| As the molten steel flow in the thin slab continuous casting mold at high casting speed is in the intense unsteady turbulence. The velocity distribution of molten steel contained within the solidifying shell of a continuous casting machine is very influential on the distribution of inclusion particles, which is important to the internal cleanliness and quality of the steel. Understanding the unsteady flow structures in this process is an important step in avoiding failures and decreasing defects.The Reynolds-averaged two-equation turbulence model, has been extensively adopted in previous studies. However, the approach, limited by its nature, is not suited for studying the time evolution of unsteady flow structures triggered by flow instabilities. It is possible, in theory, to directly resolve the whole spectrum of turbulent scales using an approach known as direct numerical simulation(DNS). However, DNS is not feasible for practical engineering problems involving high Reynolds number flows. The cost required for DNS to resolve the entire range of scales is prohibitive. In large eddy simulation(LES), large eddies are resolved directly, while small eddies are modeled with the sub-grid scale(SGS) stress model. Large eddy simulation(LES) thus falls between DNS and RANS in terms of the fraction of the resolved scales. Currently, LES has been well approved in applying to both homogeneous turbulence and free shear flow.The asymmetrically distribution of defects in the slab has been found in recent flaw detection of slab. However, the symmetrical distribution detected in flow field of the mold of continuous casting calculated by RANS is failed to explain what has shown in flaw detection of slab.The large eddy simulation was used to study the turbulent flow in the mold by some scholars at home and abroad. But due to the computer capacity and lack of understanding of the meet conditions of the large eddy simulation, the actual flow of the molten steel in the mold was not well reflect by the results of their large eddy simulation.This paper takes the lead in using the fine mesh of meeting the requirements of large eddy simulation to simulate the flow in the mold. On the basis, a kind of large eddy simulation model for calculating the unsteady turbulent flow is presented. And this model is verified by the Particle Image Velocimetry(PIV) experimental measurement, Ultrasonic testing results and RANS(k-ε model) results. Characteristics of the unsteady turbulent flow in the thin slab continuous casting mold were obtained by using this model.The velocity distribution results show that the turbulent asymmetric distribution was revealed even the nozzle in the centre position. The turbulent flow is transient and random, and the obvious characteristics of vortex flow are not center symmetrical. And the vortex is located at the lower velocity side adjacent to the SEN. With the development of the unsteady turbulent flow, the unsteady flow of the molten steel in the thin continuous casting mold presented periodic antisymmetry deviation distribution; the period was about20seconds.The formation, development, shedding and fracture process of the large eddy coherent structures are given in the evolution of vortex flow. Much biger vorticity gradient is found between the coherent vortex and the flow of out of turbulent region. And there is strong vortex diffusion which makes the molten steel of out of turbulent region flow into turbulent development process.The influence of casting speed on the flow of the molten steel in the mold shows that the most suitable casting speed should be about5.5m/min. Both too high and too low of casting speed are detrimental to the improvement of the quality of slab.With Reynolds-averaged simulation(RANS) contrast, the transient turbulence flow in the mold which is got from the large eddy simulation is asymmetric. And more random vortexes are captured and more transient flow details are forecasted. The essence of turbulence is deeply understanded from the large eddy simulation. |
PDF Full Text Request