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

Aim:- The aim of this project is to simulate the Full Hydro simulation in an ICE engine, using the inputs provided. By post processing the results, we intend to obtain vital information about the engine, namely compression ratio, combustion efficiency, power and torque for the engine.

Introduction:- In the previous project, a No Hydro simulation was performed on the IC Engine. Using the same case setup, a full Hydro set up will be performed, with the aid of tools in Converge such as Spray and Combustion modelling. The theory pertaining to the Full Hydro Simulation will be explained in the Case Setup under relevant headings.

Geomtery:- The same geometry which was used for the "No Hydro" simulation was used for this simulation.

Case Setup:-

1.Application type:- Application type was set to Crank Angle based IC engine. The values for geometry specifications such as bore dia, rpm etc were obtained from the Engine Geometric parametes provided in the question

2. Materials

    - Gas Simulation:- Default values used

      

     - Parcel Simulation- When we are modelling a simulation which has fuel spray in it, the fuel spray is represented as a  Lagrangian Parcel. A langrangian based spray model is referred to as a parcel. In this case setup IC8H18 is the fuel spray.

      

      - Global Transport Parameters:- Default value retained

      

      - Reaction Mechanism

       therm.dat and mech.dat values loaded into the profile.

      - Species

        IC8H18  has been added to the 'Parcel'

       

3. Simulation Parameters

      -Run Parameters:- Simulation Mode set to Full Hydrodynamic

       

       -Simulation time Parameters:- 

         Values set as shown below

        

      - Solver Parameters- Default PISO scheme and Density based solver  used.

         

4. Regions and Events

A region is a collection of boundaries used to differentiate different areas in the boundary. Regions are useful for a variety of reasons

    - Used to specify initial conditions by regions

    - Control Flow between regions (via events)

    - Output results on a region by region basis

    -Apply grid control techniques by regions

    -Map solutions by regions

    -Specify combustion models by region

    -Determine time step by region specific CFL numbers.

Events control when flow is permissible between regions.

In this case set up there are four regions that have been set up, namely Cylinder region, Intake 1, Intake 2 and Exhaust regions

    1) Cylinder region

         - Temperature - 1360 K (Value provided in the question)

         - Pressure-185731 Pa (Value provided in the question)

         - Computation of Species mass concentration:- It is assumed that stoichiometric combustion of IC8H18 takes place in              the combustion chamber

         

            In the above balanced equation, the coefficients of the products are in moles. However, CONVERGE accepts only       mass fractions. Moles were converted into mass fraction with the help of an excel spreadsheet.

Species	Moles	Molecular Weight (g/mol)	Mass  (g)(Mass*Mol Weight)	Mass Fraction
CO2	8	44.01	352.08	0.192304403
H2O	9	18.01528	162.1375	0.088558734
N2	47	28.0134	1316.63	0.719136864
			1830.847

These values were inserted into species.

       

       2) Intake Port 1:- In this port air fuel mixture is introduced

          Mass fraction of species is computed from comparable air fuel mixture used for SI engine.

 

          

          2) Intake Port 2- Air is introduced in through this port. So the standard composition of sir fuel mixture is used through this port

           

       4) Exhaust Port- It is assumed that whatever is products from combustion chamber is exhausted through the port, so composition of products through the port is the same as that of combustion chamber

 

  

 

     

5.Boundary Conditions

 The component of the ICE engine was classified into different region as shown below

 

The boundary condition for each of the component has been set as shown below.

- Piston:- Boundary Type- Wall

    

 - Inflow:- Boundary Type- Inflow

    

- Liner:- Boundary Type- Wall

  

- Cylinder Head:- Boundary Type- Wall

 

- Exhaust Port:- Boundary Type- Wall

 

- Outflow:- Boundary Type- Outflow

- Exhaust Valve:- Boundary Type- Wall, Movement as per exhaust_lift.in

- Exhaust Valve Boundaries:- Boundary Type- Wall. Movement as per exhaust_lift.in

- Exhaust Valve Bottom Boundaries:- Boundary Type- Wall. Movement as per exhaust_lift.in

- Intake Port:- Boundary Type- Wall

- Intake Bottom:- Boundary Type- Wall. Movement as per intake_lift.in

- Intake Angle:- Boundary Type- Wall. Movement as per intake_lift.in

- Intake Valve Top:- Boundary Type- Wall

- Spark Plug Terminal :- Boundary Type- Wall

- Spark Plug:- Boundary Type- Wall

- Intake Port:- Boundary Type- Wall

 

 

6. Physical Modelling

     -Spray Modelling:-

Spray modelling is required because solving fluid flow equations describing spray motion directly is not computationally feasible. So a modelling approach is adopted which captures the physics of spray motion.

