Week 11: Project 2 - Emission characterization on a CAT3410 engine

Aim: Project 2 - Emission characterization on a CAT3410 engine

Objective

Your job is to run simulations with both these pistons and characterize the emissions (Soot, Nox, and UHC)

• Create cut-plan animations showing Soot, Nox, and UHC and compare them between the omega and the open-w pistons. 
  • Upload animations on Youtube and provide the Youtube link for animations in your report.

• Compare the IMEP and Power values graphically

Introduction

The Deisel Engine is an IC Engine in which the ignition of the fuel is caused by the ignition of the air at the elevated temperature of the air in the cylinder due to the mechanical compression, thus the diesel engine is a so-called compression-ignition engine. The air temperature in the cylinder is elevated to such a high degree that atomized diesel fuel injected into the combustion chamber ignites simultaneously.

Application

The application of diesel engines includes

• Passenger cars
• Commercial vehicles and lorries
• Railroad rolling stock
• Watercraft
• Aviation
• Non-road diesel engines
• Stationary diesel engines

Case-setup

Make engine sector surface: The Converge provides a convenient option to simulate the sector surface engine, as the whole diesel engine will be tedious to simulate and time-consuming, and moreover, the results from the sector surface are reliable. In this challenge, the diesel engine sector surface is produced by using two-piston profiles which are omega piston and, open w piston.

The surface generated by the Converge are shown below

Omega piston

Open w piston

The boundaries are flagged automatically by using the sector surface.

In the Application Type the Crank angle based IC engine is selected and the following information was given to the Converge

In the Gas simulation, Redlich Kwong Equation of State was selected with a critical temperature of 133K and critical pressure of 3770000Pa. Therm.dat and the gas.dat were uploaded in the respective domain.

In the Parcel simulation Diesel 2 was selected as the predefined liquid and , the graph of liquid properties as a function of temperature was plotted.

The Turbulent Prandtl number and the Turbulent Schmidt number were set to 0.9 and 0.78 respectively under Global transport parameters.

In the Reaction mechanism, the mech.dat file is uploaded with the following available elements and the species

In the Species, Diesel 2 was selected under Parcel and, HIROY_SOOT and the NOx with Schmidt number as 0.78 were selected under Passive.

In Run parameters, the Transient solver is selected, the Crank angle-based engine simulation is selected under Temporal type. The Simulation mode is Full Hydrodynamic. The Gas flow solver is Compressible. The Liquid flow solver is Incompressible. In Misc. Use shared memory is not selected, the rest of the selections and the values are kept as default.

Simulation time parameters

Solver parameters [Transient]

The NS solver scheme is PISO and the solver type is Density-based and, the rest are default settings by Converge.

Boundary conditions

The boundaries were flagged and their conditions are tabulated as under 

Regions and initialization

The In_cylinder_region was created with the following parameters as shown in the below figure

Physical Models

Setting up of Turbulence modeling

The RNG k-epsilon model was selected and its default values are used.

 

Spray modeling

The Converge can model both liquid and gaseous sprays and it has a model through which it captures how the spray is taking place inside the combustion chamber through liquid atomization, drop breakup, collision, coalescence, turbulent dispersion, and drop evaporation. The usage of models illustrates the fact that it is completely impossible to simulate the spray by solving the equation.

The liquid sprays are made up of a bunch of parcels, which is a spherical structure that contains a lot of drops, and together it constitutes a fuel spray. This modeling is termed Langrangian modeling or DPM, the information travels from the gas phase (Eulerian phase) to the liquid phase (Langrangian phase). The heat, momentum, mass transfer occur between the two mediums and they are taken into account by adding suitable source terms in the transport equation. The Converge introduces parcels into the Injector domain, these parcels are made of tiny droplets which are having the same physical properties(radius, velocity, temperature, etc). The Converge solves the equation on a per parcel basis rather than on a drop basis (since drops are identical).

Sub Grid Modeling 

Once the parcel is injected into the domain the big parcel droplet breaks into the tiny droplets which are known as a Primary Breakup. These droplets are again broken into more tiny droplets which are known as a Secondary Breakup. When these droplets react with the ambient air their shape gets affected which is known as Drop Drag. After the Drop Drag, the size of the droplets gets reduced further by Evaporation. The fluctuating velocity in the gas phase is governed by the Turbulent Dissipation.

Setting up of Spray Modeling

The Spray Modeling can be set up by following these steps Case setup>Physical Models>Spray modeling. For setting up of Parcels Case Setup>Materials>Species>Parcel and select DIESEL 2. For setting up of Parcel Distribution the Converge assumes that the injection is conical, Distribute parcels evenly throughout the cone is selected. For setting up of Evaporation model the Converge expects us to tell that at what rate the radius of the droplet is decreasing, and the rate at which the mass is being transferred, the liquid droplet vaporizes and there is an additional gas mass that needs to be added as the source term in the species equation. The Converge provides two models for evaporation that is Frossling and Chiang models. The Frossling model is selected. The maximum radius of the ODE droplet has been set up by Converge as 1000m which is being divided into the number of spheres which is 15. If the radius of the droplet exceeds the maximum radius then Converge will assume radially varying temperature distribution. If the drop radius does not exceed the maximum radius, Converge will assume uniform temperature distribution.

