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.
Geometry :
The surface preperation and boundary flagging of the geometry are done in the previous half of the project.
Once the desired error-free geometry is ready, the full hydro case can be set up using the information given in the challenge.
Case set up for Full Hydro Simulation :
The application type is set to Crank angle based as we are simulating an IC engine cylinder.
The measurements for the cylinder are specified by refering to the given data file.
Cylinder Bore : 0.086 m
Stroke : 0.09 m
Connecting rod length : 0.18 m
Crank speed : 3000 RPM
Specify the reference boundaries as given below, so that the motion of the simulations takes place at the right boundaries.
Import the mech.dat and therm.dat in the materials tree to specify the reactions taking place between the materials inside the cylinder during combustion.
In the run parameters, set up a transient simulation and select No Hydrodynamic solver as simulation mode. The gas flow solver is set to be compressible.
The simulation time parameter is set from -480 degree to 240 degree as the simulation is made to run for a total of 720 degree angle.
Boundary Condition :
The liner, cylinder head, spark plug, spark plug terminal, exhaust port, Intake port, Inflow & Outflow are set as fixed non moving walls with different temperature profiles which are taken from the given data file.
Liner temperature - 450K
Head temperature - 450K
Spark Plug temperature - 550K
Spark Plug electrode temperature - 600K
Exhaust ports temperature - 500K
Exhaut outflow - 1 bar
Exhaust outlflow temperature - 800k
Exhaust species concentration - N2(0.71913), CO2(0.19235), H2O(0.088525).
Intake port - 1 temperature 425K
Intake port - 2 temperature 425K
Inflow pressure - 1 bar total pressure
Inflow temperature - 363K
Inflow species - Air
The piston is set to be a moving wall and it is considered to be following the piston profile.
Piston temperature - 450K
The exhaust valve top, bottom and angle are also taken as moving walls following the profile of a given data file (Exhaust_lift.in).
Exhaust valve top temperature - 525K
Exhaust valve angle temperature - 525K
Exhaust valve bottom temeprature - 525K
Similarly the intake port is specified to follow Intake_lift.in.
Intake valve top temperature - 480K
Intake valve angle temperature - 480K
Intake valve bottom temeprature - 480K
Region, Initialization & Events :
The geometry is split into 4 different regions namely, Cylinder region, Intake port-1, Intake port-2 & Exhaust port.
All the regions have different temperature and pressure values.
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
N2 - 0.71913
CO2 - 0.19235
H2O - 0.088525
Exhaust Port :
Pressure - 1.03737 bar
Temperature - 1215K
N2 - 0.71913
CO2 - 0.19235
H2O - 0.088525
The events is set for three different pairs of regions.
The cylinder head -- Intake port-1 is set as a valve event following intake_lift.in as the motion profile.
The cylinder head -- Exhaust port is also set as a valve event following exhaust_lift.in as the motion profile. Both of these events are set under cyclic events.
Under permenent events, Intake port-1 -- Intake port-2 are set to open event permenently.
Turbulence & Physics modeling :
The first step in physics modeling is to set up the injection of fuel into the cylinder at a specific interval of time.
Spray Modeling :
CONVERGE includes state-of-the-art physical models for a wide variety of phenomena, including chemistry, turbulence, multiphase fluid flow, spray, and radiation, and it automatically couples these models so you don’t have to. If you’re looking for a shorter runtime, you can choose one of CONVERGE’s simplified combustion models. If accuracy is paramount, CONVERGE’s detailed chemical kinetics solver will help you obtain the results you need. For turbulence modeling, you can choose from a host of both RANS and LES models. For spray modeling, CONVERGE can capture physical processes from liquid injection and spray breakup to drop/wall interaction and evaporation. CONVERGE also includes several volume of fluid modeling options to simulate multi-phase flows.
Select the spray modeling parameter in the physics models. In the spray modeling we are using O'Rourke model to initialise the spray of fuel.
The species to be injected is selected as the fuel element IC8H18, which is selected as the parcel particle in the materials properties.
The following parameter values are introduced for the specifications of fuel injection.
Fuel flow rate = 7.5e-4 Kg/second
Injection start time = -480.0
Injection duration = 191.2
Fuel temperature = 330K
The Kelvin-Helmholtz (KH), Rayleigh-Taylor (RT) & Discharge coefficient models are being used for the injection.
Here, we are using four nozzles for spraying the fuel into to the cylinder with similar specifications as follows.
Nozzle diameter = 250 micro-meter
Circular injection radius = Nozzle radius
Spray cone angle = 10
To initialise this process, Coordinates of the injectors are needed to decide the location at which the fuel is being emerged into the cylinder. The injectors will be located in the import region-1 which is located near to the cylinder.