For liquid sprays, Converge uses a Langrangian solver to model discrete parcels and the Eulerian solver to model the continuous fluid domain. Heat, momentum and Mass transefer occur between the discrete and continuous phases via source terms in the transport equation.

Converge introduces parcels (collection of drops) into the domain at the injector. Each parcel represents a group of identical drops (same, radius, velocity, temperature). CONVERGE solves for radius, velocity on a per parcel basis.

Parcel undergo several Physical process.

- Primary Breakup

-Secondary Breakup

-Drop Drag

- Collision and coalesence

-Turbulent dispersion

-Evaporation

When fuel is injected, the liquid particles interact with the surrounding air. This interaction causes creation of a drag forces that results in propagation of disturbances. If the liquid particle is large enough it  breaks into smaller particles which then break up into even smaller particles (Primary and secondary breakup). When the smaller secondary particles interact with ambient air, they undergo one or more of the other physical phenomena, namely drop drag, collision and coalesence, Turbulent dispersion and Evaporation.

For the purpose of the Full Hydro Case simulation, the following approach was adopted.

   - General Setup

        i) Parcel Distribution- Distribute Parcels evenly throughout cone

        ii) Turbulent Dispersion - O Rourke Model (For gasoline engine, O Rourke model gives better results)

        iii) Evaporation Model enabled

        iv) Frossling model selected

        v) Evaporation source:- Source all  base parcel species selected (This implies that IC8H18 (liquid) converted to IC8H18 (gas)

        vi) Temperature Discretization enabled

        vii) Radius above which 1 D  heat equation will be solved = 1000 m

         viii) No of FV cell per spray parcel =15

         ix) Thermal conductivity- Physical

- Collision Break up and modelling

     i) Collision model :- NTC collision used

     ii) Collision outcomes :- Post collision outcomes

     iii) Use a collision mesh option enables

     iv) Drop drag model- Dynamic drop drag

      v) Dynamic drag values set as shown below

  

- Wall Interaction:-

 For Wall Interaction, default values have been retained

- Injector Modelling

  i) Injected Species :- Parcel Species set to IC8H18 with mass fraction set to 1.0. Injection rate shape profile was imported.

- Models

Kelvin- Helmholtz model and Rayleigh- Taylor Model were selected as these are the commonly used models for solid cone sprays. The default constants were retained. Discharge coefficient set to 0.8.

The default constants control how the liquid particles breakup. The stability equation governs the process however do not capture the finer details. The constants govern how long it takes for breakup and size of the child droplets.

-Injector configuration

i) Injection temporal type - CYCLIC

ii) Cycle period -  640

ii) Start of Injection= -480 degree (Given)

ii) Injection duration = 191.2 degree (Given)

iii) Total injected mass is calculated as shown below

    Fuel flow rate = 7.5 e-4 kg/s

    Engine RPM=3000 

    Engine RPS = 3000/60 =50 rps

    Engine Degree Per Second= 50 * 360= 18000

    Time per degree= 1/18000= 0.00005555555

    Time for 720 degrees= 0.00005555555*720 = 0.04 seconds

      Fuel flow rate= 7.5 e-4*0.04=3 e-05 kg/sec

 

-Nozzles

   i) Nozzle Location and orientation -Cartesian

   ii) Nozzle diameter=0.00025m (Given)

   iii)Circular Injection -0.000123m (Injection dia= Nozzle radius)

    iv)Nozzle location set as per provided location

    v) Spray Cone Angle -10 degree (Usually between 10-15 degrees)

 

 

Computation of Maximum Injection pressure:- 

Peak Injection Pressure = 4.29908 (bar)

Peak Injection Velocity New = 29.7017 (m/s)

Validation of Nozzle Locations. To ensure the newly created nozzles were inside the computational domain, the Tools-Validate Nozzle location was selected upon which the below item was displayed.

 

-Combustion Modelling:- 

Combustion facilitates energy transfer in an engine. The Combustion model which is used for IC engines is the SAGE detailed Chemistry Solver. SAGE solver is the most predictive and accurate way to model combustion. It models ignition and laminar flame propagation quite accurately.

The SAGE model uses local conditions to calculate reaction rates based on the principle of chemical kinetics. it solves detailed chemical kinetics during combustion and determines kinetically limited phenomena such as engine knock and emissions. It reads in a reaction model in CHEMKIN format and solves ODE's to determine reaction rate. It also computes the rate of change of species concentration using the Arrehnius law. At each time step, the Chemistry solver calculates new species mass fraction immediately prior to solving transport equation. System of elementary reactions solved using CVODES.

 

Solving SAGE equations is a computationally slow process. In order to decrease the computational time, CONVERGE includes the following options

- Increase minimum cell temperature and minimum HC species mole fraction to reduce the number of cells in which combustion calculations are performes

- Use Start time and end time to limit SAGE to specific time intervals

-Limit SAGE to specific regions.

