What is 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. The oxidizer, likewise, could be a solid, liquid, or gas but is usually a gas (air). New chemical substances are created from the fuel and the oxidizer during combustion. 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, containing 21% oxygen and 78% nitrogen, the exhaust can also include nitrous oxides (NOX, nitrogen + oxygen). The exhaust temperature is high because of the heat 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. Soot, for example, is a form of solid exhaust that occurs in some combustion processes.
Heat is generated when the fuel and oxidizer are turned into exhaust products during combustion. Interestingly, some source of heat is also necessary to start combustion. Gasoline and air are present in your automobile fuel tank, but combustion does not occur because there is no heat source. Since heat is both required to start combustion and is itself a product of combustion, we can see why combustion occurs 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.
Three things must be present for the combustion: a fuel to be burned, a source of oxygen, and 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 fuel available, the amount of oxygen available, or the heat source.
Combustion Simulations in ANSYS Fluent Software
The theory behind the combustion process can be explained through two major fields of Mechanical Engineering; Mass transfer and heat transfer. As was mentioned in the previous paragraph, combustion is the product of fuel and air burning in the presence of heat. Mixing fuel and air and generating new chemical species can be solved using the mass transfer equation coupled with Navier-stokes equations. The heat generated by this process and its effect on the ambient medium can also be analyzed using the coupled energy and Navier-stokes equations.
The combustion process in ANSYS FLUENT software can be simulated using the species module. The three frequently used models that can be exploited to simulate the combustion in this module are Premixed Combustion, non-premixed combustion, and partially premixed combustion.
The premixed combustion model has the characteristic that fuel and oxidizer are mixed molecularly before combustion. The transfer and expansion of flame occur from hot products to cold reactants. Flame expansion rate, or flame velocity, depends on the internal flame structure, and the turbulence distorts the laminar flame shape and accelerates flame development. It should be noted that a premixed combustion model must be used for the turbulent reactive flame simulation if used from a Mixture Fraction perspective. Also, if the Reaction Progress variable is used, the premixed model should be used.
The following simulation carried out by MR-CFD experts uses the Premixed model to simulate combustion in a porous zone:
The non-premixed combustion model has features such that the fuel and oxidizer enter the reaction zone from separate flow paths, meaning they are not premixed before entering the chamber, such as diesel internal combustion engines and liquid coal furnaces, heat transfer, or reactant diffusion from either side to the flame sheet, will distort the laminar flame shape and enhance mixing, and may simplify combustion to a mixing problem and eliminate problems associated with nonlinear average reaction rates. In this model, we define the Mixture Fraction, which denotes the mass fraction derived from the fuel flow (f symbol), which is the local mass fraction of the burnt and unburned fuel elements (such as C, H …) in various gaseous species (such as CO2, H2O, O2 …).
The following simulation carried out by MR-CFD experts uses the Non-Premixed model to simulate combustion:
Partially Premixed Combustion
Partially premixed combustion systems are premixed flames with non-uniform fuel-oxidizer mixtures (equivalence ratios). Such flames include premixed jets discharging into a quiescent atmosphere, lean premixed combustors with diffusion pilot flames and/or cooling air jets, and imperfectly mixed inlets. The partially premixed model implemented in ANSYS Fluent is a simple combination of the non-premixed and the premixed models.
MR-CFD, an Expert in the Field of Combustion Simulations
With several years of experience in simulating various problems in various CFD fields using ANSYS Fluent software, the MR-CFD team is ready to offer extensive modeling, meshing, and simulation services. Simulation Services for combustion simulations are categorized as follows:
- Combustion in annular, can and double annular combustor
- CFD simulation of CH4 (methane) combustion in a vortex flame chamber
- Simulation of two and four-stroke engines in internal combustion engine using dynamic mesh method
- Combustion and explosion of methane in a circular tube
- Combustion of Keresan in a gas turbine combustion chamber
- Methane combustion in a vortex flame chamber
You may find the related products in the aforementioned categories in our CFD shop by clicking on the following link:
Our services are not limited to the mentioned subjects, and the MR-CFD team is ready to undertake different and challenging projects in the combustion field ordered by our customers. You can consult with our experts freely and without charge and order your project by sending the problem details to us using the following address.
By outsourcing your project to the MR-CFD as a CFD simulation freelancer, you will not only receive the related project’s files (Geometry, Mesh, …), but also you will be provided with an extensive tutorial video demonstrating how you can create the geometry, mesh, and define the needed settings(pre-processing, processing and post-processing) in the ANSYS Fluent software all by yourself. Additionally, post-technical support is available to clarify issues and ambiguities.