Challenge on Combustion

 

                                                      COMBUSTION 

Aim

Part 1: To Perform a combustion simulation on the combustor model and plot the variation of the mass fraction of the different species.

Part 2:To add the water content in the fuel from 5% to 30% by mole and observe the effect of it on the results.

 

Introduction

Combustion is a chemical process in which a substance reacts rapidly with oxygen and gives off heat. The original substance is called the fuel, and the source of oxygen is called the oxidizer. The fuel can be a solid, liquid, or gas, although for airplane propulsion the fuel is usually a liquid.

The products of a complete combustion reaction include carbon dioxide (CO₂) and water vapor (H₂O). The reaction typically gives off heat and light as well. The general equation for a complete combustion reaction is:

Fuel + O₂ → CO₂ + H₂O

In an internal combustion engine (ICE), the ignition and combustion of the fuel occurs within the engine itself. The engine then partially converts the energy from the combustion to work. The engine consists of a fixed cylinder and a moving piston. The expanding combustion gases push the piston, which in turn rotates the crankshaft. Ultimately, through a system of gears in the powertrain, this motion drives the vehicle’s wheels.

 

                     

There are two kinds of internal combustion engines currently in production: the spark ignition gasoline engine and the compression ignition diesel engine. Most of these are four-stroke cycle engines, meaning four piston strokes are needed to complete a cycle. The cycle includes four distinct processes: intake, compression, combustion and power stroke, and exhaust.

Spark ignition gasoline and compression ignition diesel engines differ in how they supply and ignite the fuel.  In a spark ignition engine, the fuel is mixed with air and then inducted into the cylinder during the intake process. After the piston compresses the fuel-air mixture, the spark ignites it, causing combustion. The expansion of the combustion gases pushes the piston during the power stroke. In a diesel engine, only air is inducted into the engine and then compressed. Diesel engines then spray the fuel into the hot compressed air at a suitable, measured rate, causing it to ignite.

 

 Theory 

Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is to be contrasted with thermodynamics, which deals with the direction in which a process occurs but in itself tells nothing about its rate. Chemical kinetics includes investigations of how experimental conditions influence the speed of a Chemical reaction and yield information about the reaction's mechanism and transition states, as well as the construction of mathematical models that also can describe the characteristics of a chemical reaction.

Factors affecting reaction rate includes:

•  Nature of the reactants
• Physical state
• Surface area of solid state
• Concentration
• Temperature
• Catalysts
• Pressure
• Absorption of light

On burning the fuel Soot particulates and nitrogen oxides (NO_X) from diesel engine exhaust have been causing serious problems to human health and the global environment. NO contributes not only to the production of acid rain but also to the production of photochemical smog in a reaction with hydrocarbons while under the influence of sunlight. Fine soot particulates (C₈H to C₁₀H), which contain mutagenic hydrocarbons, can easily reach far down into lung tissues when inhaled and therefore have a detrimental impact on human health. Diesel engines are the primary power source of vehicles used in heavy duty applications.

These include two wheelers, buses, large trucks, and inside-highway construction and mining equipment. Furthermore, diesel engines are becoming an increasing part of the light duty vehicle market worldwide. In India, 100% of heavy duty vehicles, 60% of light-duty commercial vehicles and 20% of passenger cars are diesel powered. Diesel exhaust is inherently low in the concentration of CO and unburned HC, for NO and particulate matter are being objectionable to be removed from the exhaust. Since the reduction of both NO and soot particulate emissions to the allowed level cannot be accomplished by engine modifications alone, after-treatment activity for the simultaneous reduction of emissions should be developed.

                                      

For the emission of the toxic exhaust gas components by engines ,One of the solutions is supplying water into the combustion chambers of engines.stringent emissions regulations for diesel engines requires a way to reduce NOx and soot emissions. Most emissions reduction strategies reduce one pollutant while increasing the other. Water injection is one of the few promising emissions reduction techniques with the potential to simultaneously reduce soot and NOx in diesel engines. While it is widely accepted that water reduces NOx via a thermal effect, the mechanisms behind the reduction of soot are not well understood. The water could reduce the soot via physical, thermal, or chemical effects.

