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

Objectives:

1. To make the engine sector surface by providing the bowl profile. (sector is made to be computationally efficient, rather than simulating the entire domain).
2. To set up the case for spray, combustion, and emissions.
3. To post-process the results and perform a comparative study for the selected bowl configurations.
4. To evaluate engine performance parameters for both piston bowl sector(omega and open-w) geometries of a CAT3410 engine.
5. To evaluate the eission characterization of Soot, Nox and UHC & comparison

Geometry: The geometry of the piston-cylinder of the engine is as shown below.

The above geometry does not consist of any exhaust and inlet valves which is a closed cycle system. Moreover, for efficient computational simulation and to reduce time a sector(60 deg) of the above geometry is solved. To extract the sectors of the engines "Make engine sector surface" option from Tools was set up from where the bowl profiles of both the geometries(omega and open-w) are loaded and the sectors are obtained.

Bowl profiles of open-w geometry

Bowl profiles of omega geometry

 

Prior to extraction of the bowl profile of piston,  the geometric parameters of the engine and sector are set.

Parameter                                                        Dimensions

Bore                                                                   0.13716 m

Stroke                                                                0.1651 m

Connecting Rod                                                0.263 m

RPM                                                                   1600

Sector Angle                                                       60
Compression Ratio                                             17.5

The snapshots above represents the "Make engine sector surface" tool box in converge. After assigning the above cylinder geometrical parameters such as bore, stroke, con rod length, the surface.dat file was imported  and also imported was  the bowl profile. After extraction  the geometry is created with the specified angle. Both sectors are 60 degree angled. e. The sectors can be pulled based on spray to angle ratio. Suppose if the cylinder has 8 nozzles then 360/8 gives 45 deg. Or if 6 nozzles then 360/6 = 60 deg. This sector with 60 deg is 1/6 th of the whole geometry. This is done for smooth running of simulation and for the computational efficiency.

 This is how the extraction of sector surface looks like.

Open -W geometry

 

Omega geometry

Boundary flagging of open w

Boundary flagging of omega

Spray Modelling: There are two types of spray modelling techniques that Converge uses: Gaseous and Liquid Sprays. Gaseous sprays are modelled using Eulerian solver which is the cell-centred solver. For Liquid sprays, Lagrangian Solver is used to model discrete parcels and the Eulerian solver to model continuous fluid domain. These lagrangian parcels need to be transferred to the Eulerian or gas phase domain where heat, momentum and mass transfer is facilitated through source term in the species transport equations. The various links that Converge uses to get from Eulerian to Lagrangian and capture the physical processes is known as Sub-Grid Modelling. Lagrangian specification of the flow field is a way of looking at fluid motion where the observer follows an individual fluid parcel as it moves through space and time. Plotting the position of an individual parcel through time gives the pathline of the parcel. Heat, momentum & mass transfer occur between the discrete and continous phases via source terms in transport equations.

A parcel represents a collection of identical fuel drops that has the same properties. These parcels are introduced into the domain of the injector which statistically represent the entire spray field. These spherical parcels need to capture the liquid sprays which they do by undergoing several physical processes:

Primary Breakup: As soon as parcels are injected through the nozzle, it undergoes the first breakup which is known as Primary breakup.

Secondary Breakup: These primary spray field undergoes a secondary breakup where the parcels become even smaller.

Drop Drag: Parcels encounter turbulence which leads to the deformation of drops causing the vapourization to either become faster or slower.

Evaporation: This further reduces the size of drops.

Collision & Coalescence: The drops collide and breakup or they may coalesce to form larger drops.

Turbulent Dispersion: It states how the fluctuating velocity in the gas phase affects the drop.

Evaporation Model: The spray evaporation that takes place needs to be considered and for that converge uses certain standard pre-created models such as Frossling, Chiang, or with boiling. In this project, the Frossling model is used and it does not need to provide the particular equations as Converge takes care of it. Converge asks for evaporation source to simulate multi-component vaporization. The source used in this project is "Source all base parcel species" which denotes multi-component liquid species evaporate into the base species. As the evaporation source is specified, the maximum radius of ODE droplet heating needs to be specified as well, its basically a temperature discretization parameter which will control if drop temperature is uniform or radially varying. If the drop radius exceeds maximum value the converge assumes a radially varying temperature or else uniform temperature distribution is assumed.

