Stress Analysis for Various Reactor Blade Diameters of a Mixing Process 1,2 NORASIKIN MAT ISA, 1 NURUL HASRIM MOHD NAZRI, 1,2 AZMAHANI SADIKIN, 1,2 SITI MARIAM BASHARIE 1 Faculty of Mechanical Engineering and Manufacturing UniversitiTun Hussein Onn Malaysia, 86400 Parit Raja, Johor, Malaysia 2 Center for Energy and Industrial Environment Studies (CEIES) UniversitiTun Hussein Onn Malaysia, 86400 Parit Raja, Johor, Malaysia sikin@uthm.edu.my, hasrim@gmail.com, azmah@uthm.edu.my, mariam@uthm.edu.my Abstract:- In this study, the pressure distribution and stress occurred on the 6-blades 45 pitch blade turbines during the mixing process in reactor have been investigated. Three dimensional of computational fluid dynamic have been carried out by using the standard k-ε turbulence model. The reactor used has a cylindrical shape with a diameter T=0.202 m and the liquid height was kept equal to the reactor diameter. The study has been restricted to the turbulence regimes. Analysis concern the effect of three blades diameter at C=T/4 from the bottom of the reactor to the stress distribution. The results showed the smallest diameter installed near the bottom of reactor produced the less stress distribution on the blade surfaces. The simulation model clearly showed the region of highest stress concentration at the blades hub fillet region. While, the critical wall shear of the blades occurred at the blades tips. Increasing the blades diameter resulted the highest stress distribution at the blades surfaces. Keywords:-Stress distribution, reactor, wall shear blades, pressure distribution, CFD 1 Introduction Transesterification or mixing is an important process in biodiesel production. This process is very complex and it produces from convection and turbulent exchanges in a stirred tank reactor[1]. For the mixing performance, to make sure the mixing blade perform in optimum capability,correct location and liquid coverage is essential.incorrect position of mixing blade may hampermixing performance and be detrimental to theperformance life of the mixer drive [2]. In the mixing process, the main characteristic that should to emphasize is the construction of mixing blade. Mixing is the very complex stages in biodiesel production. If the process operates in the unsuitable condition, it will affect the quality of biodiesel. At this stage, the consideration is not only focus on the feedstock purely, but to produce the good quality of biodiesel, the construction of mixing blade in terms of angle, position, diameter and material of the blade will totally effect the quality of biodiesel production. The blade were designed with the objectives of reducing drag at leading edge as well as over the blade, maximizing axial and minimizing radial flow through the impeller, minimizing the extent of the trailing vortices at the tips of the blade and the associated turbulent generation. Every blade design will give an impact to the flow characteristics and stress distribution during working operation [2]. 2 Literature Review During mixing process, the blade is rotating and forces the solid particle to move in circulation flow. Therefore it causedthe stress and force acting to the blade surface. In the working process, the pressure from the circulation blade movement, shaft rotating through the coupling, the rotating motor through the impeller and the force resistant between ISBN: 978-960-474-372-8 216
solid particles to the blade will influence in the several factor which is kinetic energy, potential energy and pressure energy. By considering all factors, it will generate bending and tensile stress on the surface of the blade structure[3]. Stress is synonyms with the resistance that occurs from the force acting at the structure. Geometric discontinuities will create a large variation of stress locally, and at the same time it will produce a significant increase in stress especially at the critical region. The high stress due to the geometric discontinuity is called as stress concentration. For the mixing blade, the stress concentrations which arise in the fillet of a fan blade and it will formulated in 2D dimensions as a problem without body force. Due to the centrifugal force, the only load to consider is a static load because it evenly simulated by a static load and distributed across the shank at reasonable distance outboard of the fillet. Finally, the resistant is resist by two equal static reactions on the blade shoulder [4]. Failure is the common issue in the engineering of structure development. For the general situation, fatigue failure is the structure damage that occurs when the material is subjected to cyclic loading in the continuous working operation. This will occur when the nominal stress