Investigation of Flame Structure and Soot Formation in a Single Sector Model Combustor Using Experiments and Numerical Simulations Based on the LES/CMC Approach

2017 ◽  
Author(s):  
Andrea Giusti ◽  
Epaminondas Mastorakos ◽  
Christoph Hassa ◽  
Johannes Heinze ◽  
Eggert Magens ◽  
...  
Keyword(s):  
Numerical Simulations ◽  
Volume Fraction ◽  
Soot Formation ◽  
Flame Structure ◽  
Soot Volume Fraction ◽  
Moment Closure ◽  
Quantitative Prediction ◽  
Lean Burn ◽  
Soot Precursors ◽  
Wide Range

In this work a single sector lean burn model combustor operating in pilot only mode has been investigated using both experiments and computations with the main objective of analyzing the flame structure and soot formation at conditions relevant to aero-engine applications. Numerical simulations were performed using the Large Eddy Simulation (LES) approach and the Conditional Moment Closure (CMC) combustion model with detailed chemistry and a two-equation model for soot. The CMC model is based on the time-resolved solution of the local flame structure and allows to directly take into account the phenomena associated to molecular mixing and turbulent transport which are of great importance for the prediction of emissions. The rig investigated in this work, called Big Optical Single Sector (BOSS) rig, allows to test real scale lean burn injectors. Experiments, performed at elevated pressure and temperature, corresponding to engine conditions at part load, include OH-PLIF and PDA and have been complemented with new LII measurements for soot location. The wide range of measurements available allows a comprehensive analysis of the primary combustion region and can be exploited to further assess and validate the LES/CMC approach to capture the flame behaviour at engine conditions. It is shown that the LES/CMC approach is able to predict the main characteristics of the flame with a good agreement with the experiment in terms of flame shape, spray characteristics and soot location. Finite-rate chemistry effects appear very important in the region very close to the injector exit leading to the lift-off of the flame. Low levels of soot are observed immediately downstream of the injector exit, where a high amount of vaporized fuel is still present. Further downstream, the fuel vapour disappears quite quickly and an extended region characterised by the presence of pyrolysis products and soot precursors is observed. The strong production of soot precursors together with high soot surface growth rates lead to high values of soot volume fraction in locations consistent with the experiment. Soot oxidation is also very important in the downstream region resulting in a decrease of the soot level at the combustor exit. The results show a very promising capability of the LES/CMC approach to capture the main characteristics of the flame, soot formation and location at engine relevant conditions. More advanced soot models will be considered in future work in order to improve the quantitative prediction of the soot level.

Numerical Investigation of Flame Structure and Soot Formation in a Lab-Scale Rich-Quench-Lean Burner

2018 ◽  
Author(s):  
Andrea Giusti ◽  
Savvas Gkantonas ◽  
Jenna M. Foale ◽  
Epaminondas Mastorakos
Keyword(s):  
Numerical Simulations ◽  
Soot Formation ◽  
Mixture Fraction ◽  
Flame Structure ◽  
Swirling Flows ◽  
Operating Conditions ◽  
Moment Closure ◽  
Combustion Model ◽  
Ethylene Flame ◽  
Soot Precursors

