Turbocharger Volume-metric Meshing
TURBOCHARGER INTRODUCTION
A turbocharger, colloquially known as a turbo, is a turbine-driven forced induction device that increases an internal combustion engine's efficiency and power output by forcing extra compressed air into the combustion chamber. This improvement over a naturally aspirated engine's power output is due to the fact that the compressor can force more air—and proportionately more fuel—into the combustion chamber than atmospheric pressure alone.
Turbochargers were originally known as turbosuperchargers when all forced induction devices were classified as superchargers. Today the term supercharger is typically applied only to mechanically driven forced induction devices. The key difference between a turbocharger and a conventional supercharger is that a supercharger is mechanically driven by the engine, often through a belt connected to the crankshaft whereas a turbocharger is powered by a turbine driven by the engine's exhaust gas. Compared with a mechanically driven supercharger, turbochargers tend to be more efficient, but less responsive. Manufacturers commonly use turbochargers in trucks, cars, trains, aircraft, and construction equipment engines. They are most often used with Otto cycle and Diesel cycle internal combustion engines.
The objective of a turbocharger is to improve an engine's volumetric efficiency by increasing the density of the intake gas (usually air) allowing more power per engine cycle. The turbocharger's compressor draws in ambient air and compresses it before it enters into the intake manifold at increased pressure. This results in a greater mass of air entering the cylinders on each intake stroke. The power needed to spin the centrifugal compressor is derived from the kinetic energy of the engine's exhaust gases. In automotive applications, 'boost' refers to the amount by which intake manifold pressure exceeds atmospheric pressure. This is representative of the extra air pressure that is achieved over what would be achieved without forced induction. The control of the turbocharger boost has changed dramatically over the 100-plus years of their use.
CHALLENGE OBJECTIVE
The aim of this challenge is to clean the given turbocharger geometry and mesh the model as per the given length parameters for the CFD application. As well as assign a part ID and material ID for the component.
GEOMETRY:
Imported CAD geometry of turbocharger is shown below:
Initially, there is a glitching surface in the compressor casing. As known from previous working knowledge its duplicate surface formed while importing the model. These might be due to the reference frame, reference plane or geometry used by CAD engineer. To ensure that, we can hide the compressor casing geometry and we can see a model like:
which shows clearly there is a duplicate face in it. We can just delete that entire face from our model. As noted the geometry consists of various parts like:
- Blades
- Compressor casing
- Impeller
- Inlet casing cover
- Shaft rotor
- Turbo casing
CLEANUP:
A geometry check is done as the first step to figuring out the issues (single/tripe cons) in the model. Firstly, there is a single con in the compressor casing cover inlet. In this, the small extrusion out of the circular face is not required for this geometry. To eliminate this error, one can project the cylindrical surface onto the circular face which makes a tripe con in the intersection and as the next step the small surface can be deleted from the model. Another method is to make a cut at the circular and the cylinder face which will make the two different faces using Intersect option. The geometry after the Intersect tool is used is shown below:
Now small extrusion can be deleted as it formed a new face on it. Next, there is a single cons error in the compressor casing outlet face. The image is shown:
In this, there as small extrusion in both cylindrical and conical surfaces which need to be deleted. Intersect tools can be used for this case, which makes them into four different faces and it can be deleted from it. The image after clean up is shown below:
Next error is in the compressor casing inlet phase. The error is shown below:
This issue can be fixed by creating new surfaces using a fitted algorithm. The geometry after creating a new face is shown below:
Next error is in inlet casing cover. One end of the inlet casing doesn't have a surface which covers the entire model as shown below:
New faces are tried to be created in the single con area. But due to the curvature in the model, the surface created intersects with the interior of the casing as well in both fitted and coons algorithm. This kind of surface violates our geometry requirement. So the next step can be done is to mirror the face from the other side. But, the other side of the model is having a path to casing outlet, so mirroring another side of geometry as well doesn't hold good for this model. The next option is to create a few hot points in the region and then create individual surfaces without intersecting the interior of the casing. By picking Topo - Hot points - Parametric option, new hot points can be created according to the curve. In this case, five hot points are created on either side of the model and individual surfaces are created. The image shown below shows the procedure for creating hot points and created surface on one end of the model.
The next step is to fix the rotor shaft and blades geometry. The idea here is to make the blades and rotor a single geometry which helps in applying boundary conditions further to it. Initially, the rotor shaft is a single component and the other tow stages of the blade are in a different component. To make them a single volume, the faces in between them is to be deleted and make into a single volume. The image below explains tow shared faces in between the two blade stages:
From here all the mid faces to be deleted to make a single volume. Initially, the blade face circular face doesn't have any contact with the blades on it. So cuts can be made in between the blades to make them easy to delete and make them a double con geometry.
BEFORE CUT
AFTER CUT
As well as in between the blade stage 1&2 there is a common face sharing which makes them a different component. So Top - Faces - Topo tool is used which makes them a triple con as shown in the image below, then the faces in between can be deleted to make it a single volume.
Now all the errors are fixed from the model and the image is shown below in the wireframe model.
BEFORE CLEAN UP
AFTER CLEAN UP
The next step is to create PID for the model, all the different parts are renamed and initial PID manager image is shown below:
In this manager box, there is two default PSHELL property which doesn't contain any model in it. These might be the deleted duplicate faces property ID, these things can be deleted manually or using compress option all the unwanted property and material ID will be deleted. But one had to make sure its good to do compress because after compress option is used undelete tools won't work and impossible to regain the deleted faces. Since this model is prepared for CFD analysis, thickness and material property don't have any significance in it as the only volume will be extracted out of it. The element length for different parts in the model are:
- Blade Stage 1&2 - 1mm
- Compressor casing - 5mm
- Impeller - 2mm
- Inlet casing cover - 5mm
- Shaft rotor - 1mm
- Turbo casing - 5mm
Surface meshed model is shown in the image below:
Now compressor casing inlet and outlet, inlet casing cover inlet and outlet surfaces should be created and respective property ID is created to make the model ready for meshing. The image is shown below with faces created and the respective property ID is assigned to it.
Again the unmeshed eight faces (newly created) are meshed using free mesh option in Meshing bar. Now as a final step, the volume mesh of the model should be created. In the V-Mesh tab, Volumes - Define tool is picked, which analyzes the model and shows the possible volume elements that can be created out of the geometry. As from the result of Define, there are eight possible volume mesh can be created from this model. By inspection each of those, it has volume for compressor casing, inlet casing, turbo casing outlet which are not required for this model. Since CFD mesh is to be prepared only interior volume mesh is sufficient for the simulation, so the unwanted faces are deleted from the dialogue box itself. Next in the V-Mesh module - Unstructured mesh - Tetra CFD mesh is chosen for volumetric mesh creation. The created volume mesh is shown below:
LINK FOR ANSA FILE: https://utdallas.box.com/s/j3l30mlyb5owkyogsyhvumhn2fvmzpsi