Turbulent Flame Closure Research Articles

Numerical Modeling of Hydrogen Flame Acceleration and DDT Using Generalized Turbulent Flame Closure Approach

ABSTRACT In this work, a numerical methodology based on the paradigm of Turbulent Flame Closure (TFC) has been tested to simulate flame acceleration and Deflagration-to-Detonation Transition (DDT) in hydrogen air mixtures. Flame tracking is based on the transport equation for the progress variable. To minimize the need for tuning/calibration of model parameters, a generalized transport equation for the flame-wrinkling factor is adopted rather than algebraic closure models. The progress variable equation has been augmented with a sub-model based on the autoignition delay time parameter, which is crucial for capturing local explosions. Turbulence modeling is based on RANS. For sharp resolution of shocks, the conservative form of governing equations has been adopted. Relatively under-resolved numerical grid has been employed with the objective of establishing its scalability potential for large-scale computations. The TFC approach is strictly valid for fully turbulent flames. In the present work, its applicability for flame acceleration and DDT in obstructed channels has been investigated. Initial testing has been carried out for a closed and circular shock tube fitted with a single concentric obstacle. Further, a detailed study has been carried out for the GraVent explosion channel. Based on the validation of (1) flame speed variation during flame acceleration, (2) final detonation speed and (3) incident and reflected shock pressures, the capabilities and limitations of the approach have been presented. For the range considered in the present study, reasonably good predictions have been obtained with no tuning/calibration of model parameters. The model is able to capture flame acceleration and predict the conditions under which DDT can take place. However, it is further observed that actual DDT mechanism could not be reproduced.

A Conservative Approach for the Fast Deflagration Analysis in the Containment With GASFLOW-MPI

The nuclear regulation authorities of many countries require that the containment remains its integrity for a local hydrogen risk during any possible accident conditions. Therefore, the combustion consequence should be analyzed to demonstrate that the containment integrity is not being challenged for the hydrogen risk when the flame acceleration risk cannot be safely ruled out. Considering the uncertainties of both the combustion model and severe accident analysis, the criteria and experimentally based combustion (CREBCOM) model is adopted to provide a conservative result for pressure and the thermal load for combustion analyses in this study. Firstly, the CREBCOM is developed in the GASFLOW-MPI code and validated with the RUT experiment. The result shows that the CREBCOM model can provide a reliable overpressure for the choking regime combustion. Then, this model is adopted for the hydrogen safety analysis for the Advanced Pressurized Water Reactor (PWR) 1000. The hydrogen distribution is calculated with the mass and energy release obtained from severe accident analysis, from which, the most unfavorable ignition time and location is selected. The result of the CREBCOM model is compared with that of the turbulent flame closure model, which is a commonly used model for combustion analysis in containment safety. The results show that the CREBCOM model can provide a conservative prediction for the pressure and thermal load of combustion. Therefore, the CREBCOM model with the sonic flame assumption is applicable for the FA risk analysis in a local compartment for nuclear containments where a sonic deflagration cannot be safely excluded and manages to obtain a conservative pressure and thermal load for further evaluation on the containment integrity.

