Large eddy simulation of hydrogen-air propagating flames

The future use of hydrogen as a clean fuel and an energy carrier brings in safety issues that have to be addressed before community acceptance can be achieved. In this regard, availability of accurate modeling techniques is very useful. This paper presents large eddy simulations (LES) of propagating turbulent premixed flames of hydrogen-air mixtures in a laboratory scale combustion chamber. A Dynamic flame surface density (DFSD) model where the reaction rate is coupled with the fractal analysis of the flame front structure, is implemented and tested. The fractal dimension is evaluated dynamically based on the instantaneous flow field. The main focus of the current work is to establish the LES technique as a good numerical tool to calculate turbulent premixed hydrogen flames having an equivalence ratio of 0.7. Developing this capability has practical importance in analyzing explosion hazards, internal combustion engines and gas turbine combustors. The results obtained with the DFSD model are compare well with published experimental data. Further investigations are planned to examine and validate the LES-DFSD model for different flow geometries with hydrogen combustion

LES modelling of premixed deflagrating flames in a small scale vented explosion chamber with a series of solid obstructions

In this study, simulations of propagating turbulent premixed deflagrating flames past built in solid obstructions in a laboratory scale explosion chamber has been carried out with the Large Eddy Simulation (LES) technique. The design of the chamber allows for up to three baffle plates to be positioned in the path of the propagating flame, rendering different configurations, hence generating turbulence and modifying the structure of the reaction zone. Five important configurations are studied to understand the feedback mechanism between the flame-flow interactions and the burning rate. In LES, the sub-grid scale (SGS) reaction rate should be accounted for by an appropriate model which can essentially capture the physics. The present work has been carried by using the flame surface density (FSD) model for sub-grid scale reaction rate. The influence of the flow on turbulence and flame 2 propagation as a result of the in-built solid obstructions is also examined. The impact of the number and the position of such baffle plates on the generated overpressure, flame speed and structure are studied. Results from the simulations are compared with experimental data for five configurations and they show good agreement

An assessment of large eddy simulations of premixed flames propagating past repeated obstacles

This paper presents an assessment of Large Eddy Simulations in calculating the structure of turbulent premixed flames propagating past solid obstacles. One objective of the present study is to evaluate the LES simulations and identify the drawbacks in accounting the chemical reaction rate. Another objective is to analyse the flame structure and to calculate flame speed, generated overpressure at different time intervals following ignition of a stoichiometric propane/air mixture. The combustion chamber has built-in repeated solid obstructions to enhance the turbulence level and hence increase the flame propagating speed. Various numerical tests have also been carried out to determine the regimes of combustion at different stages of the flame propagation. These have been identified from the calculated results for the flow and flame characteristic parameters. It is found that the flame lies within the ‘thin reaction zone’ regime which supports the use of the laminar flamelet approach for modelling turbulent premixed flames. A sub-model to calculate the model coefficient in the algebraic flame surface density model is implemented and examined. It is found that the LES predictions are slightly improved due to the calculation of model coefficient by using sub-model. Results are presented and discussed in this paper are for the flame structure, position, speed, generated pressure and the regimes of combustion during all stages of flame propagation from ignition to venting. The calculated results are validated against available experimental data

Calculations of explosion deflagrating flames using a dynamic flame surface density model

Explosion deflagrating flames in a small scale vented chamber, with repeated obstacles are simulated using the large eddy simulation (LES) technique for turbulent reacting flows. A novel dynamic flame surface density (DFSD) combustion model, based on the laminar flamelet concept has been used to account for the mean chemical reaction rate. All cases considered here start with a stagnant, stoichiometric propane/air mixture. Three configurations with two baffle plates and a solid square obstacle, at different axial locations from the bottom ignition centre are examined. Numerical calculations of explosion generated pressure histories; flame characteristics such as structure, position, speed and acceleration are validated against published experimental data. Influence of the relative position of baffles plates with respect to the origin of the ignition are examined and discussed. Qualitative comparisons of the computed reaction rate are also made with images of Laser Induced Fluorescence from OH measurements. Good agreement obtained between numerical predictions and experimental measurements confirms the applicability of the newly developed dynamic model to predict the dynamics of explosion deflagrating flames

LES modelling of propagating turbulent premixed flames using a dynamic flame surface density model

A Dynamic flame surface density (DFSD) model, developed recently from experimental images for transient turbulent premixed flames, is implemented and tested using the large eddy simulation (LES) modelling technique. Numerical predictions from DFSD model are compared with those predicted using the flame surface density (FSD) sub-grid scale (SGS) model for reaction rate. In the SGS-DFSD model, dynamic formulation of the reaction rate is coupled with the fractal analysis of the flame front structure. The fractal dimension is evaluated dynamically from an empirical formula based on the sub-grid velocity fluctuations. A laboratory scale combustion chamber with inbuilt solid obstacles is used for model validation and comparisons. The flame is initiated from igniting a stichiometric propane/air mixture from stagnation. The results obtained with the DFSD model are in good comparisons with experimental data and the essential features of turbulent premixed combustion are well captured. It has also been observed that the SGS-DFSD model for reaction rate found to capture the unresolved flame surface density contributions. Further investigations are planned to examine and validate of the SGS-DFSD for different flow geometries

