Modeling of a walking beam furnace using CFD - methods

In steel industry the walking beam furnace is used to heat up and reheat steel slabs for the rolling process. In order to meet the demands for high quality steel a very homogeneous temperature profile in the slabs is required. Using Computational Fluid Dynamics (CFD) in ANSYS Fluent 15, a walking beam furnace was investigated to analyze the causes of inhomogeneous temperature distribution in the steel slabs. Due to different sizes in length scale, starting with burner nozzles with a diameter of about 10 mm to dimensions of the whole oven with over 50 meters, about 220x106 tetrahedron cells were necessary to resolve the geometry. On the basis of this model size it is unavoidable to find strategies to reduce the calculation time and consumption of computational power. Therefore the walking beam furnace was modeled as a steady state process. To avoid high requirements of computational power for solving chemical reactions a steady laminar flamelet model for non-adiabatic combustion was used [1]. For this purpose a flamelet was generated for the specific mixture of natural gas, blast furnace gas and air as oxidizer. In order to avoid the need of a dynamic mesh or a complex movement algorithm for the slab motion and a transient model of the oven the steel girder were mapped as a continuous band. The motion of the band was modeled by a Multi Reference Frame (MRF) with a specific velocity, resulting in a mass flow through the furnace similar to the real one. To describe turbulent effects the realizable k-ε-model was used with one modification. For a more accurate description of the flame acceleration the turbulent constant C2ε was edited according to literature data from 1.9 to 1.8 [2, 3]. For this purpose the Discrete Ordinate Model (DOM) was used in combination with a Weighted Sum of Gray Gases Model (WSGGM) to describe the emissivity and absorption of radiation from CO, CO2, H2O und CxHy [4,5]. To set the boundary conditions for thermal properties of the oven wall a User Defined Function (UDF) was developed and implemented into the ANSYS Fluent 15 calculation [6]. The model should be used to start an improvement process to analyze the impacts of different operating conditions to get a better insight into the process.