Larger drops have a significant difference between the internal and external surface temperature of drops. This difference is ignored for drops of small size. Maximum radius for Ordinary Differential Equation droplet heating controls if the drop temperature is uniform or radially varying. Converge solves heat equation ODEs on the droplets by discretizing to create spherical Finite Volume Cells. Since it is time consuming, a maximum radius is specified where if the drop exceeds this value, Converge will assume a radially varying temperature and if the drop radius is small than the max value, a uniform temperature distribution will be assumed.

Spray Penetration and Distribution: Converge writes the liquid and vapor penetration lengths at each output interval, it calculates vapor penetration length for each nozzle. For each cell that meets the criteria that fuel vapor mass fraction exceeds the user-specified value or cell size does not exceed user-specified bin size, converge calculates the center of the nozzle to the center of the cell. The parcel distribution is done in two ways where the parcels are either distributed evenly throughout the cone or clustered near the center of the cone. Depending upon the requirement either distribution can be selected, for this project, even distribution is selected.

 

 

Penetration length

Just as how LPL is calculated, Converge calculates the Vapor Penetration Length (VPL) for each nozzle. The solver calculates fuel vapor mass fraction in each cell inside the spray cone. For each cell that meets the below two criterias, distance from center of nozzle to cell center of vapur mass is calculated.

• The fuel vapor mass fraction exceeds the user specified Fuel Vapor Mass Fraction.
• The cell size does not exceed the user specified Bin Size. A bin size of atleast twice the size of the cells in the spray cone must be present.

 

Parcel distribution

Converge assumes that injection is conical for solid cone sprays. The injector introduces parcels by either clustering them near cone center or distributing them evenly throughout the cone. By various imaging techniques it has been shown that gasoline is 'evenly distributed throughout cone.

When an injector sprays the fuel, the liquid fuel on contact with the outer atmosphere vapourises. This vapourization depends on the ambient temperatures. The liquid fuel would only penetrate upto a certain length due to high temperatures inside the combustion chamber. This length is known an the Liquid Penetration Length. It is the distance that encompasses the calculated penetrated spray mass that is arrived at by multiplying the mass fraction by total liquid mass in the domain. LPL is not calculated along the spray axis of the nozzle but along the injector axis.

Collision/Coalescence & Drop Drag: Drop Collision options allows to select how the drops will interact with each other. No Collision means that the two parcels will never collide and will only pass by each other. There are two other models, O'Rourke Numercial Algorithm and NTC Algorithm which dictate how the parcels will interact among themselves.If in a spray simulation parcels collide with parcels only in the same cell, it can lead to grid sensitivity, which is why converge has an adaptive collision mesh to reduce the grid sensitivity, Once collision mesh is checked, the parcels can collide across grid cells, requiring no additional mesh or any other setup. In this project, it is done using a standard NTC collision model & dynamic drop drag calculates the drag coefficient to account for variation in drop shape as shown in spray breakup. In the figure shown below, it is observed that parcels with collision mesh look more reasonable & natural, Othe drop options are spherical drop drag & No drop drag.

In a typical spray simulation, parcels can collide only with parcels in the same cell which can lead to grid sensitivity. By using an Adaptive Collision Mesh, this grid sensitivity can be reduced. It will also allow parcels to collide across grid cells.

When parcels enter through gas phase, they encounter drop-drag whose effects can be introduced in the simulation. If Spherical Drop Drag is selected, the drag co-eficient will be calculated assuming the drop is a perfect sphere. However, Dynamic Drop Drag Model is selected which calculated drag co-efficient by taking into account the influence of valocity and drag on drop shape.

 

Under Spray-Wall Interaction model, Wall Film was selected. This model takes a hybrid approach where the drop forms film on top of solid objects as well as encounters thermal breakup on hitting the wall. After defining all the parameters for parcels, the next step was to add an injector and 4 nozzles through which the liquid fuel would be sprayed. Injector species which is IC8H18 with mass fraction of 1 was added. Rate-shape is defined that would tell when and for how long the injector sprays fuel. Converge takes the total mass of the fuel specified and total specified time. Using this data it injects the mass of fuel in that specified duration.

