Challenge on Combustion

Project Title: Combustion model simulation

Objective:

1. Study the combustion model
2. Study the methane-air combustion reaction
3. Study the different gas component of after combustion
4. Study the NOx and soot formation
5. Perform the parametric study by changing the fuel-air combination

Theory:

What is combustion?

To move an aeroplane or a model rocket through the air, we must use a propulsion system to generate thrust. Different types of aircraft use different types of propulsion devices, but all aircraft rely on some type of engine to generate power. Rocket engines, internal combustion, or piston engines, and jet engines all depend on the burning of fuel to produce power. Burning fuel is called combustion, a chemical process that we study in middle or high school.

Because combustion is so important for aircraft and rocket propulsion, we will review the fundamentals. 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 aeroplane propulsion the fuel is usually a liquid. The oxidizer, likewise, could be a solid, liquid, or gas, but is usually a gas (air) for aeroplanes. For model rockets, solid fuel and oxidizer is used.

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 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.

During the combustion process, as the fuel and oxidizer are turned into exhaust products, heat is generated. Interestingly, some source of heat is also necessary to start combustion. Gasoline and air are both present in your automobile fuel tank, but combustion does not occur because there is no source of heat. Since heat is both required to start combustion and is itself a product of combustion, we can see why combustion takes place very rapidly. Also, once combustion gets started, we don't have to provide the heat source because the heat of combustion will keep things going. We don't have to keep lighting a campfire, it just keeps burning.

To summarize, for combustion to occur three things must be present: a fuel to be burned, a source of oxygen, and a source of heat. As a result of combustion, exhausts are created and heat is released. You can control or stop the combustion process by controlling the amount of the fuel available, the amount of oxygen available, or the source of heat.

Problem Statement:

Part I

Perform a combustion simulation on the combustor model and plot the variation of the mass fraction of the different species’ in the simulation using line probes at different locations of the combustor as shown in Fig. You need to plot for CO2, H2O, CH4, N2, O2, NOx emissions & Soot formation.

Part II

As you must have observed from the above simulation, the Nox and soot is getting formed at the outlet of the combustor. Such formation has harmful effects on the environment and humans. The stringent government norms also demand the least formation of Nox and soot and to satisfy those requirements, you need to check the effect of adding the water in the fuel.

In this part, you need to add the water content in the fuel from 5% to 30% by mole and observe the effect of it on the results. It is necessary to provide line plots and contours to prove your claim.

You can use the parametric study approach if required.

Solution:

Geometry

we considered 2D geometry for simulation. It consist inlet for fuel and air and outlet. i am considered axisymetrical model for simplicity.

Meshing:

Hex mesh is used for this model.

The local mesh is used near the inlet for better accuracy in the simulation

Mesh outline

Total mesh count in the simulation is 21400 and element is 21661

Orthogonal quality of the simulation

Element quality of the simulation

Setup

Pressure base steady-state solver is used.

In 2D space, Axisymmetric model is used.

In turbulence model k-epsilon Standard, Standard Wall Function is selected

To solve the combustion I activated Species transport model.

in reaction, Volumetric option is activated.

For fuel methane-air-2step fuel model is selected.

For turbulence-chemistry interaction, eddy-dissipation function is selected

To capture the NOx formation NOx model is activated

For soot measurement soot model is activated

Boundary condition

Inlet

velocity is 0.5 m/s

in species, O2 mass fraction is 0.23 selected

Fuel inlet

velocity 80 m/s

Species CH4 mass fraction 1 is selected

Outlet

Pressure outlet is 0 pa (gauge pressure)

Results

Convergence

Simulation is run for 1500 iteration

Co mass fraction

Co2 mass fraction

H2o mass fraction

NOx mass fraction

Soot mass fraction

N2 mass fraction

O2 mass fraction

Temperature contour

Co mass fraction along with a length of the combustion

Co2 mass fraction along with a length of the combustion

H2o mass fraction along with a length of the combustion

NOx mass fraction along with a length of the combustion

Soot mass fraction along with a length of the combustion

A parametric study by changing the fuel composition

NOx count Vs Water content in fuel

As water content is increasing NOx count reduces.

Soot count Vs Water content in fuel

As water content is increasing Soot count reduces.

CO count Vs Water content in fuel

As water content is increasing CO count reduces.

CO2 count Vs Water content in fuel

As water content is increasing CO2 count reduces.

H2O count Vs Water content in fuel

As water content is increasing H2O count reduces.

N2 count Vs Water content in fuel

As water content is increasing N2 count reduces.

O2 count Vs Water content in fuel

As water content is increasing O2 count increase.