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An intake manifold is a system of passages which carry the fuel mixture from the carburetor to the intake valves of the engine. It is used to distribute the fuel into the cylinder ports. Gambit and fluent are used to study the turbulent flow for manifold. Initially model has to create on gambit as per dimension.
Model must be meshed to study the turbulent flow, because we can study the effect of the flow at every node. Meshing gives very accurate value, so it will be very easy to analysis and to make any necessary improvements. The meshed model will be saved as intake.msh. Once the model is been meshed in gambit, the further analysis will be done in fluent.
FluentÂ is the CFD solver which can handle both structured grids (rectangular grids with clearly defined node indices) and unstructured grids. Unstructured grids are generally of triangular.
Intake.msh file is copied to working directory and 2d fluent is started. This file will be opened in fluent. To analysis the given model the following steps to be followed.
STEP 1 : GRID
1) Select File ïƒ Read ïƒ Case & Data. Browse to find the intake.msh which is been saved previously in working directory. Fluent will read in the grid geometry and mesh that were previously created by Gambit. Some information is displayed on the main screen. If all went well, it should give no errors, and the word done should appear.
2) Select Grid ïƒ check. This procedure checks the integrity of the mesh. Make sure the reported minimum volume is a positive number. In the below picture we can see the values in volume statistics, all the values are positive so we can proceed to change scale for the model.
3) Select Grid ïƒ Scale. The gird must be correspond to the actual physical dimension. In the below picture we can notice in the domain extents, which gives the information about dimension. If the dimensions are not as per actual physical dimension, then the grid has to be scaled with proper units. In this case, the geometry was created in mm and needs to be scaled. In scale grid dialog box we can observe an option grid was created in, we need to change it to mm and then press scale. So, that all values will be generated in mm.
4) Select Display ïƒ Grid and press display. A new graphical display window opens up showing the grid. Now this is ready to analysis flow in intake.
STEP 2 : MODEL
In the main fluent window select Define ïƒ Models ïƒ Solver. In the Space region of the Solver window, choose 2D and ok to accept the setting. The Solver window should disappear.
Select Define ïƒ Models ïƒ Viscous. In the model region of the viscous model window, choose k-epsilon [2 eqn] and ok to accept the setting. Panel will be closed
The Reynolds number can be calculated at the inlet using a built-in calculator in FLUENT.
Re = (U Ã- Di Ã- p)/Î¼ (2.1)
Di = the inlet diameter (0.07814m).
U = the velocity at inlet (15 m/s).
This expression can be evaluated in the FLUENT console window by entering the above equation using the corresponding values in the console as shown below:
(/ (* 15 0.07814 1.225) 1.7894e-5)
Press Enter key. The value of Re will be reported as 80240.444 showing that the flow is turbulent.
STEP 3 : MATERIAL
In the main fluent window, Define ïƒ Materials ïƒ Fluent Database. Select fluid from material type drop down list and air from the fluent fluid Materials drop down list. Close the Materials window.
STEP 4: BOUNDARY CONDITIONS
In the main fluent window, Define ïƒ Boundary conditions and a new boundary condition window will pop up. Boundary conditions for velocity inlet and outlet must be defined.
Boundary condition for velocity inlet
Select Inlet from zone and velocity inlet from type in boundary condition dialog box.
Then click set, the velocity Inlet panel opens.Specify a value of 15 for Velocity Magnitude (m/s). Select intensity and hydraulic diameter from turbulence specification method. Specify a value of 4 for Turbulent Intensity and a value of 0.07814 for Hydraulic Diameter. Click OK to close Velocity Inlet panel.
Boundary condition for outlet1
Select outlet1 from zone and pressure outlet from type in boundary condition dialog box. Then click set, the pressure outlet panel opens. Keep pressure gauge value to default. Select intensity and hydraulic diameter from turbulence specification method. Set backflow turbulence intensity to 5percent. Hydraulic diameter is set to 0.01826.
These values are correspond to outlet 1, same boundary conditions are specified for remaining outlets, but only change will be there hydraulic diameters.
STEP 5: SOLUTION
Solution control are set by selecting Solve ïƒ Controls ïƒ Solution controls. Default parameters are selected in solution control panel.
Flow is initialized by selecting solve ïƒ intialize. Select inlet in the Compute From drop-down list in solution initialization dialog box. The values of all the variables based on the boundary conditions at inlet will get updated. Click Init and close the panel. The flow will get initialized with appropriate values of velocity and turbulence parameters.
After flow is initialized residual plotting is should be analysed. So, those results will be plot in a graph which gives better picture. Plotting of residual is enabled by selecting Solve ïƒ Monitors ïƒ Residual. In the Residual Monitors window that pops up, turn on Plot in the Options portion of the window. The Print option should already be on by default. Here, Print refers to text printed in the main fluent window, and Plot causes the code to plot the residuals on the screen while the code is iterating.
At this point, and every so often, it is wise to save our work. In the main fluent window, select File ïƒ Write ïƒ Case & Data. If not on by default, turn on the option to Write Binary Files (to save disk space). To save even more disk space, the files can be compressed by adding a "gz" at the end of the file name. In the Select File window that pops up, name the file "intake1.cas.gz". Click OK to write the file onto working directory.
