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Combustion

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

Combustion

It combines one or more combustible substances such as hydrocarbons with an oxidizing substance such as oxygen in the process of combustion, which results in heat production and light, thereby increasing the ambient temperature. An example of a combustion reaction is as follows:

CH_4 + 2O_2 → CO_2 + 2H_2 O + energy

Combustion Chamber

The combustion chamber consists of an air inlet, a fuel injector, and a burner pipe that forms the combustion process within itself by mixing air and fuel.

combustion chamber

One of the most crucial sciences in automobile, space, and aerial industrial is related to combustion and reaction science. The internal combustion engine of automobile burn gasoline (C8H18) and heat released from this combustion caused the temperature of reactant reached about 2138 and based on ideal gas approximation the volume of reactant reach about seven times of primary volume (constant pressure) or pressure reaches about seven times (constant volume). Increasing pressure and volume of gases leads to piston movement. The philosophy of combustion in jet engine combustion chamber is a bit different. The main target from burning of JP4 (Keresan-gasoline blend) is increasing in volume of product and in constant area of jet tube predicting velocity should be increased. Although some combustion energy derived to rotate the turbine.  The thrust of jet is directly related to velocity (thrust is equal to mass flow rate multiplied by velocity).  Combustion is science of reaction of hydrocarbons the concentration of product and heat released.combustion chamber

We are experienced in the field of computational fluid dynamic simulation of combustion. You can see a summary of our experience in CFD simulation of combustion in portfolio in our website. The first simulation we have done is related to simulation of combustion in combustion chamber of jet engine, two strokes and 4 stroke engine. The type of combustion simulation in jet engine and two-stroke and 4 stroke is different. In jet engine combustion chamber, we use eddy-dissipation model for modeling turbulent chemistry reaction. Because we know the nature of this reaction, simulation is steady and rate of reaction is not very important. We simulate an infinite reaction rate. In the other word when reactant (hydrocarbons fuel and oxygen) reach each other the reaction will start. But in internal combustion engine, especially for automobile, time of reaction is crucial and could not simulate by eddy dissipation model. The rate of reaction is infinite will calculate using Arrhenius model and we use spark ignition model to control the time of combustion. By using spark, we prepare the needed activation energy of this reaction. The magnitude of spark energy and location of spark is significant for controlling the reaction time and shape.combustion chamber

The main parameters we investigate in designing combustion chambers is the effect of equivalence ratio and direct and magnitude of velocity on combustion performance. The equivalence ratio is the ratio of air/fuel stoichiometric to air/fuel actual, and if this ratio is below than one, the mixture is lean and if this magnitude is higher than one the mixture is rich. This is just theory of being rich or lean. If mixing does not perform well maybe we have rich mixture although we have rich mixture in the paper. Well mixing is critical and we use various method for this purpose. Using vortex generator, using fan for increasing turbulence and using tangential air injection in cylindrical chamber for increasing residence time reactant and increasing turbulence is some of these methods.

Also changing velocity in constant equivalence ratio is an essential factor. Velocity magnitude and direction will control the residence time and turbulence intensity and finally the affect on combustion performance.

Some of the experience in the area of combustion simulation can be seen in the following

  • combustion in annular, can annular, can and double annular combustor
  • CFD simulation of CH4 (methane) combustion in a vortex flame chamber
  • Simulation of two and four-stroke engine in internal combustion engine using dynamic mesh methodcombustion chamber
  • 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

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, energy equation should be active.

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 for the turbulent reactive flame simulation if used from a Mixture Fraction perspective, a pre-mixed combustion model must be used, also if the Reaction Progress variable is used, the premixed model should be used.

non-premixed combustion

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 pre-mixed 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, and …) in various gaseous species (such as CO2, H2O, O2, …).

Chemical equilibrium model

Chemical equilibrium mixing can be:

  • chemical equilibrium
  • close to chemical equilibrium (steady diffusion flamelet)
  • significantly different from chemical equilibrium (unsteady diffusion flamelet)

The chemical equilibrium model can have a more realistic prediction of flame temperature by incorporating the effects of intermediate species and the dissociation reactions. This model can also define the RFL value of the fuel flow in addition to defining the reaction pressure. It should be noted that if the secondary flow is used, in addition to defining the rich flammability limit of flow, the rich secondary flammability limit can also be defined, while using experimental or field fuel flow, It is not possible to define the rich flammability limit of the fuel flow. The Fluent software can calculate the composition in the rich range using equilibrium, but for fractions greater than this, the chemical calculates the equilibrium and suspends the mixture based on the mixture of fuel (not burning) with the rich composition. If the value of the rich combustion limit is equal to one, the equilibrium calculations are performed at a full interval of the mixture fraction, and if the value is less than one, the equilibrium calculations are suspended whenever the mixture fraction values of the fuel flows or secondary exceed the limits. The system operating pressure is also used to calculate the density using the ideal gas law. If in non-adiabatic mode, the system pressure changes significantly over time or in the workspace, the compressibility effects option must be enabled.

Adiabatic or non-adiabatic energy behavior

The non-adiabatic model is used for cases such as radiation or wall heat transfer, the entry of multiple fuels at different temperatures, the entry of multiple oxidizers at different temperatures and for liquid fuel, coal particles or heat transfer to Inert particles. However, if the adiabatic model is used, the energy equation does not need to be solved, and the system temperature is obtained directly from the mixture fraction and the inlet temperatures of the fuel and oxidizer.

Secondary Stream

The combination of a single fuel and a single oxidizer does not require the use of a secondary stream. Secondary flows can be used in cases such as two dissimilar gas fuels, mixed fuel from dissimilar liquid and non-similar fuels, mixed fuel from coal and liquid fuel, coal combustion, and combining a single fuel with two dissimilar oxidizers.

Empirical Stream

An alternative method of defining the composition of fuel or secondary stream that is used when the components of an individual species are unknown. For example, this option is used to simulate coal combustion or simulations containing complex hydrocarbon mixtures.

Defining the composition of streams

In the boundary section of the non-primixed model, it is necessary to define each of the gaseous species present in the fuel and oxidizing flows and the secondary flows (if any) existing as reactants, but the gaseous species reaction products and reaction intermediates are automatically obtained by the Fluent software. One can then define the molar ratio or mass ratio of each fuel stream and inlet oxidizer. It is also possible to define the inlet temperature value of the fuel and oxidizer flow and the secondary flow in the chamber.

Species Boundary Conditions

In the non-primixed model, the definition of the mass fraction of species at the boundaries is not needed and only the values of the mean mixture fraction (f) and the variance or variability of the mixture fraction (f´2) at the boundaries should be defined. For example, in the simulation of the reaction of a fuel stream with an oxidizing stream, the value of the mixture fraction shall be taken to be equal to one for the inlet for the fuel stream and for the inlet for the oxidizer or secondary stream equal to zero. The value of the variance of the mixture fraction at the inputs is usually assumed to be zero.

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