Numerical study of propane and hydrogen turbulent premixed flames in a small scale obstructed chamber

This paper presents numerical study of turbulent premixed flames characteristics of two different fuel/air mixtures, namely propane and hydrogen. The flames under study are propagating past solid baffle plate(s) in a small-scale combustion chamber. The chamber design allows for up to three baffle plates to be inserted followed by a square obstacle to promote the generation of turbulence. The test cases considered in this paper examine various configurations of the baffles and one central obstacle at a fixed equivalence ratio of 0.8. An in-house computational fluid dynamics (CFD) model is used to numerically evaluate the characteristics of the flame propagation. The large eddy simulation (LES) technique is used for turbulence flow modelling. Three different flow configurations with various obstacles positioning are used to highlight the generated overpressure and flame speed. The numerical results are then validated against published experimental data to confirm the capability of computational models in capturing the features of hydrogen and propane flames. A conclusion is drawn that different configurations affect the generated peak overpressure as well as the flame structure. It was also concluded that hydrogen flames generated a significantly greater peak overpressure inside the combustion chamber when compared to propane

Numerical study of the characteristics of turbulent premixed flames

Turbulent premixed flames are present in a wide range of applications. The nature of these flames transitioning from initially quasi-laminar to fully turbulent results in difficulty when providing numerical predictions. The presence of obstructions in the path of the premixed flame further deforms the transient flame front and amplifies the severity of the event. Large eddy simulations (LES) techniques are applied for simulations in this study to investigate turbulent premixed combustion. The LES technique continues to evolve as a prevailing numerical tool for the modelling of unsteady flame propagation. The challenge of applying LES for predictions of turbulent premixed flames is in part due to difficulty modelling the thin transient flame front. Results presented in this thesis apply a dynamic flame surface density (DFSD) model for estimations of the filtered reaction rate. Model parameters are automatically calculated based on the resolved flame front characteristics.The LES-DFSD model is applied to examine turbulent premixed flame propagation past consecutive solid obstructions in a lab-scale combustion chamber. Numerical results presented in this thesis focus on model predictions for hydrogen-air mixtures and obstacle configurations using multiple reported experimental test cases. Sensitivity of numerical results to parameters such as grid resolution and filter width are also inspected. The novelty of this research is within investigating hydrogen flame interaction with obstructions of varied area blockage ratios (ABRs). The numerical model is found to be successful in reproducing published experimental data. Numerical results for key combustion events such as the maximum rate of pressure rise, peak overpressure and flame speed well represent experimental data. Further, the LES-DFSD model replicates the transient flame structure observed in experimental images. The effect of obstruction ABR as well as baffle location and frequency is investigated. A brief comparison between hydrogen and propane mixtures finds that hydrogen flames generated greater overpressures at higher flame speeds when compared with propane.</div

LES – DFSD modelling of vented hydrogen explosions in a small-scale combustion chamber

Accidental explosions are a plausible danger to the chemical process industries. In the event of a gas explosion, any obstacles placed within the path of the flame generate turbulence, which accelerates the transient flame and raises explosion overpressure, posing a safety hazard. This paper presents numerical studies using an in-house computational fluid dynamics (CFD) model for lean premixed hydrogen/air flame propagations with an equivalence ratio of 0.7. A laboratory-scale combustion chamber is used with repeated solid obstacles. The transient compressible large eddy simulation (LES) modelling technique combined with a dynamic flame surface density (DFSD) combustion model is used to carry out the numerical simulations in three-dimensional space. The study presented uses eight different baffle configurations with two solid obstructions, which have area blockage ratios of 0.24 and 0.5. The flame speed, maximum rate of pressure-rise as well as peak overpressure magnitude and timing are presented and discussed. Numerical results are validated against available published experimental data. It is concluded that, increasing the solid obstacle area blockage ratio and the number of consecutive baffles results in a raised maximum rate of pressure rise, higher peak explosion overpressure and faster flame propagation. Future model development would require more experimental data, probably in a more congested configuration

LES-DFSD studies of lean-burn turbulent premixed hydrogen flames

Large eddy simulation (LES) based turbulence modelling technique may face challenges when employed to predict turbulent reacting flows. The Dynamic Flame Surface Density (DFSD) model for turbulent premixed combustion applied in this research adapts to produce accurate results based on the information obtained from transient flames. A LES – DFSD model has been developed and validated against experimental data for lean-burn premixed hydrogen flames propagating past repeated obstacles and a solid obstruction of varied area blockage ratio (ABR). The rate of pressure rise, peak overpressure magnitude, flame speed and other flame characteristics have been successfully reproduced numerically for flow configurations. It was found that having a combustion filter-width which is 6 to 7 times the smallest computational grid cell size produced the best overpressure and flame speed results. The numerical results from the LES – DFSD co-simulations produced good agreement with available experimental data