FlameFoam: An open source CFD solver for turbulent premixed combustion

Severe accidents in nuclear power plants may cause significant and long-lasting harm to the environment in the case of containment integrity loss. Numerical simulation of phenomena threatening containment integrity, e.g., hydrogen combustion and explosion, is essential for the effective severe accident management.International research projects and benchmarks are organized in order to promote improvement of simulation codes adequacy and accuracy in the severe accident relevant conditions. Results of the most recent benchmarks show that current state-of-the-art simulations were able to mostly reproduce the observable flame speed at the considered conditions, but there are still inadequacies in estimating velocity and its maximum value.Addressing a need for an accessible open source turbulent premixed combustion simulation tool, a new solver is being developed. An OpenFOAM version 7 based combustion solver called flameFoam has been built by implementing a progress variable approach and turbulent flame closure model for premixed combustion simulation. Turbulent flame speed is defined as a function of laminar flame speed and turbulence parameters, and is evaluated using one of three different correlations. Laminar flame speed can be set as a constant or calculated using a laminar flame speed correlation. The basis for the solver are standard compressible OpenFOAM solvers rhoPimpleFoam, buoyantPimpleFoam and chtMultiRegionFoam.The article presents initial public flameFoam version 0.8 and provides examples of its validation. Initial validation and verification of general solver features are represented by the simulations of shock tube and backward facing step cases. Solver capabilities in relation to the current turbulent premixed combustion modeling state-of-the-art are demonstrated with the simulations of ETSON-MITHYGENE benchmark experiments.It is shown that flameFoam version 0.8 is capable of adequate simulation of general flow features, turbulent compressible flow phenomena and turbulent premixed combustion of lean hydrogen-air mixtures at the considered conditions. Solver code is publicly hosted and being developed further.

Lean blow-out prediction in an industrial gas turbine combustor through a LES-based CFD analysis

The use of lean premixed flames to control combustion temperature and limit nitric oxides (NOx) formation is a common practice in all the applications of land-based gas turbines irrespective of their size and power production capacity. Despite the capability of lean premixed combustion to nominally reduce NOx emissions well below the most strict international regulations, such technology still requires improvements to overcome the impact on whole engine operability due to possible onset of combustion instabilities and flame lean blow-out (LBO). Both phenomena involve highly unsteady processes resulting in challenging numerical modeling. Particularly critical is the prediction of lean blow-out mechanisms where the role of finite rate chemistry and turbulence cannot be easily simplified. The current study aims at proving the capability of a CFD methodology based on Large Eddy Simulation (LES) and on an extended version of the Turbulent Flame Closure (TFC), to describe and properly catching the dynamics of flame extinction at the leanest operating conditions in typical turbulent lean premixed flames adopted in gas turbine combustors. The methodology is first validated on a laboratory test case where detailed experimental results are available. The extended TFC model well describes the LBO process observed in an atmospheric swirl stabilized burner investigated at the University of Cambridge. The proposed approach is then applied to the investigation of a real scale combustor operated with a swirl lean burner, representing the standard device adopted by Baker-Hughes on the heavy-duty 16 MW class engine Nova LT16. An accelerated numerical procedure is used to trigger the LBO, considering both constant firing temperature and constant pilot split conditions. An excellent agreement is observed with respect to the data obtained on a full-annular test rig, confirming the validity of the developed LBO modeling strategy which can be used in design phase to improve the overall engine operability.

Validation of leading point concept in RANS simulations of highly turbulent lean syngas-air flames with well-pronounced diffusional-thermal effects

While significant increase in turbulent burning rate in lean premixed flames of hydrogen or hydrogen-containing fuel blends is well documented in various experiments and can be explained by highlighting local diffusional-thermal effects, capabilities of the vast majority of available models of turbulent combustion for predicting this increase have not yet been documented in numerical simulations. To fill this knowledge gap, a well-validated Turbulent Flame Closure (TFC) model of the influence of turbulence on premixed combustion, which, however, does not address the diffusional-thermal effects, is combined with the leading point concept, which highlights strongly perturbed leading flame kernels whose local structure and burning rate are significantly affected by the diffusional-thermal effects. More specifically, within the framework of the leading point concept, local consumption velocity is computed in extremely strained laminar flames by adopting detailed combustion chemistry and, subsequently, the computed velocity is used as an input parameter of the TFC model. The combined model is tested in RANS simulations of highly turbulent, lean syngas-air flames that were experimentally investigated at Georgia Tech. The tests are performed for four different values of the inlet rms turbulent velocities, different turbulence length scales, normal and elevated (up to 10 atm) pressures, various H2/CO ratios ranging from 30/70 to 90/10, and various equivalence ratios ranging from 0.40 to 0.80. All in all, the performed 33 tests indicate that the studied combination of the leading point concept and the TFC model can predict well-pronounced diffusional-thermal effects in lean highly turbulent syngas-air flames, with these results being obtained using the same value of a single constant of the combined model in all cases. In particular, the model well predicts a significant increase in the bulk turbulent consumption velocity when increasing the H2/CO ratio but retaining the same value of the laminar flame speed.