- Set resolve option to Only resolve if temperature changes by a specified value.

- Use analytical Jacobian to pass analytically calculated Jacobian to the solver.

- Multizone models are used to reduce number of SAGE calculations.

For the purpose of the simulation, the case was set up as shown below

The SAGE detailed chemistry solver parameters were set as follows.

The General setting values were set as shown below.

The combustion was set to start a few crank angles before ignition. The end time for combustion was set considering the opening of the  exhaust valve.

- Spark Plug modelling

In a typical spark discharge, the Voltage rises between two electrodes until there is a breakdown in the spark gap. The first stage is called the breakdown phase in which the mixture between the electrodes is ionized into plasma. The breakdown phase is follwed by the arc phase in which thin cylindrical plasma expands largely due to heat conduction and diffusion. The arc phase is followed by the glow discharge phase, energy storage device will dump its energy into discharge circuits. Time scales in the breakdown phase are significantly lower than the arc/glow phase. In converge simulation we specify two phases that briefly overlap.

 

In order to model the operation of the Spark Plug, source/sink modelling option was enabled. Two spherical sources were created with radius 0.5mm. The center of the sphere location was taken from the spark location

   - Shape

The general values were set as shown below. The spark start and end time were set from the values provided.

7. Grid control

 CONVERGE uses a number of tools for controlling grid size during a simulation. Fixed embedding refines a grid at specified locations and times. Grid scaling coarsens or refines the base grid size. Adaptive Mesh Refinement automatically changes grid based on  fluctuating and moving conditions.

- Base grid size was set to 0.005

- Adaptive Mesh Refinement

 Adaptive mesh refinement increases grid resolution based on curvatures (second derivatives in field variables). It can be permanent or activated at specific times.It is can be activated on a region by region basis.It can be activated for both scalar and vector fields.

CONVERGE calculates the Sub grid Scalar (SGS) field as

                            `phi'=phi- barphi`

where  `phi''= sub grid scalar field

           'phi'= actual field

           'barphi'= resolved field

CONVERGE uses truncated infinite series to approximate sub grid scalar field for any scalar.

where 'dx_k' is the spacing for a given rectangular cell.

For the full hydro set up two types of AMR is used

1. Velocity

Maximum no of embedding level is 3

SGS criterion is 1m/s

Whenever the curvatures exceed 1m/s, the regions would be automatically refined. 

Timing control type is Permanent.

- Temperature.

Max embedding level 3.

SGS criterion 2.5 K.Whenever the curvatures exceed 2.5K, the regions would be automatically refined. 

Timing control type- Cyclic 

-Fixed Embedding

Fixed embedding allows to locally refine mesh in the domain where a finer detail is critical to the accuracy of the solution.

An embedding scale is specified which indicates how CONVERGE will refine the grid in that location

dx_embed= dx_base/(2^(embed scale))

For the Full Hydro setup Fixed embedding was used at the below location

1. Intake Valve angles:-

    Entity type- Boundary

    Mode - Permanent

    Embed Scale = 4

    Grid size at refined locations = 0.005/2^4 =0.0003125

          

2. Exhaust Valve angles

  Entity type- Boundary

    Mode - Permanent

    Embed Scale = 4

    Grid size at refined locations = 0.005/2^4 =0.0003125

3. Liner Region

Entity type- Region

    Mode - Permanent

    Embed Scale = 2

    Grid size at refined locations = 0.005/2^2=0.00125

- Spark Plug

 

Entity type- Sphere

    Mode - Cyclic

    Embed Scale = 5

    Grid size at refined locations = 0.005/2^5=0.00015625

 

-Large Spherical

Entity type- Sphere

    Mode - Cyclic

    Embed Scale = 3

    Grid size at refined locations = 0.005/2^3=0.000625

- Injector

8.Results:

a) Compression Ratio: 

In a reciprocating engine the piston oscillates between the Top dead center and the bottom dead center. These correspond to the minimum and maximum values respectively. The ration between these two volumes is known as compression ratio.

 

From the above graph we, note that

Maximum Cylinder Volume = 5.74186 X10^-4 m^3

Minimum Cylinder Volume =5.73996 X 10^-5 m^3

`Compression Ratio = (5.74186 X 10^-4)/(5.73996 X 10^-5)``= 10.0

b)Calculating the Combustion effieciency of the Engine

Combustion effieciency is defined as the energy released by the burnt fuel to the theoretical energy content of the fuel

`eta_(combustion)=(Integrated Heat Release Rate)/(m_f*LHV)`

Integrated Heat Release Rate= 1241.15 J

m_f= Mass of Fuel, kg/sec= 3x10^-5 kg

LHV= 43.4 MJ/kg

 

 

 

 `eta_(combustion)= 1241/(3 X 10^-5 X 43.4 X10^6`= 95.3 %

c) Power & Torque Consumption

Power is defined as the rate of doing work.

