This paper introduces in detail the effect of changing the cooling water supply direction on the cooling water temperature field and the copper wall temperature field. The simulation results can accurately express the real heat transfer conditions of the mold, and provide a reference for optimizing the mold water gap design and cooling process parameters.
Key words: mold heat transfer; cooling process; numerical simulation
The convective heat transfer behavior between the cooling water and the copper wall is directly affected by the flow velocity and temperature distribution of the cooling water in the water gap. Reasonable water slot width and arrangement form make the cooling water in the water slot get a higher flow velocity. A high-speed turbulent flow is formed in the water gap to ensure a good convective heat transfer state between the cooling water and the copper wall, so that the crystallizer can be effectively cooled, and the temperature of the copper wall is kept lower than the recrystallization temperature to ensure the safe and stable operation of the crystallizer.
Continuous casting crystallizer process structure parameters
Table 1 is the process structure parameters of the continuous casting crystallizer used in the simulation calculation.
| project | value | project | value |
| Mold copper tube material | Phosphorus deoxidized copper(TP2) | cast steel | Q235 |
| Mold copper tube length,mm | 900 | Cooling water inlet temperature,℃ | 35 |
| Copper tube Mold Thickness,mm | 13 | pouring temperature,℃ | 1540 |
| Water seam thickness,mm | 4 | pulling speed,m/min | 3.5 |
| Cooling water flow rate,m/s | 13 |
Establishment of mold copper wall and cooling water model
Using ANSYS-FLOTRAN to carry out numerical simulation analysis on the established finite element model, discuss the influence of the change of water supply direction on the fluid temperature field and the influence on the temperature distribution of copper pipes, and provide a theoretical basis for the actual production process in the future Guidance and Basis References.
Model simplification and assumptions
According to the heat transfer characteristics of the copper wall and cooling water of the continuous casting mold, combined with the previous research experience [1-5], the following assumptions are put forward to simplify the model:
1) Extract the slab heat flow calculated by coupling simulation and apply it to the hot surface of the copper plate of the model;
2) The influence of crystallizer vibration on the solidification and heat transfer of molten steel is not considered;
3) Ignoring the deformation of the mold copper plate, it is in good condition during use;
4) Considering the gravity effect of cooling water, the physical parameters of water are input in the program;
5) Define the outlet pressure boundary condition of the cooling water, set the pressure to 0.3Mpa;
6) The boundary condition of the solid walls on both sides of the water joint is a non-slip wall;
7) The cooling water joints are subjected to grid encryption processing, which is conducive to the convergence of the cooling water model;
8) The crystallizer cooling water is an incompressible fluid, and the flow state is turbulent;
9) The cooling water flows in at the lower end of the water slot and flows out at the upper end, and the cooling water speed and temperature are applied at the inlet of the water slot.
Two-dimensional finite element model of copper wall cooling water
According to the characteristics of heat transfer between the copper wall of the continuous casting mold and the cooling water, the model is simplified and assumed, and the two-dimensional mathematical model of the mold and the cooling water is established by using the two-dimensional longitudinal slice method.
To mesh the model, select the FLUID141 unit, set the field number to 1, and divide the grid of the cooling water area; then select the PLANE55 unit, set the field number to 2, and continue to mesh the solid area, using MAPPED way to divide the grid.
The established grid model is shown in Figure 1. The grid density of the cooling water slot is greater than that of the copper wall of the mold. It is beneficial for model analysis to simulate the final convergence of cooling water. And it can be seen that in the structure of the crystallizer, the height of the copper wall of the crystallizer is higher than the height of the cooling water gap, and part of the uppermost and lowermost ends of the copper wall are not directly cooled by the cooling water.
Fig.1 Two-dimensional finite element model
Analysis method of the model
In this study, FLOTRAN is used to complete the flow-thermal steady-state bidirectional coupling analysis of the mold copper plate and cooling water. In ANSYS-FLOTRAN, it is stipulated that the material number of the cooling water fluid area is 1, and the material number is 2 is the crystallizer copper plate. The cooling water and the crystallizer copper plate are distinguished by different material numbers, and the indirect coupling method is used for flow-heat two-way Coupling analysis.
Initial and boundary conditions
The density of the crystallizer copper plate is 8900kg/m3, the thermal conductivity is 390w/(m/℃), and the specific heat capacity is 380J/(kg·K).
Since the crystallizer cooling water flows fast in the cooling water channel and the water channel is narrow and long, it can be considered that the cooling water inside it is in a state of incompressible turbulent flow. The flow of water is considered as a forced convection heat transfer process.
Define the boundary conditions of the cooling water. At the lower inlet of the cooling water finite element model, define the initial velocity and temperature of the cooling water. The water velocity is 10m/s, and the water temperature is 308k. At the upper outlet of the cooling water finite element model, define For the outlet pressure boundary condition of the cooling water, set the pressure to 0.3MPa; set the other solid wall boundary conditions as no-slip walls. The standard k-ε equation model is selected for solution.
