Height combustor

Figure 12.3 shows some computational examples of nonreactive and reactive turbulent flows in a combustor with the bluff-body flame holder. The size of the combustor in Fig. 12.3 is 35 x 8 cm. The characteristic height and length of the bluff body is H = 2 cm. The left boundary is set as inlet, right boundary as outlet, and the upper and lower boundaries as rigid walls. 

Figure 12.4 The comparison of predicted mean temperature fields in long and short combustors at t = 14.9 ms (a), 22.1 (6), and 58.1 ms (c) after ignition behind a bluff body. Boundary condition at outlets is ABC of Eq. (12.19). Mean velocity at the inlet Uin = 10 m/s. Other conditions are po = 0.1 MPa, To = 293 K, fco = 0.06 J/kg, lo = 4 mm. A set of graphs below compares mean absolute velocity distributions in the different cross-sections (/ to VII) of both combustors (from left to right x = 0, 80, 100, 112, 135, 235, and 330 mm). Solid line — short combustor, dashed line — long combustor, j/max is the height of the corresponding cross-section of the combustor...

Figure 12.5 Calculated mean temperature fields in combustors with a set of similar open-edge V-gutter flame holders of height H = 3 cm and apex angle of 60°. The isoterms divide the entire temperature interval from the initial temperature To to combustion temperature Tc into 10 uniform parts and correspond to t = 27.5 ms. The combustor is 1 m long and the distance between the planes of flame holders is 0.05 m. Flame holders are shifted in longitudinal direction by OH (no shift) (a), IH (6), 2H (c), 3H (d), and 5H (e). Combustion of stoichiometric methane-air mixture at the mean inlet velocity Ui = 20 m/s, po = 0.1 MPa, To = 293 K, ko = 0.24 J/kg, /o = 4 mm. The lower and upper boundaries of the computational domain are the symmetry planes...

Table 12.1 Calculated characteristic reaction time tc, residence time tr, and the Mikhelson number Mi for the combustion of the stoichiometric methane-air mixture in a combustor with the open-edge V-gutter flame holder of height H and apex angle 60° at the mean inlet velocity Uin. Also presented is the maximum approach-stream velocity Um- Signs and correspond to stabilized flame and unstable flame, respectively... 

In beds of both coarse and fine solids one may observe a somewhat different solid distribution with height—a distinct difference between dense and lean regions and a sharp dense phase surface, as shown in Fig. 20.14. This behavior is more typical of fluidized combustors, not catalytic reaction systems. 

Example 5.4 Design a geometrically similar laboratory-scale cold model fluidized bed to simulate the hydrodynamics of a large-scale fluidized bed combustor. Also specify the operating conditions for the cold model. The combustor is a square cross section column with a width of 1.0 m and a height of 6 m. The fluidized bed combustor is operated at a temperature of 1,150 K, a superficial gas velocity of 1.01 m/s, and a bed height of 1.06 m. Particles with a density of2,630 kg/m3 and a diameter of677ptm are used for the combustor. The cold model is operated at a temperature of 300 K. Air is used for both the cold model and hot model fluidized beds. The physical properties of air are... 

The combustor was composed of three fusion-bonded silicon wafers. Hydrogen was added to the air flow by 76 injector holes of 30 pm diameter. The mixture then entered the annular-shaped combustion chamber through 24 combustor inlet ports of340 pm diameter and left the device through a circular exhaust. The dimensions of the combustion chamber were 5 and 10 mm diameter at a height of 1 mm, which corresponds to a volume of 66 mm3. The fabrication of the device was performed by dry isotropic and anisotropic etching (Figure 2.42). 

Based on the two-phase model for axial gas-solid distribution, the steady operation of the combustor can be described by using N compartments in series. Therefore, for the ith compartment with height Az at combustor height z, according to mass, momentum and heat conservation, a set of operation equations can be established as follows. 

Figure 36 is an illustration of a CFBC boiler retrofitted from a stoker-fired boiler. Also, at the bottom is a bubbling bed operated at a higher velocity of 3.5 m/s. A tube bundle is installed at an inclination of 15° in the dense bed, which extends to a height of 1.5 m from the air nozzles. A channel separator is also used, and the separated solid particles are returned through an air seal to the combustor. 

The mathematical model for char combustion described in the previous two sections is applicable to a bed of constant volume, i.e., to a fluidized bed of fixed height, Hq, and having a constant cross-sectional area, Aq. The constant bed height is maintained by an overflow pipe. For this type of combustor operating for a given feed rate of char and limestone particles of known size distributions, the model presented here can predict the following ... 

The most commonly encountered height for industrial CFB combustors is 20 m. For one pass, the fraction of devolatilisation is given in Table S. 

The combustor is cylindrical with a 1.14 m internal diameter in the fluidised bed region, 1,0 m surface area and a 4 m height. It consist of ... 

This formula was verified in the tube combustor by experimentally determining the ultimate gain of a linear phase-shift controller to be 4.2 and then using a pulsed controller with the same phase shift and various pulse heights. By observing the amplitude of the ultimate limit cycle, the ultimate gain iuit can be computed at each pulse height. The plot in Fig. 18.2 shows that this computation yields the expected value of 4.2. 

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