【作者】 赵明；

【导师】 范爱武；

【作者基本信息】 华中科技大学 ， 动力工程， 2020， 硕士

【摘要】 射流火焰具有简单易控制的优点,是最常用的一种燃烧方式。与常规尺度的射流火焰相比,微尺度射流火焰的燃烧特性会发生诸多变化。例如,流动状态由湍流变为层流,火焰形状变得规则;由于面体比的增大,火焰与壁面之间的传热加强等。此外,在重力环境下,浮升力对微射流火焰也具有一定的影响。由于微射流火焰的稳定性较差,尤其是在极端工况,如非常低的燃料流量下,浮升力对它的影响可能会更大。因此,本文采用数值模拟的方法研究不同燃料速度和无伴流空气的情况下浮升力对大空间和受限空间内微射流氢气火焰的结构、燃烧效率、温度分布、流场等方面的影响,并对无浮升力条件下套管内径等几何参数对微射流氢气火焰的影响进行研究。首先,研究了浮升力对自由大空间微射流火焰的影响。与无浮升力的情况相比,浮升力会加剧火焰侧面的冷空气流动,使火焰被压缩,火焰的高度和宽度都变小。同时高温区变得狭小,火焰温度略低。在中、高射流速度下,有浮升力时尽管火焰体积减小,但局部热释放速率更大,燃烧效率并未降低。但在极低燃料速度下,浮升力使火焰散热增加,燃烧效率减小,给燃烧带来负面影响。然后,研究了浮升力对小套管中微射流火焰的影响。发现浮升力会显著地提高空气的供给量。有浮升力时的燃料速度下限远低于无浮升力情况。前者的火焰在所有速度下都位于喷管出口上方,随着速度增大,原本封闭的火焰顶部会逐渐打开。而后者的火焰在低速时位于喷管出口下方,随着速度增大,火焰向上移动,在所有速度下火焰顶部都有较大缺口。在相同燃料速度下,有浮升力时的燃烧效率、火焰温度、排气温度都显著高于无浮升力时的情况。对比低速和高速下浮升力对燃烧的影响后发现,燃料速度越小,浮升力对燃烧特性的影响程度越大。最后,研究了套管射流燃烧器的结构参数(套管内径、喷管内径和套管长度)对燃烧状况的影响。结果显示,套管内径越大,火焰体积越大。增大套管内径可以提高引射空气量,使燃烧效率升高,同时火焰温度、排气温度以及外壁面的平均温度均会升高。减小喷管内径,燃料速度下限和上限都变小,在相同燃料流量下其火焰较为狭长,引射系数更大,燃烧效率更高。因而火焰温度、排气温度和外壁面温度都更高。减小喷管出口下方的套管长度可以轻微地提升燃烧效率。减小喷管出口上方的套管长度在低速下能提高燃烧效率,但在高速下反而降低燃烧效率。总体而言,套管长度对燃烧效率的影响与套管内径和喷管内径相比较小。更多还原

【Abstract】 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.更多还原