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]
Combustion Modeling :
CONVERGE’s SAGE detailed chemistry solver uses local conditions to calculate reaction rates based on the principles of chemical kinetics. This solver is fully coupled to the flow solver, but the chemistry and the flow solvers parallelize independently of one another, which speeds up the simulation. With the appropriate mechanism, the SAGE solver can predict a wide range of cases (e.g., a variety of fuels [premixed, non-premixed, partially premixed, multiple fuels], emissions modeling, and unique phenomena such as end-gas auto-ignition). With its accuracy and robustness, CONVERGE can perform predictive modeling instead of merely confirming experimental results.
The next step in the modeling is the combustion modeling parameter. The values for the combustion properties like start time, end time & species is to be specified as given in the image below.
The combustion model here is taken as the SAGE model. The turbulence model is taken as the RNG K-Eppsilon model.
Source Modeling :
Then the source of power should be specified in order to ignite the incoming fuel during the compression of the fuel. For this select source modeling under the physics modeling.
In the source modeling add two sources where both the sources have a spherical shape.
Spark location = -0.003 0 0.0091
Spark radius = 0.0005m
The spark location and radius specifies the spherical volume where the ignite starts.
The time interval for the spark is given in degree.
There are two sources acting for a period of 10 degrees, where for a duration of 0.5 degree both the sources act simontaneously.
This source will ignite the fuel when it is compressed inside the cylinder.
Grid :
The grid size is taken to be 0.004 m and fixed embeddings are done at critical points.
The valve angles are provided with a scaling of 3 as they are moving so close to the walls of the cylinder head.
The mesh inside the cylinder is also refined to a scale of 2 as the combustion can be captured properely.
Fixed Embedding :
To refine that critical area, we can use Fixed Embedding around the Throttle Boundary.
fixed embedding is used to refine the grid at specific locations in the domain where a finer resolution is critical to the accuracy of the solution. Fixed embedding allows the rest of the grid to remain coarse to minimize simulation time.
For each fixed embedding, you must specify an embedding scale that indicates how CONVERGE will refine the grid in that location. The embed_scale parameter, which must be a positive integer, scales the base grid sizes (dx_base, dy_base, and dz_base) according to
dx_embed = dx_base/2^embed_scale.
Note that CONVERGE requires two-to-one connectivity between cells, i.e., a cell with an embed_scale of 2 can be adjacent only to a cell with an embed_scale of 1, 2, or 3. To maintain the required connectivity, CONVERGE provides cells with intermediate embed_scale values as necessary.
The volume around the ignition source has to be refined to capture the heat transfer more accurately. Two spheres with radius 3mm & 1mm are used around the ignition to undergo embedding.
Likewise some volume around the nozzles of the injector are also embedded using injector embedding option.
Adaptive Mesh Refnement :
CONVERGE’s innovative Adaptive Mesh Refinement (AMR) technology automatically adjusts the grid at each time-step, adding cells in areas with complex phenomena and eliminating cells that are not needed to yield accurate results. This strategic refinement ensures that CONVERGE can resolve flame fronts and high-velocity flows while minimizing the overall cell count.
This Adaptive Mesh Refinement (AMR) process adds local refinement in areas of steep field variable gradients and is now also being pursued by many of the other leading CFD software OEMs. The company calls the process "Autonomous meshing" and argues the following benefits:
By eliminating the process of manual mesh creation, inspection and refinement, time is saved
Accuracy is improved through higher grid density where it is needed and where manual meshing strategies might not allocate it
AMR provides a robust method for capturing large or rapid boundary motions
AMR provides run-time savings due to using an optimized number of computational cells
Consistency and standardization can be achieved through the use of a consistent, algorithmic strategy for mesh production.
Adaptive mesh refinement is done to capture the places at which the properties of the calculation are critical.
The refinement is made to be adaptive for velocity and temperature.
The parameters for both the properties are specified so that the mesh adapts the values from the specification to refine the places where the values tend to be within the specified parameter.
Using these specifications, the calculation for the flow simulation can be done using cygwin.
Once the calculation are complete the properties of the fluid flow can be post processed using para view.
Using these information & outputs, we can calculate the engine's performance.
Expected motion of the Engine :
an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four separate strokes are termed:
Intake: Also known as induction or suction. This stroke of the piston begins at top dead center (T.D.C.) and ends at bottom dead center (B.D.C.). In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing vacuum pressure into the cylinder through its downward motion. The piston is moving down as air is being sucked in by the downward motion against the piston.
Compression: This stroke begins at B.D.C, or just at the end of the suction stroke, and ends at T.D.C. In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke (below). Both the intake and exhaust valves are closed during this stage.