Measurements and LES calculations of turbulent premixed flame propagation past repeated obstacles

Measurements and large eddy simulations (LES) have been carried out for a turbulent premixed flame propagating past solid obstacles in a laboratory scale combustion chamber. The mixture used is a stoichiometric propane/air mixture, ignited from rest. A wide range of flow configurations are studied. The configurations vary in terms of the number and position of the built-in solid obstructions. The main aim of the present study is two folded. First, to validate a newly developed dynamic flame surface density (DFSD) model over a wide range of flow conditions. Second, to provide repeatable measurements of the flow and combustion in a well-controlled combustion chamber. A total of four groups are derived for qualitative and quantitative comparisons between predicted results and experimental measurements. The concept of groups offers better understanding of the flame-flow interactions and the impact of number and position of the solid baffle plates with respect to the ignition source. Results are presented and discussed for the flame structure, position, speed and accelerations at different times after ignitions. The pressure-time histories are also presented together with the regimes of combustion for all flow configurations during the course of flame propagation

An LES-DFSD study of transient premixed propane/air flames propagating past obstacles

Simulation of deflagrations requires accurate modelling of transient premixed flames. An initial laminar flame kernel is subjected to progressive stretch by obstacle generated turbulence as it propagates downstream. Large eddy simulation (LES) based flame surface density models may encounter difficulties when a predefined model constant is used for given turbulence characteristics. The reliable description of flame wrinkling considerably determines the transient flame behaviour. In this work, we applied a dynamic flame surface density (DFSD) model to automatically adjust the model parameter based on the instantaneous resolved flame information, whilst recovering laminar flame propagation in the absence of sub-grid turbulence. In the work we have successfully verified the LES DFSD approach against recent experimental data of stoichiometric propane/air deflagrating flames past obstacles. The overpressure, flame front speed and other flame features have been correctly reproduced by the model at various stages of flame propagation for distinct obstacle configurations. The wrinkling factor was identified as an informative parameter to understand the flame dynamics and pressure build-up mechanisms, and the significant contribution from the sub-grid flame surfaces have been demonstrated. By constructing the combustion regime diagram for LES, we have demonstrated that the flame wrinkling is fully resolved during the early propagation, and the flame lies in the thin reaction zone regime when the pressure peak is reached. Through comprehensive parametric studies, we found that ignition modelling has a considerable impact on the time taken to reach the peak pressure yet a much weaker effect on the peak magnitude. A negative correlation was identified between the Smagorinsky constant and the maximum pressure. Early-stage overpressure and flame evolutions were confirmed to be grid-independent, while the pressure peak is slightly mesh-sensitive. It was found that the magnitude of the dynamic parameter adequately self-adjusts to varying grid and filter widths. The satisfactory agreements with experiments and the robust findings ensured by the sensitivity studies indicate the predictive capabilities and the benefits of the LES-DFSD model

Large eddy simulation of hydrogen-air premixed flames in a small scale combustion chamber

While hydrogen is attractive as a clean fuel, it poses a significant risk due to its highreactivity. This paper presents Large Eddy Simulations (LES) of turbulent premixed flames of hydrogeneair mixtures propagating in a small scale combustion chamber. The sub-grid-scale model for reaction rate uses a dynamic procedure for calculating the flame/ flow interactions. Sensitivity of the results to the ignition source and to different flow configurations is examined. Using the relevant parameter from the calculations, the flames are located on the regimes of combustion and are found to span the thin and corrugated flamelet regimes, hence confirming the validity of flamelet modelling. The calculations are compared to published experimental data for a similar configuration. It is found that both the peak overpressure and flame position are affected by the number of baffles positioned in the path of the flame and this is consistent with earlier findings for hydrocarbon fuels. Also, the LES technique is able to reproduce the same flame shape as the experimental images. A coarse study of sensitivity to the ignition source shows that the size of the ignition kernel does not affect the flame structure but influences only the time where the peak overpressure appears while moving the ignition source away from the base plate leads to a decrease in the peak overpressure

LES modelling of explosion propagating flame inside vented chambers with various built-in solid obstructions

This paper presents large eddy simulations (LES) of the transient interaction between propagating turbulent premixed flames and solid obstructions mounted inside a laboratory scale combustion chamber. Interactions between the flame movement and the obstacles found to create both turbulence by vortex shedding and local wake/recirculation whereby the flame is wrapped in on itself, increasing the surface area available for combustion and the rate of local reaction rate. Accounting the influence of such local events in order to predict overall flame spreading speed, flame behaviour and the generated overpressure as a measure of reaction rate are extremely useful in combustion analysis in order to develop new models. The rise in the reaction rate due to the local nature of the flow and the increase in overall pressure due to the enhanced turbulence flame interactions as the flame travels through the unburned fuel air mixture are presented and discussed. The main focus of the current work is to establish the LES technique as a good numerical tool to calculate turbulent premixed propagating flames of propane/air mixture having equivalence ratio of 1.0, which is of practical importance in analysing explosion hazards and gas turbine combustors