Week 10 - Simulating Combustion of Natural Gas.

Simulation of combustion of the natural gas

Aim : To simulate the combustion of natural gas and plot for CO2, H2O, CH4, N2, O2, NOx emissions & Soot formation at the outlet.

Objectives : 1. Use the appropriate boundary conditions and combustion models related to combustion.

                     2. Post process the results and collect the contours of pollutant species at the outlet.

Theory : 

Combustion : 

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 oxidizer, likewise, could be a solid, liquid or gas.

During combustion, new chemical substances are created from the fuel and the oxidizer. These substances are called exhaust. Most of the exhaust comes from chemical combinations of the fuel and oxygen. When a hydrogen-carbon-based fuel (like gasoline) burns, the exhaust includes water (hydrogen + oxygen) and carbon dioxide (carbon + oxygen). But the exhaust can also include chemical combinations from the oxidizer alone. If the gasoline is burned in air, which contains 21% oxygen and 78% nitrogen, the exhaust can also include nitrous oxides (NOX, nitrogen + oxygen). The temperature of the exhaust is high because of the heat that is transferred to the exhaust during combustion. Because of the high temperatures, exhaust usually occurs as a gas, but there can be liquid or solid exhaust products as well. Soot, for example, is a form of solid exhaust that occurs in some combustion processes. 

Modelling the non-premixed combustion using FLUENT : 

In non-premixed combustion, fuel and oxidizer enter the reaction zone in distinct streams. This is in contrast to premixed systems, in which reactants are mixed at the molecular level before burning. Examples of non-premixed combustion include pulverized coal furnaces, diesel internal-combustion engines and pool fires.

Under certain assumptions, the thermochemistry can be reduced to a single parameter: the mixture fraction. The mixture fraction, denoted by , is the mass fraction that originated from the fuel stream. In other words, it is the local mass fraction of burnt and unburnt fuel stream elements (C, H, etc.) in all the species (CO , H O, O , etc.). The approach is elegant because atomic elements are conserved in chemical reactions. In turn, the mixture fraction is a conserved scalar quantity, and therefore its governing transport equation does not have a source term. Combustion is simplified to a mixing problem, and the difficulties associated with closing non-linear mean reaction rates are avoided. Once mixed, the chemistry can be modeled as being in chemical equilibrium with the Equilibrium model, being near chemical equilibrium with the Steady Laminar Flamelet model, or significantly departing from chemical equilibrium with the Unsteady Laminar Flamelet model.

NOx formation and modelling in ANSYS FLUENT :

The ANSYS FLUENT NOx model provides the capability to model thermal, prompt, and fuel NOx formation as well as NOx consumption due to reburning in combustion systems. It uses rate models developed at the Department of Fuel and Energy at The University of Leeds in England as well as from the open literature. NOx reduction using reagent injection, such as selective noncatalytic reduction (SNCR), can be modeled in ANSYS FLUENT along with an N O intermediate model which has also been incorporated.

To predict NOx emissions, ANSYS FLUENT solves a transport equation for nitric oxide (NO) concentration. When fuel NOx sources are present, ANSYS FLUENT solves additional transport equations for intermediate species (HCN and/or NH ). When the N O intermediate model is activated, an additional transport equation for N O will be solved. The NOx transport equations are solved based on a given flow field and combustion solution. In other words, NOx is postprocessed from a combustion simulation. It is thus evident that an accurate combustion solution becomes a prerequisite of NOx prediction. For example, thermal NOx production doubles for every 90 K temperature increase when the flame temperature is about 2200 K. Great care must be exercised to provide accurate thermophysical data and boundary condition inputs for the combustion model. Appropriate turbulence, chemistry, radiation and other submodels must be employed.

To be realistic, one can only expect results to be as accurate as the input data and the selected physical models. Under most circumstances, NOx variation trends can be accurately predicted but the NOx quantity itself cannot be pinpointed. Accurate prediction of NOx parametric trends can cut down on the number of laboratory tests, allow more design variations to be studied, shorten the design cycle, and reduce product development cost. That is truly the power of the ANSYS FLUENT NOx model and, in fact, the power of CFD in general.

NOx Formation and Reduction in Flames:

In laminar flames, and at the molecular level within turbulent flames, the formation of NOx can be attributed to four distinct chemical kinetic processes: thermal NOx formation, prompt NOx formation, fuel NOx formation, and intermediate N O. Thermal NOx is formed by the oxidation of atmospheric nitrogen present in the combustion air. Prompt NOx is produced by high-speed reactions at the flame front, and fuel NOx is produced by oxidation of nitrogen contained in the fuel. At elevated pressures and oxygen-rich conditions, NOx may also be formed from molecular nitrogen (N ) via N O. The reburning and SNCR mechanisms reduce the total NOx formation by accounting for the reaction of NO with hydrocarbons and ammonia, respectively.