Combustion: Also known as power or ignition. This is the start of the second revolution of the four stroke cycle. At this point the crankshaft has completed a full 360 degree revolution. While the piston is at T.D.C. (the end of the compression stroke) the compressed air-fuel mixture is ignited by a spark plug (in a gasoline engine) or by heat generated by high compression (diesel engines), forcefully returning the piston to B.D.C. This stroke produces mechanical work from the engine to turn the crankshaft.
Exhaust: Also known as outlet. During the exhaust stroke, the piston, once again, returns from B.D.C. to T.D.C. while the exhaust valve is open. This action expels the spent air-fuel mixture through the exhaust valve.
What is the compression ratio of this engine?
Compression ratio, in an internal-combustion engine, degree to which the fuel mixture is compressed before ignition. It is defined as the maximum volume of the combustion chamber (with the piston farthest out, or bottom dead centre) divided by the volume with the piston in the full-compression position (with the piston nearest the head of the cylinder, or top dead centre).
A compression ratio of six means that the mixture is compressed to one-sixth its original volume by the action of the piston in the cylinder. The maximum possible ratio based on cylinder dimensions may not be achieved if the intake valve closes after the piston begins its compression stroke, as this would cause backflow of the combustible mixture from the cylinder. A high ratio promotes efficiency but may cause engine knock.
Using the plot for volume of the cylinder region, we can find the maximum volume and the minimum volume which can be used to calculate the compression ratio.
Compression ratio = 0.0005742195/5.7539186e-5
The compression ration of the engine here is 9.98
Why do we need a wall heat transfer model?
Internal combustion engines are now extremely optimized, in such ways improving their performance is a costly task. Traditional engine improvement by experimental means is aided by engine thermodynamic models, reducing experimental and total project costs. For those models, accuracy is mandatory in order to offer good prediction of engine performance.
Modelling of the heat transfer and wall temperature is an important task concerning the accuracy and the predictions of any engine thermodynamic model, although it is many times an overcome task. In order to perform good prediction of engine heat transfer and wall temperature, models are required for accomplish heat transfer from hot gases to engine parts, heat transfer inside each engine part, and also heat transfer to coolant and lubricating oil.
Why can't we predict the wall temperature from the CFD simulation?
The detailed Computational Fluid Dynamic (CFD) modeling methodology was developed using FLUENT. Eulerian two-phase flow model is used to model the flow and heat transfer phenomena. In order to gain the peak wall temperature accurately and stably, the effect of different turbulence models and wall functions are investigated based on different grids. Results show that O type grid should be used for the simulation of CHF phenomenon. Grids with Y+ larger than 70 are recommended for the CHF simulation because of the acceptable results of all the turbulence models while Grids with Y+ lower than 50 should be avoided. To predict the dry-out position accurately in a fine grid, Realizable k-ε model with standard wall function is recommended. These conclusions have some reference significance to better predict the CHF phenomena of vertical pipe. It can also be expanded to rod bundle of Boiling Water Reactor (BWR) by using same pressure condition.
So the wall temperature obtained using the CFD calculations are the temperature of the fluid near to the wall of the cylinder, which is not the best result. Thus the wall heat transfer model is used to predict the wall temperature.
Calculate the combustion efficiency of this engine
In real engine applications, the combustion process is incomplete. This means that not all the energy content of the fuel supplied to the engine is released through the combustion process. There are several factors which can influence the combustion process, the most important being the fuel-air intake and fuel atomization (size of droplets). The fuel inside the cylinder needs air (oxygen) to burn. If there is not enough oxygen available, not all the fuel is burnt, therefore only a partial energy is released from combustion (e.g. around 96 %). If we analyze the exhaust gas of an Internal combustion engine, we can see that it contains both incomplete combustion products (carbon monoxide CO, nitrogen oxides NOx, unburnt hydrocarbons HC, soot PM) and complete combustion products (carbon dioxide CO2 and water H2O).
The combustion efficiency ηc is defined as the ratio between the energy released by the burnt fuel and the theoretical energy content of the fuel mass during one complete engine cycle.
Where,
C.v = 44000 KJ/Kg
mf = 3e-5 Kg.
Combustion efficiency ηc = 1.241 (KJ) / 44000 (KJ/Kg) * 3e-5 Kg
ηc = 0.94 (i.e) 94%
Use the engine performance calculator, to determine the power and torque for this engine.
To open performance calculator go to Line plotting in converge CFD.
Go to Tools --> Engine calculators --> Engine performance.
Give the thermo.out file for the cylinder region as the input file for calculating the engine's performance.
Power Calculation :
To calculate power we can use the formula; Power = Work/sec
From performance calculator we can take;
Work = 486.646 Nm
Start time = -120.09 deg
End time = 120.11 deg
Total duration = 240.199 deg
Total rotation per minute RPM = 3000 (Given)
Rotation per second = 3000/60 = 50.
Total degree per second = 50*360 = 18000
Time in sec = 240.199/18000 = 0.01334 sec.
Power = 486.646/0.01334
Power = 35130.88 Kj = 35.13 KW
Torque Calculation :
power = (2*pi*N*T)/60
T = power * 60/ (2*pi*N)
N = 3000 RPM.
T = 35.13 *1000* 60 / (2*3.14*3000) = 2107.8/18840
Torque(T) = 111.879 Nm.
What is the significance of ca10, ca50 and ca90?
Combustion phasing angles of 10%, 50% and 90% of heat release (CA10, CA50 and CA90) are determined by integrating the AHRR. Crank angles between CA10 and CA90 (CA10–90) are calculated to analyze combustion duration.
The CA10 phase retards from −2◦ CA to 7◦ CA when the spark timing changes from −15◦ CA to 0◦ CA, and they have basically a linear correlation. The CA50 phase and CA90 phase retard as well, but the CA50 phase retarding magnitude increases and the CA90 phase retarding magnitude decreases when the spark timing approaches 0◦ CA. The combustion duration of CA10–90 prolongs when spark timing changes from −15◦ CA to −5 ◦ CA, and it remains the same with spark timings ranging from −5 ◦ CA to 0◦ CA.
Earlier spark timing away from TDC could generate
hot jet from the PC and induce the MC mixture ignition earlier, which coupling with the cylinder compression speeds up the combustion of MC pre-mixture. Shorter combustion duration and quicker heat release lead to increasing RI. The RI with −15◦ CA spark timing is 4.6 MW/m2 when the CA50 is 5 ◦ CA.
Plots of flow properties at the combustion chamber :
From the pressure plot, we can see that the pressure only varies a little until the ignition occurs. Once the ignition starts, the pressure inside the combustion chamber increases at a faster rate and reaches a maximum value when the chamber volume is minimum. Then after the power stroke, the piston moves apart increasing the volume which in turn decreases the pressure to the initial value.
The volume of the combustion chamber follows a uniform wave form where the volume increases and decreases at the same rate. The piston follows a linear too & fro motion which controls the volume of the combustion chamber.
The temperature graph at the beginning have a low temperature which gets decreasing further as after an exhaust stroke the high temperature emissions take place by decreasing the temperature inside the chamber.
Once the chamber gets all the exhaust gas out, the temperature inside the chamber remains unchanged for a certain period of time until the ignition starts. Once the source of heat is released to the fuel particles from the spark plug, the temperature spreads to all the fuel particles there by increasing the temperature inside the chamber.
Emission of different gases :
This plot shows us the amount of emission occurring in an IC Engine. We can see various materials of gas getting released at the exhaust port. The main gas particles are plotted in the graph above.
We can see a high amount of CO2 emission at the end of the power stroke. The NOX emissions can also be read through this graph and proper actions can be taken if there are any emissions higher than the limited amount.
Post processing the combustion chamber :
Meshing :
The embedding given to the base grids gives us a finer and neat finish along some critical areas where the calculated values may be more important.
The embedding at the nozzles is clearly shows a refined mesh in the first image. Similarly the embedding around the spark plug is also refined which can be clearly seen in the second image.
Other embeddings and the Adaptive mesh refinement can be clearly seen through animation where we can see the refined mesh changing according to the sgs parameter which is specified for both velocity and temperature values during the calculation.
From the animation we can see that after ignition the meshes near the walls gets refined as the sgs parameters specified matches the requirement when the temperature suddenly raises near the wall.
Particle spray animation :
The valve movements and the piston movement is clearly following the data given during the case set up. The spray of the fuel particles are timed so that the particles gets into the chamber when the compression takes place.
As the inlet valves move to open up the space between the inlet port and the combustion chamber, the fuel particles moves through the opening into the chamber.
Temperature animation inside the chamber :
The combustion chamber initially has a medium temperature as the exhaust stroke ends. The temperature in the exhaust port can be found to be maximum at the beginning. This is because of the high temperature emission of the exhaust gases through the exhaust port.
The intake port has a minimum value as it sprays the fuel particles into the chamber. During expansion the temperature inside the chamber gradually decreases and attains a minimum value.
Once all the fuel particles enters the chamber the inlet valve closes and the piston moves up to compress the fuel. At this time the temperature is gradually increasing.
At the time of ignition there is a sudden increase in the temperature which spreads through out the chamber and the heat is transferred to the walls of the combustion chamber.