Transient Simulation of Fluid Flow Through a Throttle Body
Transient Simulation of Fluid Flow Through a Throttle Body
The aim of this project is to perform a Transient simulation for fluid flow through a throttle body. Converge CFD software is used for the project. For Transient analysis, the throttle body (Butterfly Valve) is moved to an angle of 25 degrees in 2ms and kept in that half-open position for next 8ms.
Wide Open Throttle Valve Position
The most significant difference between steady-state and dynamic simulation is that steady-state assumes that variables are constant with respect to the time. This means that in steady-state there is no accumulation in the system so the overall mass and energy input matches its output. For transient simulation, the objective is to capture the flow variations occurring at each instance as the flow conditions are changing due to valve motion.
Meshing Technique
Converge CFD uses a cut-cell cartesian method. One of the most obvious benefits of the cut-cell Cartesian approach is the fact that you have a body-fitted mesh. This ultimately means that your mesh fits perfectly with your geometry, reducing the number of inaccuracies that propagate through the rest of the domain. Because of this, the cut-cell Cartesian approach makes handling complex and moving geometries much easier and accurate by cutting the cells rather than stretching and compressing the cells to fit. The other benefit of the cut-cell Cartesian method is added cell refinement at the wall, as this will have a positive impact on accuracy. Mesh refinement can be performed at locations where the flow properties need to be captured accurately. A method called \"Fixed Embedding\" is deployed in this case with the region near the throttle body being the region of interest.
Scaling factor of 3 has been used with 2 layers of fixed embedding. The base grid used in this simulation is 2e-3 in x, y and z-direction. This means that after scaling has been applied for the mesh near the throttle valve region, the mesh will be of value 6e-3. The refined mesh near the throttle valve and the base mesh can be seen in Fig.2 and Fig.3 respectively.
Mesh Refinement near Throttle Valve
Base Grid Mesh
The video below shows how the mesh moves with the movement of geometry and thus maintaining the desired mesh refinement.
Time Parameters
The simulation is run for 10ms. The time interval between the flow inlet and flow outlet is around 2.5ms. This value is computed by analyzing the average velocity of fluid through the pipe and the length of the pipe. The initial/minimum time step is taken as 1e-09 and a maximum time step of 1. This lower time step is taken because of the dynamic time-stepping algorithm is done by Converge software. As long as the initial and boundary conditions are correct, the converge makes sure that the time-step value is updated for each time step as the simulation progresses. This makes sure that at each time-step, the flow parameter values are accurate. This means that Converge will reduce the time-step as the simulation moves ahead and starts to converge.
Boundary Conditions
A pressure difference of around 0.1 bar is maintained between inlet and outlet which drives the fluid flow.
Inlet: Temperature = 300K ; Pressure = 1.1 bar
Outlet: Pressure = 1.2 bar
Throttle Body: Rotating Wall
Elbow: Wall
Results
The simulation is run in parallel. The converge uses MPI programming to parallelize the solver, which means that the solver is equally distributed to given no of processors in the working computer. In the present problem, the solver is run with 4 processors which equally distributing the cell structure to each of the processors.
Velocity Profile
The discharge has to remain constant for fluid flow through a throttle body. This means that to satisfy the continuity equation (Area*Velocity=constant), velocity increases in sections where the flow area is reduced. A maximum flow velocity of around 160m/s is obtained when Valve is rotated through an angle of 25 degrees.
The Fluid Flow Profile through the throttle valve is shown in videos below:
Pressure Profile
The Pressure Profile for Throttle Valve is the most important as it can be used to determine the thickness, material and other physical properties the Valve Body should have to bear the pressure load. Initially, the pressure gradient across the throttle valve was not much because the valve was in open position. However, as the valve rotates, the fluid flow is obstructed which results in conversion of Kinetic Energy into Pressure Energy resulting in higher Pressure Gradient across the Throttle Body. The video below shows the changes in Pressure Profile as Fluid Flow occurs: