COMPUTATIONAL FLUID DYNAMICS (CFD) FOR THE OPTIMIZATION OF PRODUCTS AND PROCESSES

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1 THE INTERNATIONAL CONFERENCE OF THE CARPATHIAN EURO-REGION SPECIALISTS IN INDUSTRIAL SYSTEMS 7 th EDITION COMPUTATIONAL FLUID DYNAMICS (CFD) FOR THE OPTIMIZATION OF PRODUCTS AND PROCESSES Franz, Haas DI Dr. techn., University of Applied Science CAMPUS 02, 8021 Graz, Austria Manfred, Pauritsch DI Dr. techn., University of Applied Science CAMPUS 02, 8021 Graz, Austria Matthias Knabl DI (FH), INTECO GmbH, 8600 Bruck a. d. Mur, Austria Abstract: At first the principles of flow simulation are described and a new conceptual design to carry out projects using Computational Fluid Dynamics (CFD) is introduced. Two project examples in the fields of mechanical and electrical engineering show the various possibilities of application and the benefit of CFDcalculations. The first example is the study of a copper profile, which is used in ESR (Electro Slag Remelting) plants. The elimination of a water cooling system leads to significantly lower production costs of the conductor parts. Another project shows the CFD-analysis of an electronic housing to optimize the internal flow and to avoid the damage of electronic systems as a result of a wrong cooling concept. Key words: Computational Fluid Dynamics, Simulation, Electro Slag Remelting, Electronic Cooling 1. FUNDAMENTALS The abbreviation CFD (Computational Fluid Dynamics) is used for special finite element software, which analyses flowing media (liquids or gases). CFD-simulations yield to the main fluid parameters (e.g. pressure, temperature, flow velocity, mass flow). A new aspect is to combine CFD with process simulation for chemical and biochemical processes. 1 In comparison to the static FEM (finite element method) the flow simulation process is more complex und needs higher computer performance. In addition basic knowledge in the field of fluid mechanics is necessary for the correct interpretation of the results. The user must be familiar with the three basic types of heat transfer, thermal conduction, convection and radiation. System simplifications and the consideration of symmetries are important factors to reduce computing time. The CFD-packages usually comprise three distinct elements, namely preprocessing (geometry specification, model selection, parameter specification and grid generation), numerical solution procedure and postprocessing (visualization and data treatment). 1 At first a new conceptual design will be introduced to improve the transfer process from the real problem to the simulation model. 1 Lim, Young-il/Jorgensen, Sten Bay (2006): Distributed Dynamic Models and Computational Fluid Dynamics, WILEY-VCH Verlag, Weinheim, p

2 1.1 Preprocessing The operation preprocessing provides the solver with all necessary information for the calculation. The figure 1 shows as a practical approach an overview of all decisions and definitions which are essential in this phase. In the first step called structure idealization it must be selected, whether it is an internal (e.g. crankcase of a motor with cooling channels) or external flow (e.g. windmill rotor). In the case of an internal flow the inlet and outlet openings have to be closed by so called lids. These parts must be added to the existing assembly and boundary conditions should be defined at their inner surfaces. The external flow volume, also called Computational Domain, is used for grid generation and influences both the result quality and the computing time. Structure idealization Flow condition Internal flow External flow Medium Gas Liquid Steam Consideration of thermal conduction and radiation Consideration of time Compressibility Gravity Turbulence Rotation Fig. 1: Preprocessing of CFD-simulations Typical flow systems have a complex geometry, furthermore the fluid also influences the whole system in different ways. For example forces and moments are generated within the structure as a result of deflections and the fluid pressure. The fluid temperature also causes a higher material temperature and as a consequence thermal deformations of the concerned parts. If it is necessary to consider the heat transfer in solids, radiation and the dependency of time, the according parameters have to be defined. Similar to all simulators the quality of results is very much depending on the accuracy of the material parameters. Concerning the fluid type liquids, gases and steam have to be distinguished. Of special interest is the Newtonian viscosity and the possible consideration of compressible fluids. The correct determination of boundary conditions provides the integration of the simulation model into the environment. Pressure and velocitiy define a given flow condition at the open components in the case of an internal flow. Thermal conditions can be applied to faces of given temperature or heat generation.

3 Meshing is the last preprocessing activity, whereas the number and the distribution of the elements have a major impact on the result precision. Since the mesh density must be as fine as possible in regions of high temperature or pressure, the software allow the user to control the meshing. This may be carried out globally by choosing a certain element size or with local mesh refinements. 2 Boundary conditions Flow Boundary conditions Thermal Geometry Symmetry Heat source Lids for open faces Computational Domain Physical Values Pressure Temperature Thermal output Temperature Velocity 1.2 Numerical solution Meshing Mesh refinement Fig. 2: Definition blocks for boundary conditions Physical principles are often formulated in terms of partial differential equations. Solutions of these equations depend on the boundary conditions. For instance the flow of incompressible fluids is described by the continuity equation and the Navier-Stokes equation for constant viscosity. Direct solutions exist only for simple geometrical shapes, so numerical solutions are sought in engineering. 3 To prepare the calculation with the CFD-software FloWorks values of special interest are defined as goals which variation during the iteration will be controlled permanently. The calculation is finished when there is no significant difference of all goal values from one iteration step to the next. Results of computational mechanics simulations include errors that are introduced at three stages: 3 Creating mathematical models of real structures Representing mathematical models using numerical models Simulating numerical models on computers 2 Fröhlich, Peter (2005): FEM-Anwendungspraxis, Vieweg Verlag, Wiesbaden, p Raphael, B., Smith, I.F.C. (2003): Fundamentals of Computer Aided Engineering, WILEY-VCH Verlag, p

4 1.3 Postprocessing After the calculation has been finished the results are illustrated in many different ways. Cut plots, surface plots and the visualization of trajectories are some examples to interpret the results. It is also possible to generate a report automatically which includes all information about the analysing task, the chosen parameters and the computer hardware configuration. In this way the documentation work can be minimized and the comparison of FE studies is simplified. 2. CONDUCTOR FOR ELECTRO SLAG REMELTING PLANT The intention of one research project at CAMPUS 02 is the optimization of the conductor profiles, which supply electro slag remelting plants (ESR) with currents of about 18 ka. To produce steel of highest quality the original steel block is being remelted by the use of electric current and a liquid slag operating as an ohmic resistance. 4 Figure 3 shows an ESR plant that was developed and built by INTECO, the global market leader in this field. Fig. 3: ESU-plant Breitenfeld/Mitterdorf, Austria 4 The conductors are made of water cooled copper profiles. The thermal energy of the water is delivered to cooling towers outside the factory floor without any utilization. The simulation task is to find out, if it is possible to replace the water cooling system by aluminium made cooling elements, which reduce the temperature by the air flow. This method is well known in the field of electronics and the required dimensions, the number and the allocation of the elements should be found out. A further advantage of this system is the reduction of the production costs by eliminating expensive shaping and welding processes that are necessary to manufacture the cooling channels. 4 Knabl, Matthias (2007): Projektbeschreibung Hochstromschiene, Inteco GmbH

5 2.1 Simulation studies This new cooling concept has been simulated as a model with the whole profile length of 3 meters and in addition only as a section of 500mm in length to reduce computing time. All three types of thermal transfer (conduction in solids, convection and radiation) have to be considered, the time dependent calculation is not necessary because of the stationary operating mode. The maximum number of cooling elements and one thermal relief can be seen in figure 4. As the distance between the cooling fins is only a few millimetres the mesh must be very fine. 2.2 Results Fig. 4: Air temperature distribution of the new cooling concept The cut plot of the air temperature (see figure 4) shows which regions are influenced by the thermal load. The figure 5 gives an overview about the results by varying the air flow velocities. If the velocity increases from 0 to 1m/s, a temperature drop of about 10 C will be achieved. This decrease is the same both in the case of the profile without any cooling activity and with the mounted cooling parts. The effect of the new cooling system is very successful because the temperature reduction is about 84 C. This leads to conductor temperatures of 40 C to 50 C, which are significant lower than the limit value of 70 C. 140,00 120,00 temperature [ C] 100,00 80,00 60,00 temperature without cooling parts temperatue with cooling parts 40,00 20,00 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 flow velocity [m/s] Fig. 5: Maximum air temperature by varying the flow velocity

6 3. ELECTRONIC APPLICATION All new developements in the communication technology are charcterized by the fact that increasing power rates are placed at the smallest possible volumes. So it is possible that fast CPU-systems are destroyed by their own generation of heat. In order to make the cooling components smaller you have to change the medium (e.g. water) or optimize the existing thermal concept. Therefore CFD-simulations can help to make the design of electronic cooling systems more efficient. Figure 6 shows the PCB of an actual CAMPUS 02 project, where power dissipation up to 10W occur. Due to simulation results the thermal concept includes one fan and eight holes to minimize the internal temperature. Figure 7 shows the typical air flow at 7W power dissipation and 20 C room temperature. The simulation results could also be verified by laboratory measurements. Fig. 6: Printed Circuit Board (PCB) Fig. 7: Trajectory plot of air velocity 4. CONCLUSION Computational Fluid Dynamics is a very useful method of optimizing thermal management in a variety of applications. This paper shows practical examples in the field of mechanics an electronics where significant improvements could be achieved by the use of commercial CFD-software. The effort in using any flow simulation software can be minimized if you follow the preprocessing concept that is presented in this paper. 5. REFERENCES Fröhlich, Peter (2005): FEM-Anwendungspraxis, Vieweg Verlag, Wiesbaden, p. 51 Knabl, Matthias (2007): Projektbeschreibung Hochstromschiene, Inteco GmbH Lim, Young-il/Jorgensen, Sten Bay (2006): Distributed Dynamic Models and Computational Fluid Dynamics, WILEY-VCH Verlag, Weinheim, p Raphael, B., Smith, I.F.C. (2003): Fundamentals of Computer Aided Engineering, WILEY-VCH Verlag, p