The understanding of the processes involved in soot formation and oxidation is a critical factor for a reliable prediction of emissions in aero-engines, particularly as legislation becomes increasingly stringent. This work studies the flame structure and soot formation in a lab-scale burner, which reproduces the main features of a Rich-Quench-Lean (RQL) combustor, using high-fidelity numerical simulations. The investigated burner, developed at the University of Cambridge, is based on a bluff-body swirl-stabilised ethylene flame, with air provided in the primary region through two concentric swirling flows and quenching enabled by means of four dilution jets at variable distance downstream. Measurements for different air split between the two inlet swirling flows and dilution ports, and different height of the dilution jets, indicate noticeable differences in the soot tendency. Numerical simulations have been performed using Large-Eddy Simulation with the Conditional Moment Closure combustion model and a two-equation model for soot, allowing a detailed resolution of the mixing field and to directly take into account the effect of turbulent transport on the flame structure, which has been shown to have an important effect on the soot formation and evolution. The main objective of this work is to study the flow field and mixing characteristics in the burner’s primary region, in order to improve the understanding of the mechanisms leading to the soot behaviour observed in the experiment at different operating conditions. Results show the key role of mixing in determining the level of soot in the burner, with the soot production mainly related to the extension of the flame zone characterized by a rich mixture, with pyrolysis products and soot precursors. The presence of additional dilution air seems to improve the oxidation and leads to a leaner mixture in the primary combustion region whereas the air added through the outer swirl stream seems to have less impact on the mixture formation in the primary region. Analysis of the solution in mixture fraction space shows the importance of residence time for the soot formation and highlights the existence of a range of values of mixture fraction, between 0.1 and 0.2, where the soot production terms are maximum. High residence times and local air-to-fuel ratio in the range of high soot production should be avoided to decrease the level of soot mass fraction in the burner.

scholarly journals Investigation of Flame Structure and Soot Formation in a Single Sector Model Combustor Using Experiments and Numerical Simulations Based on the Large Eddy Simulation/Conditional Moment Closure Approach

2018 ◽  
Vol 140 (6) ◽  
Author(s):  
Andrea Giusti ◽  
Epaminondas Mastorakos ◽  
Christoph Hassa ◽  
Johannes Heinze ◽  
Eggert Magens ◽  
...  
Keyword(s):  
Large Eddy Simulation ◽  
Soot Formation ◽  
Flame Structure ◽  
Moment Closure ◽  
Conditional Moment ◽  
Lean Burn ◽  
Eddy Simulation ◽  
Conditional Moment Closure ◽  
Soot Precursors ◽  
Large Eddy

In this work, a single sector lean burn model combustor operating in pilot only mode has been investigated using both experiments and computations with the main objective of analyzing the flame structure and soot formation at conditions relevant to aero-engine applications. Numerical simulations were performed using the large eddy simulation (LES) approach and the conditional moment closure (CMC) combustion model with detailed chemistry and a two-equation model for soot. The CMC model is based on the time-resolved solution of the local flame structure and allows to directly take into account the phenomena associated to molecular mixing and turbulent transport, which are of great importance for the prediction of emissions. The rig investigated in this work, called big optical single sector rig, allows to test real scale lean burn injectors. Experiments, performed at elevated pressure and temperature, corresponding to engine conditions at part load, include planar laser-induced fluorescence of OH (OH-PLIF) and phase Doppler anemometry (PDA) and have been complemented with new laser-induced incandescence (LII) measurements for soot location. The wide range of measurements available allows a comprehensive analysis of the primary combustion region and can be exploited to further assess and validate the LES/CMC approach to capture the flame behavior at engine conditions. It is shown that the LES/CMC approach is able to predict the main characteristics of the flame with a good agreement with the experiment in terms of flame shape, spray characteristics and soot location. Finite-rate chemistry effects appear to be very important in the region close to the injection location leading to the lift-off of the flame. Low levels of soot are observed immediately downstream of the injector exit, where a high amount of vaporized fuel is still present. Further downstream, the fuel vapor disappears quite quickly and an extended region characterized by the presence of pyrolysis products and soot precursors is observed. The strong production of soot precursors together with high soot surface growth rates lead to high values of soot volume fraction in locations consistent with the experiment. Soot oxidation is also very important in the downstream region resulting in a decrease of the soot level at the combustor exit. The results show a very promising capability of the LES/CMC approach to capture the main characteristics of the flame, soot formation, and location at engine relevant conditions. More advanced soot models will be considered in future work in order to improve the quantitative prediction of the soot level.

