OCR Text |
Show where Ts = To at x=xM (or t=O) and Ts = Tsi ,the ignition temperature, at x=O. For the rate of heat generation,Z, the heterogeneous reaction on the particle surface is taken into account and Q is the rate of heat loss due to convection and radiation. Then, 2 E Z = nDpo C 2 A exp ( - - ) o C RT (2 ) s 2 Q = nDpo [h (T - T ) + Q ] 9 s r (3) Tsi ,the ignition temperature~ used in eqn.(l) is estimated from the thermal ignition theoryl . That is, Tsi is assumed as the value of Ts satisfying the two simultaneous equations of Z = Q and dZ/dT = dQ/dT . Further, the gas temperature, T ,a function s s ~ of the distance from the burner exit, is estimat:ed from the following heat balance equation as, d dT dT Cd 2 [Apc ( _9 )] + pVC ( - _9) + (_) nDpoh (T - T ) = 0 dx p dx p dx M S 9 (4 ) o where T = T at x=xM and T = T (or dT /dx = (Tf-T )/L) at x=O. 'h 9 0 h h 9 1 S~ f f ,,9 f b 9 WJ. t respect to t e t erma OJ. uSJ.VJ. ty 0 tur uS 1 ence,l\'\ ., t h e following equation demonstrated by Mizutani2 ) is adopted: A = [k / ( pC)] + [1 AU] p L (5 ) Consequently, the two differential equations ,eqns.(l) and ( 4 ) , are sol ved numerically under the initial and boundary conditions specified. In this calculation, the parameter to be iterated is the gas velocity, V resulting in the flame propagation velocity. Some factors are assumed, examples being lL=4 mm, U=0.8 m/s and A=10 referred to from ref.2. Fig.4 shows the flame propagation velocity in relation to the coal concentration for some particle sizes. For the particle size ,10 ~m, the calculated flame propagation velocity becomes in the order of the actual reported values (about 10-15 m/s). Referring to this theoretical consideration, a single burner test was conducted where the gas veloci ty and the coal concentration in the dense flow were examined, which is mentioned later in this paper. 2.Flow analysis by using cold model We examined the flow characteristics of coal and air stream around the concentrator in a plastic model burner simulating a coal firing burner of 500 kg/h capacity. Fig.5 shows the concentrator we used. We defined the shape 3 |