Super Charger Simulation Project

Purpose

To simulate roots type supercharger with 4 different boundary conditions at the outlet (Array of the pressure ratio = [1.1 1.5 2 2.2]Bar). To get supercharger performance data (Total pressure ratio vs mass flow rate). Furthermore, see the importance of the total energy solver.

Theory

Supercharger is a device which increases the pressure and density of incoming air in order to increase the power of the IC engine. Power required for supercharger is provided by belt, chain or gear connected to the IC engine's crankshaft.

• Types

There are 2 main types of supercharger according to the method of the gas transfer: positive displacement and dynamic compressors. Positive displacement compressors and blowers deliver almost constant pressure increase at all engine speeds. Dynamic compressors do not deliver pressure increase at low RPMs but at high speeds pressure increases exponentially.

As we said Positive displacement pumps deliver constant pressure increase at all engine RPMs, It means that this type of supercharger loses it's importance at high engine RPMs.

There are some types of positive displacement superchargers.

• Roots https://en.wikipedia.org/wiki/Roots-type_supercharger
• lysholm twin-screw https://en.wikipedia.org/wiki/Rotary-screw_compressor#Superchargers
• sliding vane https://en.wikipedia.org/wiki/Powerplus_supercharger
• scroll-type (G-Lader) https://en.wikipedia.org/wiki/Scroll-type_supercharger

Dynamic compressors rely on accelerating air at high speedf and then exchanging that velocity to pressure by diffusion or slowing down the flow.

Major types of dynamic compressors

• Cenrtifugal https://en.wikipedia.org/wiki/Centrifugal-type_supercharger
• Multi-stage axial-flow https://en.wikipedia.org/wiki/Axial_compressor

• Comparing different type superchargers

Simulating root type supercharger using Converge Cfd

• Geometry

• Nohydro animation

• Case set up

we are using air(real gas) as an operating fluid.

• Run parameters (Total energy solver)

• Time parameters

• Boundary Conditions

Inlet and Outlet

Outlet Pressure Matrix

Lobes (Roots) Boundary Condition

Wall Boundary Condition

• Initialization

• Turbulence Modeling

• Base grid

I wanted to set the element size lower or gererate fixed embedding on the lobe surfaces because gap size between the lobes and casing is lower than 2mm. Unfortunately it would be computationaly pretty expensive.

• POST PROCESSING

Total pressure

Mass flow at the inlet

Mass flow at the Outlet

As you can see mass flow rate decreases as we increase total pressure at the outlet.

Velocity at the Outlet

Total cell count

As we see, there is a little fluctuation in total cell count, this is because we have the rotating geometries in our case, while geometry is changing it's position converge cfd mehing system generates mesh elements due to pairing.

• 1.1 bar outlet pressure case

• 1.5 bar outlet pressure case

• 2 bar outlet pressure case

• 2.2 bar outlet pressure case

The animation above shows pressure and velocity contours, you can see that velocity magnitude in some areas is too high, it's caused due to compressible flow, when air travels through the lobes you can clearly see that area of the moving air is decreased dramaticaly, meaning it's compressed, leading fluid to high values of speed, however at the outlet fluid velocity is droped. there is no doubt that velosity drop is significant which is the reason why we have pressure increase in the supercharger, In other words kinetic energy of the fluid is converted to pressure (thermal) energy.

• simulating without total energy solver

• Temperature comparison between total pressure and internal pressure solvers

when we are including total energy solver it means that mechanical work done on fluid will be included which then will be converted to heat, that's why we have much lower temperature value while using internal energy solver.

• density comparison

As the temperature of the fluid increases the density decreases, that's why we have higer density values without total energy solver.

• Mass convergence

we can assume that mass flow has been converged, however if we need more tight tolerance value only one way to get it is to increase the simulation time or refine the mesh.

• Performance data

As we see while pressure at the outlet increases, mass flow drops.

Conclusion

We successfully simulated all the cases, learnt the working principle of a supercharger, Furthermore we saw how some physical and performance parameters changed due to pressure ratio increase. Moreover, figured out what is the the difference between solving with and without total energy solver.