Numerical studies of turbulent premixed flame interaction with repeated solid obstacles

This paper presents numerical simulations of hydrogen and propane turbulent premixed flames interaction with repeated solid obstructions. The laboratory-scale combustion chamber used in this study is equipped with three solid baffles which promote the generation of turbulence and a square obstacle located downstream from the ignition source. The test cases considered have two different area blockage ratios (ABR) of 24% and 50%, respectively. The large eddy simulation (LES) turbulence modelling technique is used. The numerical simulations are carried out using an in-house computational fluid dynamics (CFD) model. Two different flow configurations are examined, both using three consecutive baffles to identify the subsequent effects and the sensitivity of each fuel to increasing the ABR. These effects are studied using the nature of the flame-obstacles interaction, generated combustion overpressure and resultant flame speed. The modelling capability is confirmed by validating the numerical results against published experimental data. Conclusions are drawn that increasing the ABR increases the combustion overpressure, rate of pressure rise and flame speed. It is also concluded that the larger obstacle has a significant effect on the propagating flame structure and that hydrogen flames are more sensitive to an increased ABR and produce a significantly higher peak overpressure

Numerical studies of premixed hydrogen/air flames in a small-scale combustion chamber with varied area blockage ratio

The increasing use of hydrogen as a renewable source of energy underlines the need to be able to assess the safety risks involved in the event of an accidental explosion. This paper presents numerical studies for hydrogen/air propagating flames at an equivalence ratio of 0.7 in a laboratory-scale combustion chamber equipped with turbulence generating baffles and a solid square cross section obstruction. The large eddy simulation (LES) modelling technique is used with an in-house computational fluid dynamics (CFD) model for compressible flows to study the flow turbulence and the flame propagation characteristics. The study is carried out using four different baffle arrangements and two different solid obstructions with area blockage ratios of 0.24 and 0.5. Results for the generated peak overpressure and the timing at which it occurs following ignition are considered as the primary safety factors. The time histories of the flame speed and position relative to the ignition source are validated against published experimental data. Good agreement is obtained between numerical results and experimental data which enables further predictions where measurements are limited in the study of vented hydrogen explosions. It was concluded that adding successive baffles and increasing the area blockage ratio escalates the maximum rate at which pressure rises and raises the generated peak explosion overpressure.<br

Numerical study on the influence of blockage ratio on hydrogen turbulent premixed flames in a small scale obstructed chamber

© 2020 ASME Although hydrogen is a clean and renewable fuel, there is still a need to understand and evaluate the potential risks posed in the event of an accidental explosion. This paper presents large eddy simulation (LES) numerical analysis for lean hydrogen premixed flames propagating inside a small laboratory combustion chamber with built in solid obstructions. The small-scale chamber is 0.625 litres in volume with three removable turbulence generating baffles and a square solid obstacle. A lean equivalence ratio of 0.7 is selected in this study. The LES model is utilised to investigate the influence of obstruction configuration and area blockage ratio on the flame characteristics and the generated combustion overpressure. The LES turbulence technique is used with an in-house computational fluid dynamics (CFD) model for compressible flows. The numerical simulations are carried out with various arrangements of the baffle plates and a solid obstacle to examine the effects of the area blockage ratio and generated turbulence on the flame structure and generated over-pressure. Two different area blockage ratios of 0.24 and 0.5 are studied. Four configurations with different baffle arrangements are studied to examine the resulting turbulence effects on the generated overpressure, flame position-time traces and flame transient speed following ignition. Direct comparisons are made between the different baffle/flow configurations to identify the various effects of an increased area blockage ratio. Numerical results showing the flame structure at various time windows after ignition are presented and compared with published experimental images. High speed laser induced fluorescence (LIF-OH) images of the reaction zones obtained from the experiments at a rate of 5 kHz provide the flame position data and convey the impact of the turbulence generated by the baffles and solid obstacle on the propagating flame structure [1]. The pressure is recorded at a rate of 25 kHz using a piezo-electric pressure transducer in the base plate of the chamber [2]. The rise in over-pressure as a result of increased turbulence due to additional baffles and an increased area blockage ratio is found to be consistent with experimental data. This is also found to be consistent for the flame position-time and speed-time traces across all configurations. Main points of interest such as the peak over-pressure, maximum rate of pressure rise and the flame propagation trends are also observed along with variations in flame speed as the flame interacts with the baffles and obstacles. Validation of the numerical results against available published experimental data conveys good agreement confirming the ability of the numerical model to predict numerical results for an increased area blockage ratio. Further numerical simulations are also carried out for flame/flow parameters where experimental data is unavailable due to physical limitations. Satisfactory agreement between numerical results and experimental data endorses further predictions for computational models in studying vented hydrogen explosions where there is an increased risk or limited experimental data