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Chemical Reactions

MR-CFD experts are ready for Chemical Reactions analysis, consulting, training, and CFD simulation.

Chemical Reaction


We use the Species transport model to define the combustion process inside the chamber. we calculate the continuity and momentum equations for all species in this model. The reaction usually is between air and fuel. Since we do the composition of fuel and air in a volume, so we use the Volumetric option.

Eddy-Dissipation should be used since the criterion of the combustion reaction is assumed to be based on the rate of mixing between fuel and air. Also, since the reaction contains energy sources, the Diffusion Energy Source option is used. Also, it is possible to produce contaminants during a chemical reaction. In some reaction, the created nitrogen oxides, including NO and NO2, are called NOx as reaction pollutants. One of the main causes of the growth rate of these nitrogen oxides is the excessive rise in temperature inside the combustion chamber. Also, the energy equation should be active.

Fuel Cell

A fuel cell is a device that generates electricity through a chemical reaction. Fuel cells consist of two electric poles, or electrodes called the cathode and the anode. In fact, chemical reactions take place inside these electrodes and generate electricity. In addition, each fuel cell also has an electrolyte and a catalyst, so that the task of electrolyte is to move charged particles between the electrodes, and the task of catalysts is to increase the rate of reaction at the electrodes. One of the important features of these fuel cells is the production of electricity with the lowest pollution levels. Hydrogen is the main fuel input to the cell, but oxygen is also needed to form the reaction. Finally, most of the oxygen and hydrogen entering the cell is discharged as a safe product (water). Since each fuel cell generates only a very small amount of direct current, it is usually attempted to use a large number of cells in large batches called the stack. The advantage of fuel cells compared to other sources and methods of electricity generation is that the fuel cell generates electricity by chemical reaction without the need for a combustion reaction; therefore, these fuel cells are more efficient and work less costly. The disadvantage of these cells is that despite their relatively simple operation, they are very difficult to build.

The main purpose of fuel cells is to generate electricity. In fuel cells, chemical reactions are the main cause of electricity production. Each fuel cell consists of two anode and cathode channels that are housed in collectors called cathode collector and anode collector so that these collectors are located on both sides of the fuel cell. (Top and bottom or left and right) and five layers between these two collectors. These layers include the anode gas diffusion layer, the anode catalyst, the electrolyte membrane, the cathode gas diffusion layer, and the cathode catalyst, respectively. The mechanism of action of these cells is that the hydrogen atoms enter the collector anode and are ionized in the catalyst region in a chemical reaction. This means that the hydrogen atoms are positively charged and produce free electrons. Then the electrons separated by hydrogen form the electric current inside the wire. The current flows back through the wire to the fuel cell again to form a circuit. On the other hand, oxygen enters the cathode collector and reacts with the electrons returning through the circuit as well as the positive hydrogen ions passing through the electrolyte membrane, resulting in It will be the production of water that drains out of the cell. In general, it can be concluded that electricity is generated as long as hydrogen and oxygen fuel cell inputs are provided. In general, the most important part of the manufacture of cells is electrolytes selection. Electrolytes play a main role in the cells, in that they must allow certain ions to pass between the cathode and the anode, and if mistakenly free electrons or other excess particles crossing the electrolyte, it will disrupt the chemical reaction. Fuel cells have different types of applications, such as PEMFC, SOFC, and Electrolysis.

The fuel cell with a proton exchange membrane, or PEMFC (polymer electrolyte membrane fuel cells), is composed of a polymer electrolyte formed into a thin, permeable plate. Its efficiency is between 50% and 60% and its operating temperature is about 80°C and has an output power of between 50 and 250 kW. The electrolyte used is solid, flexible, impermeable, and non-crackable. These types of cells are capable of operating at low temperatures and have in-house and in-vehicle applications, but their fuel needs to be purified and purified.

Solid oxide fuel cells (SOFCs) use a metal oxide that has a hard ceramic composition. The efficiency of these cells is about 60 percent and their operating temperature is approximately 1000 degrees Celsius and has an output power of 100 kW. Since these cells operate at high temperatures, their use is limited.

Governing Equations

In a fuel cell, the hydrogen combustion reaction is actually divided into two electrochemical half-reactions:

In the first reaction, because the electrons are released by the reaction, they are oxidized; while in the second reaction, because the electrons are consumed in the reaction, they are reduced. Also, the anode is the electrode where the electron is released, while the cathode is the electrode in which the electron is consumed.

The energy equation for fuel cells is as follows:

According to the above relationship, the bulk of the energy source consists of resistive (ohmic) heating or joule heating, which is the reaction heating resulting from the formation of water, electrical work, and latent heat.

Based on the fuel cell voltage being determined, the current density (Ampere per surface unit) is calculated, as the cell voltage can be calculated based on the defined current density. In the electrochemical modeling section, calculations of hydrogen oxidation rate and oxygen reduction rate are performed. These electrochemical processes behave as heterogeneous reactions occurring on the catalytic plates within the two-layer catalyst on both sides of the membrane layer. The driving force behind these reactions is the difference between the solid potential and the electrolyte or membrane potential. Hence, two potential equations are solved in the PEM model, one being the equation for electron transfer (e-) along with the solid conductive material i.e. the flow collectors and the porous medium solid grids, and the other Protonic (H +) ionic transfer, written as follows:

In the above reactions, σ denotes electrical conductivity (1 / ohm. Meter), φ denotes electrical potential (volts), R denotes the volumetric current (A / m3), sol as the solid symbol and mem as membrane symbol.

Mass Transfer

Mass transfer occurs when mass is transferred from one phase to another. Mass transfer is used for phenomena such as cavitation, evaporation and distillation, chemical reactions, welding, and so on.

Species transport model

The Species Transport model is used to simulate:

  • The transport of gaseous species
  • The chemical reaction of the gaseous species
  • The combustion process

There are several models for gaseous species including:

  • species transport
  • premixed combustion
  • non-premixed combustion
  • partially premixed combustion


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