The Penetration tells us that how much the liquid has penetrated in the combustion chamber from the injector which depends on the ambient temperature in the combustion chamber. If the ambient temperature is 800K, or more the liquid vaporizes into the gas phase, so after some time the steady-state is achieved and there is no further penetration. It is quite possible to achieve the penetration length using the three parameters, "Liquid fuel mass fraction for calculating spray penetration" for example if we calculate the mass fraction from where the fuel is injected and we reach the point which is 95% of the mass fraction of the fuel then the value is termed as Liquid fuel mass fraction for calculating spray penetration. "Bin size for calculating vapor penetration", "Fuel vapor mass fraction for calculating vapor penetration". In order to calculate the VPL, the cell is chosen which should contain the minimum amount of vapor in terms of the mass fraction which is 1e-3 anything less than that should be considered that the cell has no vapor. The value of the Bin size is selected in such as way that the cell size is two times smaller than the Bin size especially in the location of the spray.

Mass diffusivity can be expressed mathematically by 

`rho_(gas)**D=1.293D_0(frac{T_(gas)}{273})^(n_0-1)`

`D_0` `&``n_0` values can be selected from the drop-down menu provided by the Converge.

In the Collision model, we have three types of algorithm which is No collision, O'Rourke collision, NTC collision. Based upon the engine and user experience the type of model is selected. Similarly in the collision outcomes, there are two types of outcomes which are O'Rourke collision outcomes, Post-collision outcomes which can be selected on the basis of the engine and user experience. The collision mesh is an important option that is selected. When we are going to perform a collision calculation we need another mesh apart from the computational mesh. This feature is independent of the gas phase grid and is used for performing collision calculations and improves the accuracy further.

Drop drag model: When the parcel is entering through the gas phase it encounters air resistance. In the spherical drop drag model, the drag coefficient calculation assumes that the shape of the parcel is spherical. The dynamic drop drag model accounts for the variation of the shape of the parcel based on the velocity and the drag. It invokes the TAB model to determine the drop distortion.

In the Spray wall interaction model, the Converge provides three options which are Rebound/Slide, Vanish, Wall film. For Diesel Engine simulation Rebound/Slide model is selected.

Injector setup

In Spray modeling, the Injector is selected and then the injector is added. The Edit option is selected from the same dialogue box. The Parcel species is chosen as DIESEL2 with a mass fraction of 1. A trapezoidal profile injection rate shape is provided which is shown below

The Kelvin-Helmholtz model(KH) and Rayleigh-Taylor model are selected for the solid cone sprays. The discharge coefficient model is selected and the discharge coefficient value is 0.8.

Time/Temp/Tke/Eps/Mass/Size

Nozzles

The polar type nozzle location and orientation are selected. The information of the nozzles are listed below

Nozzle diameter (m): 0.000259

Nozzle radial position (m): 0.00097 

Circular injection radius (m): 0.0001295

Spray cone angle (deg): 9

Nozzle axial position (m): 0

Nozzle azimuthal position (deg): 0

Combustion modeling

There are several simplified models which converge uses to simulate the combustion process in an Engine, which are SAGE detailed chemistry solver and Simplified combustion models. The SAGE model is the most predictive and accurate way to model ignition and laminar flame propagation. The Simplified model is less predictive and computationally less expensive than SAGE and is applicable for specific applications. The SAGE solver utilizes the values of equivalence ratio, temperature, and pressure to solve the reaction rates based on the chemical kinetics. The Converge calculates the rate of change of species concentration for all the species from the mech.dat files. The Converge uses the species transport equation to solve ODE. The rate of change of species terms is added as the source term in the species transport equation and computes how the species mass fraction changes during the combustion.

The equation is summed as 

`frac{delbarrhotildeY_m}{delt}+frac{delbarrhotildeu_jtildeY_m}{delx_j}=frac{del}{delx_j}(barrhoD_tfrac{delbarY_m}{delx_j})+tildeomega_m`

`frac{delbarrhotildeY_m}{delt}+frac{delbarrhotildeu_jtildeY_m}{delx_j}` is the generic species transport equation, it dictates how the species which are the part of the computational domain is conserved.

`tildeomega_m` is the source term which is calculated by chemistry solver which is solved by CVODES (ODE solver)

SAGE solver setup

Start time: In an IC engine simulation the start time denotes when the combustion will start, i,e, when the spark will ignite or when the fuel is going to be injected.

End time: In an IC engine simulation the end time denotes that when the exhaust valve will open.

The SAGE can be turned off in certain regions to speed up the solver.

