Week 10: Project 1 - FULL HYDRO case set up (PFI)

Aim :

To set up a combustion simulation with the details given in the challenge and to study different properties of combustion by post processing the results obtained by the calculation of the full hydrodynamic set up of the given geometry.

Objective :

1. What is the compression ratio of this engine?

2. Why do we need a wall heat transfer model? Why can't we predict the wall temperature from the CFD simulation?

3. Calculate the combustion efficiency of this engine

4. use the engine performance calculator, to determine the power and torque for this engine.

Theory :

Internal combustion engine (ICE) burn a mixture of fuel and air (oxygen), in an appropriate ratio, in order to produce mechanical power. In a four-stroke internal combustion engine the combustion process occurs after the mixture of fuel-air has been induced into the cylinder, properly compressed and a spark generated (in case of a gasoline/petrol fuel).

The start of combustion process is linked to the position of the piston in the cylinder and usually happens before the piston hits the Top Dead Centre (TDC). As a result of the combustion process, the pressure generated in the cylinder rises and its transmitted as a torque to the crankshaft through the piston and connecting rod.

Combustion can be defined as a rapid oxidation process of a fuel, which generates heat and light. With other words, combustion converts the energy stored in the chemical bonds of the fuel into heat. In an engine, combustion occurs as a flame that propagates in the cylinder. A flame (fire) occurs if three components are present: fuel, oxidizing agent (oxigen) and heat. Depending on the type of the fuel, the heat for the occurrence of the flame comes from: a spark (gasoline/petrol engines) or heated
(compressed) air (diesel engine).

The main product of an internal combustion engine is mechanical power, this is why we use them as propulsion systems. Together with mechanical power, as a by-product, we also get exhaust gases. Ideally, if the combustion process is complete, the exhaust gases should only be carbon dioxide and water vapour. In reality, mainly due to incomplete combustion, the exhaust gases also contain pollutant emissions: oxides of nitrogen, unburnt hydrocarbons, carbon monoxide, soots/particles, polyaromatics, aldehydes ketones and nitro-olefins.

Description :

The geometry looks as below:

                                                            Fig 1. PFI Engine Geometry 

The next step involves in running diagnosis for the imported geometry. This procedure is followed to make sure that there is no error in the fluid domain (i.e., with a closed fluid geometry). The the diagnostic tools is runned, the following errors are observed:

                                    Fig 2. Error observed in the geometry when diagnosis tool is used.

                      Fig 3. Diagnosis dialog box

From the above figure we can observe the list of error that are found in geometry:

1. Intersections               : 1574 
2. Nonmanifold Problems  : 5
3. Open Edges                 : 32
4. Normal Orientation       : 214

Intersections: CONVERGE cannot use a surface containing triangles that intersect because an intersection would violate the concept of a consistent inside/outside orientation of the surface. This type of problem can be difficult to detect in a surface.CONVERGE Studio graphically represents parallel triangles that have been moved too close together and thus are considered intersecting.

                                                                                 Fig 4. Intersection Errors

Nonmanifold Problems: With the exception of special conjugate heat transfer cases, every edge in the geometry needs to be shared by exactly two triangles. If more than two triangles share a common
edge, thus forming a T-junction, this edge is a nonmanifold edge. It is impossible to have distinct inside and outside directions when a surface contains nonmanifold edges.

                                                                               Fig 5. Nonmanifold Errors

Open Edges: In a closed triangulated surface, each triangle shares each of its three edges with exactlyone other triangle (except in special Conjugate Heat Transfer simulations). If an edge is part of only one triangle, this creates what is called a open edge.

                                                                          Fig 6. Openedges error

Normal Orientation: CONVERGE requires the surface to have a properly defined inside (solved fluid side) and outside (non-fluid side). The normal vectors of all triangles (as defined by the right-hand rule) must point toward the fluid. One extra normal orientation step is required when a surface consists of multiple independent surfaces, as in the case of an object completely immersed within a fluid domain, such as a sphere immersed in (but not connected to) a box. For the immersed sphere, the normal vectors of the triangles must point away from the center of the sphere, since the solved fluid portion of the domain is outside of the sphere. The normal vectors of the box triangles must point inward, toward the sphere.

