Combustion Simulation of non-premixed model using Ansys Fluent

Combustion Simulation of the non-premixed model

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 are 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.

Theory:

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 methane combustion, pulverized coal furnaces, and diesel (compression) internal-combustion engines.

General terminologies:

Chemical Kinetics: The Study of entire Reaction mechanism subjects to different operating conditions.

Intermediary Species: The reaction between two material such as air and fuel.

Elementary Reaction: The reaction between intermediary species.

Possible types of Combustion:

• Based on mixing:
• Non-Premixed Combustion(Direct injection or late injection):

Example: In Diesel (CI) engine, the fuel just injects before the combustion starts. Hence the mixing is not uniform.

• Premixed Combustion (Carburator in SI engine):

Example: In the SI engine there will be a uniform mixture of air and fuel. Hence the mixing is uniform.

• Partially-Premixed: In between non-premixed and Premixed combustion.
• Based on Phase:
• Fluid Phase-Volumetric reaction.
• Wall-Surface reaction.
• Porous region.

Equations solved:

• Mass
• Momentum
• Species Transport equation.

When you choose to solve conservation equations for chemical species, ANSYS FLUENT predicts the local mass fraction of each species, , through the solution of a convection-diffusion equation for the th species. This conservation equation takes the following general form:

where Yi is the net rate of production of species by chemical reaction and Si is the rate of creation by addition from the dispersed phase plus any user-defined sources. An equation of this form will be solved for species where is the total number of fluid phase chemical species present in the system.

Diffusion:

Diffusion is the net movement of anything from a region of higher concentration to a region of lower concentration.

It combines both laminar and turbulent diffusion.

Typiclly for Engineering applications like IC engines, gas turbines, burners, etc, the turbulent diffusion overcomes the laminar diffusion. So we use Fick’s law (Dilute approximations) to modeled the diffusion coefficient.

Fick’s law:

The Fick's diffusion law postulates that the diffusive flux goes from a high-concentration area to a low-concentration area proportional to the concentration gradient.

Molecular diffusion from a microscopic and macroscopic point of view. Initially, there are solute molecules on the left side of a barrier (purple line) and none on the right. The barrier is removed, and the solute diffuses to fill the whole container. Top: A single molecule moves around randomly. Middle: With more molecules, there is a clear trend where the solute fills the container more and more uniformly. Bottom: With an enormous number of solute molecules, randomness becomes undetectable: The solute appears to move smoothly and systematically from high-concentration areas to low-concentration areas. This smooth flow is described by Fick's laws.

Dilute approximation:

Each fluid has a different diffusion rate with respect to other fluid.

Combustion Source calculation:

Finite Rate chemistry:

• In this, we look for laminar flow.
• To calculate the rate of production of each and every species after diffusion of a species into some other species.
• Slow but accurate.

Mixing Based model

• The K-epsilon Eddy dissipation model is used to calculate the time scale required for the mixing once some turbulent value is reached.
• The time scale for mixing depends on the rate of turbulence, eg If there is more turbulent then the time scale required for the mixng is less and if there is ales turbulence then the time scale required for the mixing is more.
• Fast but requires User experience.
• Finite Rate Eddy dissipation Model:
• Combination of both Finite rate Chemistry and Eddy dissipation model.
• In a single cell, when air and fuel interact right away then the combustion occurs. But it is not suitable when we want the ignition delay.
• At this time, depends upon the nature of the flow, the solver will automatically choose Finite rate chemistry (for laminar flow) or Eddy dissipation model.
• In turbulence dominating flow, the reaction rate predicted by the Eddy dissipation model will be smaller, so that the mixing time required for the Combustion increases when we get ignition delay.

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

SIMULATION:

Steps in Simulation:

Open Ansys Workbench. Drag and drop Fluid Flow (Fluent) onto Project Schematic.

Here, we will see Steps we need to follow to solve a particular engineering system from a CFD standpoint.

1. Geometry: Spaceclaim, where we import the geometry.
2. Mesh: Ansys Mesh, where Boundary naming, element sizing and mesh generation with required grid size will be done.
3. Setup: Ansys Fluent, where actual case setup will be done.
4. Solution: Ansys Fluent, where runtime results visualization done.
5. Results: CFD-Post, where Post Processing of Results will be done.

Import Graphics card geometry into Spaceclaim, Spaceclaim is CAD cleanup software that can be used to prepare CAD models for simulation setup. It could be for CFD or FEA.

As we can see above figure is the Solid model.

If we simulate the entire Geometry, this will be more time consuming and expensive, hence we assume that the flow is Axisymmetric, meaning if we take out any slice of the body then any change in the radial direction can be completely neglected. The radial direction is our circumference of the circle.

Split the 3D body in the XY plane into 1/4^th.

Copy the faces and paste it into the new XY plane to convert it into 2d geometry

Mesh Generation

Name Selection:

Mesh element size 5mm.

Capture Proximity to more refines the mesh near the gaps.

Assign the number of cells across the gap 3.

Fluent Setup:

Setting up physics: Select k-epsilon, Standard model.

Model-> Species-> Species transport ->Reaction type -> Volumetric.

Option ->Inlet diffusion. Enable diffusion energy source.

Mixture Properties: Methane air. When we want to add water content into the fuel, use methane-air 2-step mixture to allow water in the fiuel.

Terbulence Chemistry interaction->Eddy dissipation->Ok.

Solver: Pressure-based as Mach number is less than 3.

Assign Time as Steady-state Solver.