scholarly journals Effect of the Preheated Oxidizer Temperature on Soot Formation and Flame Structure in Turbulent Methane-Air Diffusion Flames at 1 and 3 atm: A CFD Investigation

Energies ◽  
2021 ◽  
Vol 14 (12) ◽  
pp. 3671
Author(s):  
Subrat Garnayak ◽  
Subhankar Mohapatra ◽  
Sukanta K. Dash ◽  
Bok Jik Lee ◽  
V. Mahendra Reddy
Keyword(s):  
Mole Fraction ◽  
Air Temperature ◽  
Reaction Rate ◽  
Volume Fraction ◽  
Diffusion Flames ◽  
Soot Formation ◽  
Flame Structure ◽  
Soot Volume Fraction ◽  
Computational Domain ◽  
Pressure Elevation

This article presents the results of computations on pilot-based turbulent methane/air co-flow diffusion flames under the influence of the preheated oxidizer temperature ranging from 293 to 723 K at two operating pressures of 1 and 3 atm. The focus is on investigating the soot formation and flame structure under the influence of both the preheated air and combustor pressure. The computations were conducted in a 2D axisymmetric computational domain by solving the Favre averaged governing equation using the finite volume-based CFD code Ansys Fluent 19.2. A steady laminar flamelet model in combination with GRI Mech 3.0 was considered for combustion modeling. A semi-empirical acetylene-based soot model proposed by Brookes and Moss was adopted to predict soot. A careful validation was initially carried out with the measurements by Brookes and Moss at 1 and 3 atm with the temperature of both fuel and air at 290 K before carrying out further simulation using preheated air. The results by the present computation demonstrated that the flame peak temperature increased with air temperature for both 1 and 3 atm, while it reduced with pressure elevation. The OH mole fraction, signifying reaction rate, increased with a rise in the oxidizer temperature at the two operating pressures of 1 and 3 atm. However, a reduced value of OH mole fraction was observed at 3 atm when compared with 1 atm. The soot volume fraction increased with air temperature as well as pressure. The reaction rate by soot surface growth, soot mass-nucleation, and soot-oxidation rate increased with an increase in both air temperature and pressure. Finally, the fuel consumption rate showed a decreasing trend with air temperature and an increasing trend with pressure elevation.

OH PLIF and Soot Volume Fraction Imaging in the Reaction Zone of a Liquid-Fueled Model Gas-Turbine Combustor

2004 ◽  
Author(s):  
Terrence R. Meyer ◽  
Sukesh Roy ◽  
Sivaram P. Gogineni ◽  
Vincent M. Belovich ◽  
Edwin Corporan ◽  
...  
Keyword(s):  
Gas Turbine ◽  
Reaction Zone ◽  
Large Scale ◽  
Volume Fraction ◽  
Soot Formation ◽  
Flame Structure ◽  
Soot Volume Fraction ◽  
Gas Turbine Combustor ◽  
Planar Laser ◽  
Model Gas Turbine Combustor

Simultaneous measurements of OH planar laser-induced fluorescence (PLIF) and laser-induced incandescence (LII) are used to characterize the flame structure and soot formation process in the reaction zone of a swirl-stabilized, JP-8-fueled model gas-turbine combustor. Studies are performed at atmospheric pressure with heated inlet air and primary-zone equivalence ratios from 0.55 to 1.3. At low equivalence ratios (φ < 0.9), large-scale structures entrain rich pockets of fuel and air deep into the flame layer; at higher equivalence ratios, these pockets grow in size and prominence, escape the OH-oxidation zone, and serve as sites for soot inception. Data are used to visualize soot development as well as to qualitatively track changes in overall soot volume fraction as a function of fuel-air ratio and fuel composition. The utility of the OH-PLIF and LII measurement system for test rig diagnostics is further demonstrated for the study of soot-mitigating additives.

