The Standard Enthalpy of Combustion and Combustion Models
Key Takeaways
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The standard enthalpy of combustion quantifies the heat released during combustion under standard conditions.
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For calculating the standard enthalpy of combustion for complex fuel mixes, Hess’s law is an ideal approach.
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Chemical reaction-based or fluid flow-based combustion models can ideally represent the variations in temperature, momentum, or flow structure.
Combustion is the result of a chemical reaction between fuel and an oxidant (air) which releases heat energy in the process of the formation of the products. This is an important energy concept in engineering applications with a wider use in rockets, aircraft, power plants, and internal combustion engines. The standard enthalpy of combustion quantifies the heat released during combustion, the reference to which is the deciding factor when identifying ideal fuels for the above-mentioned applications.
In this article, we will discuss the standard enthalpy of combustion and analyze its importance in the design of combustion systems through ideal CFD (computational fluid dynamics) modeling.
The Standard Enthalpy of Combustion
Let us look at the following reaction:
This is a combustive reaction where butane reacts with oxygen to form carbon dioxide and water as products. The heat energy released during this process is the enthalpy of combustion. For butane, this value is -2878 kJ/mol. In general, the numerical value for the enthalpy of combustion can be determined using the following expression:
In this equation, Hc is the enthalpy of combustion and Hp and Hr represent the enthalpy of the product and reactant at a specific condition.
When the enthalpy is measured for completely oxidizing 1 mole of fuel under standard conditions, it is called the standard enthalpy of combustion. The standard condition in enthalpy refers to the temperature of 298K and 1 bar pressure. The value for the standard enthalpy of combustion is important in finding the total calorific value of the fuel used in the combustion system.
The enthalpy of combustion of many fuels, including hydrogen (-285.8kJ/mol), carbon (-393.5kJ/mol), methane(-890.0kJ/mol), are readily available values from common experimentations. In the case of incomplete combustion, however, the value for the enthalpy of combustion doesn’t provide much meaning. Additionally, for many fuel mixtures, direct calculations for the standard enthalpy of combustion can be difficult. One solution in such a case would be the application of Hess’s law.
Computational Combustion Modeling
The illustration of the standard enthalpy of combustion has been ideal in understanding the calorific value of the fuel during combustion, the associated enthalpy change, and the chemical mixes. A precise CFD combustion model should be able to reflect these factors so as to accurately represent chemical and physical changes.
A more direct approach for CFD modeling would be to solve the energy balance equation for products and reactants. However, when associated with fluid flow, combustion can be influenced by momentum, heat transfer, flame structure, and intensity (laminar or turbulent). Some models that can account for these factors for combustion modeling include:
- Eddy breakup model
- Flamelet model
- Favre averaging
- PDF (probability distribution function) transport model
- Flame surface density model
The heat produced during combustion affects its environment with changes in temperature, volume, flow pattern, momentum, etc. With a CFD model accurately representing these changes from a combustive reaction, a reasonable prediction can be made about how fluid flow affects fuel power and engine efficiency.
Complex Combustion Analysis With CFD Tools
With combustion influencing the physical and chemical properties of the associated elements and surroundings, uncertainties can be evident during system design. While we may start with the calculation of the standard enthalpy of combustion, the model should include a thorough analysis of combustion kinetics. A good CFD model means engineers and system designers will be able to:
- Accurately represent the combustion reaction
- Replicate the variations in temperature, momentum, and flow structure
- Solve transport equations and heat transfer
Fidelity from Cadence provides a complete set of combustion analyses for the design of internal combustion engines and propulsion systems. This includes the calculation and analysis of enthalpy of combustion, heat transfer, fluid flow regimes, and ideal air-fuel ratio, with the help of chemical reaction-based or fluid flow-based models. Through the analysis of different combustion factors, efficiency can be maintained during the design of critical fuel systems.
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