exerted on the value of ultimate tensile stress limit will cause the structure failure. During the working operation, the blade suffers by the stress distribution and pressure.as a result, it will induced degradation which may be natural or accelerated due to different causes and increasing the risk of failure occurring [5]. Commonly, fatigue failure occurs due to the several factors such as high mechanical stresses, high thermal stresses and operation environment. For high mechanical stresses, the fatigue will cause due to the centrifugal force, vibratory and flexural stresses. For the high thermal stresses, it depends on the thermal gradients of the stress. While for the operation environment, it will occurs due to the high operating temperature, fuel and air contamination and characteristics of solid particles[6]. In the mixing process, the viscosity and the volume of the fluid will created the resistance for mixing blade to blending the feedstock. Due to the resistance, it will generate high stress acting at the blade surface until it reach at the maximum stress limit and the blade become failure. For the blade failure, there are many different factors will influence the lifetime of the blade. Generally, there are several kinds of failure were found due to the working operation such as broken blades, cracked blades and un-cracked blades. In addition, based on the previous study in the fatigue failure analysis, the most common failure mechanism always occur in blade structure is forced vibrations, those caused by transient operating conditions in the working fluid [7]. 3 Methodology For the method, the main essential aspect that should have to concern is on the modeling of the blade model for biodiesel reactor. The data for this study was simulated by using ANSYS-Fluent software. For the construction of blade model for biodiesel reactor, SolidWorks 2012 software is used. Three impellers diameter was used where D=T/3, D=T/2 and D=3T/4 at clearances from the bottom of C=T/4. A pre-process was used to analyze the stress distribution with a tetrahedral mesh.providing a suitable unstructured mesh with inflation mesh for the geometry is strongly tied to the choice of the turbulence model. For solver setup, Fluent was used to simulate the steady state 3-dimensional turbulence model. The simulation fluid used in this work is a Fatty Acid Methyl Ester (FAME) that had a density of 843 kg/m 3 and viscosity of 0.00272 kg (m s). The blades was rotated with 300 rpm and all solid boundaries with a no-slip condition. Most simulation required about 1000 iteration to convergence. The segregated solver was utilized with the implicit formulation and the relative velocity formulation. The following discretization schemes were employed in the solution process: second-order for pressure, second-order upwind for the momentum equation and SIMPLE for the pressure-velocity coupling. Future studies used the simulation from SolidWorks 2012 to simulate the stress concentration, blades deflection and also factor of safety of the blades. ISBN: 978-960-474-372-8 217
Figure 1: Geometry of the reactor 4 Result and discussion 4.1 Blade pressure From the study,by increasing the diameter of the blade, it was slightly generate the high pressure occurred at the blade surface area. During the working operation condition, the contact between blade and fluid particles will caused the energy was dissipated due to the high friction between two contacting surface. Critical pressure was concentrated at the fillet of the blade as showed in the Figure 2. However, if the diameter of blade is too large, the contactbetween surface area and the fluid become wide and it will produce higher pressure resistance between blade and fluid. Figure 2b and Figure 2c show the pressure distribution on the blade with D=T/2 and D=3T/4. Normally, the small area on the blade hub fillet region was exposed to the maximum pressure during the mixing operation. This condition due to the non-uniform distribution of pressure field at this area. (b) (c) Figure 2: Pressure distribution at blade for condition(a) D=T/3, (b) D=T/2 and (c) D=3T/4 at C=T/4 4.2Wall shear (a) Wall shear was exerted to the blade during the mixing process. In the observation, the highest wall shear also occurs when the blade diameter was increased. These phenomena also due to the increasing of the surface area that affected from the contacting surface. Figure 3 shows the wall shear occurred on the blade with D=3T/4 which created the highest value of wall shear compared to the both diameter of D=T/2 and D=3T/4 installed in the reactor. Result shows that the wall shear occurred at the surface area of the blade and also the shaft. But, the most critical wall shear occurred at the tip of the blade. As known, the blade was contacting with the fluids particle caused the energy dissipated because the blades is not purely elastic. Then, this condition will produced the friction between contacting ISBN: 978-960-474-372-8 218