    Work = 468.646 Nm

     Duration= 120.11+120.09= 240.2 degree

     RPM=3000

     RPS= 3000/60=50

     1 revolution/sec= 360 degrees

      50  revolution/sec= 50 *360= 18000 degree

      Time (sec)= 240.2/18000=0.0133444 sec

       Power = Work/sec= 468.646/0.0133444 =35119.301 J/sec

        Power = 35.119 KW

Torque

 The equation for Power is given below

`P=(2piNT)`

 N= revolution per sec

T= Torque

Re arranging for Torque

'T=(P)/(2piN)'

= 35.119*1000/(2*pi*50)

=111.878 Nm

The PV diagram for the engine is given below. Area under the PV diagram signifies the work done.

 

d) Need for Wall Heat Transfer model

Heat loss through the cylinder walls  has a detrimental impact on the performance of the engine. Some of the undesirable effects include reduction in volumetric efficiency, incomplete combustion etc. The description of heat transfer in an ICE is a challenging task, considering the different systems (intake and exhaust ports, coolant circuit, lubricant oil subsystem), the different heat transfer mechanisms (convection, conduction and radiation), and the rapid and unsteady changes that take place inside the cylinder

So there is a need to accurately model this phenomena. Due to the complexity involved in a mathematical solution, there is a need to adopt suitable modelling

 

e) Why we cannot use CFD  to predict wall temperature?

 (CFD) can provide precise and instantaneous information about the flow within the engine (temperature, pressure, velocity distributions), but require temperature boundary conditions for the engine walls, which are typically assumed as being constant throughout the engine cycle. This will affect the combustion process, thus leading to inaccurate results  Imposing appropriate wall temperature boundary conditions is not an easy task, and may require some iterative process. However, such calculations with given surface temperature are not sufficient when the focus is on analyzing the heat transfer within the engine. The heat flux and gas properties of the CFD are used as boundary conditions for the heat conduction calculation of the solid regions. 

f) Significance of  CA10, CA50 and CA90

The CA10, CA50 and CA90  are defined as positions of the crank angle at which cumulative heat release rate reaches 10%, 50 % and 90%. Physical significance of CA10 is that it is used to signify the start of ignition in a engine, while CA 50 signifies the end of pre mixed combustion and start of diffusion combustion of fuel used in the engine. Greater the difference between CA10 and CA50, higher is the duration of combustion and vice versa.

 

g) Other Plots

i) Cylinder Pressure:- The plot for cylinder pressure as function of crank angle is given below. It is noted that the cylinder pressure peaks during the compression stroke (compression occurs between -120 degrees to 120 degrees).

2) Mean Temperature:- The plot for peak mean temperature in the cylinder region as a function of crank angle is given below. It is noted that peaks of temperature and pressure co-incide and occur during the combustion stroke.

3)Liq Spray Drops (Cylinder region):-It is noted that maximum liquid droplets present in the cylinder is around 450000. This decreases after some time due to evaporation and combustion. Eventually this drops to a nearly zero value . From this we infer that combustion is nearly complete in the cylinder.

A similar trend is observed for IC8H18, The mass peaks at a certain crank angle after which it decreases due to evaporation and combustion and reduces to a near zero value. This indicates that combustion process does not leave any unburnt fuel in the engine.

4) Emissions:- The emission quantities of various gases as a function of crank angles is presented below.

 

h)Animations:

i) Meshing:- From the below plots we can see that regions of the mesh get refined in accordance Adaptive Mesh refinement and Fixed embedding criteria that were set during the case set up stage

 - Overall Mesh

    

 - Spark Plug region 

    

   Valves

   

 

i) Temperature:- 

 

In the above animation we can observe the variation of temperature with the over the course of the four stroke combustion process in the IC engine. As expected, during the combustion process, the temperature in the cylinder reaches the maximum. During the exhaust stroke,temperature in the exhaust port/valves registers an increase.

ii) Spray Modelling

The fuel injection in the engine during the four stroke combustion process is captured in the below animation. The movement of spray particles through intake ports and to through the cylinder is observed.

 

9) Conclusion

In this project a detailed simulation of IC engine has been carried out. In the case setup, spray and combustion modelling were setup. Movement of piston, intake and exhaust valves were created using regions and events. Mesh refinement techniques such as AMR and Fixed embedding were adopted to obtain finer results.

After simulation and post processing, engine performance parameters such as compression ratio, power and torque were computed and are listed below

Compression Ration=10.1

Power = 35.119 Kw

Torque= 111.878 Nm

Apart from this, other aspects such as cylinder mean temperature and pressure, liquid particles and emissions were studied as a function of a crank angle. Also, animations for temperature and spray modelling were examined,