5) Solution is to be started by requesting iteration for 200 initially. In the main fluent window, select Solve ïƒ Iterate to open up the Iterate window. Change Number of Iterations to about 200 and click Iterate.
The solution converges in about 82 iterations with default convergence criteria, while the graphics display window plots the residuals as a function of iteration number. The residuals may rise at first, but should slowly start to fall, and should eventually level out. The residual plot is shown below.
6) Point surfaces is created to check whether the flow of variables have reached a constant value or not, monitor the average values for some of the relevant parameters. In this case, track velocity magnitude, pressure, and turbulence kinetic energy over three points in the flow domain. Select surface ïƒ points. Specify a value of -0.0595476 for x0 and a value of 0.02164832 for y0 in coordinates in point surface dialog box. New surface is named point-1 and Click Create.
Similarly, two more monitor points are created for following values.
Point x0 y0
Point - 2 0.001064202 0.02015189
Point - 3 0.03839816 0.01478293
7) At the first point surface vertex average velocity magnitude is to be monitored. Select Solve ïƒ Monitors ïƒ Surface monitors. Number of surface monitor is to be increased to 3. Plot should be enabled at all corresponding three monitors. Click on define for all monitors.
Define surface monitor window pops up for surface monitor 1 and select Velocity... and Velocity Magnitude in the Report of drop-down list. Under Surfaces, select point-1 and select Vertex Average in the Report Type drop-down list and then click OK to accept the settings.
Similarly, set the monitors for static pressure and turbulent kinetic energy at point - 2 and point - 3 respectively as show below in picture.
For point - 1
For point - 2
For point - 3
8) Set second order upwind scheme for momentum. Select Solve ïƒ Controls, solution control window pops up. Select second order upwind for momentum, under discretization and click ok.
9) Disable the convergence criteria for all the equations.
For a better convergence, the iterations need to be performed till all the monitors
flatten out. Therefore, disable the convergence criteria. In the main fluent window select Solve ïƒ Monitors ïƒ Residual monitors. Select none in convergence criterion under residual monitors' window. This disables the convergence check for all the equation and click ok.
10) Process of iteration is carried out till all the monitors reach a constant value. Below graph show the iterations for all 3 surface points. As we can see from the graph 3 points have achieved constant value at 1500 iteration. Figure 1 represents residuals history for iterations. Residual are continuity, x velocity, y velocity, k and epsilon. Figure 2 represents convergence history of velocity magnitude on point 1. Figure 3 represents convergence history of static pressure on point 2, and figure 4 represents convergence history of turbulent kinetic energy (k) on point 3.
STEP 6 : POST PROCESSING
1) In the main fluent window, select Display ïƒ Vectors. Adjust the scale factor as needed to make the velocity vectors clearly visible. Select Velocity in the Vectors drop-down list and select Velocity... and Velocity Magnitude in the Color by drop-down list under vectors and click display.
Below figure shows the velocity vector colored by velocity magnitude. Where dark blue is a point where there is more velocity of fuel mixture i.e. 3.08e+02.
2) In the main fluent window, select Display ïƒ Contours. Under contours of retain the default options i.e. pressureâ€¦ and static pressure. Enable filled under option and click display. Below figure shows the contours of static pressure. Where red color is a point where there is more pressure of fuel mixture i.e. 1.50e+02. Right click on the contour to display the pressure at particular point.
3) In the main fluent window, select Define ïƒ Injections ïƒ Create. In the injection properties window a number of parameters are to be described to inject the particle. Select Surface in the Injection Type drop-down list. Under Release from Surfaces, select inlet and click OK to accept the settings.
4) To actually track the particle path, select Display ïƒ Particle Tracks. Select injection-0 from release from injection under particle tracks and then click display. A line drew showing the particle path.
As we can see different colors in a line starting from blue to red, where blue represents start point and end point. Where start point is 0.00e00, where this a origin for a particle. We can observe that the particle tracks end before reaching the outlet.
In each step, particles travel a distance and there is a limit on the total number of steps. By default it is set to 500.
To get the complete particle track, you need to set the maximum number of steps such that the particles escape through the outlet. It can be achieved by changing the Max, Number of Steps in the Discrete Phase Model panel. To change the maximum number of steps, select Define ïƒ Models ïƒ Discrete phase. Set the number of steps to 2000 under tracking parameters in discrete phase window. Then click ok to accept setting.
After changing a particle numbers, then again display particle track. The track is shown below. Now we can observer the particles pass through the outlet.
7) Path of the particles through outlet 1,2,3 and 4 is recorded. These trajectories passing through outlet is written to file. Select Report ïƒ Discrete phase, under release from injection select injection-0. Select all the outlet surfaces under boundaries and click compute. This will write out four .dpm files containing the information of particles that pass through the corresponding outlet.
Conclusion: From the above analysis we can observe the impact of velocity and pressure of the fuel air mixture on the manifold. With the help of the above data we can make necessary improvements in the manifold, like distance of outlet from inlet (velocity can be varied) and design of the manifold to withstand pressure. We can observe that most of the pressure is concentrated at the junctions of the outlet.
Reference: Tutorial notes - Andy E Young