Design And Simulation With Cfd And Fea

CHAPTER 6

DESIGN AND SIMULATION WITH

CFD AND FEA

6.1 INTRODUCTION

The modules of AxSTREAM like AxCFD and AxSTRESS tool analyses the turbomachines and overcome the problem of reverse blade engineering and optimized the design of axial flow turbine. A comparison of aerodynamic computational results using CFD analysis, vibration analysis, stress analysis and 3Daerofoil analysis in AxSTREAM with precisely final aerodynamic efficiency estimation of different design is made with CFD simulation.

6.2 FINITE ELEMENT ANALYSIS OF AN AXIAL FLOW TURBINE

In AxSTREAM, the AxSTRESS tool is used to analyze stress, elongation and vibrational analysis of axial flow turbine. AxSTREAM provides structural and modal analysis with embedded FEA tool named AxSTRESS and Campbell diagram calculation. It builds mesh automatically in embedded mesh-generator using selected presets. The AxSTRESS tool analysed and optimized. The stress and vibrational analysis are important and essential to design a high efficient and proper balancing turbomachines. In the gas turbine, the inlet temperature and velocity are high so the centrifugal and thermal stress of blade is very high. To reduce that stress stacking of blade profile is done. By AxSTRESS tool calculating von mises stresses of blade as each stage with frequency analysis.

6.2.1 Stress Analysis of Stator Blade

Like rotor blade, the vibrational stress analysis also carried out in stator blade with the help of AxSTRESS tool of AxSTREAM turbomachinery suite. By this the static and thermodynamic analysis to be visualized and analyzed for high efficient stator blade, that helps the better overall design of an axial flow turbine. The diagram allows us to analyses the critical section of the blade. Fig.6.1 shows the Von mises stress distribution in stator blade by AxSTRESS tool the stress distribution to be optimized for different mode and frequencies.

Fig. 6.1 Von mises stress in stator 1_1

In Fig. 6.1 von mises stress distribution is shown for stator 1_1. The result clearly indicates that the von mises stresses are higher near the leading edge and at the hub section. Exhaust gas force being the driving force, exerts loads on the blade due to gas pressure from the pressure side of the blade profile resulting in bending stresses. This is because of the higher temperature and higher pressure at inlet of the stage. When the pressure and temperature decreases the stresses are also going to decrease as on the next stages.

Table 6.1 shows the values of thermal and static analysis of stator. In short the duty of designer is to keep the maximum value of Von Mises stress induced in the material less than its strength.

Table 6.1: Thermal and Static Analysis

6.2.2 Stress Analysis of Rotor Blade

As rotor is consider in AxSTRESS tool, thermodynamic and kinematics analysis and designing criteria to be included of rotor design. The stress distribution with a different plane of rotor design to be analysed and visualized as well as the overall design analysis of rotor blade is to be carried out. Fig. 6.4 shows the von mises stress distribution in the rotor 2_1.

Fig. 6.4 Von mises stress in Rotor 2_1

In turbine inlet temperature is very high so the thermal stresses value also higher. The vibrational and structural frequencies change take place. To reduces the vibration and stresses in rotor blades the blade tapering and stacking are to be done. AxSTRESS tool give the overall idea of stress and vibration of each stage for axial turbine as thermal and static analysis to be considered.

Table 6.2: Thermal and Static analysis

It is good to analyze and reducing the stress in a particular blade so that the visualization of that blade is essential to optimize stresses and improving overall total-to-total efficiency. The AxSTRESS tool provides the simulation and visualization of stresses and frequencies for turbomachines with different planes, rotations and designing component. An example for this highlights the stress distribution with vibrational analysis of an axial flow turbine.

6.3 CFD ANALYSIS OF AN AXIAL FLOW TURBINE

In CFD analysis various analyses like potential flow path analysis, kinematic flow path analysis, mixing parameter and thermodynamic flow path analysis can be discussed. CFD investigations are used to clarify the characteristics of fluid flow in detail that are otherwise invisible to experimental measurements. 

6.3.1 3D Flow Calculations Objectives

The main objectives of 3D flow calculations are

1) Evaluate designed flow path quality.

