Combustion Dynamics Research Articles

Inlet buzz phenomenon driven by flow choking in high-enthalpy Mach 4.5 flows using circular scramjet models

Inlet buzz driven by flow choking in circular dual-mode scramjets is experimentally investigated in high-enthalpy Mach 4.5 flows. Circular scramjet models with two different diverging combustors are used to investigate the correlation between the buzz frequency and flow choking driven by thermal and non-thermal loading. Flow and combustion dynamics are characterized by wall pressure histories, high-speed flame chemiluminescence, and flow luminosity to quantify and visualize the unsteady shockwave and flame propagations driven by fluid-combustion interactions. The circular geometry shows distinct physics from fluid/combustion dynamics due to the absence of corner boundary-layer effects that can distort flame propagation in rectangular geometries. It is observed that the 2° combustor can be thermally choked inside the diverging section while the 5° combustor can only be choked by additional non-thermal loading from its excessive area expansion. Correspondingly, different cyclic oscillations develop from the interactions between fluid and combustion during an unstart event due to the different characteristics of the choking. The 2° combustor is temporarily unchoked during a buzz cycle and the scramjet has a lower fundamental buzz frequency from the extended time to establish flow choking in the diverging section. In contrast, the 5° combustor is always choked during the buzz and the scramjet model has higher buzz frequencies that can be predicted by resonance modes of an acoustic duct with an open- and a closed-end.

Modeling with WFDS Combustion Dynamics of Ornamental Vegetation Structures at WUI: Focus on the Burning of a Hedge at Laboratory Scale

ABSTRACT This study is part of an ongoing project on modeling vegetation fires and their impacts on dwellings at wildland-urban interfaces. As a first step, fire tests were conducted at laboratory scale on both wooden wool litter and rockrose non-natural hedge. To this end, a 0.5 m x 0.5 m x 1 m hedge reconstituted from rockrose branches was burned under a Large Scale Heat Release (LSHR) apparatus. A litter of wooden wool was used before and under the hedge to mimic an ignition from an underneath burning herbaceous layer. These fire tests provided experimental data for mass loss and heat release rate, which were used as a basis for comparison with the simulations performed by using the Wildland-urban interface Fire Dynamics Simulator (WFDS). To numerically model the rockrose hedge, the distribution of the different particle classes (leaves, twigs of various diameters) was determined as well as their physicochemical properties. The comparison of the experimental data and predicted quantities showed a good agreement, whether for wooden wool burnings alone or mixed wooden wool and rockrose hedge.

Characterization of flow field and combustion dynamics in a novel pressurized side-wall quenching burner using high-speed PIV/OH-PLIF measurements

A novel, pressurized side-wall quenching (SWQ) burner test rig enabling investigations on flame-wall interactions (FWI) of a fully premixed V-shaped flame at conditions mimicking some characteristics of practical devices, e.g. increased pressures and Reynolds numbers, is introduced within this study. Two operating cases featuring turbulent flow conditions at atmospheric and 3 bar operating pressure are defined and characterized based on velocity fields and flame front positions provided by simultaneous high-speed particle image velocimetry (PIV) and planar laser-induced fluorescence of the OH radical (OH-PLIF). For non-reacting conditions, profiles of mean velocities and mean velocity fluctuations are measured in addition to near-wall flow fields at locations where FWI occurs for chemically reacting conditions. For reacting conditions, near-wall flows are characterized in the vicinity of the quenching zone. Based on measured fluctuations of the streamwise velocity component, the turbulent flame is classified at the transition from the wrinkled to the corrugated flamelet regime. Using the derived flame front positions, the instantaneous flame-wall interaction is classified into SWQ-like, head-on quenching (HOQ)-like and multi-zone quenching events and the transition between these scenarios is identified by means of OH-PLIF image sequences. A statistical analysis of the flame front topology is performed based on the flame brush and deduced frequency distributions. Statistically significant differences between both operating cases are confirmed by chi-squared homogeneity tests.