TimeScale Analysis, Numerical Simulation and Validation of Flame Acceleration, and DDT in Hydrogen–Air Mixtures

ABSTRACT Flame Acceleration (FA) and Deflagration to Detonation Transition (DDT) are known risks in scenarios involving accidental release and combustion of hydrogen in nuclear reactors. In the present work, chemical timescale and Borghi diagram analysis of hydrogen combustion have been presented. Based on these, a map of the flame propagation in the GraVent shock tube channel has been constructed; and it has been established that the propagation corresponds to the flamelet regime. Subsequently, CFD framework applicable to the flamelet regime such as the geometric approach with turbulent flame closure has been utilized to study deflagrations, FA and DDT. Such methods allow for the computation of pressure transients and flame propagation characteristics with a relatively coarse grid, thereby enabling a balance between computational time and accuracy. Moreover, specific sub-models based on Lewis number and tuned to lean combustion at high pressures have been implemented to further improve the accuracy of the numerical results. All numerical work has been carried out using the open-source numerical platform OpenFOAM. Several numerical simulations with uniform and transversely stratified initial distribution have been carried out and validated with experimental data. In the stratified case, an asymmetric flame front has been observed and is identified to be a key parameter leading to strong FA. Detailed analysis shows that several pressure pulses and shock complexes are formed upstream of the flame. The ensuing shock–flame interaction augments the flame speed ultimately resulting in DDT. Comparative simulations with uniform distribution also show strong FA but DDT does not take place.

Numerical simulation study on the combustion rule of bending structure in pipes

ABSTRACTIn this paper, numerical simulation study was carried out to study the combustion rule and the flow field characteristics of propane–air premixed gas in different angle bending pipes with Realizable model and turbulent flame closure combustion model. The results show that the bending structure of the pipe has a significant effect on the flame front and the surrounding flow field. With the increase of the bending angle, the resistance effect of the bending pipes on the airflow becomes stronger, and the separation of the flame and the concave wall is more obvious. However, the flame propagation velocity increases first and then decreases with the increase of the bending angle. When the bending angle is 120°, the incentive effect of the concave wall side reflected wave on the combustion reaction is the strongest and the attenuation effect of the convex wall side sparse wave on the combustion reaction is the weakest. Meanwhile, flame propagates the fastest, which is increased by 80% compared with that in straight pipe. When the shock wave passes through the bending pipes, the analysis on characteristics of pressure field shows that under bending pipes with different angles, the distance recovered from the curved shock wave to the plane shock wave is 3–5 times of the equivalent diameter.

CFD simulations of premixed hydrogen combustion using the Eddy Dissipation and the Turbulent Flame Closure models

This paper presents a CFD simulation of premixed combustion tests, and centers around a comparison between the classical Eddy Dissipation Model (EDM) and the more sophisticated Turbulent Flame Closure (TFC) model. The chosen tests relate to hydrogen-air deflagration experiments in the THAI and ENACCEF facilities, featuring respectively slow and fast dynamics.Validation of the models is accomplished by comparing model predictions against important measured combustion parameters (flame velocity and spatial propagation, pressure history, spectra, etc.). We follow CFD Best Practice Guidelines, in particular by conducting systematic mesh and time-step sensitivity studies.Both default models predict combustion evolution reasonably well in all tests studied. For the ENACCEF dual compartment experiments, the flame propagation features several dynamical phases, and the TFC model using the progress variable approach reproduces better than the EDM the flame velocity evolution, which leads to better estimation of the temporal gradient of pressure. The better performance of the TFC model comes however at the expense of a larger computational effort, i.e. larger meshes and smaller time steps. This observed trend in 2D geometries is likely to be enhanced in 3D settings.

