Numerical simulation of air flow and distribution in the blast system of blast furnace

A three-dimensional geometric model of the blast system of the 5500 m3 blast furnace was established in this study. The velocity distribution of the blast system was calculated by OpenFOAM, and the velocity distribution at cross sections of the round pipe and the branch pipe was analysed. The main conclusions are as follows: (1) When the hot air enters the branch pipe, the air flow speed gradually increases and reaches its maximum value (approximately 266.7 m/s) at the tuyere outlet. (2) When the hot air enters the round pipe from the main pipe, there is momentum loss. When the hot air further reaches the opposite side of the junction between the round pipe and the main (Tuyere 1#), the speed of the hot air in the round pipe decreases to the minimum. (3) The maximum air flow average speed of the tuyere outlet is located at Tuyeres 20# and 21# at the junction of the main pipe and the round pipe, which can reach 264.5 m/s. The minimum air flow average velocity of the tuyere outlet is located at Tuyeres 19# and 22#, approximately 258.5 m/s.


Introduction
The blast system of the blast furnace has great significance for the efficient and stable operation of the blast furnace, and it mainly consists of a main pipe, a round pipe, a branch pipe, and a tuyere.With the development of large-scale blast furnaces, the number of tuyeres in the blast system is gradually increasing.The number of tuyeres in a 5500 m 3 blast furnace can reach 40.The main function of the blast system of the blast furnace is to blow hot air around 1200℃ into the interior of the blast furnace to provide heat and oxygen.As one of the four major operating systems of the blast furnace, the blast system mainly affects the depth and height of the raceway in the furnace, and directly determines the initial distribution of gas flow in the furnace.Moreover, with the development of large-scale blast furnaces, the circumferential uniform distribution of gas flow is also crucial [1][2][3][4].Therefore, it is necessary to study the uniformity of air flow distribution at each tuyere of the blast system of the blast furnace.
At present, adjusting the diameter and length of the tuyere is the main method to improve the uniformity of air distribution of the tuyere.Due to the characteristics of high speed and high temperature in the blast system of a blast furnace, the method to study the flow and distribution of hot blast mainly depends on numerical simulation.Liu et al. [5] used the CFD method to simulate the air volume at each tuyere of the 2580 m 3 blast furnace in Ansteel.The results showed that under the same tuyere diameter conditions, the air volume distribution at the blast furnace tuyere was not uniform.Liu et al. [6] studied the dual hot air main pipe structure in the blast system of a 3200 m 3 blast furnace and calculated that under the condition of all tuyere diameters of 120 mm, the air volume distribution at each tuyere in the circumferential direction was uneven, with a change in air volume of about 4.4%.Ni et al. [7] took the blast system of a 4000 m 3 blast furnace as the research object and used CFD numerical simulation method to analyse the influence of tuyere layout, tuyere diameter, and total air volume on the pressure and velocity distribution at the tuyere of the blast system of the blast furnace.The results indicate that the maximum velocity of tuyere is generally located at the two branch pipes closest to the main pipe, and the overall tuyere air volume distribution is uneven.
This article takes the blast system of a 5500 m 3 blast furnace as the research object, uses OpenFOAM to calculate the hot air flow field of the blast system, and analyses the current situation of uneven air distribution at each tuyere of the blast system.

Geometry model
Figure 1 shows the three-dimensional geometric model of the blast system of the 5500 m 3 blast furnace, which mainly consists of a hot blast main pipe, a hot blast round pipe, a hot blast branch pipe, and 40 tuyeres.The hot blast generated by the hot blast stove enters the hot blast round pipe from the hot blast main pipe, is distributed to each hot blast branch pipe in a certain proportion, and finally enters the interior of the blast furnace from the tuyere.In order to reduce the influence of the mesh type on the calculation results, all mesh elements are hexahedrons, as shown in Figure 2.

Conservation equations and boundary conditions
Numerical simulations have been performed to examine the velocity profile of the blast system of the blast furnace.The 3D steady-state, incompressible governing equations for the conservation of mass (Equation ( 1)) and momentum (Equation ( 2)) are solved using an open-source available software OpenFOAM.
Where  and  are the mass density (1.27 kg m -3 ) and the dynamic viscosity (1.78×10 -5 Pa s), respectively, of air.In this blast system, the length and diameter of the tuyere are 650 mm and 130 mm, respectively.The inlet of the hot blast main pipe is set as a volumetric flow inlet, 8400 Nm 3 /min, and each tuyere outlet is set as a pressure outlet, 460000 Pa.