 

Solving and Modelling

Geometry:(Convert 3D to 2D)

                                 

2D geometry:

             

Meshing:

         

                                                                                              

Axis                                                                                                                                       Wall

       

Outlet                                                                                                                                       Air inlet

    

Fuel inlet

  

 

Solver Setting

 

• Solver Setting K- Epsilon Standard
• Axisymmetic
• Energy Equation on
• Steady State
• Pressure-based solver
• Transport Species with volumetric reaction along with inlet diffusion and Diffusion energy Source turned on.Mixture properties as methane -air 2 and eddy disscipation as turbulence chemistry interaction
• Turn on Soot and NOx
• 1500 iteration

             

Boundary Conditions:

     Fuel inlet  velocity 80 m/s with CH4 and O2 on proportions specified at 300k

     air Inlet   velocity 0.5 m/s with mass fraction of oxygen as 0.23 at 300k

     axis and wall

 

Part 1:plot for CO2, H2O, CH4, N2, O2, NOx emissions & Soot formation. 

The following line are created on the 2D plot from origin at 

line 1 = 0.1 

line 2 = 0.25 

line 3 = 0.4 

line 4 = 0.6 

line 5 = 0.7 ,respectively

Residual plot:

       

temperature plot: (max temp=2312k)

      

animation file:https://drive.google.com/drive/u/1/folders/1FaVCLIIgjrieL2DfeJHf-SDxM-QU7lPI

 

Mass fraction Plots:

CH4                                                                                                                                                         CO

  

 

CO2                                                                                                                                                          N2

  

 

O2                                                                                                                                                          H2O

   

 

Soot(Area weighted average=0.001517)

    

NOx:(Area Weighted Average=5.8798*10^(-5))

 

Charts;

line 1 = 0.1 =series1

line 2 = 0.25 =series 2

line 3 = 0.4 =series 3

line 4 = 0.6 =series 4

line 5 = 0.7=series 5

Soot:

  

 

NOx;

  

 

Mass fractions of CH4                                                                                                                                      CO

     

 

CO2                                                                                                                                                              H20

    

 

N2                                                                                                                                                                O2

    

 

Part 2;

water content in the fuel from 5% to 30% by mole 

To reduce Soot and NOx formation water is added.The below proofs claims it's effect.

 

A)95% fuel, 5% water (Same conditions as above simulation)

But in the fuel inlet: Species are given Fuel as 0.95 and water as 0.05

Temperature plot;(max temperature 2300 k )

   

 

mass fraction Plots;

CH4                                                                                                                                                            CO

   

 

CO2                                                                                                                                                            H2O

        

 

N2                                                                                                                                                        O2

   

 

NOx;Area weighted average=4.6285*10^(-5 )

  

 

Soot;Area weighted average=0.0012109

 

 Charts;

Soot;

   

 

NOx;

  

 

CH4                                                                                                                                                CO

   

 

CO2                                                                                                                                                H2O

  

 

N2                                                                                                                                                 O2

  

 

 

B) 90 % fuel and 10 % water(fuel 0.9,water 0.1 as mass fraction)

Temperature plot;(max temp 2289k)

Mass fraction plots;

CH4                                                                                                                                       CO

   

 

CO2                                                                                                                                          H2O

  

 

N2                                                                                                                                              O2

 

 

Soot;Area weighted average=0.0010761

 

NOx;(Area weighted average =3.6484*10^(-5))

Charts;

soot;

  

 

NOx;

  

 

CH4                                                                                                                                          CO

  

 

CO2                                                                                                                                        H2O

   

 

N2                                                                                                                                             O2

   

 

C)85% fuel and 15 % water(0.85 CH4 and 0.15 H2O)

 

Temperature Plot;(Max temp 2274k)

 

Mass fraction Plots;

CH4;                                                                                                                                                        CO

  

CO2;                                                                                                                                          H2O

   

 

N2                                                                                                                                                        O2

 

 

Soot;Area weighted average=0.000542748

 

 

NOx;Area weighted average=2.803522*10^(-5)

 

 

Charts;

Soot

  

NOx;

 

 

CH4                                                                                                                                                                 CO

  

 

CO2                                                                                                                                                   H2O

  

 

N2                                                                                                                                                         O2

     

 

D)80% fuel and 20% water(0.8 CH4 and 0.2 H20);

Temperature plot;(max temp 2261k)

  

 

mass fraction plots;

 CH4                                                                                                                                                                   CO

    

 

CO2                                                                                                                                                                  H2O

  

 

N2                                                                                                                                                                  O2

  

 

Soot(Weighted average of soot=0.000421757)

  

NOx;Weighted average of NOx=2.0814*10^(-5)

  

 

Charts;

Soot;

  

 

NOx;

  

 

CH4                                                                                                                                                         CO

     

 

CO2                                                                                                                                                        H2O

  

 

N2                                                                                                                                                             O2

    

 

E)75% fuel and 25% water (0.75 CH4 and 0.25 H20 as mole fraction in fuel inlet)

Temperature Plot;(max temperature=2245k)

 

 

mass fraction pots;         

CH4;                                                                                                                                                         CO

   

 

CO2                                                                                                                                                         H2O

    

 

N2                                                                                                                                                            O2

     

 

Soot;Area weighted average=0.0003024687

  

 

NOx;Area weighted average=1.48227*10^(-5)

 

Charts;

Soot;

  

 

NOx;

  

 

CH4                                                                                                                                                 CO

    

 

CO2                                                                                                                                               H2O

    

 

N2                                                                                                                                       O2

    

 

F)Fuel 70 % and Water 30%(CH4 0.7 and H2O 0.3 as massfraction in fuel inlet)

Temperature plot;(max temp=2224k)

  

Mass fraction plots;

CH4                                                                                                                                 CO

   

 

CO2                                                                                                                              H2O

   

 

N2                                                                                                                                                             O2

   

 

Soot;Area weighted average=0.00015626

 

NOx;Area weighted average=1.02485*10^(-5)

 

 

Charts;

Soot;

  

 

NOx;

  

 

 

CH4                                                                                                                                      CO

    

 

CO2                                                                                                                                    H2O

    

 

N2                                                                                                                                            O2

   

 

Result

  

Fuel Mass fraction	Mass fraction of Nox (Area weighted avg)	Mass fraction of Soot(Area Weighted avg)	Max Temperature (K)
1	5.8798*10^(-5)	0.001517	2312k
0.95	4.6285*10^(-5 )	0.0012109	2300k
0.9	3.6484*10^(-5)	0.0010761	2289k
0.85	2.803522*10^(-5)	0.000542748	2274k
0.8	2.0814*10^(-5)	0.000421757	2261k
0.75	1.48227*10^(-5)	0.0003024687	2245k
0.7	1.02485*10^(-5)	0.00015626	2224k

 

From the given table we can see as the percetage of water content increases the NOx and Soot Reduces by a significant margin.

for 0% water(100% fuel)- mass fraction of NOx=5.8798*10^(-5) 

for 30% water-mass fraction reduces to 1.02485*10^(-5)

ie it's an 5.73 times decrease from initian mass fraction

 

Coming to soot;0% water mass fraction of soot=0.001517 and at 30% water content it reduced to 0.00015626

  ie 0.001517/0.00015626  =9.7

  that is 9 fold decrease in soots.

Also indicate the fact that the maximum temperature reduces on addition of water but by mere degrees.

 ie 2312-2224=88 deg.

 

Ploting the graph:

  

           

We can definetly see the reduction in mass fraction of Nox and soots.

Conclusion

By the results we can finalize that addition of water with fuel can significantly reduce the NOx and Soot formation by many folds.

 

Reference

https://www.jstor.org/stable/44721075?seq=1 The Effect of Water on Soot Formation Chemistry

https://www.ck12.org/chemistry/combustion-reaction/lesson/Combustion-Reactions-MS-PS/#:~:text=Combustion%20is%20commonly%20called%20burning,vapor%20(H2O).&text=The%20general%20equation%20for%20a,CO2%20%2B%20H2O

https://www.sciencedirect.com/science/article/pii/S1876610217337724

https://link.springer.com/article/10.1007/s00773-015-0303-8