Modeling Study on Initial Soot Formation at High Pressures

2000 ◽  
Vol 214 (4) ◽  
Author(s):  
D. Hu ◽  
M. Braun-Unkhoff ◽  
P. Frank
Keyword(s):  
High Pressures ◽  
Volume Fraction ◽  
Soot Formation ◽  
Mechanistic Model ◽  
Soot Volume Fraction ◽  
Observation Time ◽  
Chemical Kinetic Model ◽  
Reflected Shock ◽  
Soot Precursors ◽  
Modeling Study

Detailed modeling of soot formation was carried out using different complex models. Main effort was put on modeling particle inception. The experimental data base consisted of soot volume fraction measurements observed in laminar premixed ethene flames for pressures between 5 and 20 bar and behind reflected shock waves for the pressure range of 4.5 to 55 bar with the fuel ethene and benzene, respectively. Due to the observation time scale in the shock wave experiments (a few milliseconds) induction period and initial soot formation zone are temporally well resolved. The present study investigates the formation and growth of the polycyclic aromatic hydrocarbon (PAH) and the adjacent section of particle formation by PAH coagulation. For the models the same relatively large reaction gas phase mechanism up to the first aromatic ring structure was used. PAH formation was described either by applying linear lumping of the well known HACA sequence (model I) or by using a detailed chemical kinetic model for species ranging from benzene to coronene (model II). Particle inception and growth was described by applying a standard mechanistic model which comprises coagulation and surface growth. The modeling study showed that growth of soot precursors and particle inception predicted by the PAH reaction mechanism (model II) gives better agreement with measured soot volume fration profiles than the HACA mechanism alone (model I). From the study it was also deduced that a simplified PAH growth model based on reactive coagulation of a relevant growth species (model III) gives similar results. From the comparison with the high pressure flame experiments it was also shown that the modified soot model is able to predict soot formation and oxidation in the post flame zone.

Analytical Formulation-Based Soot Modelling in Ethylene Laminar Jet Diffusion Flames

2021 ◽  
Author(s):  
Amit Makhija ◽  
Krishna Sesha Giri
Keyword(s):  
Volume Fraction ◽  
Diffusion Flames ◽  
Soot Formation ◽  
Mixture Fraction ◽  
Soot Volume Fraction ◽  
Smoke Point ◽  
Computational Time ◽  
Detailed Model ◽  
Conservation Equations ◽  
Oxidation Rates

Abstract Soot volume fraction predictions through simulations carried out on OpenFOAM® are reported in diffusion flames with ethylene fuel. A single-step global reaction mechanism for gas-phase species with an infinitely fast chemistry assumption is employed. Traditionally soot formation includes inception, nucleation, agglomeration, growth, and oxidation processes, and the individual rates are solved to determine soot levels. However, in the present work, the detailed model is replaced with the soot formation and oxidation rates, defined as analytical functions of mixture fraction and temperature, where the net soot formation rate can be defined as the sum of individual soot formation and oxidation rates. The soot formation/oxidation rates are modelled as surface area-independent processes. The flame is modelled by solving conservation equations for continuity, momentum, total energy, and species mass fractions. Additionally, separate conservation equations are solved to compute the mixture fraction and soot mass fraction consisting of source terms that are identical and account for the mixture fraction consumption/production due to soot. As a consequence, computational time can be reduced drastically. This is a quantitative approach that gives the principal soot formation regions depending on the combination of local mixture fraction and temperature. The implemented model is based on the smoke point height, an empirical method to predict the sooting propensity based on fuel stoichiometry. The model predicts better soot volume fraction in buoyant diffusion flames. It was also observed that the optimal fuel constants to evaluate soot formation rates for different fuels change with fuel stoichiometry. However, soot oxidation strictly occurs in a particular region in the flame; hence, they are independent of fuel. The numerical results are compared with the experimental measurements, showing an excellent agreement for the velocity and temperature. Qualitative agreements are observed for the soot volume fraction predictions. A close agreement was obtained in smoke point prediction for the overventilated flame. An established theory through simulations was also observed, which states that the amount of soot production is proportional to the fuel flow rate. Further validations underscore the predictive capabilities. Model improvements are also reported with better predictions of soot volume fractions through modifications to the model constants based on mixture fraction range.