2) Compare results between 3D CFD, Streamline solver (other experiments and calculations if applicable)

3) Check presence of separations, supersonic shocks etc.

AxCFD tool provides the simulation and visualisation of thermodynamic parameters, kinematic parameters and velocities. In this simulation process pressure and Mach number variation is shown of three stage axial flow turbine. AxCFD tool also provide the velocity vector of each stage.

6.3.2 3D Flow Analysis with AxCFD

To evaluate designed blades in 3D CFD express flow analysis, AxCFD was embedded into AxSTREAM as an automated mesh-generator. Different software provides CFD solutions but in AxSTREAM with few seconds provides all parameters and different view analysis of axial flow turbine with precisely calculated design data. The quality of mesh can be increased from 0 to 1. The meshing of the blade can be view with solid structure and without solid structure for better visualization. Fig. 6.7 shows the meshing of 1st stage stator.

Fig. 6.7 3D CFD messing

K-ɛ Model is considered for CFD analysis on blade to blade stream surface. The K-epsilon model is one of the most common turbulence models. It has two equations model which it includes two extra transport equations to represent the turbulent properties of the flow. This allows a two equation model to account for history effects like convection and diffusion of turbulent energy.

The first transported variable is turbulent kinetic energy, κ. The second transported variable in this case is the turbulent dissipation, ɛ. It is the variable that determines the scale of the turbulence, whereas the first variable, κ, determines the energy in the turbulence.

6.3.3 CFD Simulation Result

AxCFD tool provide results like potential flow analysis, mixing, kinematic and thermodynamic within a minute. These results are very important to design a high efficient axial flow turbine. The main advantage of AxCFD tool is it takes couple of minutes to converge the solution.

Fig. 6.8 Conversion calculations

In three stage axial flow turbine designing of mixing flow and parameters at stator and rotor combined area and velocity vectors role in that area. In axial flow turbine the inlet temperature direct affect the efficiency of turbine so that the variation in temperature with CFD analysis and visualization is very important as high efficient gas turbine design considered. Fig. 6.9 shows the pressure variation in 1st stage stator.

Fig. 6.9 AxCFD Mach number contour

In real world application one cannot know the exact heat flux at every point on the surface, but approx schemes can be used to calculate the integral. AxCFD tool provide the heat flux variation in the blade. Fig. 6.10 shows the enthalpy variation and heat flux distribution in the 1st stage rotor.

Fig. 6.10 Enthalpy with heat flux contour

6.3.4 Potential Flow Analysis

In potential flow analysis centroid, moments of inertia are to be discussed. It is important for mass balancing and proper efficient axial flow turbine design.

Table 6.3: Potential Flow Analysis

6.3.5 Mixing Parameters Analysis

In three stage high pressure axial flow turbine mixing take place as each stage of stator and rotor of 1st stage etc. the mixing parameters like area, density, pressure and temperature of stator and rotor are essential for designing, modeling and simulation of axial flow turbine. The mixing parameters help for computational fluid analysis, reducing loss factor and increase the overall efficiency. This table shows that the mixing parameter of stator and rotor of first stage.

Table 6.4: Stator and Rotor Mixing Parameters

The next one shows that the mixing parameters at stator and rotor of each stage by AxCFD tool. It gives clear cut and overall idea of design at stator and rotor mixing position. This table highlights the distances and the radius as pass through the axial turbine channel so that the kinematic and thermodynamic properties take place.

Table 6.5: Kinematic and Thermodynamic Parameters

6.3.6 Kinematic CFD Results

With help of AxCFD tool of AxSTREAM calculating kinematic properties are very easily and precisely of axial turbine. Normally kinematic properties include all type of velocities like radial, axial, relative and circumferential velocities.

Table 6.6: Kinematic CFD Analysis

6.3.7 Thermodynamic CFD Results

The thermodynamic properties like static and stagnation temperature and pressure for axial turbine stage to be analyzed with the tool AxCFD.

Fig. 6.11 Pressure contour via CFD

This thermodynamic properties help to design axial flow turbine part like stator and rotor.

Table 6.7: Thermodynamic CFD Analysis