Local gas velocity

Consider Fig. 4.32, a graph of flame velocity 5L as a function of distance, for a wave inside a tube. In this case, the flame has entered the tube. The distance from the burner wall is called the penetration distance dv (half the quenching diameter dT). If iij is the mean velocity of the gas flow in the tube and the line labeled (7, is the graph of the velocity profile near the tube wall, the local flame velocity is not greater than the local gas velocity at any point therefore, any flame that finds itself inside the tube will then blow out of the tube. At a lower velocity u2, which is just tangent to the SL curve, a stable point is reached. Then u2 is the minimum mean velocity before flashback occurs. The line for the mean velocity % indicates a region where the flame speed is greater than the velocity in the tube represented by in this case,... 

The combustion air is introduced at two levels, 60 to 70 percent being introduced through the distributor and 30 to 40 percent above the bed. This staged entry results in the lower reaches operating substoichiomet-rically, which helps to reduce NO emissions but tends to reduce the fluidizing velocity at the base of the combustor. To compensate for this and to increase solids mixing by increasing the local gas velocity, the portion of the combustor below the secondary air entry points is tapered. 

Hence, once the local gas velocity and drag coefficient are given, the variation of the particle velocity along the bend can be obtained by numerical integration of Eq. (11.51). 

For gas-solids two-phase flow, owing to the presence of solids, measurement of gas velocity is difficult. Zhang (1990) designed an isokinetic momentum probe to measure gas velocity with reduced experimental error caused by the presence of the solids. Figure 30 shows measured results of local gas velocity under various operating conditions. 

From these diagrams, it can be seen that gas velocity in the fast fluidized bed diminishes along the radial direction from the center to the wall. Local gas velocities in the center are much higher than the corresponding superficial gas velocities, and radial heterogeneity of gas velocity increases with increase in superficial gas velocity and solids circulation rate. The local gas velocity can be estimated by using the following equations ... 

For many cases in drying, the heat-transfer coefficient is proportional to Ug, where Ug is an appropriate local gas velocity. For flow parallel to plane plates, the exponent n has been reported to range From 0.35 to 0.8. The differences in exponent have been attributed to differences in flow pattern in the space above the evaporating surface, particularly whether it is laminar or turbulent, and whether the length is sufficient to allow fully developed flow. In the absence of applicable specific data, the heat-transfer coefficient for the parallel-flow case can be taken, for estimating purposes, as... 

Often coal ash deposit effects are inter-related. For example, slagging will restrict waterwall heat absorption changing the temperature distribution in the boiler which in turn influences the nature and quantity of ash deposition in downstream convective sections. Ash deposits accumulated on convection tubes can reduce the cross-sectional flow area increasing fan requirements and also creating higher local gas velocities which accelerate fly ash erosion. In-situ deposit reactions can produce liquid phase components which are instrumental in tube corrosion. 

Erosion of convective pass tubes, while not a function of deposits, is caused by the abrasive components in flyash, Flyash size and shape, ash particle composition and concentration, and local gas velocities play important roles concerning erosion phenomenon. 

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