Reacting flow

Reacting flow

Flows with reactions are widely used in various industries, and their study and analysis are very important to us. One of the most widely used examples of reactive flows is the flow inside the combustion chamber. Combustion occurs in combustion chambers when fuel and air combine, releasing much energy as heat. Combustion chambers are one of the main types of equipment in power plant cycles. Due to the importance of reactive flows in industry, models have been developed to simulate these flows using CFD. In this section, we introduce these types of flows, and with some of the models available, we will also learn about Fluent software.

Reacting flow

A chemical reaction is a chemical transfer process in which a group of elements or chemical compounds are transformed into another group of elements or chemical compounds with different properties. In chemical processes, the arrangement of atoms in molecules is mixed, and new molecules with new electron arrangements are created. New molecules have different properties compared to previous molecules. Of course, according to the law of Conservation of mass, atoms are neither created nor destroyed in a chemical reaction, but their arrangement is changed, and new molecules are produced.

Chemical reactions are always associated with energy exchange. This energy exchange can be in sound, heat, and light. Some chemical reactions are exothermic, and some are endothermic. Exothermic reactions include combustion.

Reacting flow

Chemical reactions can be oxidation reactions, direct combination or synthesis reactions, chemical decomposition or decomposition reactions, substitution or single substitution reactions, double substitution reactions, acid-base reactions, combustion, and hydrolysis reactions. But in general, the four main types of reactions are direct combination, decomposition reaction, single substitution reaction, and double substitution reaction.

In some cases, the reaction happens in a flow. That is, a chemical reaction accompanies the flow. For example, we can refer to the combustion chamberŲŒ In the combustion chamber, air flows from one side, and fuel enters from the other side and is mixed. After mixing with a spark, a combustion reaction occurs, and much heat is released. These types of flows are called reacting flows. Flows with chemical reactions can be stable, unstable, slow, turbulent, multi-phase, cyclic, etc.

combustion chamber  CFD

In some reactions, before the reaction, the reactants are completely mixed together, which is called premixed. In some other cases, partial mixing is done (Partial-Premixed); in others, the reactions are not mixed at all (non-Premixed). Different reactions can occur with the amount of heat and other compounds depending on the reactants’ mixing. For example, in the combustion reaction, the amount of fuel and air mixing can affect the completeness of the reaction and produce products with different pollution levels. Therefore, CFD software has developed simulation models for all three states mentioned above.

Simulation of reactive flows with CFD

Flows with reactions are widely used in industry. Therefore, studying and simulating these flows is very important for us. As mentioned, due to the importance of this issue, models have been developed to simulate these flows in CFD software. One of the most powerful software for combustion simulation is Fluent software. In the following, we will review some models developed in Fluent for combustion simulation.

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SPECIES TRANSPORT

Using this section in Fluent, you can solve the species transfer equation for any reaction. This module is for simulating all reactions, such as endothermic and exothermic. But the rest of the Fluent modules are only for simulating the heat reaction, i.e., combustion. Using convection, diffusion, and reaction source terms for each chemical species, this model can simulate different species’ calm and turbulent flows and the possible reaction between them. In this model, it is possible to simulate several simultaneous reactions and all types of volumetric reactions, surface reactions, reactions on the surface of particles, and reactions in the porous medium.

In this model, reactions with 700 different species can be simulated.

NON-PREMIXED COMBUSTION

As mentioned before, one of the types of combustion reactions is a non-premixed reaction. Fuel and air enter the combustion chamber separately and are not premixed in this reaction. The model (Non-Premixed Combustion) has been developed to simulate this type of combustion reaction. Diesel internal combustion engines, pulverized coal furnaces, and pool fires are examples of non-premixed combustion.

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PREMIXED COMBUSTION

Premixed combustion is when the reactants are mixed before entering the combustion chamber. Aerobic internal combustion engines, combustion chambers of gas turbines, and explosions caused by gas leaks include premixed combustion. Fluent software has several models for simulating premixed combustion. Modeling premixed combustion is much more complex than non-premixed combustion. This is because the occurrence of combustion in a thin layer accompanied by flame propagation is according to the intensity of the flow disturbance. The overall flame propagation rate in subsonic flows is obtained based on the laminar flame speed and turbulent vortices. The slow flame speed is also calculated based on the rate of change in the number of chemical species and heat released from upstream to the reactants.

To model a laminar flame, it is necessary to solve the internal structure of the flame, such as molecular transport and diffusion processes and chemical kinetic details. Due to the very low flame thickness, which is usually in the order of millimeters or the less, accurate resolution of the flame is often not cost-effective (requires a lot of time and powerful hardware). Notably, in turbulent flows, the flame is wrinkled under the influence of turbulence, and its effective speed changes. Large vortices wrinkle the flame, and microscopic vortices (smaller than the flame thickness) change the flame structure.

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PARTIALLY PREMIXED COMBUSTION

The third type of combustion reaction is partial-premixed combustion. In this combustion model, fuel and air are mixed unevenly and partially. This combustion model has its advantages and is a modern combustion process intended to be used in internal combustion engines of cars and other motor vehicles of the future. This module is developed in Fluent based on Non-Premixed and Premixed concepts. Therefore, the limitations of the above two methods are the same as those of the Partially Premixed method.

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