant changes on flow and flame characteristics. The emission of NOx is also similar to prototype by using Da<sc>i</sc> law. But Re law could keep flow characteristics under non-combustion state. It is suggested that Dai law is suitable for lean premixed swirl combustor geometry scaling.

Bifurcation in Rotating Detonation Engine Operation with Continuously Variable Fuel Injection Location

Sensitivity of rotating detonation engine operation and performance to the location of fuel injection was characterized using continuous variation of the injector geometry during an experiment. The fuel injector location was translated in relation to the oxidizer throat in a jet-in-crossflow configuration to study the influence of injection dynamics on combustor operation. The injector was translated toward and away from the stationary oxidizer throat for a range of operating conditions. High-speed measurements of pressure and flame chemiluminescence intensity, along with thrust, were used to characterize the detonation wave propagation in the experiment and evaluate combustor performance. Higher wave speeds were observed at injector locations closest to the oxidizer throat, with decreased wave speed and eventual transition to deflagrative operation occurring at locations farther downstream. Variation in fuel injection location induced bifurcations in the number of waves, resulting in corresponding changes in wave speed and gross thrust. Hysteresis was observed in these quantities as the direction of injector translation was reversed. Active translation promoted detonative operation of the experiment at conditions and configurations that hitherto operated only in a deflagrative mode with fixed combustor geometry.

A novel technique to optimize combustor geometry for micro thermophotovoltaic system by combining numerical simulation and machine learning

Optimizing the combustor geometry of micro thermophotovoltaic system can improve combustion conditions of the fuel and performance of the system. To accelerate the optimization process, a novel technique is proposed in this work by combining numerical simulation and machine learning. The influences of combustor geometric parameters on flame shape, wall temperature and combustor performance are investigated via numerical simulations which have been validated by experiment. Results show the length and diameter of inlet passage are important factors in affecting the flame shape. Also, all geometric parameters have great impacts on the performance of combustor. The data from simulations are then used to train the artificial neural network and the output of the network is compared with the simulations to determine whether the optimization is successful. Finally, the radiation power of combustor is optimized to 34.51 W from its initial value of 24.8 W using only 197 data samples.