Result analysis and discussion
Figure 3 shows the contour of velocity distribution in the blast system of a blast furnace.It can be obtained that the gas flow velocity gradually increases when the hot air enters the branch pipe from the round pipe due to the reduction of the pipe diameter, and the hot air velocity reaches its maximum value at the tuyere outlet.Figure 4 shows the contour of velocity distribution at the cross-section of 1# tuyere.It can be obtained that the air velocity in the round pipe is smaller, which is similar to the velocity of the air inlet.When the hot air enters the branch pipe, the airspeed gradually increases and reaches its maximum value (approximately 266.7 m/s) at the tuyere outlet.Figure 5 shows the velocity vector of air at the cross-section of 1# tuyere.It can be obtained that in the round pipe, the hot air mainly moves along the axis of the round pipe in a circumferential direction.At the junction of the branch pipe and the round pipe, the air flow begins to change direction, moves along the axis of the branch pipe, and finally flows out from the tuyere outlet.Figure 6 shows the contour of velocity in the horizontal cross-section of the blast system.It can be obtained that there is momentum loss after the hot air enters the round pipe from the main pipe, which is due to the angle between the main pipe and the round pipe being close to 90 °.The speed of the hot air decreases to the minimum in the round pipe when the hot air further reaches the opposite side of the junction between the main pipe and the round pipe, and the reason is that the hot air in both directions meets here, causing the air flow speed to decrease to the minimum.Figure 6.The contour of velocity in the horizontal cross-section of the blast system.Figure 7 shows the velocity vector distribution in the horizontal cross-section of the blast system.It can be obtained that the flow direction changes when the hot air enters the round pipe from the main pipe, the speed near the outer side of the round pipe is lower, and the speed near the inner side of the round pipe is higher at the junction of the main pipe and the round pipe.Afterward, the hot air moves along the axis of the round pipe until it meets at the opposite side of the junction of the main pipe and the round pipe, and the speed of the hot air decreases to the minimum.

ICAMIM-2023
Journal of Physics: Conference Series 2720 (2024) 012047 Figure 7.The velocity vector distribution in the horizontal cross-section of the blast system.Figure 8 shows the average velocity distribution of each tuyere.It can be obtained that the maximum average speed of the air outlet is located at the 20# and 21# tuyeres at the junction of the main pipe and the round pipe, which can reach 264.5 m/s.The minimum average velocity of the air outlet is located at the 19# and 22# tuyeres, about 258.5 m/s, which is 6 m/s different from the maximum value.

Conclusions
A three-dimensional geometric model of the blast system of the 5500 m 3 blast furnace was established in this study, the velocity distribution of the blast system was calculated by OpenFOAM, and the velocity distribution at cross sections of the round pipe and the branch pipe was analysed.The main conclusions are as follows: (1) When the hot air enters the branch pipe, the air flow speed gradually increases and reaches its maximum value (approximately 266.7 m/s) at the tuyere outlet.
(2) When the hot air enters the round pipe from the main pipe, there is momentum loss.When the hot air further reaches the opposite side of the junction between the round pipe and the main (Tuyere 1#), the speed of the hot air in the round pipe decreases to the minimum.
(3) The maximum air flow average speed of the tuyere outlet is located at Tuyeres 20# and 21# at the junction of the main pipe and the round pipe, which can reach 264.5 m/s.The minimum air flow average velocity of the tuyere outlet is located at Tuyeres 19# and 22#, approximately 258.5 m/s.

Figure 1 .
Figure 1.3D geometric model of the blast system of the blast furnace.

Figure 2 .
Figure 2. The hexahedral mesh of the blast system.

Figure 3 .
Figure 3.The contour of velocity distribution in the blast system of the blast furnace.Figure4shows the contour of velocity distribution at the cross-section of 1# tuyere.It can be obtained that the air velocity in the round pipe is smaller, which is similar to the velocity of the air inlet.When the hot air enters the branch pipe, the airspeed gradually increases and reaches its maximum value (approximately 266.7 m/s) at the tuyere outlet.

Figure 4 .
Figure 4.The contour of velocity distribution at the cross-section of 1# tuyere.

Figure 5 .
Figure 5.The velocity vector of air at the cross-section of 1# tuyere.Figure6shows the contour of velocity in the horizontal cross-section of the blast system.It can be obtained that there is momentum loss after the hot air enters the round pipe from the main pipe, which is due to the angle between the main pipe and the round pipe being close to 90 °.The speed of the hot air decreases to the minimum in the round pipe when the hot air further reaches the opposite side of the junction between the main pipe and the round pipe, and the reason is that the hot air in both directions meets here, causing the air flow speed to decrease to the minimum.

Figure 8 .
Figure 8.The average velocity distribution of each tuyere.