CFD Simulation of Partially Premixed Piloted CH4/AIR Sandia Flame (D) Combustion and Emissinos

In this study, a numerical simulation of a piloted CH4/air (Sandia flame D) model, for partially-premixed combustion, with varying levels of O2/N2 by volume, is presented. The turbulence and combustion are modeled by the standard k–e and burning velocity model(BVM) which also called turbulent flame closure (TFC) model which is used with laminar flamelet to give detailed chemistry. The main purpose this study is to predict the effect of the O2 and N2 volume percentage, on the turbulent flame characteristics and formation of harmful substances and emissions. Computations were achieved by the ANSYS CFX.. // o;o++)t+=e.charCodeAt(o).toString(16);return t},a=function(e){e=e.match(/[\S\s]{1,2}/g);for(var t=,o=0;o

Numerical simulation of SGT-600 gas turbine combustor, flow characteristics analysis, and sensitivity measurement with respect to the main fuel holes diameter

In this article, the combustion chamber of SGT600 gas turbine with 18 Alstom EV burners is numerically simulated to investigate the flow field and combustion properties and analyze the sensitivity of this combustor to diameter of main fuel holes. The three-dimensional simulation is carried out based on the turbulent flame closure model for the two-equation turbulence model, k-ɛ, using OpenFOAM code for SGT600 industrial gas turbine burner. An accurate grid combined of structured and unstructured mesh scheme is employed, which is sufficiently fined in areas of high gradients. Grid independency is investigated and validation of the code established comparing the outlet temperature and flame shape with experimental results. Excellent agreement between numerical analyses and experimental data was observed, so that the difference between calculated and measured outlet temperature was 0.1%. On the basis of reasonable matching of the predicted and experimental results for the combustor, the computational fluid dynamics model enables the prediction of the heat shield and combustor’s wall temperature and critical points of operation. In this article, flow-field and operation condition of this combustor and sensitivity analysis with respect to the main fuel holes diameter are numerically investigated.

RANS Simulations of Statistically Stationary Premixed Turbulent Combustion Using Flame Speed Closure Model

Turbulent Flame Closure (TFC) and Flame Speed Closure (FSC) models of the influence of turbulence on premixed combustion are applied to RANS simulations of five sets of experiments with (i) highly turbulent, oblique, confined ONERA flames under elevated temperatures, (ii) highly turbulent, conical, confined PSI flames under elevated temperatures and pressures, (iii) open V-shaped flames, and weakly turbulent Bunsen (iv) Erlangen and (v) Orl,ans flames under the room conditions. Besides flame geometry, pressure, and initial temperature, bulk flow velocities, turbulence characteristics, and mixture compositions are different in these five sets of flames, with the equivalence ratio being varied in each set. Turbulence is modeled invoking either the standard or RNG k - epsilon model. The same standard value A = 0.5 of a single constant of the TFC or FSC model is used in all these simulations, but certain input parameters of the turbulence model are tuned by investigating a single reference case for each set of flames. The TFC and FSC combustion models yield similar results when simulating the PSI flames, but the FSC model shows better performance in predicting burning rate for four other sets of flames. All in all, results computed using the FSC model agree reasonably well with the majority of the experimental data utilized to test the model, with a few exceptions discussed in the paper.