Data-Driven Model Learning and Control of RCCI Engines based on Heat Release Rate

Reactivity controlled compression ignition (RCCI) technology not only offers high thermal efficiency but also produces low nitrogen oxides (NOx) and soot emissions. However, it is imperative to control the combustion in RCCI engines to prevent high pressure rise rates and combustion instability. In this study, a model-based control framework is developed to optimize the RCCI operating mode. To this end, the effects of variations in the premixed ratio, start of injection timing and fuel equivalence ratio on the combustion dynamics are analyzed by examining the heat release rates. Three distinct heat release rate patterns are identified together with two transition zones. Heat release rate traces are grouped together as a function of fractions of early and late heat release rates. Based on a classification algorithm, the fractions of early and late heat release rate are identified as scheduling variables for the data-driven modeling of an RCCI engine. Linear regression is used to model the fractions of early and late heat release. These models are then used to train linear parameter varying (LPV) models using least-squares support vector machine (LS-SVM). Using the learned LPV model, a model predictive controller (MPC) scheme is then developed for a 2-liter 4-cylinder RCCI engine to control combustion phasing (CA50) and indicated mean effective pressure (IMEP) while limiting the maximum pressure rise rate (MPRR) to avoid engine knocking. The simulation results show that the designed controller is capable of limiting MPRR below 6 bar/CAD while tracking CA50 and IMEP with average errors of 1.2 CAD and 6.2 kPa, respectively.

Experimental Investigations on NOx Emission and Combustion Dynamics in an Axial Fuel Staging Combustor

Experimental Investigations on NOx Emission and Combustion Dynamics in an Axial Fuel Staging Combustor

Combustion Dynamics of Crude and Upgraded Thermal Deoxygenation Oils in a Compression Ignition Engine

Combustion Dynamics of Crude and Upgraded Thermal Deoxygenation Oils in a Compression Ignition Engine

Experimental investigation on effects of injection distribution on combustion instability in a model rocket combustor

Self-excited combustion instabilities of transverse modes were experimentally investigated in a rectangular multi-injector model combustor, operating with the bipropellants O2/CH4. The propellants were injected through a linear array of five oxidizer-centered shear coaxial injectors into the combustor. High-amplitude limit cycles obtained in hot-fire tests were analyzed in detail. Different combustion instability modes, including first and second width modes, were observed in cases with three different injection distribution schemes. Hence, the injection distribution strongly determined the combustion dynamics. One insight can be gained that the stable combustion could be achieved by properly designing the propellants' injection distributions.

Three-dimensional radiation thermometry for dynamic combustion field based on multi-CCD synchronous coupling (invited)

Three-dimensional radiation thermometry for dynamic combustion field based on multi-CCD synchronous coupling (invited)

TDLAS Tomography System for Online Imaging and Dynamic Process Playback of Temperature and Gas Mole Fraction

Tunable diode laser absorption spectroscopy (TDLAS) tomography enables online diagnosis of dynamic flames by imaging its temperature variations and exhaust gases, owing to its rapid response. Each tomographic image requires huge laser absorption spectrum data along multiple laser paths. For a high temporal resolution, the data sampling rate needs to be as high as possible which challenges the hardware implementation of parallel data acquisition of multiple channels. An online TDLAS tomography system was designed on field programmable gate array (FPGA) and PC for temperature and gas mole fraction distributions measurement. An on-chip parameter extraction module based on digital lock-in amplifier (DLIA) extracts absorption spectrum parameters at 1000 frames per second (FPS), and the framerate of real-time reconstruction on host PC can reach up to 302 FPS. A high-speed buffer on FPGA termed as transient spectrum recorder continuously records the latest five seconds of raw data. Both the playback of absorption spectrum parameters and raw data provide a way for detailed and in-depth understanding of the dynamic combustion process. Experiments were carried out by using an alcohol burner to evaluate the online monitoring and offline playback abilities of the system.

Dynamic Combustion Process of Agglomerated-Reduced Propellant with Organic Fluoride Additive

Dynamic Combustion Process of Agglomerated-Reduced Propellant with Organic Fluoride Additive

Parametric analysis of core-noise from a realistic gas-turbine combustor for cruise and take-off conditions

Combustion noise from a realistic gas-turbine combustor geometry is investigated parametrically using a hybrid simulation framework that combines large-eddy simulation and a linearized Euler formulation. The effect of operating conditions on the relative noise-source contributions arising from direct and indirect noise is examined by considering take-off and cruise conditions. To quantify the predictions of the combustion flow field, comparisons with available experimental data for velocity and spray droplet diameter are performed. Analysis of combustion dynamics shows that tonal combustion noise from a thermoacoustic instability is present at both operating conditions, which is consistent with experimental data. The unsteady combustion and dilution are the main sources for the fluctuations of pressure, temperature and mixture composition at the combustor exit. While the thermo-chemical properties do not significantly change between the two operating conditions, the simulation results show that the level of unsteadiness in the flow field is significantly higher for the take-off condition. This leads to an overall increase in the noise emissions by up to 20 dB at take-off compared to the cruise condition. Indirect noise arising from compositional inhomogeneities is found to exceed the entropy noise for both operating conditions. Phase cancellation between composition noise and temperature-induced noise is shown to result in an overall reduction of the indirect combustion-noise emissions.