Injector Models: The constants under Kelvin-Helmholtz model control how the parcel will breakup. Stability equation or dispersion will tell how long it will take for the disturbance on a parcel to go unstable. Since this equation has been derived in a simple manner, putting these constants will help increase or decrease the time to breakup and also size of child droplets. When K-H model predicts the size of child droplets, it gets multiplied by model-size constant for further breakup.

Once the spray breakup, & Parcel modeling is done, Injector and nozzles need to be specified, their locations, model, radius and such. One injector can have multiple nozzles and for this project there are 4 nozzles in one injector, This is done as per the engine manufacturer's requirement based on the amount of fuels and speed at which the fuel has to be sprayed inside the chamber.

Spray Rate Graph

This graph is obtained from tools - Spray rate preview, where we can see the graph of rate shape, peneteration velocity, injection pressure etc wrt crank angle provided in rate shape.

 

 The fuel is specified for this injector as DIESEL2 with mass fraction 1 as fuel is sprayed through the injector. Kelvin-Helmholtz & Rayyleigh Taylor models are used. The discharge coefficient recommendation for DIESEL - correlation for Cv - 0.7 is set.

Collision Break up/Drag

Model-Collision Model, Drop Drag Model

Nozzle Location:

 Nozzle Diameter- 0.000259 m

Injection radius- 0.0001295 m

 Nozzle radial position -0.00097 m

Fuel and Parcel Simulation:

The fuel used in this simulation is Diesel2 / C7H16, the composition of both is same Diesel;2 in converge is more refined for simulation. Parcel simulation specifies the fuel used in the simulation.Once chosen. Converge imports all it’s properties as shown below.

Combustion Modelling:

Combustion facilitates the energy transfer in the engine. Converge contains detailed chemistry solver and simplified combustion models.

In the present project, SAGE model is used which solves detailed chemical kinetics .

 SAGE: The SAGE detailed chemistry solver is a general combustion model used to solve the detailed chemical kinetics in the combustion process. It can accurately model ignition and laminar flame propagation. It uses local conditions to calculate reaction rates based on the principles of chemical kinetics. Also, determines kinetically limited phenomenon of engine knock and emissions.

It requires a CHEMKIN-formatted input fils to solve the reaction rate ODEs. The ODE solver used is called as CVODE solver. SAGE couples with transport solver via source terms in the species transport equations. A chemical reaction mechanism is a set of elementary reactions that describe an overall chemical reaction. The combustion of different fuels can be modeled. SAGE calculates the reaction rates for each elementary reaction while the CFD solver solves the transport equations. Given an accurate mechanism, SAGE (in addition to AMR) can be used for modeling many combustion regimes (ignitions, premixed, mixing-controlled). You can use SAGE to model either constant volume or constant pressure combustion.

As SAGE is a detailed solver, it can take enormous amount of time to solve so Converge has accelerating options that require increasing minimum cell temperature and min HC species mole fraction to reduce the number of cells in which combustion calculations are performed. Using start time and end time to limit SAGE to a specific time interval and can limit the  SAGE operation to a specific region. Other options include, Setting resolve only if temperature changes by specified value ~ no greater than 2 K. & using analytical jacobian to pass an analytically calculated jacobian to solver. SAGE cal also reduce time by using multizone modeling where it groups cells into bins (zones) for which appropriate options are provided.

Full Hydrodynamic Case Setup:

Boundary conditions:

Regions and initialization: All the boundaries are assigned to the single region.

 

Grid Control Setup:

Adaptive Mesh Refinement is a method of adapting & enhancing the accuracy of a solution within certain sensitive or turbulent regions of simulation, dynamically and during the time the solution is being calculated. When solutions are calculated numerically, they are often limited to pre-determined quantified grids as in the Cartesian plane which constitute the computational grid, or 'mesh'.

 

 

Converge calculates the second order of the variable with the application of AMR embedding and while calculating if the difference is above the Sub-grid criterion, it will perform a refinement of the cells with embed level as provided. The base grid is 0.004 m.

Velocity SGS: It is permanent with an embedding level of 2 for SGS of 2 m/s.

