Experimental And Numerical Study Of Dynamics Of Premixed Hydrogen-Air Flame Propagating In Ducts

Premixed combustion of the combustible gas mixture is a very fundamental and potential subject for practical application, e.g. accidental explosions. The fundamental understanding of premixed flame propagation phenomena is essential for the development of novel analytical and numerical combustion models. Premixed flame dynamics in confined vessels is of particular importance since it provides understanding of the burning processes taking place in internal combustion engines, and explains the mechanisms behind flame acceleration that can lead to transition from deflagration to detonation. In addition, hydrogen is a promising alternative energy carrier in the future, and it is desirable to characterize the combustion behavior of its blends with air. Meantime, the development and the validation of contemporary combustion models with a wide range of applicability are important for both hydrogen combustion applications and explosion safety.This study aims to provide fundamental and in-depth investigation for premixed combustion and reliable predictive approaches for combustion engineering and explosion safety. Two primary aims is planned to be achieved in the present work. The first objective of this work is to study the premixed combustion dynamics in tubes, i.e. flame and pressure dynamics, and explain the mechanisms of the dynamics of the premixed combustion and flame. Another important target of the present study is to investigate gas explosions in tubes, and to develop and validate theoretical and numerical methods that could provide reasonable prediction of accidental gas explosions inside tubes. Laboratory experiments and CFD numerical simulations of premixed hydrogen-air flames in tubes are the basis of the thesis.In the experiments, both the dynamics of premixed hydrogen-air flames and pressure build-up at various equivalence ratios propagating in half-open and closed horizontal ducts are investigated using high-speed schlieren photography and pressure records. The high-speed schlieren device is used to record the changes both in the flame shape and position as a function of time during the combustion process. The pressure transient in the duct during the nonsteady combustion is measured using a pressure transducer. The influences of gravity, opening ratio and equivalence ratio on the flame dynamics are also examined in the experimental investigation.In the numerical simulations, the premixed combustion wave is simulated as two-dimensional (2D) or three-dimensional (3D) chemically reacting flow. A dynamic thickened flame (TF) model is applied in the2D numerical simulation to account for the premixed combustion. The chemical reaction of hydrogen and air is taken into account using a19-step detailed chemistry scheme. The3D numerical simulations are carried out using two numerical approaches. The first one is based on the same combustion technique as that in the2D simulation, namely the dynamic TF flame method. Nevertheless, a dynamically and locally adaptive mesh refinement is adopted, and tracks the location of the flame front. The hydrogen-air chemical reactions are taken into accounts using a seven-step chemistry scheme. The second one is the large eddy simulation (LES) together with a turbulent burning velocity model. The LES premixed combustion model is applied to gain an insight into various phenomena of flame and explain the experimental observations. The model accounts for the effects of four different physical mechanisms, i.e. flow turbulence, turbulence generated by flame front itself, diffusive-thermal instability, and transient pressure and temperature of unburned gas, on the premixed flame burning velocity.The experimental study shows that the premixed hydrogen/air flame in ducts undergoes more complex shape changes and exhibits more distinct characteristics than that of other gaseous fuels. One of the outstanding findings is that significant distortions happen to the classical tulip flame front after its full formation when equivalence ratio ranges from0.84to4.22in the closed duct. A distorted tulip flame is initiated as the distortions or indentations are created very near the leading tips of the tulip lips after a well-pronounced classical tulip flame is produced. The distorted tulip flame develops into a salient "triple tulip" shape as the secondary tulip cusps approach the center of the primary tulip lips and appear comparable to the primary cusp. A second distorted tulip flame appears with a cascade of secondary cusps on the primary tulip lips just before the collapse of the first one. The salient tulip flame distortions are specially scrutinized and distinguished from the classical tulip. The dynamics of distorting tulip flame is different from that of classical tulip flame. The distorting tulip flame undergoes more complex shape changes and more unstable combustion process than the classical tulip flame. The normal tulip flame can be reproduced after the disappearance of the first distortion followed by another distortion. The schlieren images and the pressure records show that the distorted tulip flame propagation can be divided into five stages of dynamics, i.e. spherical flame, finger-shape flame, flame touching the sidewalls, tulip flame and distorted tulip flame. The initiation of flame shape changes coincides with the deceleration both of pressure rise and flame front speed for flames with tulip distortions. And the formation and dynamics of both tulip and distorting tulip flames depend on the mixture composition. The gravity has a noticeable impact on the tulip flame and can make the tulip flame collapse in different way between low and high equivalence ratios. The opening ratio can significantly influence the flame dynamics in a partially open duct. When the opening ratio is smaller than0.4a remarkable distorted tulip flame can be formed. The characteristic times and the corresponding characteristic distances of flame front increase with the increase of the opening ratio.The flame dynamics observed in the experiment is well reproduced in the2D numerical simulation with TF method. The flame-induced reverse flow and vortex motion are observed both in experiment and the2D simulation. The interactions between the flame front, reverse flow and vortices in the burned gas change the flame shape and ultimately the flame front develops into a tulip shape. The pressure wave triggered by the first contact of the flame with the side walls is responsible for the periodic deceleration of the flame front and plays an important role in the formation of the distorted tulip flame. The flame and pressure dynamics observed in the experiment are well reproduced in the3D numerical simulation using the dynamic TF model. The predicted pressure dynamics in the numerical simulation is also in good agreement with that in the experiment. The close correspondence between the experiment and the numerical simulation demonstrate that the TF approach is quite reliable for the study of premixed hydrogen/air flame propagation in the closed duct. Both the tulip and distorted tulip flames can be created in the simulation with free-slip boundary condition at the duct walls, which means that the wall friction could be unimportant for the tulip and distorted tulip formation.The LES numerical simulation provides further understanding of the interaction between flame front, pressure wave and combustion-generated flow, especially when the flame acquires a "distorted tulip" shape. The dynamics of "distorted tulip" flame observed in the experiment is well reproduced by the LES. The numerical simulations show that large-scale vortices are generated in the burnt gas after the formation of a classical tulip flame. The vortices remain in the proximity of the flame front and modify the flow field around the flame front. As a result, the flame front in the original cusp and near the sidewalls propagates faster than that close to the centre of the original tulip lips. The discrepancy in the flame propagation rate can finally lead to the formation of the "distorted tulip" flame. The LES model validated previously against large-scale hydrogen/air deflagrations is successfully applied in this study to reproduce the dynamics of flame propagation and pressure build up in the small-scale duct. It is confirmed that grid resolution has an influence to a certain extent on the simulated combustion dynamics after the flame inversion.On the basis of the experimental and numerical investigation of the interaction between flame front and pressure wave, the premixed flame dynamics for hydrogen/air mixture in the closed duct is theoretically analyzed. A theoretical model of the distorted tulip flame is developed. The results predicted using the theoretcial model is in satisfactory agreement with those in the experiments and LES. The theoretical analysis demonstrates that the Taylor instability is the substantial cause of the "distorted tulip" flame.