The Analysis of the Possibility of Feeding a Liquid Catalyst to a Coal Dust Channel

This article presents the concept of a method of improving the dynamics of combustion in boilers operating in power plants, cogeneration plants, and heating plants by introducing a catalyst that is fed with a carrier in the form of droplets. Thanks to the proposed method, a greater degree of fuel burnout can be obtained, which, in turn, results in lower energy consumption in the case of producing the same amount of power. The parameters of the emitted exhaust gases and ash are also improved. The method described in the article involves the adding of a catalyst to the dust pipe of the boiler, which improves the combustion parameters. The catalyst was implemented using a sprayer/nebulizer. In order to obtain the correct flow parameters, the sprayer was modeled using CFD calculations. The calculations include trajectories, velocities and concentrations with regards to various flow parameters. Particular attention should be paid to the model of the evaporation of moving droplets. The results of these calculations enable the parameters that guarantee that the catalyst reaches the dust channel outlet in the desired form to be assessed. The analysis is an introduction to experimental research that is carried out on a medium and large scale.

High-frequency transverse combustion instabilities of lean-premixed multislit hydrogen-air flames

The present experimental investigation is concerned with combustion dynamics of lean-premixed multislit hydrogen-air flames, with the aim of addressing some of the central technical problems associated with the development of low-emission carbon-free gas turbine combustion technology. To mitigate flashback risk in relatively fast premixed hydrogen flames, we use a multislit injector assembly with a slit width of 1.5 mm, of the same order of magnitude as the characteristic thickness of lean-premixed hydrogen flames. This new perspective on the characteristic nozzle dimension makes it possible to scrutinize the dynamics of multisheet hydrogen flames under extreme conditions. We carry out extensive measurements of self-excited pressure oscillations over a broad range of operating conditions between 30 and 100 kW thermal power. The experimental datasets are analyzed using the acoustic wave decomposition method, low-order thermoacoustic network modeling, and FEM-based three-dimensional Helmholtz simulations. We show that lean-premixed multislit hydrogen flames – unlike the multi-element hydrogen-air flame ensembles considered in our earlier investigations – undergo strong high-frequency pressure oscillations between 3.1 and 3.5 kHz in excess of 25 kPa, originating from the triggering of the first tangential mode of the combustion chamber. Whereas the frequency of the first tangential mode is well defined by the square root dependence of adiabatic flame temperature, the magnitude of pressure fluctuations is observed to be intensified exponentially with increasing flame temperature up to 2200 K. We demonstrate that the transverse mode is characterized by high-amplitude spinning modes in the clockwise direction under relatively short combustor length conditions, discontinuously switching to counter-clockwise spinning modes at relatively long combustor length, and eventually transitioning to a moderate standing wave mode at the same resonant frequency. These observations demonstrate the previously unidentified complex modal dynamics induced by lean-premixed multislit hydrogen-air flames of nozzle dimension comparable to the characteristic length scale of the flame.

Machine Learning-Based Digital Twins Reduce Seasonal Remapping in Aeroderivative Gas Turbines

Abstract Aeroderivative gas turbines have their combustion set points adjusted periodically in a process known as remapping. Even turbines that perform well after remapping may produce unacceptable behavior when external conditions change. This article introduces a digital twin that uses real-time measurements of combustor acoustics and emissions in a machine learning model that tracks recent operating conditions. The digital twin is leveraged by an optimizer that select adjustments that allow the unit to maintain combustor dynamics and emissions in compliance without seasonal remapping. Results from a pilot site demonstrate that the proposed approach can allow a GE LM6000PD unit to operate for ten months without seasonal remapping while adjusting to changes in ambient temperature (4 − 38 °C) and to different fuel compositions.