Enable 2D mode. Axisymmetric.

Material Selection:

Methane-air for fuel without water.

Methane-air 2step for fuel with water content.

Properties of Material:

For Air: Density = 1.225 kg/m^3

Viscosity = 0.00089 Pa

For methane: Density = 0.657 kg/m^3

Viscosity = 1.10*10^-5 Pa.s

Boundary Conditions:

For air inlet:

Velocity=0.5m/s

Temperature=300k (Ambient air temperature)

Species: Mass fraction of 02=0.23

Fuel inlet:

Velocity=80m/s

Species: CH4(methane)=1(100% methane)

Outlet: Pressure outlet.

Enable thermal NOx model. The formation of thermal NOx is determined by a set of highly temperature-dependent chemical reactions known as the extended Zeldovich mechanism.

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

Enable prompt NOx. 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.

Select fuel species: CH4

Solving:

Hybrid initialization.

Create a temperature plot and calculate for 1000 iterations.

CFD-Post:

Enable Volume rendering.

Plotline at some distance throughout the Combustor to measure the different species and emmisions rate of the Soot and NOx.

Nitrogen oxide (NOx) is a chemical compound of oxygen and nitrogen that is formed by reacting with each other during combustion at high temperatures, mainly combustion of fuel such as oil, diesel, gas, and organic matter. In atmospheric chemistry, NOx is a generic term for the nitrogen oxides that are most relevant for air pollution, namely nitric oxide and nitrogen dioxide. These gases contribute to the formation of smog and acid rain, as well as affecting tropospheric ozone.

Soot is a mass of impure carbon particles resulting from the incomplete combustion of Hydrocarbons. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual parolysed fuel particles such as caol, cenospheres charred wood, and petroleum coke that may become airborne during parolysis and that are more properly identified as cokes or char.

Soot causes various types of cancer and lung disease.

Plots and Results for CO2, H2O, CH4, N2, O2, NOx emissions & Soot formation:

At a distance of x=01,0.2,0.3,0.5m & 0.7m

CH4(Methane)

CO2(carbon dioxide)

H20(Water)

N2(Nitrogen)

O2(Oxygen):

NOx(Nitrogen-oxide)

Soot:

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

Part 2:

Need for water in the fuel

Compression ignition (CI) engines are the most fuel-efficient internal combustion engines with an overall efficiency greater than spark-ignition engines. High combustion temperature and lean stoichiometric air-fuel ratios (A/F) are the main reasons for the high efficiency. However, the CI engine advantage is overshadowed by its emission characteristics like particulate matters (PM), Oxides of nitrogen (NOx), Soot, and Oxides of carbon (CO2 & CO). These diesel fuel emissions, especially NOx emission is dangerous to human health. Hence stringent regulations are required to control the NOx and Soot emission from diesel engines. Low engine pollution has been achieved by adopting pre-formation and/or post-formation emissions control techniques like biodiesel, particulate filters, catalyst converters, and the introduction of water in the combustion chamber. Out of these, water in diesel has been found to be the best economical solution that can introduce in the CI engine without retrofitting.

Engine combustion chamber processes consist of the air intake, compression, fuel injection, evaporation, mixing of fuel and air, combustion, exhaust gas generation, expansion, and exhaust, as shown in Figure 1. These processes are repeated continuously. The engine power is obtained by transforming the cylinder pressure generated by this series of processes into piston reciprocal movement. The fuel consumption and emissions are affected by moment-to-moment changes of the in-cylinder conditions. The high-temperature gas in the combustion chamber is the main factor of heat stress, which must be considered to design a reliable engine. As a result, proper understanding and optimization of the series of processes described above are important in developing a highly reliable engine with low emissions.

Mechanism of Micro explosion:

As in the case of using water, it is possible to expect NOx reduction at the same time, due to the reduction of soot from the micro-explosion phenomenon, caused by the intense evaporation of water, and the effect of promoting combustion through free radicals, and the decrease of the combustion chamber temperature due to the latent heat of evaporation of water. As shown, in the micro-explosion phenomenon, the difference in the boiling point between diesel and water upon fuel injection in a combustion chamber results in the evaporation of water droplets and a more thorough splitting of fuel droplets. As a result, secondary atomization contributes to enhancing the air-fuel mixture and reducing incomplete combustion.

Plots and Results for CO2, H2O, CH4, N2, O2, NOx emissions & Soot formation:

At a distance of x=01,0.2,0.3,0.5m & 0.7m

When the Water content is 5%,15%, and 30% of Mass fractions:

CH4(Methane)

Mass fraction of Methane(fuel) reduces when the water is added.

CO2(carbon dioxide)

The CO2 emissions reducing slightly when water is added

H20(Water)

N2(Nitrogen)

There is no effect on N2 when water is added.

O2(Oxygen):

There is no effect on O2.

NOx(Nitrogen-oxide)

There is a reduction in the NOx emission when the water is added in the fuel.

Soot

There is a reduction in Soot emission when water is added in the fuel.

Effect of adding water on the different species:

Species->	CH4	CO2	H2O	N2	O2	NOx	Soot
Effect->	decreases	decreases	increases	No effect	No effect	decreases	decreases

Conclusion:

• As the water is added with the fuel, there is a reduction of emissions such as NOx, Soot, and CO2 to a great extent.
• As the water content increases, there is a reduction in the species such as NOx, Soot, CH4, CO2, while H20 increases and there is no effect on N2 and O2.
• Hence the emissions can be controlled by the application of the water in the fuel.