                                                                 Fig 7. Normal Orientation error

These errors are cleared using basic geometric clean up tools. Once all these errors are cleared, we should get dialog box as shown below to make sure that the simulation run:

      Fig 8. Diagnostc Tool box after clearing errors

 Once these errors are clarified the next step involved is to split the boudaries and grouped using different boundaries:

           Fig 9. Boundary Flagging - view tool bar

                                     Fig 10. Geometry after boundary flagging is complete.

Once these steps are completed as per the above methods, we are at stage where the geometry preparation is completed and the next procedure is to setup the case for sunning the simulation.

The following are the input parameter for the IC Engine Simulation:

i. Engine Geometric Parameters

1. Bore =0.086 m
2. Stroke = 0.09 m
3. Connecting rod length = 0.18 m
4. RPM = 3000

ii. Run parameters

1. Simulation mode: No Hydro

iii. Simulation time parameters

1. Start time : -520 deg
2. end time : 120 deg

iv. Boundary conditions

1. Piston temperature - 450K
2. Liner temperature - 450K
3. Head temperature - 450K
4. Spark Plug temperature - 550K
5. Spark Plug electrode temperature - 600K
6. Exhaust ports temperature  - 500K
7. Exhaut outflow - 1 bar
8. Exhaust outlflow temperature - 800k
9. Exhaust species concentration - Calculate the stoichiometric condition
10. Exhaust valve top temperature - 525K
11. Exhaust valve angle temperature - 525K
12. Exhaust valve bottom temeprature - 525K
13. Intake port - 1 temperature 425K
14. Intake port - 2 temperature 425K
15. Inflow pressure - 1 bar total pressure
16. Inflow temperature - 363K
17. Inflow species - Air
18. Intake valve top temperature - 480K
19. Intake valve angle temperature - 480K
20. Intake valve bottom temeprature - 480K

v. Initial conditions

• Intake port - 1 (closer to combustion chamber)
• ic8h18 - 0.025508
• o2 - 0.20157
• n2 - 0.77292
• Temperature - 390k
• pressure - 1 bar
• Intake port - 2 (away from combustion chamber)
• Air
• Temperature - 370K
• Pressure - 1 bar
• Cylinder
• Pressure - 1.85731 bar
• Temperature - 1360K
• Stoichiometric composition

vi. Injection Parameters

1. Fuel flow rate = 7.5e-4 Kg/second
2. Injection start time = -480.0
3. Injection duration = 191.2
4. Fuel temperature = 330K

vii. Nozzle positions

     Nozzle diameter = 250 micro-meter

     Circular injection radius = Nozzle radius

     Spray cone angle = 10

• Nozzle 0
• center 0.0823357 0.00100001 0.07019
• Align Vector -0.732501 0.210489 -0.647408
• Nozzle 1
• center 0.0823357 -0.00099999 0.07019
• Align Vector -0.732501 -0.210489 -0.647408
• Nozzle 2
• center 0.0823357 -0.0004 0.07019
• Align Vector -0.5 -0.2 -0.647408
• Nozzle 3
• center 0.0823357 0.0003 0.07019
• Align Vector -0.5 0.2 -0.647408

viii. Spark Ignition Parameters

• Start of spark = -15 deg
• Spark duration = 10 deg
• Spark location = -0.003 0 0.0091
• Spark radius = 0.0005m

  The case setup is setup using the above parameters as follows:

i. IC Engine (crank angle-based)

                                       Fig 11. IC engine dialog box

ii. Gas simulation

                                     Fig 12. Gas simulation dialog box

iii. Reaction Mechanism

                    Fig 13. Reaction Mechanism

iv. Run parameters

                                                               Fig 14. Run Parameter

v. Simulation time parameters

                                                            Fig 15. Simulation Parameters

vi. Boundary

                                                              Fig 16. Boundary Conditions

vii. Regions and Initilization

                                                            Fig 17. Regions definition

viii. Base grid

                           Fig 18. Base Grid size

ix. Adaptive mesh refinement

                                                                 Fig 19. Velocity based AMR

                                                             Fig 20. Temperature based AMR

x. Fixed embedding

                                                             Fig 21. Fixed Embedding

With the above parameters the case setup for IC engine is complete and is ready to be exported and run No-Hydro Simulation (i.e., simulation where the transport eqations are not solved but only the mesh is generated).

                                                 Fig 22. Exporting Input Files to run simulation

Once all the Input files are exported, it is ready for running the simulation using cigwin.The simulation summary is as follows once completed:

                                Fig 23. Simulation summary for No-Hydro

And the mesh movement is as follows:

          Vid 1. PFI Engine mesh movement - No-Hydro Simulation 

After completing No-Hyro simulation, the setup is the altered to run for full hydro simulation.

