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 typi- cal 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 di- ameter 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∗) 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∗ 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 mini- mum 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.