Thermal NOx formation :

The formation of thermal NOx is determined by a set of highly temperature-dependent chemical reactions known as the extended Zeldovich mechanism. The principal reactions governing the formation of thermal NOx from molecular nitrogen are as follows:

A third reaction has been shown to contribute to the formation of thermal NOx, particularly at near-stoichiometric conditions and in fuel-rich mixtures:

Prompt NOx formation and modelling :

The presence of a second mechanism leading to NOx formation was first identified by Fenimore and was termed "prompt NOx''. There is good evidence that prompt NOx can be formed in a significant quantity in some combustion environments, such as in low-temperature, fuel-rich conditions and where residence times are short. Surface burners, staged combustion systems, and gas turbines can create such conditions.

Prompt NOx Mechanism :

Prompt NOx is most prevalent in rich flames. The actual formation involves a complex series of reactions and many possible intermediate species. The route now accepted is as follows:

 

A number of species resulting from fuel fragmentation have been suggested as the source of prompt NOx in hydrocarbon flames (e.g., CH, CH , C, C H), but the major contribution is from CH (Equation  13.1-16) and CH , via

Prompt NOx formation factors : 

Prompt NOx formation is proportional to the number of carbon atoms present per unit volume and is independent of the parent hydrocarbon identity. The quantity of HCN formed increases with the concentration of hydrocarbon radicals, which in turn increases with equivalence ratio. As the equivalence ratio increases, prompt NOx production increases at first, then passes a peak, and finally decreases due to a deficiency in oxygen.

Modelling approach :

Creation of the Geometry :

The given 3D geometry is computationally costly. It means when it is discretized, the resulting elements are high in number and the simulation takes a lot of time. However, this can be bypassed by taking the advantage of the axisymmetric nature of the geometry.

The given geometry is a hollow cylinder which is fromed by the revolution of a "U" around an axis.

Given 3D geometry : 

The given solid has three components which are concentric cylinders. In order to get a 2D geometry, the solid has to be cut by any of the two planes - XY / XZ. 

The resultant 2D looks as follows : 

This geometry has to be split again using a XZ plane to get the final geometry.

The faces in the green and grey are to be merged into a single face which looks as follows. This is the final 2D geometry that is considered for simulation.            

The names of the boundaries shown in the above have to be declared in the mesher and the appropriate parameters like air velocity at the inlet etc. have to be declared in the setup component of the FLUENT.

Meshing the Geometry :

The face has to be load into the Mesh component of the analysis system. 

An element size of 1 mm has to be given in the mesh size field. The final mesh looks as follows  :

Details of the Mesh : 

Element size : 1mm         Number of nodes : 17516          Number of Elements : 17007

Quality parameters : 

1. Element size : 

The maximum element quality is 0.999 which is very high and signifies that the mesh is a good one.

2. Aspect Ratio : 

For atleast 90% of the elements the aspect ratio is 1 which is an indication of the cells which are near perfect rectangles. 

Setting up the case : 

Two cases, one in which no water is added and the one in which water is added from the fuel inlet ranging from 5% to 30%.

 - -- - -------------- - ---- - --- -- - - ----------------- -- - --.... .. . -- ----------------------------------------------------------------

Setting up case.1 : (Without addition of water)

Fuel is the Natural gas which has Methane (CH4) as the main constituent. In order to specify Methane as the fuel, methane-air-2 step is selevted in the species.   

Step.1 : When FLUENT loads the case, the Axisymmetric condition is selected in the general tab. As shown a pressure based solver with steady state conditions is used for the simulation.

Step.2 : Choosing the turbulence model

The standard K-epsilon model with standard wall functions is choosen to model the turbulence.

Step.3 : Setting up the species :

Enable the species transport option and check the box to enable the volumetric option. Also enable the inlet diffusion and Diffusion energy source. In the mixture properties tab choose the methane-air 2 step from the list. Click on apply and then on OK.

Step.4 : Modelling the NOx emissions

Check all the boxes in the formation and in the fuel species select everything except CO2. 

Step.5 : Modelling the SOOT 

Enable the one-step model and in the species definition select CH4 as fuel and O2 as the oxidant. Click on apply and then on close.

Boundary conditions : 

1. Air inlet :

Air flows through the inlet with a speed of 0.5 m/s. In the species tab we specify the mole fraction of air to be 0.23

2. Fuel inlet :

CH4 - The primary component is the fuel which has the velocity of 80 m/s. In the species tab, the mole fraction of the CH4 is set to 1. 

3. Outlet :

Pressure outlet with the gauge pressure of 0 Pa. Temperature at the outlet : 300 K

4. Axis : Axisymmetric boundary condition.

5. Wall  : Wall boundary condition i.e. No slip velocity condition.

Step.6 : Initialize the solution using Hybrid initialization 

Step.7 : Create a report definition which provides the temperature at the outlet as well as plots the temperature at the outlet.