Numerical investigation of a dual-stage off-gas burner to support high pressure and high temperature solid oxide fuel cell/gas turbine (SOFC/GT) hybrid systems

The objective of this study is to investigate the challenges associated with burning these off-gases and to develop a simple yet efficient combustor to support solid oxide fuel cell/gas turbine hybrid power systems. As a result of operating of the fuel cell under conditions of high fuel utilization, a solid oxide fuel cell produces high temperature exhaust gases on the anode side that are of low heating value, and depleted air with low oxygen content on the cathode side. Anode off-gases characterized by heating values less than 2 MJ/m3 and cathode off-gases with as little as 9 vol % oxygen concentration present a challenge for achieving stable combustion. In this study, the focus is on the development of a dual-stage combustor for low-calorific solid oxide fuel cell off-gases at combustor inlet temperatures up to 1100 K. In addition to taking advantage of high temperature off-gases to support the stable combustion of these low heating value off-gases, the influence of elevated-pressure combustor operation (set by the solid oxide fuel cell operating pressure) is investigated. Initially, a chemical reactor network is developed and used to simulate mixing and flow characteristics of a typical gas turbine combustor operating on the aforementioned fuel/air mixtures at the elevated operating conditions. These results are then used to inform a dual-stage burner design in the form of a computational fluid dynamics model for simulations incorporating both flow and combustion dynamics. The final parameters that are used to evaluate the final design are criteria pollutant emissions such as carbon monoxide and oxides of nitrogen, as well as suitable temperature profiles across the burner.

Study of Displacement Characteristics of Fire Flooding in Different Viscosity Heavy Oil Reservoirs

A field test in the Xinjiang oilfield in China shows that the viscosity of heavy oil has a certain influence on the combustion dynamics and injection-production performance of fire flooding. The experiment in this study uses a one-dimensional combustion tube to study the temperature, gas composition, and air injection pressure and the production performance of the fire flooding of heavy oil with different viscosities. The results show that the oil viscosities of 1180–22500 mPa·s can achieve stable combustion, and the O2 content of the gas produced during the stable combustion stage is <0.5%. The higher the viscosity of the heavy oil, the higher the temperature in the burned zone and the smaller the range of the temperature increase in the unburned zone. The air injection pressure will increase rapidly until a stable seepage channel is formed, and then, it will drop to a level close to the formation pressure. High-viscosity heavy oil requires a higher air injection pressure and will remain in the high-pressure stage for a longer period of time. Low-viscosity heavy oil has a low water cut in the early stage of fire flooding, a large oil production rate, and a low and stable air–oil ratio. The water cut of high-viscosity heavy oil increases rapidly in the early stage of fire flooding and then decreases gradually, so a good air–oil ratio can only be obtained in the middle and late stages of fire flooding. Thus, fire flooding may be more suitable for application in common heavy oil and some extra heavy oil reservoirs with lower viscosities.

Business Roundup

BUSINESS Business Roundup ShareShare onFacebookTwitterWechatLinked InRedditEmail C&EN, 2021, 99 (42), p 19November 22, 2021Cite this:C&EN 99, 42, 19AbstractBaker Hughes has invested in Ekona Power, a Canadian start-up developing a low-carbon process for making hydrogen via methane pyrolysis. Ekona uses combustion and high-speed gas dynamics to turn methane into hydrogen and solid carbon. Ascribe Biosciences , a crop protection company started in a Cornell University lab, has raised $2.5 million in seed funding to commercialize a broad-spectrum biopesticide. The funding will expand field trials for wheat, soy, potatoes, and other crops. Givaudan , a Swiss flavor and fragrance company, is acquiring the US-based fragrance creator Custom Essence, which specializes in natural fragrances. Custom Essence has annual sales of about $40 million. Codexis , an enzyme engineering firm, has invested $10 million in Molecular Assemblies, which is developing an enzymatic process for DNA synthesis. Codexis is helping to improve the DNA polymerase enzymes that the process uses. Jubilant Biosys will provide discovery chemistry to the San Diego–based cancer drugView: PDF | Full Text HTML

Comparison of ethylene combustion mechanisms for the simulation of a supersonic combustion experiment