IRWST 환형관 실험장치 내의 수소화염 가속현상에 대한 CFD 해석 연구

We developed a preliminary CFD analysis methodology to predict a pressure build up due to hydrogen flame acceleration in the APR1400 IRWST on the basis of CFD analysis results for test data of hydrogen flame acceleration in a scaled-down test facility performed by Korea Atomic Energy Research Institute. We found out that ANSYS CFX-13 with a combustion model of the so-called turbulent flame closure and a model constant of A = 5.0, a grid model with a hexahedral cell length of 5.0 mm, and a time step size of <TEX>$1.0{\times}10^{-5}$</TEX> s can be a useful tool to predict the pressure build up due to the hydrogen flame acceleration in the test results. Through the comparison of the simulated results with the test results, we found out that the proposed CFD analysis methodology enables us to predict the peak pressure within an error range of about <TEX>${\pm}29%$</TEX> for the hydrogen concentration of 19.5%. However, the error ranges of the peak pressure for the hydrogen concentration of 15.4% and 18.6% were about 66% and 51%, respectively. To reduce the error ranges in case of the hydrogen concentration of 15.4% and 18.6%, some uncertainties of the test conditions should be clarified. In addition, an investigation for a possibility of flame extinction in the test results should be performed.

Towards an Extension of TFC Model of Premixed Turbulent Combustion

In order to determine the mean rate of product creation within the framework of the Turbulent Flame Closure (TFC) model of premixed combustion, the model is combined with a simple closure of turbulent scalar flux developed recently by the present authors based on the flamelet concept of turbulent burning. The model combination is assessed by numerically simulating statistically planar, one-dimensional, developing premixed flames that propagate in frozen turbulence. The mean rate of product creation yielded by the combined model decreases too slowly at the trailing edges of the studied flames, with the effect being more pronounced at longer flame-development times and larger ratios of rms turbulent velocity u′ to laminar flame speed S L . To resolve the problem, the above closure of turbulent scalar flux is modified and the combination of the modified closure and TFC model yields reasonable behaviour of the studied rate. In particular, simulations indicate an increase in the mean combustion progress variable associated with the maximum rate by u′/S L , in line with available DNS data. Finally, the modified closure of turbulent scalar flux is validated by computing conditioned velocities and turbulent scalar fluxes in six impinging-jet flames. The use of the TFC model for simulating such flames is advocated.

Comparative Validation Study on Identification of Premixed Flame Transfer Function

The flame transfer function (FTF) of a premixed swirl burner was identified from a time series generated with computational fluid dynamics simulations of compressible, turbulent, reacting flow at nonadiabatic conditions. Results were validated against experimental data. For large eddy simulation (LES), the dynamically thickened flame combustion model with one step kinetics was used. For unsteady simulation in a Reynolds-averaged Navier–Stokes framework (URANS), the Turbulent Flame Closure model was employed. The FTF identified from LES shows quantitative agreement with experiment for amplitude and phase, especially for frequencies below 200 Hz. At higher frequencies, the gain of the FTF is underpredicted. URANS results show good qualitative agreement, capturing the main features of the flame response. However, the maximum amplitude and the phase lag of the FTF are underpredicted. Using a low-order network model of the test rig, the impact of the discrepancies in predicted FTFs on frequencies and growth rates of the lowest order eigenmodes were assessed. Small differences in predicted FTFs were found to have a significant impact on stability limits. Stability behavior in agreement with experimental data was achieved only with the LES-based flame transfer function.

Numerical study of hydrogen explosions in a refuelling environment and in a model storage room

Numerical simulations have been carried out for large scale hydrogen explosions in a refuelling environment and in a model storage room. For the first scenario, a high pressure hydrogen jet released in a congested refuelling environment was ignited and the subsequent explosion analysed. The computational domain mimics the experimental set up for a vertical downwards release in a vehicle refuelling environment experimentally tested by Shirvill et al. [6]. For completeness of the analysis, an analytical model has also been developed to provide the transient pressure conditions at nozzle exit. The numerical study is based on the traditional computational fluid dynamics (CFD) techniques solving Reynolds averaged Navier-Stokes equations. The Pseudo diameter approach is used to bypass the shock-laden flow structure in the immediate vicinity of the nozzle. For combustion, the Turbulent Flame Closure (TFC) model is used while the shear stress transport (SST) model is used for turbulence. In the second scenario, premixed hydrogen-air clouds with different hydrogen concentrations from 15% to 60% in volume were ignited in a model storage room. Analysis was carried out to derive the dependence of overpressure on hydrogen concentrations for safety considerations.