scholarly journals The influence of aliphatics on soot inception modelling

2021 ◽  
Author(s):  
Nemanja Ceranic
Keyword(s):  
Volume Fraction ◽  
Diffusion Flames ◽  
Soot Formation ◽  
Fluid Dynamic ◽  
Soot Volume Fraction ◽  
Surface Reactivity ◽  
Complex Nature ◽  
Laminar Diffusion Flames ◽  
Laminar Diffusion ◽  
Polycyclic Aromatic

Soot models have been investigated for several decades and many fundamental models exist that prescribe soot formation in agreement with experiments and theories. However, due to the complex nature of soot formation, not all pathways have been fully characterized. This work has numerically studied the influence that aliphatic based inception models have on soot formation for coflow laminar diffusion flames. CoFlame is the in-house parallelized FORTRAN code that was used to conduct this research. It solves the combustion fluid dynamic conservation equations for a variety of coflow laminar diffusion flames. New soot inception models have been developed for specific aliphatics in conjunction with polycyclic aromatic hydrocarbon based inception. The purpose of these models was not to be completely fundamental in nature, but more so a proof-of-concept in that an aliphatic based mechanism could account for soot formation deficiencies that exist with just PAH based inception. The aliphatic based inception models show potential to enhance predicative capability by increasing the prediction of the soot volume fraction along the centerline without degrading the prediction along the pathline of maximum soot. Additionally, the surface reactivity that was used to achieve these results lied closer in the range of numerically derived optimal values as compared to the surface reactivity that was needed to match peak soot concentrations without the aliphatic based inception models.

Prediction of Soot Formation Trends in Turbulent Kerosene-Air Diffusion Jet Flames With Elevated Operating Pressure

2017 ◽  
Author(s):  
Pravin Nakod ◽  
Saurabh Patwardhan ◽  
Ishan Verma ◽  
Stefano Orsino
Keyword(s):  
Environmental Protection Agency ◽  
Volume Fraction ◽  
Soot Formation ◽  
Mixture Fraction ◽  
Soot Volume Fraction ◽  
Operating Pressure ◽  
Jet Flames ◽  
Emission Standard ◽  
Radial Profiles ◽  
Air Diffusion

Emission standard agencies are coming up with more stringent regulations on soot, given its adverse effect on human health. It is expected that Environmental Protection Agency (EPA) will soon place stricter regulations on allowed levels of the size of soot particles from aircraft jet engines. Since, aircraft engines operate at varying operating pressure, temperature and air-fuel ratios, soot fraction changes from condition to condition. Computation Fluid Dynamics (CFD) simulations are playing a key role in understanding the complex mechanism of soot formation and the factors affecting it. In the present work, soot formation prediction from numerical analyses for turbulent kerosene-air diffusion jet flames at five different operating pressures in the range of 1 atm. to 7 atm. is presented. The geometrical and test conditions are obtained from Young’s thesis [1]. Coupled combustion-soot simulations are performed for all the flames using steady diffusion flamelet model for combustion and Mass-Brookes-Hall 2-equation model for soot with a 2D axisymmetric mesh. Combustion-Soot coupling is required to consider the effect of soot-radiation interaction. Simulation results in the form of axial and radial profiles of temperature, mixture fraction and soot volume fraction are compared with the corresponding experimental measured profiles. The results for temperature and mixture fraction compare well with the experimental profiles. Predicted order of magnitude and the profiles of the soot volume fraction also compare well with the experimental results. The correct trend of increasing the peak soot volume fraction with increasing the operating pressure is also captured.

