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Rules of Thumb Mechanical Engineers P2

For many problems, 2D or axisymmetric analysis is used. This may require adjusting the heat transfer coefficients. Consider the bolt hole in Figure 13. The total surface area of the bolt hole is nDL, but in the finite element model, the surface area is only DL. In FEA, it is important the total hA product is correct. | 30 Rules of Thumb for Mechanical Engineers 2D Analysis For many problems 2D or axisymmetric analysis is used. This may require adjusting the heat transfer coefficients. Consider the bolt hole in Figure 13. The total surface area of the bolt hole is TtDL but in the finite element model the surface area is only DL. In FEA it is important the total hA product is correct. Therefore the heat transfer coefficient should be multiplied by 7t. Similarly for transient analysis it is necessary to model the proper mass. If the wrong mass is modeled the component will react too quickly too little mass or too slowly too much mass during a transient. The user should keep in mind the limitations of 2D FEA. Consider the turbine wheel in Figure 14. The wheel is a solid of revolution with 40 discontinuous blades attached to it. These blades absorb heat from the hot gases coming out of the combuster and conduct it down into the wheel. 2D FEA assumes that temperature does not vary in the tan- Figure 13. Convection coefficients must be adjusted for holes in 2D finite element models. gential direction. In reality the portions of the wheel directly under the blades will be hotter than those portions between the blades. Therefore Location A will be hotter than Location B. Location A will also respond more quickly during a transient. If accurate temperatures in this region are desired then 3D FEA is required. If the analyst is only interested in accurate bore temperatures then 2D analysis should be adequate for this problem. Blades Figure 14.2D finite element models cannot account for variation in the third dimension. Point A will actually be hotter than point B due to conduction from the blades. Transient Analysis Transient FEA has an added degree of difficulty because boundary conditions vary with time. Often this can be accomplished by scaling boundary temperatures and convection coefficients. Consider the problem in Figure 15. A plate is exposed to air in a cavity. This cavity is fed by