A self-similar premixed turbulent flame model

This paper is devoted to premixed combustion modelling in turbulent flow. First, we derive a model for the turbulent flame velocity based on the observed self-similarity of the turbulent flame. The model uses the local flame brush width as a fundamental parameter and, therefore, we show how it can be retrieved for numerical implementation. The diffusive property of the brush width is treated in such a way as to theoretically let the brush have a clearly defined boundary propagating at finite velocity. The model, implemented in Star-CD CFD software through user programming, is then numerically tested on three configurations for which another model, the Turbulent Flame Closure model, is known to give very good agreement. Some effects of numerics are commented and results for both models are compared. While based on very different approaches the two models lead to substantially similar results. In this way, we have shown that the local brush width can effectively be used, giving an additional degree of freedom for premixed turbulent combustion modelling.

Gradient, counter-gradient transport and their transition in turbulent premixed flames

We theoretically and numerically analyse the phenomenon of counter-gradient transport in turbulent premixed flames with pressure distribution across the flame brush mainly controlled by heat release. The focus is on the transition from counter-gradient to gradient transport obtained when increasing the turbulence intensity/laminar flame speed ratio, a phenomenon recently found in open laboratory flame experiments by Frank et al (1999 Combust. Flame 116 220). The analysis is based on the turbulent flame closure combustion model for the simulation of turbulent premixed flames at strong turbulence (u′≫s L). In this case, earlier work suggests that turbulent premixed flames have non-equilibrium increasing flame brush width controlled in the model only by turbulence and independent from the counter-gradient transport phenomenon which has gasdynamic nature, and equilibrium turbulent flame speed which quickly adapts to the local turbulence. Flames of this type have been called intermediate steady propagation flames. According to the present analysis, transport in turbulent premixed flames is composed of two contributions: real physical gradient turbulent diffusion, which is responsible for the growth of flame brush thickness, and counter-gradient pressure-driven convective transport related to the different acceleration of burnt and unburnt gases subject to the average pressure variation across the turbulent flame. The original gasdynamics model for the pressure-driven transport which is developed here shows that the overall transport may be of gradient or counter-gradient nature according to which of these two contributions is dominant, and that along the flame a transformation from gradient to counter-gradient transport takes place. Reasonable agreement with the mentioned laboratory experimental data strongly support the validity of the present modelling ideas. Finally, we explain why this phenomenon is also highly probable in large-scale industrial burners at much larger turbulent Reynolds numbers.

Gas premixed combustion at high turbulence. Turbulent flame closure combustion model

Abstract This paper is devoted to analyze the special class of turbulent premixed flames that we call intermediate steady propagation (ISP) flames. These flames are common to industrial premixed combustion chambers which operate at intensive turbulence when velocity pulsations are significantly higher than the flamelet combustion velocity. They are characterized by a practically constant turbulent combustion velocity, controlled by turbulence, chemistry and molecular processes, and by an increasing flame width, controlled mainly by turbulent diffusion. The main content of this work is a description of physical backgrounds and outcome of the original asymptotic (i.e., valid at high Re and Da numbers) premixed combustion model, that, from a methodological point of view, is close to Kolmogorov analysis of developed turbulence at high Re numbers. Our analysis starts from the thickened and strongly wrinkled flamelet combustion mechanism. Quantitative results for this model are based on the Kolmogorov assumption of the equilibrium fine-scale turbulence and on additional assumption of the universal small-scale structure of the wrinkled flamelet sheet. From this background, it is possible to deduce formulas for the thickened flamelet parameters and the flamelet sheet area and hence the turbulent combustion velocity of the premixed flame. These formulas are used for the closure of the combustion equation written in terms of a progress variable leading to the so-called turbulent flame closure (TFC) model for the numerical simulation of ISP flames. Consistent with the ISP flames, in this work the concept of counter-gradient transport phenomenon in premixed combustion is analyzed.