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Understanding Enthalpy of Combustion for CFD Modeling

Key Takeaways

  • Enthalpy of combustion defines the value of heat released when a fuel is burned.

  • The calorific value of enthalpy of combustion is a critical factor in making the selection of fuel for a specific application. 

  • The low and high heating values indicate the relative efficiency of the combustion engine. 

Engine explosion

Combustion is a common phenomenon observable in real life when burning fuel or coal, which results in the release of heat energy. This concept has been utilized by engine manufacturers for automotive or thermal power plants, where the heat released from the combustive reactions is used to work the heat engine. The enthalpy of combustion is related to the value that defines the amount of heat given off during the process. 

The representation of the combustive process in the design of these aircraft or vehicle engines involves solving various energy and heat transfer equations. However, this calculation requires analysis of different stages of reactions at different time steps. Maintaining accuracy in these calculations can be attributed to computational fluid dynamics (CFD) simulations, which can represent the combustion process and calculate the energy involved. 

What Is Enthalpy of Combustion?

In general terms, the enthalpy of combustion is the amount of heat released when the fuel is burned in oxygen under controlled conditions. Let us take the following reaction as an example. 

Combustion of carbon in oxygen to form carbon dioxide

In this reaction, carbon is burnt completely in oxygen, which releases heat to form carbon dioxide. Its enthalpy of combustion is calculated at -393.50 kJ per mole, indicating an exothermic reaction. The calorific value of enthalpy of combustion of the fuel is an important determinant for selecting a specific fuel for a specific application. For instance, liquid hydrogen is used as a fuel for rocket engines. The heat released during the combustion of this fuel acts as the kinetic energy source to induce thrust.  

Low and High Heating Values

The calorific value for the heat of combustion can also be called the heating value. Depending on the provision for condensing or cooling after combustion, the enthalpy can be categorized into low or high heating values.

  • The high heating value (HHV) of the fuel indicates the total amount of energy produced during combustion. This value can be calculated by bringing down the temperature to its original, pre-combustion state. 
  • The lower heating value (LHV) is obtained once the latent heat of vaporization of the vapor formed due to combustion is subtracted from the high heating value.  

These values are important to consider when analyzing the engine efficiency of a heating system. For instance, LHV is used in systems with no secondary condenser. The water is converted into a gaseous state and the total heat released through the burning of the fuel is LHV. In this case, not all the heat released can be used, resulting in energy waste. On the contrary, HHV should be considered for advanced combustion engines where multiple condensers are used. HHV can condense the water vapor to extract the maximum amount of energy.

HHV and LHV can be calculated by developing a relationship between them with the help of the following equation:

Relation between HHV and LHV

Note that:

Hv - the heat of vaporization

n(H2O, out) - number of moles of water vaporized during combustion

n(fuel, in) - number of moles of the combusted fuel 

Relation With the Enthalpy of Formation

Combustion and formation are two interlinked phenomena. When the fuels combust, energy is released as the bonds rearrange to form a new product. The energy expended to form this product is the enthalpy of formation, which can also help in the calculation of enthalpy of combustion using the following equation:

Relation between the enthalpy of combustion and formation

Note that:

ΔHc is the enthalpy of formation

ΔHf is the enthalpy of formation for product ‘p’ and reactant ‘r’

CFD Modeling of the Combustion Process

The combustion reaction is subjected to changes in the physical and chemical attributes of the associated fuels. When designing a combustion system, it is important that these changes be ideally represented. CFD modeling of the combustion process can properly implement these complex phenomena to accurately provide information about the rate of combustion, enthalpy change, and the associated heat transfer from the fluid flow. This can be done by developing a fluid flow model or chemical equilibrium model. With advanced CFD packages to support these simulations, the enthalpy of combustion and its meaning in critical engineering applications can be easily analyzed.

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