Converge software is used to simulate PFI engine various parameters and efficiency of the engine. To simulate the engine properly various models have been introduced in the simulation so as to replicate the actual combustion cycle computationally. 

Following Models were used during the simulation:

1.Spray Model:

For Liquid Spray Converge uses Lagrangian solver to model discrete parcels and the Eulerian solver to model continuous fluid domain. Heat, momentum and mass transfer occur between the discrete and the continuous phases with the help of source term in the transport equation.

Converge introduces parcels into the domain at the injector. Each parcel represents identical drops (radius, velocity, temperature) and as a result converge solves for parcel basis rather than per-drop basis. These parcels statically represent entire spray field.

Also, spray modelling uses sub grid models to model various physics of fuel spray after it interacts with the air in the chamber. The interaction of fuel parcels with the air results in breakup of the parcels and leads to various physical interaction provided in the above diagram. Sub grid model takes care of these interactions so as to provide accurate results closely matching the experimental

 

                                                                Fig 24. Spray Parcel Formation

                                              Fig 25. Spray Parameter

2.Combustion Modelling:

Converge contains detailed chemistry solver and simplified combustion model to provide the result for combustion. The detailed chemistry solver provides accurate way to compute combustion which can be easily compared with experimental data. Due to this the computational time taken by detailed chemistry is more as compared to simplified combustion model.

Sage Solver:

It uses the local condition to calculate reaction rates based on principles of chemical kinetics. Each mesh cell present inside the engine has a unique value for various thermo-physical properties, which the SAGE solver uses to calculate the reaction rates. The solver uses Arrhenius rate law equation to solve the rate of change of species concentration. data.

                                                                   Fig 26. Combustion modelling

 Once these additional parameters are added to the case setup, the inputs files are agin exported and full hydro simulation is made to run using cigwin.

                       Vid 2. PFI Simulation for Full Hydro

RESULTS:

1. Time Mapping:

                                                                                                     Fig 27. Time Mapping 

The figure indicates the time period in which different process takes place during the simulation process in a graphical representation.

2. Mass Flow Rate

                                                                          Fig 28. Mass Flow rate into the engine(region 0)

The graph indiates the flow of mass into the cylinder takes place during the intake stroke of the engine.

3. Pressure

                                                                            Fig 29. Pressure Developed in the Engine

From the graph it is clear that there is a lot of pressure building up in the cylinder during compression stroke helping in increasing temperature of the fuel for proper combustion of  fuel and later which drops back.

4. Density

                                                                                               Fig 30. Density curve

Since Pressure and density are inter-related (i.e., directly proprtional) with the increase in the pressure, density also increases.

5. Temperature

                                                                                            Fig 31. Temperature Curve

From the graph it is evident that the temperature inside the pistion increases significantly during the copression stoke of the engine cycle.

6. Engine Performance Calculator:

                                                                   Fig 32. Engine Performance Calculator

CONVERGE provides a useful tool which calculates the performance of the engine using library commands.

1. Compression Ratio

Compression ratio is defined as the ratio of maximum volume to the minimum volume in the cylinder of an IC engine. Mathematically it can be written as:

Compression Ratio = Maximum Volume / Minimum Volume = v$\frac{{}_{max\; /\; v}}{{}_{min}}$

	=	static compression ratio
	=	displacement volume
	=	clearance volume

Compression Ratio = $9.99$

2. Efficiency of Engine:

Efficiency = Output Energy / Input Energy

where Input Energy = mass of fuel * Calorific value of the fuel = 3*10e-5 * 45 MJ

          Output Energy = Heat Released = 1240 J

Hence, Efficiency = 91.8518%

3. Power and Torque Calculation:

From Energy Performance Calculator we get, 

Work = 468.646 Nm

Duration = 240.199 deg

Crank angle speed = 3000 rpm

Time for every 240.199 deg = 0.01334 s

Power =  468.646/0.01334 = 35.130 KW

Torque [P =`(2piNT)/60]  = 111.8 Nm

4. Significance of Ca10, Ca50 and Ca90.

Ca10 - crank angle degree for 10% of cumulative combustion heat release = 6.83717 deg

Ca50 - crank angle degree for 50% of cumulative combustion heat release = 18.4623 deg

Ca90 - crank angle degree for 90% of cumulative combustion heat release = 31.701 deg