Hydrocarbon fuels have certain advantages for high speed flight over liquid hydrogen, but are difficult to simulate accurately due to the expense and complexity of the available reaction mechanisms. This paper selects 5 reduced-order chemical reaction schemes for hydrocarbon combustion and applies them to the numerical simulation of an ethylene fuelled supersonic combustion experiment. The experiment consists of an axisymmetric diffuser-cavity-combustor model flowpath, tested at the University of Queensland's T4 reflected shock tunnel at an equivalent Mach 8 flight condition and dynamic pressure of 100 kPa. Data obtained from pressure sensors on the walls is compared to a set of 3D Computational Fluid Dynamics simulations of the flowpath, which employ Reynolds-Averaged Navier-Stokes turbulence modelling and quasi-laminar chemical reactions. Simulations without fuel injection are first performed to assess the underlying validity of the numerical modelling, and the subsequent wall pressure predictions match the results of a no-fuel experiment to within the ≈ 18 kPa margins of uncertainty. In the fuel-on cases however, two shockwaves are observed to have discrepancies in their positions of up to 60 mm, and some of the pressure predictions are outside of the ≈ 25 kPa mean uncertainty of the fuel-on measurements. Of the five tested mechanisms, three correctly predicted ignition of the fuel and two did not, the latter providing a poor comparison to experimental data. The three successful mechanisms produced very similar results to each other, which broadly replicate the pressure rise seen in the experiments but not the precise location of the combustion induced shockwaves. The results imply that any of these three schemes could be used, with caution, to roughly predict ignition behaviour and combustion dynamics as long as conditions are similar enough to those considered in this paper. Future work should be directed at relaxing this caveat by additional testing at different conditions and applying the same mechanisms to other experimental geometries.

Reactive molecular dynamics of pyrolysis and combustion of alternative jet fuels: A ReaxFF study

The current work presents the simulation of the pyrolysis and combustion of alternative jet fuels by reactive force field molecular dynamics methods. A comparison has been done between saturated hydrocarbon farnesane and two unsaturated compounds, α-farnesene and β-farnesene, all of them obtained by the fermentation of sugars present in sugarcane juice. The pyrolysis and combustion mechanisms were elucidated for all the three species at a specified temperature. Significant differences have been observed among the compound reactions during the decompositions. Using a first-order approach, the Arrhenius parameters of the global process were obtained with three different temperatures, held constant over time. For the pyrolysis, the obtained activation energies for farnesane, α-farnesene, and β-farnesene were 132.55, 117.28, and 112.88 kJ mol−1, respectively, and for the combustion, the obtained activation energies were 71.63, 37.99, and 37.98 kJ mol−1, respectively. These data are compatible with the results found in the literature for hydrocarbon fuels. A detailed computational study of all three compounds was performed using the B3LYP/6–311 + G(d,p) approach in the gas phase. Analysis of structures, NBO charges, FMOs, MEP plots, and global reactivity parameters unequivocally supports the simulation results obtained using the ReaxFF code, proving noticeably higher potential reactivity of α- and β-farnesenes compared to farnesane, and furthermore higher reactivity of β-farnesene compared to α-farnesene.

Investigation on breakup characteristics of multicomponent single droplets of nanofluid and water-in-oil emulsion using a pulse laser

Multicomponent liquid fuels such as nanofluids and water-in-oil emulsion have been used to control combustion dynamics and pollutant emissions. Similarly, experimental studies on the micro-explosion phenomena of multicomponent liquid fuels have been conducted to enhance the performance of spray combustion. However, the differences in their breakup mechanisms have not been clarified. In this study, breakup characteristics were investigated by introducing irradiation from a pulse laser on a single liquid droplet of n-dodecane and its mixture with nanoparticles or dispersed water droplets. First, the properties of the nanoparticle fluid and the water-in-oil emulsion were compared. After that, various breakup and dynamic behaviors were categorized into three regimes based on the location of breakup due to energy absorption, which is determined by the additive concentration. That is, there was a self-focusing regime (F-regime), an absorption regime (A-regime), and a transient regime between them (T-regime). In addition, detailed breakup characteristics were classified into four-modes depending on the energy level: a no variation mode, an oscillation and puffing mode (OP), a swelling and breakup mode (SB), and an intensive breakup mode (IB). Transitions between the regimes or the modes are described in a map and their criteria were discussed. Two representative characteristics of the secondary droplets (e.g., number and size) were investigated. Different trends appeared according to the concentration and type of additives, and some empirical scale-relationships were suggested. Finally, differences in the breakup mechanisms of the nanofluid and the emulsion were explained.