Open the temperature control equation option, and set the flow state of the fluid to turbulent flow, and keep the others as default. Next, set the fluid attribute parameters, and input the physical parameters of the cooling water into the computer in the form of a table, and the cooling water should be considered Vertically downward gravity, set the gravity to 9.8, then set the cooling water to solve the stability parameters, and set the artificial damping to 0.2.
Set the fluid property parameters, add the viscosity and thermal conductivity of the cooling water at different temperatures to the command of the program with the MP command, and the vertical gravity of the cooling water should be considered, set the gravity to 9.8, and then set the cooling water to solve the stability parameter, set the artificial damping to 0.23.
The effect of changing the direction of cooling water entering the water on the heat transfer of the crystallizer
In the actual production of the steel plant, the way in which cooling water flows into the cooling water slot of the crystallizer is usually from the lower end of the mold and flows out from the upper end of the cooling water slot of the mold. Change to upper water supply, which will affect the heat transfer of the crystallizer.
Effect of water supply direction change on temperature field of copper wall
When the water inlet direction in the water gap is changed, the temperature field of the copper wall of the crystallizer is shown in Figure 2.
Fig.2 Temperature field of mold copper wall when water supply direction is different
The influence of the change of water supply direction on the change law of copper wall temperature
When changing the direction of cooling water entering the water, the temperature change law of the hot surface and cold surface of the copper wall of the crystallizer is shown in Figure 3.
Fig.3 Temperature distribution of copper wall under different water supply directions
It can be seen from the figure that when the cooling water is changed from forward water supply to reverse water supply, the temperature of the upper part of the crystallizer copper plate, the hot surface and the cold surface have decreased to varying degrees. When the water is supplied in the forward direction, the highest temperature of the copper wall is 154.5°C in the area about 50mm below the meniscus of the hot surface, the lowest temperature is located in the contact area between the inlet of the cooling water seam and the copper wall, the inlet water temperature is 35°C, and the temperature of the upper part of the copper plate is 46.5°C; When the water is supplied in reverse, the temperature of the meniscus region of the copper wall is 146.2°C, and the temperature of the upper part of the copper plate is the lowest at 35.3°C.
The influence of the change of water supply direction on the change law of cooling water temperature
When the water supply direction changes, the temperature distribution at the upper end of the cooling water joint is shown in Figure 4.
Fig.4 Temperature distribution at the upper end of the cooling water slot
It can be seen from the figure that when the water is supplied in the reverse direction, the upper end of the water slot is the inlet water temperature of the cooling water slot at 35°C. The water temperature is 42.8°C.
Figure 5 shows the temperature change curve at the lower end of the cooling water slot when the water supply direction is different.
Fig.5 Temperature distribution at the lower end of the cooling water slot
It can be seen from the figure that when the cooling water enters from the bottom and exits from the top, the water temperature at the lower end of the water slot is 35°C; when the cooling water enters from the top and exits from the bottom, the average temperature at the lower end of the water slot is 43.7°C.
Variation law of cooling water speed in reverse water supply
Fig. 6 is the speed change curve of the middle position of the water seam along the longitudinal direction of the water seam.
Fig.6 Longitudinal velocity distribution in the middle of the water seam
It can be seen from the figure that the initial velocity of the cooling water is 13m/s at the upper end of the water slit, and the speed increases rapidly along the direction of water flow, and the water velocity reaches the maximum at 14.8m/s at a distance of 129mm from the upper end of the water slit , and then the velocity decreases along the direction of water flow, and at the lower end of the water gap, the velocity is 14.2m/s.
Conclusion
(1) When the water is supplied in the forward direction, the highest temperature point of the copper wall is near the meniscus, and the lowest temperature point is located at the position where the cold surface of the copper wall contacts the lower inlet of the water gap; when the water is supplied in reverse, the highest temperature point of the copper wall is at the copper wall At the position of the card slot at the lower end, the lowest temperature point is at the top of the copper wall.
(2) When the water supply mode is changed from the bottom water supply to the top water supply, the temperature at each position of the copper wall decreases to varying degrees. Under the same process conditions, the temperature at the position 50mm below the meniscus of the copper wall decreases by 8.3°C, and the temperature at the upper end of the copper wall decreases by 11.2°C .
(3) When the water is supplied in the reverse direction, the upper end of the water joint is the inlet water temperature of the cooling water joint at 35°C; when the water is supplied in the forward direction, the upper end of the water joint is the outlet water temperature of the cooling water joint of the crystallizer, the highest temperature is 46.3°C, and the average water temperature is 42.8 ℃.
(4) During reverse water supply, the initial velocity of the cooling water at the upper end of the water slit is 13m/s. Along the direction of water flow, the speed rapidly increases. At 129mm from the upper end of the water slit, the water velocity reaches the maximum at 14.8 m/s, and then the speed along the water flow direction decreases, and at the lower end of the water gap, the speed is 14.2m/s.
