Advanced methodology of determination water jet cooling intensity during the casting process

Companies which produce aluminum alloy ingots seek a final product without structural defects. One crucial factor is the cooling during the semi continual casting process. In the beginning of the process, most cracks are made with lengths up to 300 mm, and then, by selecting a suitable method of water cooling, the cracks are closed. A major influence on defect generation is the superheat extraction from the incoming liquid metal by the secondary water-cooling system due to direct water impingement on the ingot surface. The temperature distribution during the casting process can be simulated numerically with known boundary conditions (cooling intensity along the surface). Boundary conditions are obtained by experimental investigation and subsequent evaluation. A special experimental device was designed for measurement. The device’s main function is to ensure that the position of the mold and the sample during measurement is as it would be during the real casting process. The aluminum sample was equipped with a set of thermocouples placed along the cooling surface. The hot vertical surface was cooled down during the experiments by a flat water jet. The impact area is located in the upper part of the cooling surface. The rest of surface is cooled by water flow down along the surface. This article deals with the evaluation of this type of experiment. The boundary conditions (heat transfer coefficients) are estimated as a function of temperature and vertical position. Unfortunately, the results obtained by standard methods for solving the inverse heat conduction problem (for example, using the 2D sequential function specification method) are blurred. This is caused by the Leidenfrost effect and this special type of cooling. A sharp border between the transient and film boiling modes is created and moves down during the experiment. This article illustrates an applicable solution based on shifting computation element borders during the inverse computations. The method was tested on measured data. INTRODUCTION Various simulations for thermal processes such as cooling and hardening are commonly used. The temperature distribution or gradient history inside the material are used in order to determine the influence on the final structure, residual stress and potential for defect formation. Numerical simulations are based on solving the direct heat conduction problem using the finite element [1], difference [2] or volume [3] methods. These methods are well known and they are included in the solvers of all standard commercial software, including ANSYS and COMSOL. For each thermal simulation it is necessary to know the following inputs: Geometry Material properties – thermal conductivity, density and specific heat capacity Initial conditions – initial temperature distribution Boundary conditions – heat flux or heat transfer coefficients Most of these points are not difficult to obtain. For example, the geometry is usually determined by assignment of a studied problem. Material properties can be obtained from material databases or can be measured using standardized measuring equipment. The initial condition is usually reduced to a homogenous temperature. The last point is much more complicated. Boundary conditions can be express using empirical formulas [4], [5] in some trivial cases simply with geometry, and a special type of cooling with a short temperature range. However, common industrial applications such as spray cooling are not trivial so it is necessary to obtain boundary conditions by measurement. Cooling experiments are usually designed to be transient. A test sample with built in thermocouples is heated to an initial temperature. Then, the temperature history is recorded during the cooling process. Boundary conditions can be evaluated using the Inverse Heat Conduction Problem (IHCP) from measured temperatures. This article deals with the 2D IHCP for a highly heatconductive sample made from aluminum. The sample was cooled using a flat water jet in the impact area and by water flowing along the surface below. This configuration is common for the continuous casting of aluminum. Solving the IHCP is made more difficult by the Leidenfrost effect combined with a special type of cooling conditions. DESCRIPTION OF EXPERIMENTS Experimental measurements were designed to reproduce real aluminium casting conditions with a realistic sample material, temperature range, water flow rate and cooling 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics