Nastran Finite Element Analysis and Simulation Software
Heat Transfer Analysis and Simulation
Heat Transfer Problems in Industry
Engineering heat transfer problems occur in a wide range of products and industrial processes ranging from aerospace to consumer products and mechanical, electrical, and chemical processes. For example, in electronic products, it is important to understand thermal behavior from a single chip all the way up to the full configured product. In manufacturing, applications include mapping thermal processes in molding, mixing, curing, and drying. In vehicle design, studies of thermal analysis can be applied to many components from brakes to exhaust systems. The object of this article is to explore the heat transfer analysis and simulation capabilities of NEi Nastran FEA software and the benefits designers and engineers derive from the use of this technology.
Heat Transfer Calculations and the Typical Design Cycle
We can start this discussion by examining the typical design cycle in which the starting point is a concept. From here, the engineer must gather sufficient detail and information so that some basic heat transfer calculations can be made. This phase would involve consideration of the basic heat transfer modes of conduction, convection and radiation and a quantitative assessment of which heat transfer modes need to be considered based on the significance of their contribution. The calculations most likely would involve the classical equations for describing heat transfer and could be simple linear equations or more complex differential equations using a spreadsheet or a math program. From this series of calculations the designer would explore key aspects of the design and vary different parameters, like material selection, or heat transfer coefficients. The design would then proceed to a fabrication and test phase. Here the prototype would be built and instrumented and a test regime thought out and developed that would validate the performance of the device. From the test data, the engineer gains the information needed to judge the appropriateness of his calculations and make design improvements. This process is essentially an iterative one and is repeated until a complex combination of time, project budget, and design objectives are reached.
Benefits FEA Brings to the Traditional Design Cycle
One main benefit FEA software can bring to this traditional design cycle is the ability to cut out many of the iterations requiring physical prototyping and testing which is the most time consuming and expensive phase of the overall design process. Virtual testing with FEA software can wring out many design issues before the first prototype is built. In addition, FEA may have a number of advantages over hand calculations. Hand calculations require many simplifying assumptions that FEA does not, for example, part geometry being a major one. Real world parts often have complex geometry but can be easily represented in an FEA model. Certain other aspects of the problem may be difficult to capture with hand calculations as well. For example, the selected material’s heat transfer properties may be a function of temperature or may not be isotropic i.e. may vary with material orientation. FEA software also injects a powerful visualization component that straight forward calculations cannot match. The representation of this information through contour plots or animations can bring an engineering insight and comprehension not available through any other means. The process can bring with it a virtuous cycle that allows for the exploration of more alternatives, thereby improving the overall quality of the solution and contributing to opportunities for innovation. In summary, the ability to handle complex aspects of the problem, the many options for rich visual display of results, and the quick feedback on the impact of design changes made to FEA models bring to the design process a combination of technical comprehension, speed, and resource savings that are needed to reach high level optimized solutions.
Sample Heat Transfer Problem to Illustrate the Use of FEA Software
To illustrate the actual workings of the heat transfer capabilities of FEA software a sample analysis will be performed on a very specific part, in this case we have chosen an electrical transformer, common in most power supply units. For this problem, we want to know where the highest temperature occurs and what that temperature will be under the loading and specifications imposed. For our demonstration, we will be using Femap®, FEA modeling software to construct the model and to display the results. NEi Nastran FEA software will be used to generate the solution for Femap.
We can construct the FEA model in Femap by either importing it from a 3D CAD system and modifying it as needed, or by creating it from scratch using Femap’s 3D CAD tools. In this case, we have constructed the 3D model of the transformer in SolidWorks CAD software as shown in Figure 1. Next, we imported the model into Femap and performed the steps of defining the materials, the thermal load, and the boundary conditions. For simplicity of the model, we defined just one material property by selecting an Aluminum Alloy from the material library in Femap. Here all the material mechanical and thermal properties needed for analysis are defined. The thermal load for this model was determined by using a 70% efficiency rating for a 400 Watt transformer or 120 Watts (i.e. 30% X 400). The boundary condition would be the ambient air temperature or room temperature of 22oC. We decided to omit the radiation component and consider only conduction and convection because the radiation component will be negligibly small. The appropriate conduction and convection components are factored in by specifying the coefficients for each. In this case, the convection coefficients were chosen as 125 W/m2·K and 20 W/m2·K and the conduction coefficient is loaded with the material properties. Because forced convection is not supported by NEi Nastran, two convection properties are used to roughly model forced convection over the model. The forced convection is needed in this case to simulate a fan that is used to cool this component in a power supply. In the model, surfaces that would be most effected by the fan were selected to have the higher convection coefficient. The other faces were assigned the lower coefficient to model natural convection.
Figure 1: 3D CAD model of an AC-DC transformer is imported into Femap for heat transfer analysis.
Figure 2: The AC-DC transformer model is automatically meshed using solid elements.
Figure 3: Contour plot of the temperature distribution found by NEi Nastran in the heat transfer analysis.
The next step is to assign FEA element properties to this transformer. The choice of element property determines how the software will mesh the structure and hence handle the equations for each of these “finite elements” and solve the heat transfer equations. This is the fundamental nature of the finite element method and where the name is derived from. In essence, it is necessary to get uniform, well-shaped and behaved elements in the meshing process to get accurate results. Transitions should be smooth and gradual without sliver or distorted elements. For our model, a solid element is the most appropriate to use and provides a high quality mesh as shown in Figure 2. After the model has been meshed, an analysis can be set up so that the model is ready to be analyzed.
For the actual analysis, Femap writes a Nastran bulk data file (.nas) and that new file is opened and run in NEi Nastran. In this example, the problem was solved in less than two minutes. From there, the results were loaded back into Femap for post-processing purposes. We selected a contour plot that provides a visualization of the temperatures ranges by using different colors — red being the highest and blue the coolest. From the contour plot shown in Figure 3, we can see where the highest temperatures occur. In this plot, the hottest portions are designated by the bright red.
One of the strengths of FEA software is the ability to examine design alternatives. With the problem set up, it is an easy task to explore alternatives. For example, we might want to try providing openings in the mounting board of the transformer to improve convection cooling, or consider different materials.
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