Numerical Investigation In The Buoyancy Effects On Micro Hydrogen Jet Flames

Jet flame is one of the most commonly used combustion method because of the advantages of being simple and easy to control.Compared with normal-scale jet flames,there are many changes in the combustion characteristics of micro-scale jet flames.For example,the flow state changes from turbulent to laminar,and the shapes of flames becomes regular.As the area to volume ratio increases,the heat transfer between the flame and wall surface is enhanced.In addition,under the condition with gravity,buoyancy has a certain effect on the micro-jet flames.Due to the poor stability of the micro-jet flames,especially under extreme conditions,such as conditions at very low fuel flow velocities,the buoyancy may have a greater impact on it.Therefore,we numerically study the effect of buoyancy on flame structure,combustion efficiency,temperature distribution,and flow field of hydrogen micro-jet flames in large and limited spaces at different fuel flow velocities without air co-flow.And the effect of geometric parameters such as inner diameter of tube on hydrogen micro-jet flames without buoyancy was studied and analyzed.First,the effect of buoyancy on micro-jet flames in a large free space was studied.Compared with conditions without buoyancy,buoyancy will aggravate the cold air flow on the side of the flame,which will compress the flame and reduce its height and width.At the same time,the high temperature zone becomes narrow and the flame temperature is slightly lower.At a medium or high jet velocity,although the flame volume is small when there is buoyancy,the local heat release rate is greater,and the combustion efficiency is not reduced.However,at a very low fuel flow velocity,buoyancy increases heat loss of flames and reduces combustion efficiency,which has a negative impact on combustion.Then the effect of buoyancy on the micro-jet flames in a small tube was studied.Studies have found that buoyancy can significantly increase air supply.The extinction velocity limit with buoyancy is much lower than that without buoyancy.Flames of the former are located above the nozzle exit at all velocities.As the velocity increases,the top of the flame originally closed will gradually open.Flames of the latter are located below the nozzle exit at low velocities.As the velocity increases,the flame moves upwards.Large gap appears at the top of flames at all velocities.The combustion efficiency,flame temperature and exhaust gas temperature of conditions with buoyancy are significantly higher than the counterparts without buoyancy at the same fuel velocity.After comparing the effects of buoyancy on combustion at low and high velocities,it is found that the smaller the fuel velocity,the greater the degree of influence of buoyancy on combustion characteristics.Finally,the effect of structural parameters(inner diameter of the tube,inner diameter of the nozzle and length of the tube)of tube jet burners on the combustion was studied.The results show that the larger the tube inner diameter,the larger the flame volume.Increasing the tube inner diameter can increase the amount of entrainment air and increase the combustion efficiency.At the same time,the flame temperature,exhaust gas temperature,and average temperature of the outer wall will increase.Reduce the nozzle inner diameter,the lower and upper flammable limits of the fuel velocity become smaller.At the same mass flow rate of fuel,the flame is narrower and longer,the entrainment coefficient is larger,and the combustion efficiency is higher.Therefore,the flame temperature,exhaust gas temperature and outer wall temperature are all higher.Reduce the length of tube below the nozzle exit can slightly improve combustion efficiency.Reduce the length of tube above the nozzle exit can improve combustion efficiency at low velocities,but decrease combustion efficiency at high velocities.In general,the influence of tube length on combustion efficiency is smaller than that of tube inner diameter and nozzle inner diameter.