surface and increased the wall shear values on the blade tip. (a) Figure 3: Wall shear distribution at blade with D=3T/4 4.3 Von-Misses Stress Simulation by using SolidWorks Software 2012 has been done to determine the Von Misses stress of the blades only for three of the blade conditions. During mixing process, the blades was rotated and forces the fluids particles to move in circulation flow. It will gave the stress and force acting to the blades surfaces. From the Figure 4, increasing the blades diameter resulted the increasing of Von Misses stress values. By increasing the blades diameter, the total surfaces area also was increased. Thus, the surfaces area was exposed to the high pressure due to the contacting of the fluids domain with the blades during the mixing process. Besides that, the surfaces area was subjected to a complex loading condition because when installed the blades at the bottom, the blades exposed to the higher load condition since it needed to fluctuate the fluids domain from the bottom region of the reactor to the upper region. (b) (c) Figure 4: Stress distribution on the blades for condition (a) D=T/3, (b) D=T/2 and (c) D=3T/4 at C=T/4 4.4 Blades deflection Blades displaced under a load and created the deflections. The maximum deflections of the blades structure occurred at the same location of the maximum bending moment which is at the blades tips as showed in Figure 5. Increasing the diameter of the blades was increased the wall shear values. ISBN: 978-960-474-372-8 219
Thus, it resulted the increasing of the deflections on the blades tips. By using the smaller blades diameter reduced the possibility of failures because it only exposed to the less deflection on the blades. Thus, these condition was acceptable to apply in the mixing process. Less surfaces area of the blades reduced the loading condition at the bottom of the reactor. The simulation done by SolidWorks proved that the bigger blades not suitable to use in the mixing process because it can made the blades has critical deflection and thus exposed the blades to the failure mechanism. Rotating of the larger blades has the highest possibility in term of failures due to the working operation. (c) Figure 5: Blades deflections for condition (a) D=T/3, (b) D=T/2 and (c) D=3T/4 at C=T/4 5 Conclusion (a) In this study, the mechanical analysis about the stress imposed on blades structures was analyzed by using the Computational Fluid Dynamics (CFD). The analysis was resulted the pressure distribution on the blades surfaces where the critical pressure distribution was occurred at the small area on the blade hub fillet region which in due to the nonuniform distribution of stress field at the region. While, the critical wall shear occurred at the tip of the blade. The simulation by SolidWorks Software approved that the critical stress occurred at the small area on the blade hub fillet region. From the analysis, the condition of blades with D=T/3 was suitable to use during the transesterification process. This is because the conditions not only can reduced the possibility for the blades to failures and then can help the blades has a longer life time, but also can help to improve the efficiency of the mixing operation. (b) References [1] G.R.Kasat, A.R.khopar, V.V.Ranade, A.B.Pandit., CFD Simulation of Liquid, Phase mixing and Liquid stirred reactor. Mumbai : Elsevier Ltd, 2008, Vol. 63. 3877 3885 [2] N.J.Fentimen, N.S.Hill, K.C.Lee, G.R.Paul, M.Yianneskis., A novel Profilled Blade Impeller For Homogenization of Miscible Liquids in Strired Vessels. London : Trans IChemE, 1998, Vol. 76. 0263-8762. ISBN: 978-960-474-372-8 220
[3] L. Baoliang, H. Dan, Z.H. Feng.,Strength Of Model Analysis of Impeller on High Pressure and Small FLow of Centrifugal Pump. China : s.n., 2010. [4] R.R.Babu, K.V.Ramana, K.M.Rao., Ditermination of Stress Concentration Factors of a Steam Turbine Rotor Using Finite Element Analysis. India : Journal of Mechanical Engineering, 2009, Vol. 40. [5] A.Califano, S.Steen., Identification of ventilation regimes of a marine propeller by means of dynamic load analysis. Norway : Elsevier Ltd., 2011, Vol. 38. 1600-1610. [6] Y.jiang, M.C.Wang, G.Zhang, M.Chang., Analysis of Superalloy Turbine Blade Tip Crack During Service. Beijing China : Elsevier Ltd., 2006, Vol. 12. 1429-1436. [7] E.Poursaeidi, M.Salavatian.,Failure Analysis of Generator Rotor Fan Blades. Karaj : Esevier Ltd., 2007, Vol. 14. 851-860. ISBN: 978-960-474-372-8 221