Step.7 : Run the simulation for 1500 iterations (a guess value) and collect the results.

  - -- - ---- ----------- - - --.. ..----------------------- - - - - - -- -- --------- ---- ---- ----- ------------- ---- -- ---------------------

Case.II  : Setting uo a parametric case study where the water is added from the fuel inlet.

Just like the case I follow the steps from 1 to 5 i.e. till the boundary conditions.

In the boundary conditions, 

Navigate to the Fuel inlet boundary condition and then to the species tab. In our parametric study since the mole fraction of CH4 and H2O change in every design point the mole fractions of those species are set as input parameters.

Click on the down arrow button next to the species field where we can find the new parameter option. click on it and a new window pops up. In the definitions box we input a value of 0.95 for CH4 and check the "use as input parameter" option. 

The same procedure is followed for H2O with a variation in specifying the mole fraction. Here the mole fraction is set as 0.05.

Checking the "Use as input parameter" shouldn't be forgetted.

After this step, create report definitions for Temperature, mass fraction of NOx and SOOT.

It is important that the "create output parameter" is checked.

Same procedure is followed for seeting the report definitions for SOOT and NOx

Initialize using the Hybrid initialization option and run the simulation for 1500 iterations.

After the simulation is performed for the first design point, the input parameters for all the design points are input and an option "Update all design points" is clicked.

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Results : 

Part.I : Basic combustion simulation

a. Residuals

b. Temperature at the outlet :

c. Voumetric rendering of CO

d. Volumetric rendering of NO

e. Volumteric rendering of SOOT

f. Volumetric rendering of CH4

g. Volumetric rendering of O2

h. Volumetric rendering of H2O

In order to obtaian the variation of property values at different locations, line probes were set at four different location of 0.5 m, 1 m, 1.5 m and 2 m respectively.

i. CHART for the CH4

j. CHART for N2

k. CHART for CO

l. CHART for O2

m. CHART for the SOOT

n. CHART for the Temperature 

 - -- - -- -------------------------------- - -- - --- - - -- --- ---------------------------- -- - -- ----------------------------------------

Results for Part.2 : Parametric study

a. Volume rendering of temperature for all the cases starting from 5% water to 30% water.

b. Volume rendering of Mass fraction of pollutant Nitrous oxide (NO)

c. Volume rendering of the Mass fraction of N2O

d. Volume rendering of the Mass fraction of H2O

e. Volume rendering of Mass fraction of O2

f. Volume rendering of Mass fraction of CH4

g. Volume rendering of Mass fraction of SOOT

h. Volume rendering of Mass fraction of N2

i. Volume rendering of Mass fraction of CO

a. Temperature Chart data for all the cases

b. Chart for the Variation of Mass fraction of CH4 along the longitudinal axis of the combustor

c. Chart for the Variation of Mass fraction of CO2 along the longitudinal axis of the combustor

d. Chart for the Variation of Mass fraction of NO along the longitudinal axis of the combustor

e. Chart for the Variation of Mass fraction of NO2 along the longitudinal axis of the combustor

f. Chart for the Variation of Mass fraction of SOOT along the longitudinal axis of the combustor

g. Chart for the Variation of Mass fraction of CO along the longitudinal axis of the combustor

h. Chart for the Variation of Mass fraction of O2 along the longitudinal axis of the combustor

i. Chart for the Variation of Mass fraction of H2O along the longitudinal axis of the combustor

Parametric study results : 

The parameter 2 and Parameter 3 represent the mole fraction of CH4 and H2O respectively.

Parametric study result analysis : 

1. It can be analysed from the report that with an increase in the % of H2O addition, the concentration of the NOx and SOOT at the outlet decreases. This justifies our hypothesis that addition of water results in the decrease of SOOT and NOx emissions.

2. The overall temperature inside the combustion chamber decreases because of the addition of water.

References : 

1. combustion : https://www.grc.nasa.gov/WWW/k-12/airplane/combst1.html

2. Combustion modelling using ANSYS FLUENT : https://www.afs.enea.it/project/neptunius/docs/fluent/html/ug/node537.htm#:~:text=ANSYS FLUENT 12.0 User's Guide,Modeling Non-Premixed Combustion&text=In non-premixed combustion, fuel,the molecular level before burning.

3. Reducing the NOx and SOOT formation by adding water to the fuel : https://dieselnet.com/tech/engine_water.php#:~:text=In addition to the NOx,impressive reductions of PM emissions.

4. NOx modelling in ANSYS FLUENT : 

https://www.afs.enea.it/project/neptunius/docs/fluent/html/th/node209.htm