煤粉射流中颗粒团的传热及其对着火行为的影响

建立描述煤粉射流中典型尺寸颗粒团的传热及着火过程的瞬态数学模型,考察其在高温环境中的升温特性和着火行为。模型计算发现,颗粒团的升温过程与颗粒团尺寸明显相关,对流换热份额随着颗粒团当量直径Dc的增加而下降。在火焰温度Tf=2 000 K、烟气温度T)s=1 400 K的条件下,对于D_c=3 mm的颗粒团,对流换热量Q_(conv,sc)占总换热量的86%;Dc=10 mm时,辐射换热量Q_(rad,fp)占到60%;而D)c≈8 mm时,对流换热与辐射换热量相近。火焰辐射温度T_f的升高降低了对流换热份额,并使得Q_(conv,sc)=Q_(rad,fp)对应的临界D_c,即D_c*减小。T)f对D_c较小的颗粒团的加热和着火影响较小,对D_c较大的颗粒团的影响明显。烟气温度Ts升高时,对流换热份额升高,D_c*增大。T_s在D_c较小时对着火时间影响较大,而D_c较大时影响不再明显。煤粉浓度增加时,颗粒团着火时间先明显下降,在0.6~0.8 kg/m~3的范围内达到最小值,而后随着煤粉浓度的进一步增加而略微增加。

Heat transfer of the coal clouds in a coal-air jet and its effect on ignition behavior

A transient model was developed to describe the heat transfer and ignition processes of coal clouds with typical sizes in a coal jet，to analyze their heating process and ignition time.The results show that the heating process greatly depends on the size of coal clouds，and the fraction of convective heat transfer decreases with the equivalent diameter of particle cloud，Dc.When Tf=2 000 K，Ts=1 400 K，for coal cloud of Dc=3 mm，the amount of convective heat transfer counts for 86% of the total heat exchanged.While Dc=10 mm，radiative heat transfer is the dominant and its amount counts for 60%.The amounts of convective and radiative heat transfer are nearly equal when Dc≈8 mm.The increase of flame temperature Tf reduces both the fraction of convective heat transfer and ignition time，and the corresponding critical Dc (D*c) at which Qconv，sc=Qrad，fp.The effect is more obvious for a larger Dc.With the increase of flue gas temperature Ts，the fraction of the convective heat transfer and D*c increase.The effect of Ts is significant only for particle clouds with a small Dc.The increase of Ccoal firstly promotes the ignition of coal clouds，leading to a minimum ignition time near Ccoal=0.6-0.8 kg/m3，while impedes the ignition when Ccoal further increases due to the increase of heat capacity.