Challenge

Challenge on Combustion

AAbdul Rahman Zeeshan7 Jul 2020

AIM :

PART I

To perform combustion simulation on the combuster model and plot the variation of different species and display the species formation in the combuster model on the chart

PART II

To add water in term of mole fraction from 5% to 30% and check weather it the curbs the NOX and Soot formation and also display the plot

THEORY

A combustion reaction is a major class of chemical reactions. Combustion usually occurs when a hydrocarbon reacts with oxygen to produce carbon dioxide and water. In the more general sense, combustion involves a reaction between any combustible material and an oxidizer to form an oxidized product. Combustion is an exothermic reaction, so it releases heat, but sometimes the reaction proceeds so slowly that a temperature change is not noticeable.

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 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 the 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.

The residuals NOX and Soot are the major contributions to harmful effects on the environment and it became mandatory to reduce its formation in any applications. So with advancement in technology, CFD package paved a way to develop the to come up with ideas to solve problems using numerical integration. To make ease in computation the combustion categorized into two groups

1.Based on Mixing: Non-premixid, Premixed and Partially premixed combustion

2. Based on Phase: Fluid phase, Volumetric Reactions, Surface Reaction and Porous Region combustion Reaction

The combustion reaction used is

CH4 +ar(O2+3.76N2)--> aCO2+bH20+cN2

After solving the stoichiometric equation

ch4=1 mole

Air ie (ar(O2+3.76N2))= 2 moles of O2 + 7.52 moles of N2

molecular weight=16

molecular weigth=14

Mass fraction =Mass of element/Mass of air

Mass fraction of oxygen=32/137.8=0.23

Mass fraction of oxygen=105.8/137.8=0.77

GEOMETRY

MESHING

Case Setup

Solver: Steady State

Pressure based

Velocity Formulation: Absolute

2d Space: Axissymmetry

Viscous Turbulence Model: K epsilon, Standard, Standard Wall Function

Energy Equation Turned ON

Species Model turned ON

NOTE: Species model allows to set the parameter for the calculation of spcies transport and calculation

Boundary Conditions

Air Inlet:

Velocity :0.5m/s

Temperature:300k

Species: O2 Mass fraction-0.23

Fuel Inlet:

Velocity :80 m/s

Temperature:300k

Species: CH4 Mass fraction-1

Initialization: Hybrid

No of Iteration:1500

Results

Residuals

The nature of similar pattern are observed after 700 iteration thereby indicating that the solution approached the convergence

Temperature

The temperature near the inlet shows a very high temperature of about 4516k which is maximum in the combustor model obvious reason that combustion took place and temperature at the air inlet is least of nearly 213.65k, As we move across the length of the computer model the there is even distribution of temperature with the mixing of air and fuel and finally an outlet the temperature range between 1800-1900k

The line chart is made using 5 random location, the chart of temperature validated the results of the contour

CH4 Contour

At the inlet the mass fraction methane is as we specified but after combution is eventually decreased to nearly 0 at the outlet

Line chart of CH4 mass fraction at a different location

O2 Contour

Oxygen is a very vital element in the process of combustion, Here the oxygen's mass fraction is high at the inlet and then get reduced with combustion. The contour displays its reduction and surprisingly it didn't get reduced to nil at the outlet, the reason behind is the uneven burning of oxygen or maybe it turned into an intermediate species

Line chart of a Mass fraction O2

CO2 Contour

Co2 is a product of combustion process. Its mass fraction keeps on increasing as it can be seen in the contour

Line chart of a Mass fraction CO2

H2O Contour

Line chart of a Mass fraction H20

N2 Contour

Line chart of a Mass fraction N2

NOX Mass fraction

The mass fraction NOX is displayed in contour.It can be seen from the above figure that the mass fraction gets reduced at the outlet