Strength Analysis of the Carbon-Fiber Reinforced Polymer Impeller Based on Fluid Solid Coupling Method

Carbon-fiber reinforced polymer material impeller is designed for the centrifugal pump to deliver corrosive, toxic, and abrasive media in the chemical and pharmaceutical industries. The pressure-velocity coupling fields in the pump are obtained from the CFD simulation. The stress distribution of the impeller couple caused by the flow water pressure and rotation centrifugal force of the blade is analyzed using one-way fluid-solid coupling method. Results show that the strength of the impeller can meet the requirement of the centrifugal pumps, and the largest stress occurred around the blades root on a pressure side of blade surface. Due to the existence of stress concentration at the blades root, the fatigue limit of the impeller would be reduced greatly. In the further structure optimal design, the blade root should be strengthened.


Introduction
In the development of pumps for the corrosive, toxic, and abrasive liquids, it is important to choose suitable highstrength, corrosion resistant materials for the parts in contact with the liquids.However, even the use of high-alloy steels and alloys does not always provide the necessary safe and smooth operation of the pumps under the conditions of the chemical and other industries.Furthermore, pump components of special steels are extremely laborious to fabricate [1].Consequently, studies are being conducted on the design and utilization of new structural materials possessing chemical resistance and good mechanical properties.In the last decades, the plastic and plastic-lined pumps were developed to deliver such media due to their high chemical corrosion resistance [2], whereas the plastic is not suitable for the cases of higher stress.
The most promising material is the fiber reinforced polymer composite.For example, carbon-fiber reinforced polymers (CFRP) are distinguished from other structural plastics by their combination of low density, high modulus of elasticity, high fatigue strength, thermal stability, low friction coefficient, and high wear resistance [3].In this paper, the CFRP impeller as a key component of centrifugal pump is analyzed using one-way fluid-solid coupling method with the purpose of estimating the strength of the impeller.

Numerical Model and Methodology
The short carbon-fiber reinforced epoxy resin (one kind of CFRP) material characteristic parameters at room temperature of the impeller are shown in Table 1.Due to the long and short blades, composite impeller can improve the flow rate and increase the limited blades' correction coefficient [4,5].In this work, the long and short blades composite CFRP impeller is designed according to orthogonal method [6,7] and numerical simulation.Figure 1 shows the structure of the impeller and its finite element analysis model.The impeller's diameter is 0.18 m, and the short and long space twisted blades are equal in number and arrange alternately.
In order to improve the accuracy of analysis, oneway coupled fluid-structure simulation method [8] is used for the strength analysis of the impeller.The steady flow patterns inside a pump are calculated by a CFD software FLUENT with the impeller revolving speed 3600 r/min and  Figure 2 presents the fluid region with boundary conditions of the impeller, volute, and outlet duct.The steady numerical simulation of structure dynamics is performed to predict the averaged stress using the finite element analysis software ANSYS.

Calculation Results and Analyses
The pressure and velocity fields in a coupling are obtained from the CFD simulation.Figure 3 shows the contours of the static pressure distributed in the pump.As expected for a centrifugal pump, the static pressure increases continuously from the inner to the outer region of the impeller due to the impulse of the blades.The high pressure regions are seen close to the outside edge of the blades.Additionally, flow  circulation from the pressure to the suction side of the blades can be observed in the figure.This circulation induces a small low pressure region near the trailing edge of the blades (pressure side).
The pressure distributions on the blade surfaces are transferred to the surface of the solid blade mesh, as shown in Figure 4; those are treated as the load conditions acting of the blades.Similar to the pressure distribution in the pump, the pressure on the blades increases continuously from the inner to the outer region, and there is a complicated pressure distribution on the surface of the blade due to the complex of flow.
Figure 5 shows the equivalent stress distribution of the impeller couple caused by the flow water pressure and rotation centrifugal force of the blade.It can be seen that there are certain stress concentrations at the root of some blades.The maximum stress is 23.6 MPa at the root, which is the most adverse situation.As presented in Table 1, the minimum strength of CFRP is 536 Mpa; it is clearly indicated that the CFRP impeller can meet the of the centrifugal pump.Nevertheless, due to the existence of stress concentration at the root, the fatigue limit of the blade would be reduced greatly which leads to the destruction under the action of the alternating stress caused by the rotation of the blade.So, in the further mechanical optimal design, the blade root should be strengthened.

Conclusions
The strength of the CFRP impeller is analyzed using one-way fluid-solid coupling method.The result indicates that (1) the pressure increases continuously from the inner to the outer region of the impeller due to the impulse of the blades.The high pressure regions are seen close to the outside edge of the blades.(2) The largest stress 23.6 MPa occurred around a blade root on a pressure side of blade surface; it indicates that the strength of CFRP impeller can meet the requirement of the centrifugal pump.(3) There is stress concentration at the root of the blade, which would be reducing the fatigue limit greatly.In the further structure optimal design, the blade root should be strengthened.

Figure 2 :
Figure 2: The fluid region with boundary conditions of impeller, volute, and outlet duct.

Figure 3 :
Figure 3: The distribution of the static pressure in the pump.

Figure 4 :
Figure 4: Static pressure distribution on the blades of impeller.

Figure 5 :
Figure 5: Equivalent stress distribution of the impeller.

Table 1 :
The CFRP material parameters of the impeller.
Figure 1: The structure and finite element analysis model of the impeller.