3. CFD Simulation
3.4 Three dimension simulation
The gas flow recirculation was investigated in a three dimension model, which is presented as follows. First, the main differences between the two- and three-dimensional models are compared in Fig. 3.38, they consist mainly of the following two facts:
1. In the 2D model the flow is circulated through one rectangular channel, whose hydraulic diameter is larger; in the 3D model, the flow is circulated through two pipes, whose hydraulic diameters are definitely smaller than that of the 2D model. This results in a difference between pressure drop of the two models.
2. The ventilator power (represented by the pressure jump in the fun function during the simulation) in 2D model works everywhere along kiln length direction (1 m is defaulted by FLUENT), while this power in 3D works only in a pipe cross sectional area. To force the flow recirculation in the whole kiln segment, more ventilator power (e.g. a larger pressure jump of the fun function) is required.
76 D=1m
D=1m D=d
Figure 3.38: Comparison of two- and three-dimensional models
3.4.1 Geometry
As discussed above, a 3D simulation must be carried out. Fig. 3.39 shows the four views of the kiln segment. It illustrates two suction pipes located on the left side of the kiln in the middle along the kiln length direction, and one outlet pipe located in the middle of the right side of the kiln. The considered kiln segment has a length of 2 m, the diameter of the pipe is 0.6 m and the radius of curvature 0.5 m.
Figure 3.39: 3D-views of kiln segment
As in the simulation of the 2D vertical ventilator, distribution walls are installed under the ventilator to ensure the cross sectional flow is homogenous, which cannot be seen from Fig.
3.39. Instead, it is shown in Fig. 3.40.
77
Figure 3.40: Distribution plates in 3D model
For the 3D simulation, tetrahedral cells were used both inside the gaps and at the top and bottom of the kiln. The use of this shape in the 3D simulation provides both time efficiency and accuracy. Fig. 3.41 shows the outlet part of the gap. The growth ratio in the gap width direction is 1.2, and the interval size is 7.5 mm.
Figure 3.41: 3D cells in the gap outlet position
3.4.2 Cross section flow
Fig. 3.42 shows the velocity vector for the middle cross section of the kiln. Like the 2D simulation, it illustrates the flow recirculation in the cross section due to the work of the ventilator. It can be seen that the velocity in the ventilator pipe is much larger, approximately 40 m/s. It also shows the mean velocity inside the gaps is around 11 m/s, while the velocity at the top and bottom of the kiln is approximately 2 m/s.
78
Figure 3.42: Velocity vector in cross section for 3D model
Fig. 3.43 shows the velocity profile inside the gaps. It is clear that the profile is a typical turbulent flow velocity profile. There are some fluctuations on the velocity profile. The reason is that, for 3D simulation, the cells are not fine enough due to the simulation time. It also shows that to reach this velocity, the pressure drop for the 3D model is much larger than that of the 2D internal ventilator, which is shown in Fig. 3.18. For the 2D internal ventilator, the pressure drop is only 126 Pa, while here the pressure drop is 300 Pa.
0 10 20 30 40 50 60
0 5 10 15
Gap position [mm]
w g [m/s]
ϑ = 970°C, ΔP = 300 Pa Re = 8825
Figure 3.43: Velocity vector in cross section with 3D model
Fig. 3.44 shows the mean velocity of each gap. It can be seen that on the left side under the suction, the velocity for each gap is relative homogenous, while on the right side, due to
79
the impulse of the flow after leaving the ventilator, the velocity on the kiln outer side is higher than that of inner side, despite distribution plates installed under the ventilator. However, this velocity profile is better than that in traditional tunnel kiln. In the traditional tunnel kiln, the so-called wall effect - that almost all the gas flow will go through the gap near to the kiln wall - will be improved by the performance of the ventilator and the gas recirculation in the cross section.
0 1 2 3 4 5
-20 -10 0 10 20
Kiln position [m]
w g [m/s]
ϑ = 970°C, ΔP = 300 Pa
Figure 3.44: Velocity profile of each gap with 3D model
3.4.3 Flow along the kiln length direction
The velocity profiles along the kiln length direction are plotted in Fig. 3.45. It shows the velocity profile in the X direction in four gaps. The red and green lines are on the suction side;
the red line is the maximum value, which is the first left point shown in the Fig. 4.44, while the green line is the minimum value, which is the 8th point shown in the same figure. The pink and blue lines are the velocity profiles on the ventilator side; the pink line is the maximum value, which is the sixth point from the right side shown in Fig. 4.44, while the blue line is the minimum value, which is first point shown. It can be seen from Fig. 3.45 that the gas velocity on both sides along the kiln length direction (X direction) are not absolute homogenous. At the middle of the cross section (x = 1 m), the velocity absolute value is higher due to the suction and impulse of the ventilator. It also shows that on the suction side, the velocity difference between the different gaps is smaller than on the ventilator side. This phenomenon has also been shown in Fig. 3.44. This relatively small derivation in the gas velocity distribution can be accepted, and, when compared to the traditional tunnel kiln, this velocity profile is already much improved.
80
0 0.5 1 1.5 2
-20 -15 -10 -5 0 5 10 15
Kiln lenght direction [m]
w g [m/s]
Suction side max Suction side min Ventilator side max Ventilator side min
Figure 3.45: Velocity profile along in the kiln length direction with 3D model
3.4.4 Electrical power consumption
The electrical power consumptions for different temperatures are plotted in Fig. 3.46. It shows that for the 3D model, the electrical power consumption also increases with an increase in the gas mean temperature.
0 200 400 600 800 1000
0 500 1000 1500 2000
Gas mean temperature [°C]
P* el [W/m]
Inter. simulation Inter. fitting curve 2D verti. simulation 2D verti. fitting curve 3D simulation
3D fitting curve
Kiln Position [m]
0 10 20 30 40 50
Figure 3.46: Electrical power requirement for 2D optimal vertical ventilator
For comparison, the electrical power consumptions for the 2D internal ventilator and vertical ventilator are also plotted in Fig. 3.46. As it can be seen, the electrical power consumption for the 3D model is much higher than for the 2D model, due to the model differences discussed at the beginning of section 3.4.
81
The pressure drop coefficients for each term are summarized in table 3.4. It can be seen that the ventilator contributes the most pressure loss for the whole cross section. These values were used in Chapter 2 in the simplified mathematical model.
Table 3.4: Pressure drop coefficients with 3D model
Symbol Explanation Left Right
ξ1 90° turn over 0.1 0
ξ2 inlet 0.5 0.5
dh
⋅H
λ friction in gap 0.57 0.57
ξ3 outlet 1 1
ξ4 90° turn over 0 0.5
Sum cross section 4.74
ξ5 Ventilator 9.05
Total 13.79
The above discussed results are based on the ventilator geometry shown in Fig. 3.38, whose pipe diameter is 0.6 m and the radius of curvature is 0.5 m. However, in the real situation, a different energy-saving geometry of the ventilator can be installed to change the pressure drop coefficient ξ5and, furthermore, change the electrical power consumption.
Therefore, the sum of the fossil energy and primary ventilator power energy Etotal is calculated using ξ5 in the range of 1-9.05 as a parameter, as shown in Fig. 2.27.