Experimental Study On Dynamics And Suppression Methods Of Premixed Flame Propagation In Closed Ducts

As the rapid development of society and economy,the global demand for new clean alternative energy is increasing sharply due to the fossil energy depletions and serious environmental pollutions.Combustible gases are favored worldwide due to their clean and efficient properties.However,combustible gases also have the dangerous characteristics of flammable,explosive,and easy to leak,disaster accidents(including fire and explosion)will be easy to occur if they are not used or controlled in a proper way.Usually,the combustible gases are transported through ducts in actual industrial applications.Fire and explosion accidents occur almost annually worldwide due to the difficulties faced in gases controlling.Thus,it is of great significance to gain a further understanding on dynamics and suppression methods of premixed flame propagation in closed ducts both for safety and engineering applications of combustible gases.This thesis aims to investigate the dynamics of premixed flame propagation in closed ducts and to explore effective premixed flame suppression methods.Firstly,a comparative study upon the dynamics of typical premixed combustible gases-air flame propagation in a closed duct is conducted through the premixed flame propagation experimental system.Four different combustible gases with different chemical reactivity,including methane,natural gas,acetylene,and hydrogen are chosen.The high-speed schlieren photography system is applied to capture the flame shape changes and determine the flame tip speed,and the pressure transducer is adopted to record the pressure-time history.The results indicate that the characteristics of combustible gases influence the flame behaviors directly.Classical tulip flame forms in the equivalence ratio range of 0.79???1.30 for methane,0.72???1.44 for natural gas,0.40???1.70 for acetylene,and 0.60???5.56 for hydrogen,respectively.Distorted tulip flame forms in the equivalence ratio range of ?=1.00 for acetylene and 1.00???2.38 for hydrogen,and no distorted tulip flame is not observed for methane and natural gas.The small amount of ethane and propane that exist in natural gas accelerate the flame propagation and increase the pressure compared with pure methane.Mean,while,acetylene presents a much faster flame propagation speed and higher pressure compared with natural gas and methane,and hydrogen has the fastest flame propagation speed and maximal pressure due to it having the highest chemical reactivity.The theory proposed by Bychkov et al.for predicting the flame skirt motion coincides well with our experimental data with equivalence ratio being close to ?=1.00,and it is more suitable to predict the flame skirt motion for the high-chemical reactivity gases,such as acetylene and hydrogen.Subsequently,both of methane and hydrogen are chosen as the representatives of low_chemical reactivity and high-chemical reactivity gases to reveal the effects of single-layer metal wire mesh and multi-layer metal wire mesh on premixed flame propagation using the premixed flame suppression experimental system.The results demonstrate that metal wire mesh makes the tulip flame formation time advanced and makes the flame front inversion extent weaker compared with the case of no wire mesh.And it is attributed to the effects of metal wire mesh on advancing and weakening the interactions between flame front and pressure waves.In addition,the metal wire mesh also strengthens the disturbance flow and generates a violent combustion after the flame propagating through the suppression zone.For the single-layer wire mesh,the quenching performance does not increase continuously as mesh density increases,and this is because the anti-destructive performance is also qu'ite important in practical applications.No suppression effect on flame tip speed in the upstream duct is obtained because the decrease of gas flow speed in unburned field is balanced by the increase of combustible mixture burning velocity.Multi-layer wire mesh can attenuate the maximum flame tip speed,maximum pressure,and maximum sound effectively,and the suppression effects increase as mesh and layer increase.Then,the factors affecting the flame quenching under the effects of multi-layer metal wire mesh are analyzed systematically,including the characteristics of combustible gases,temperature and pressure,dopants in combustible gases,equivalence ratio,volume of wire mesh,arrangement style of wire mesh,and ignition position.It is found that the characteristics of combustible gases,temperature and pressure,dopants in combustible gases,and equivalence ratio can influence the flame quenching results through impacting on the laminar burning velocities.The relationship between critical quenching parameters and volume of wire mesh is revealed.The critical quenching speed increases almost linearly as the volume of metal wire mesh increases.However,the maximum critical quenching pressure maintains at a constant value of about 0.115 MPa.Metal wire mesh performs a better suppression effect on rich-fuel combustion than lean-fuel combustion.Besides,it is found that the flame quenching performance of metal wire mesh can be changed through varying the spacing between the wire mesh.Whether the flame can be quenched in the suppression zone is just the competition between the flame accelerating effects and the quenching effects of sidewalls.The ignition positions affect the flame quenching results significantly.Three different ignition positions are adopted to ensure the flame propagate into the suppression zone with three different stages.With tulip flame completely formed in ignition position 1,just before tulip flame formation in ignition position 2,and with finger shape flame in ignition position 3.It is found that the closer the ignition position is to the metal wire mesh,the easier the flame is quenched.When the flame is initiated in the closed end of the duct,tulip flame formation is essentially produced by the vortexes generated by the interactions between pressure waves and flame front.While,for the cases of flame initiating from the center of the duct,tulip flame is mainly attributed to the reverse gas flow in burnt field enhanced by the great discrepancies of combustion intensity between the flame propagating toward upstream duct and downstream duct.Meanwhile,the results demonstrate that both of the flame tip speed propagating toward upstream duct and downstream duct are of great importance in determining the pressure dynamics in the cases of flame initiating from the center of the duct.Finally,the coupling effects of inert gas and metal wire mesh on premixed flame propagation are discussed.It is found that CO2 has a more effective suppression effect on premixed hydrogen-air flame than N2.In rich fuel combustion,CO2-5%has little suppression effect on flame tip speed and pressure due to it having almost no influence on flame surface areas.However,both of the flame tip speed and pressure are reduced significantly under the effect of CO2-5%in lean-fuel cases.As the concentration increases,the suppression effect increases continuously with concentration below 25%,but becomes almost constant with concentration changing from 25%to 30%.The increase of CO2 concentration is able to suppress the hydrodynamic instability and reduce flame disturbance.As for the thermal diffusion instability,the increase of CO2 concentration in lean-fuel combustion makes Lewis number become close to unity and reduces the flame disturbance.On the contrary,thermal diffusion instability itself has a suppression effect on hydrodynamic instability in rich-fuel cases,and the addition of CO2 only contributes to strengthening this effect and reducing flame disturbance further.The coupling effects of CO2 dilution and metal wire mesh on premixed hydrogen-air flame is more effective than that using either of the two suppression agents separately.CO2 dilution promotes the suppression effects of metal wire mesh on lean-fuel combustion,and metal wire mesh improves the suppression effects of CO2 dilution on rich-fuel combustion.Moreover,the suppression effects of CO2 dilution with a higher concentration on premixed hydrogen-air flame can be still enhanced by the addition of metal wire mesh.