Animation of Velocity-SGS with Advanced Mesh Refinement:

Omega

Open-W

Temperature SGS: It is sequential with an embedding level of 2 for SGS of 5K after the fuel is injected into the cylinder.

Animation of Temperature SGS:

Omega

Open-W

Fixed Embedding Layers:

Total cells:

This is the total cells graph in accoprdance wityh meshing. It can be observed that how the mesh first decreased then increased. This graph is in accordance with AMR. Omega geometry produces slightly more highest cell count than open w geometry but toward the end it is opposite.

Animation of the movement of mesh:

 

 

Timing Map: The timing map is like a pathway that lays out all simulation timing details into something like process charts which displays which process is activated at a particular time. It is a handy tool to verify the activities in one place.

Post Processing Results and Discussions:

After completing the case setup and modeling, the simulation was run using the CYGWIN command line terminal on a 6 core machine. Once it is simulated the data is post-processed using ParaView.

In-Cylinder Pressure: The cylinder pressure contains different values at different strokes of the cylinder, the highest of which is observed in the combustion/Expansion stroke. This stroke is where the ignition of the air-fuel mixture takes place, creating very high pressure. This is where the engine power is generated. The peak pressure is 11.53 MPa and 11.25 MPa for Open-W and Omega type of piston, respectively. They are extremely close with a difference of only 2.48 %. There is also slight delay noted in omega pistons pressure rise & drop.

In-Cylinder Mean Temperature: Maximum temperatures are obtained at the combustion/expansion stroke. The temperature obtained by Omega piston is higher than the Open-w piston.

 

Open-W~Temperature animation

Omega~Temperature animation

 

Volume & Compression Ratio: The volume vs crank angle graph plotted in the cylinder region is as shown below.

 

Compression Ratio: Maximum Volume/Minimum Volume

                                                          Open W           Omega

Minimum Volume                                0.000148        0.000163

Maximum Volume                               0.002447        0.002463

Compression Ratio                              16.5338         15.1105

 PV Plot:

The I.C engine follows a diesel cycle. Contrary to the theoretical cycles, nothing is constant here,pressure rise exists in the initial combustion phase. Since only compression, expansion and  combustion stroke are simulated , the motion of  intake and exhaust plots are not available. The diesel I.C engine differs from the gasoline powered Otto cycle . This is because of the usage of highly compressed hot air to ignite the fuel rather than using a spark plug which is actually the spark ignition.

 

 

Engine Performance Calculator for both geometries.

Omega

Open W

                                                                                              Open W           Omega

Total Gross Work :                                                                 3028.53           409.28

Duration of Cycle( in degree)                                                  270.156           270.171

RPM                                                                                      1600              1600

RPS                                                                                       26.67             26.67

Time for rotation of each degree(in s)                                       1.04e-4           1.04e-4

Time for each cycle (in s)                                                         0.02815         0.02815

 Number of Cycle in every  minutes (60/time for each cycle)       2136              2135  

Net Power (work/time)  (in KW)                                               107.792         121.335

Torque =   `(P*60)/(2piN)`                                                      643.33            724.164

 

Heat release rate & combustion efficiency:

Heat release & Integrated heat release is plotted as shown below.

It gives the amount of heat i.e., the total amount of heat released from the combustion process. From the total heat release in the combustion chamber, the combustion efficiency of the engine can be determined.The heat energy released rate is very high in the omega piston as compared to open W which means the temperature is higher in omega. The lower heat release rate in open W indicates some unburnt fuel or delay in burning due to some geometrical parameters as between the two case setup there is only difference in geometry.

Integrated Heat Release Rate:

We can see that at the end of the plot the openW engine ends at a lower heat  release rate. From the engine performance parameters above & Integrated HR data we can compare  the efficiency between the two and it is seen that it is lower in open w than omega piston.

 

Combustion efficiency is defined as the ratio of total heat energy released by the burning of fuel to the total energy content of fuel mass in a cycle.