Minimum species HC mole fraction: In order to have the combustion started we need to have the right temperature, pressure, and most importantly there is a requirement for the correct combination of fuel and air mixture. If we have an HC-based fuel, in order to have the combustion started we should have the minimum amount of hydrocarbon present in the computational mesh. If the mass fraction of HC in a cell is less than 1e-08 then combustion will not take place.

SAGE acceleration options: The SAGE solver is very slow, it is three times slower than the transport equation (momentum, turbulence, and species), and solving for a refined mesh becomes very tedious. There are few things that can be done to speed up the SAGE solver. The Re-solve temperature value is set to 2K only if the temperature in the cell exceeds 2K the SAGE solver is executed. The Analytical Jacobian solver is selected.

The other technique employed by the Converge to accelerate the SAGE solver is using the Multizone model. The new computational mesh is created with `phi` and `T` space with `phi` be equivalence ratio and T be temperature. A bin is created which contains a computational mesh and the chemistry for the bin is solved and not for the individual cells which speeds up the calculation because the number of bins is substantially lower than the number of cells. Conserve NOx is selected.

Base grid 

The base grid of 4e-3 m was used for simulation

Fixed embedding

Nozzle embedding

Piston embedding

 

Cylinder head embedding

Adaptive mesh refinement

Adaptive mesh refinement is a technique that is used to refine the grids automatically based on fluctuating and moving conditions such as temperature or velocity. The feature is used to refine the grid in the flow region of interest such as flame propagation or high velocity without slowing down the simulation with a globally refined grid. There are two types of AMR, mainly sub-grid scale-based and value-based.  The AMR type and criteria are chosen in such a way that the embedding is added where the flow field is most unresolved.

AMR Group-1 is velocity type AMR in which SGS is 2 m/sec. AMR Group-2 is temperature type AMR in which SGS is 5K. For the cell whose value is greater than the SGS criterion then AMR is going to refine the mesh in that region. 

 

 Comparison between omega piston and open w piston

Pressure

 Temperature

 Integrated_HR(J)

HR_Rate(J/Time)

C7H16(Kg)

HIROY_Soot(Kg)

NOX(Kg)

HC(Kg)

 

CO(Kg)

CO2(Kg)

Animation

Spray wall interaction omega piston

C7H16

Temperature

Pressure

NOX emission

HIROY_SOOT

Spray wall interaction open w piston

C7H16(Kg)

Temperature

Pressure

NOX emission

HIROY_SOOT

Engine Performance Calculator for omega piston

 

Engine Performance Calculator for open w piston

 Comparison of IMEP values for open w and omega piston

Comparison of Power for open w piston and omega piston

Open w piston

Gross work done=415.097N-m

Duration(deg)=270.289

RPM of engine=1600

RPS of engine=26.66

Time per 1 degree=0.00010417

Combustion duration in seconds= Time per 1 degree*Duration(deg)=0.0281551

Power=Gross work done/Combustion duration in seconds=14743.2259 W

Omega piston

Gross work done=480.311N-m

Duration(deg)=270.21

RPM of engine=1600

RPS of engine=26.66

Time per 1 degree=0.00010417

Combustion duration in seconds= Time per 1 degree*Duration(deg)=0.02814778

Power=Gross work done/Combustion duration in seconds=17063.9034 W

 

 Conclusion 

• The emission characteristics for both the pistons were simulated using Converge-CFD and Cygwin command prompt terminal.
• The maximum pressure for the omega piston is 12.2974Pa and, the open w piston is 11.6737Pa.
• The maximum temperature for the omega piston is 1722.34K and, the open w piston is 1510.44K.
• Ignoring the fluctuations in the plot the heat release rate is higher for the open w piston than for the omega piston.
• The integrated heat release rate in Joules is higher for the omega piston than for the open w piston clearly indicating that the combustion efficiency for the omega piston is higher than the open w piston.
• The bar graph of the Power and the IMEP values are higher for the omega piston.
• HIROY_SOOT emission is higher for open w piston which is 1.62895e-06Kg whereas for the omega piston the soot emission is 1.20348e-06Kg.
• NOX emission is higher for the omega piston which is 6.59363e-07Kg whereas for the open w piston the NOX emission is 5.05488e-07Kg.
• The HC emission produced by the open w piston is 1.30289e-05Kg whereas the HC emission for the omega piston is 1.12324e-05Kg.
• From the C7H16 plot, it was observed that the fuel (gaseous) is more for omega piston than for open w piston which indicates that there is unburnt liquid fuel inside the engine in the case of open w piston. The piston design or the spray wall interaction can be attributed to this.
• From the spray wall interaction animation, it was observed that for the open w piston the spray is directly impinging on the bowl whereas, for the omega piston there is a spray penetration after which it impinges on the wall. Due to which there is delayed vaporization in the open w piston, an obvious reason for lower temperature and, higher emission rate in terms of SOOT and HC.
• Lastly, both the piston are able to produce similar pressure. The main hindrance lies in the spray wall interaction for the open w piston. Similarly by adjusting the spray angle and spray location the performance and efficiency of the open w piston can be enhanced.