Structural behavior and optimization of a turbofan engine fan

Structural behavior investigation of a turbofan engine fan and its optimization named thesis is based on optimization of a turbofan engine fan. Optimization is used for finding the best design of a system or design. The best design is named optimum design and it is founded or calculated by optimization methods. CFM 56-7 engine fan was been taken as an example for this optimization study. This engine is commonly used for civil aviation industry. Reducing the weight of an aircraft is so important for aviation industry because weight effect on stability, fuel consumption and structure. For this reason, it has been worked to increase the performance of an aircraft for decades and also will be done for decades. Thus, optimization has a big affect in engineering. It is needed to have an objective for optimization at least. Weight of a system, cost, time and performance can be chosen as an objective for optimization. In this study, mass was been chosen as objective function and it was been minimized. Fan rotates with 4200 rpm and it must be suitable for working condition. Thus, structural behavior of fan investigated for suitability. Fan was designed to have a safety factor of 1.1 and because of this reason stresses were taken as lower than yield stress by 1.1 safety factor. This stress criteria was been chosen as an constrain function for optimization. Another constrain function is blade tip elongation on radial direction and it is limited by a value of 30 mm. Blade tip elongations must be smaller than 30 mm. As a result of this, mass was been identified as objective function. And also stresses and blade tip elongations was been identified as constrain functions. The fan material was been selected as one of the Al 7050-T7651 and Ti-6Al-4V. Al 7050-T7651 is an aluminum based material. And Ti-6Al-4V is an titanium based material. Both these materials are commonly used in aviation industry. These materials density is low and also their strength is high contrast with their density. It was seen that Al 7050-T7651 material is more suitable than Ti-6Al-4V because it gave the best optimization results. All the structural convenience analysis done for Al 7050-T7651 material fan. Fan has 22 blades and its diameter is 1.55 m. Fan designed parametrically for optimization. This made because of finding new designs. Blade profiles and fan disk dimensions changed to get new design iterations of optimization. Static structural analysis of fan was done during optimization and ANSYS was used for this analysis. For the boundary conditions, fan disk hole was fitted by let the rotating motion on mil axis. Fan has a rotating speed of 4200 rpm for loading condition. Also a pressure was given on blades impression surfaces for loading. Pressure calculation was done by turbo machine formulas. Tetrahedral elements were used for fan mesh. Fan consists of 230520 elements and these elements quality is about 0.82. This is accepted as a good quality for ANSYS static structural analysis. Before optimization, beginning design was controlled if it ensures the constraints of the optimization. The beginning design ensures the stress and blade tip elongations constraints for Al 7050-T7651 material. However, blade tip elongations were bigger then 30 mm for Ti-6Al-4V material fan. Dimensions of fan disk and fan blade profiles were changed because Ti-6Al-4V material fan must be ensured the blade tip elongation constrain. All these changes were done by modeFRONTIER optimization program automatically. There are 13 parameters for fans disk and fan blades. 5 parameter for blade profiles and 8 for fans disk dimensions. Blade profiles have 4 section planes that were positioned in radial direction. Top and bottom plane profiles didn't change. Other profiles positions were changed in x and z directions. All parameters were identified by dimension limits and changing step in modeFRONTIER. This program changes the parameters in ANSYS automatically by using step and parameters limits. For the optimization, Simplex and MOGA II algorithms were used. Simplex algorithm is used for optimizations that have one objective. MOGA II is used for more objective optimization problems. MOGA II consists of Simplex algorithm and Pareto elitism theory. In fan optimization study, it was investigated which algorithm gives better results for optimization. In this study, 32 DOE (Design of Experiments) are identified and 150 iterations were done for optimization. Behavior of Simplex and MOGA II algorithms were also investigated in this study. Beginning design mass was obtained as 165.5 kg for Al 7050-T7651 material fan and 276.0 kg for Ti-6Al-4V material fan. Al 7050-T7651 material fan mass was reduced to 135.5 with Simplex algorithm optimization. Also with Simplex algorithm, the mass of fan made from Ti-6Al-4V was reduced to 235.1 kg. It was seen that the mass of aluminum fan was lower than the mass of titanium fan. Thus, MOGA II optimization was done for the fan of Al 7050-T7651 material and at the result, the mass of fan was obtained as 145.7 kg. The Simplex algorithm optimization gave better results than MOGA II algorithm optimization. Finally, the mass was obtained as 135.5 kg. The material of fan was selected as Al 7050-T7651. The mass of preliminary design was 165.5 kg and it was reduced to 135.5 kg by changing the blade profiles and disk dimensions. The mass was reduced %18.1 with this optimization study. After obtaining the optimum design by the Simplex optimization algorithm, the vibration fatigue analyses of the fan were done by using ANSYS software. The optimum design was controlled for the fatigue life, and vibration characteristics and resonance. The fan rotates with 4200 rpm that has a frequency of nearly 70 Hz. The optimum design is suitable for vibration conditions in the service. The life curves of Al 7050-T7651 were used for confirming the life analyses results. The optimum fan is also suitable for life constraints. As a result, an optimum design that was %18.14 lighter than preliminary design was obtained by this optimization study. Fuel consumption of aircraft will been reduced in this way. Reducing the fuel consumption will reduce the unhealthy gases from the environment. For this reason, this study will be helpful for protecting the nature of earth.