Study Of Dynamics Of Premixed Hydrogen-Air Flame Propagation In Obstructed Channels

With the continuous growth of global energy demand,the depletion of fossil energy and the increasing environmental pollution,the development and utilization of new energy have received extensive attention.Hydrogen is seen as the most promising new energy resource because of its wide source and zero pollution.However,hydrogen is a very explosive material,characterized by a high diffusion,low density,low ignition energy and wide flammability.Therefore any leakage of hydrogen into a confined space can result in a flame even detonation.From the perspective of hydrogen safety,the study on premixed hydrogen-air combustion is the base and core of preventing fire and explosion accidents.Meanwhile,premixed combustion in confined space is a typical process of internal combustion engines.It also involves the process of detonation wave,such as flame acceleration,deflagration and deflagration-to-detonation transition.As a result,the research on premixed combustion is of great significance to engineering applications.There are many studies concerning of the effect of rigid obstacles on the propagation of premixed flame in confined space,however less attention is paid to the flame propagation with some special structures laid in flame path,such as flexible obstacles and perforated plate.The comparison of flame propagation in tube with rigid/flexible obstacles and the propagation dynamics and quenching mechanism of premixed flame across perforated plate are critically important to enhance the safety of hydrogen energy.Two primary aims is planned to be achieved in the present work.There are two basic aims in the present thesis.The first one is to compare the dynamics of premixed flame across rigid/flexible obstacles,i.e.the characteristics of unburned gas flow field,flame and pressure dynamics.Another one is to study the dynamics of flame across perforated plate,the flame quenching limit and the factors affecting the flame quenching.Firstly,the premixed flames dynamics and the pressure built-up in the tube with rigid obstacles are both investigated using high-speed schlieren system and pressurerecording system.The schlieren are employed to observe the flame characteristics and flame velocity as a function of position.And pressure was recorded by pressure transducers.The influences of equivalence ratio,blockage ratio and multi-obstacles on the flame dynamics are examined in the experiments.The results show that the equivalence ratio mainly affects the laminar burning velocity,which affects the flow velocity and flame propagation.The cross-sectional channel is varied by the blockage ratio,which influences the compressive efficiency of the flame and the unburned gas flow filed in front of it.In a single rigid obstacle-lade tube,flame is mainly influenced by flow compression in the entrance of the obstacle opening and flow expansion downstream of the obstacle.At beginning,flames across the multi-rigid-obstacles nearly share the identical mechanism as it in the single rigid obstacle tube.Latter the turbulent bur ning in the spaces of successive obstacles is the main mechanism of facilitating flame acceleration.Secondly,the flame dynamic across flexible obstacles are compared with that across rigid obstacles at the same conditions.Flow filed,flame dynamics and pressure built-up in the flexible-obstacle-laden tube are significantly different compared to that in the rigid obstacle tube.Flow filed detached as it passes through the flexible obstacle,forming a shear layer attached to the tip of the obstacle and vortex located downstream of it.The difference in the mechanism of flame across the two types of obstacles are mainly dependent on the deformation of flexible obstacles.For the flexible obstacles with non-shape changed,the flame dynamics are similar.However,as mentioned above the deformed obstacles can result in totally different flow field,i.e.,shear layers and vortex.When the flame propagates in such flow field,the flow compression caused by the shear layer and the outer edge of the vortex is the main reason for flame acceleration.Meantime,flame is also affected by the instantaneous blockage ratio of flexible obstacles.The maximum pressure and the rate of pressure rise are attenuated by the flexible obstacle due to it porous structure.Lastly,the dynamic of premixed flame through perforated plate is studied in the present work.The effect of equivalence ratio,initial pressure and geometrical size on flame dynamic and pressure built-up are also tested.Four stages are identified when the premixed flame across perforated plate,namely,laminar flame,jet flame,turbulent flame and secondary flame front.Rayleigh-Taylor instability is considered to facilitate the formation of the fourth stage.When the geometrical size of perforated plate is increased,three propagation modes are obtained,i.e.,"go","quench"and"near limit".For the"quench"mode.,flame quench occurs either in the passage of a flame through the narrow holes,or immediately downstream of the perforated plate due to being over-mixed with the cold unburned gas.For "near limit"mode,it blends the features of"go"and"quench"mode,however quench is not seen though flame is cooled down by mixed the cold mixtures.In a P0-? map,the limit between"go"and"quench"mode fits linear equations.For the lean mixtures(?<1.0),the critical thickness of perforated plate dc to quench flame is smaller than 80mm;whereas,no such thickness is found in the rich mixtures.