Mass of fuel injected = 1.621e-4 kg

 Calorific value of fuel = 45.6e6 J/kg

 Integrated heat released for Open-W= 7212.06 J

Integrated heat released for Omega = 7279.46 J

Combustion Efficiancy:

Open-W -0.97568

Omega- 0.98481

Emission Characterisation:

Gaseous Fuel Mass: For the Omega piston, the larger amount of gaseous fuel is present which is burnt lately. Hence, the temperature increase will be lower for over a duration of the cycle rather than a sudden increase.With the injection of the gaseous fuel, there is a steep increase in Omega piston gaseous fuel intake which is because of the design. There is significant peneteration length in the omega w piston design which as a result allows it to vapourize in the high temperature whereas a lot of fuel impingement occurs in  openW piston, as a result high vapour is not  formed, yet as observed the fuel combustion takes place in the end due to high temperature. Another advantage of omega type design is the swirl motion it creates as observed from above attached  animation of mesh refinement - velocity in Grid control, The circular swirl helps again in vapourization of fuel which leads to higher burning in Omega piston design than in  openW.

Liquid Spray Mass:

Liquid Spray Parcels:

As can be observed from the figure, after the injection of fuel for the Open-W piston the liquid parcels do not evaporate quickly which can be observed in the animation as well. Due to delayed evaporation, the efficiency and Torque produced are less in comparison to Omega Piston.

 

Pollutants:

CO Emissions: Open-W has higher CO & HC emissions due to the presence of unburnt fuel which is shown in the animation due to fuel impingement in the Open-W piston. Higher peak is obtained by omega piston however it decreases right after combustion, but there is a significant delay in OpenW piston engine as the fuel remains unburnt for a while.

Hiroy Soot: The mass of soot calculated using the Hiroyasu model is shown below.What we can observe is ass soon as the combustion process starts soot starts to accumulate in small quantity then it reaches it's peak value when the pressure is maximum  then
dissolves and averages and becomes steady attaining a certain value. These are generated in the fuel rich zones within cylinder during combustion & come out as exhaust smoke.

The curves with circle markers belong to the Omega Case. It can be observed that the soot formed by an Omega piston engine is higher in comparison to the Open-W profile it is because the soots get oxidized at a higher temperature. The soot formed in OpenW piston is significantly higher than in Omega which is as predicted as there is unburnt fuel at the beginning of combustion in openW and hence is fuel rich zone at a high temperature. Approximately 25 - 29 % difference only because of difference in piston bowl design. At the end of the combustion as well openW leaves with a significantly higher Soot which will pass through exhaust.

Animation of Hiroy soot  in 

Omega

open-W

NOx Emissions: With the higher temperature in Omega piston and higher heat release, the NOx is significantly higher here than open W. Diesel fuels contain more nitrogen than gasolines and hence they produce higher Nox. The below graph is generated by Zeldovich Nox Model. NOx production is low in OpenW however soot is higher. The critical time period is when burned gas temperatures are at a maximum i.e between start of combustion and after a short while of the attainment of peak cylinder pressure.

Omega

 

open-W

 

Conclusions:

1. It is evident from the spray wall interaction animation, due to injector angle in the Open-W piston engine fuel impingement takes place which leads to lower Torque and Power generation in comparison to the engine with Omega piston. Due to faster combustion in Omega piston, efficiency and the power generated are higher than the Open-W piston.
2. Due to higher temperatures in Omega Pistons, Soot and NOx emissions are higher. CO and HC emissions are higher in the Open-W pistons due to unburnt fuel (i.e., due to fuel impingement which is nothing but that the fuel sticks to the profile of piston).
3. So conclusively to obtain the higher performance of the Open-W piston injector angle needs to be corrected to avoid the fuel impingement.
4. So if primary goal is to reduce NOx and soot formation then OpenW has an edge here. On the other hand OpenW geometry faces issues like unburnt fuel during combustion and due to higher fuel impingement, there is a delay in combustion due to very less vapourization of fuel as compared to omega geometry leadsing to the formation of unburnt species.
5. It is critical to perform the spray parcel & combustion modeling accurately with recommended values for smooth flow of simulation and to land into correct results, otherwise the data obtained will have no valuie.
6. Grid control parameters like embedding and AMR are also critical in Converge and should be used in an efficient way so that productive results are obtained as mesh determines the results accuracy and since AMR in specific sensitive areas with coarsening of mesh in areas which is not of major importance is critical to reduce over all simulation time for such a complex and detailed case seup.