A Numerical Study on the Influence of Hydrogen Addition on Soot Formation in a Laminar Aviation Kerosene (Jet A1) Flame at Elevated Pressure

2021 ◽  
Author(s):  
Mingshan Sun ◽  
Zhiwen Gan
Keyword(s):  
Volume Fraction ◽  
Soot Formation ◽  
Laminar Flame ◽  
Numerical Study ◽  
Soot Volume Fraction ◽  
Elevated Pressure ◽  
Condensation Process ◽  
Hydrogen Addition ◽  
Aviation Kerosene ◽  
Soot Precursor

Abstract The hydrogen addition is a potential way to reduce the soot emission of aviation kerosene. The current study analyzed the effect of hydrogen addition on aviation kerosene (Jet A1) soot formation in a laminar flame at elevated pressure to obtain a fundamental understanding of the reduced soot formation by hydrogen addition. The soot formation of flame was simulated by CoFlame code. The soot formation of kerosene-nitrogen-air, (kerosene + replaced hydrogen addition)-nitrogen-air, (kerosene + direct hydrogen addition)-nitrogen-air and (kerosene + direct nitrogen addition)-nitrogen-air laminar flames were simulated. The calculated pressure includes 1, 2 and 5 atm. The hydrogen addition increases the peak temperature of Jet A1 flame and extends the height of flame. The hydrogen addition suppresses the soot precursor formation of Jet A1 by physical dilution effect and chemical inhibition effect, which weaken the poly-aromatic hydrocarbon (PAH) condensation process and reduce the soot formation. The elevated pressure significantly accelerates the soot precursor formation and increases the soot formation in flame. Meanwhile, the ratio of reduced soot volume fraction to base soot volume fraction by hydrogen addition decreases with the increase of pressure, indicating that the elevated pressure weakens the suppression effect of hydrogen addition on soot formation in Jet A1 flame.

Soot and PAH Formation Characteristics in a Micro Flow Reactor With a Controlled Temperature Profile

2011 ◽  
Author(s):  
Ryu Tanimoto ◽  
Takuya Tezuka ◽  
Susumu Hasegawa ◽  
Hisashi Nakamura ◽  
Kaoru Maruta
Keyword(s):  
Flow Velocity ◽  
Temperature Profile ◽  
Mole Fraction ◽  
Equivalence Ratio ◽  
Flow Reactor ◽  
Volume Fraction ◽  
Soot Formation ◽  
Soot Volume Fraction ◽  
Low Flow ◽  
Micro Flow

To examine soot and PAH formation processes for rich methane/air and acetylene/air mixtures, a micro flow reactor with a controlled temperature profile was employed. In the experiment for a methane/air mixture, four kinds of responses to the variations of flow velocity and equivalence ratio were observed as follows: soot formation without a flame; a flame with soot formation; a flame without soot formation; and neither flame nor soot formation. Soot formations were observed in low flow velocity and high equivalence ratio. Starting point of soot formation shifted to the upstream side, i.e., low-temperature side, of the micro flow reactor with the decrease of flow velocity. One-dimensional steady-state computation was conducted by a flame code. In high flow velocity, low mole fraction of C2H2 and high mole fraction of OH were observed in the whole region of the micro flow reactor. Soot volume fraction did not increase in this case. On the other hand, in low flow velocity, high mole fraction of C2H2 and low mole fraction of OH were observed at the downstream side of the micro flow reactor. Soot volume fraction increased in this case. Since significant soot formation was observed at the low flow velocity and the high equivalence ratio, experiments with gas sampling were conducted for acetylene/air mixture to investigate temperature and equivalence ratio dependence of soot precursor production in such condition. Volume fractions of benzene increased with an increase of temperature. They were larger at higher equivalence ratio at the same temperature. Volume fractions of styrene increased with an increase of temperature. They were larger at higher equivalence ratio when the temperature is less than 1000 K. However the tendency was changed at 1000 K, styrene volume fraction at equivalence ratio of 7.0 was larger than that at equivalence ratio of 8.0.

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