Chemical Kinetics

Model the Chemical

and Thermodynamic Behavior

of a Combustion Process

Goal

What is the goal of chemical kinetics?

Experimental chemical kinetics ranges from fundamental kinetic experiments to studying the combustion behavior within the cylinders of an engine.

When the focus is on the many intermediate species in a combustion process, chemical kinetic modeling involves zero dimensional studies, focusing species concentrations over time, ignition dependency on

temperature and pressure. In these studies, the chemical kinetic mechanisms can be quite complex with thousands of species and reactions.

As the physical conditions get more complex, as in flames and engines, the chemical modeling must be simplified. More computational e!ort has to be paid to the physical conditions, for example one, two and three

dimensional transport and even turbulence, and less computational e!ort is allowed for detailed chemical information.

Why Combustion Modeling?

Food Power and Heat Transport

Improving the efficiency of everyday processes

Why should we study combustion?

Combustion e!ects all aspects of society, from the cooking our pizzas we each, to powering our iPods, to warming the house and to getting us to where we want to go.

In a society that is trying to use less energy and reduce global warming, the study of combustion is essential to improve the e"ciency of all these processes.

Why Chemical Kinetics?

Auto and Plant Emissions

Necessary for 'Clean' future

With an ever increasing emphasis on clean energy and clean transport, the study of chemical kinetics is crucial for understanding the chemical origins of emissions and essential in optimizing the combustion process

to reduce these emissions.

Thermal Event

Food Power and Heat Transport

Chemical Kinetics: a means to create thermal energy

In one sense, the chemical kinetics is a means to an end, the chemical process provides the means to produce thermal energy.

For many forms of modeling, the major concern is how much energy the species produce when they react.

Here, global reactions of the fuel going completely to products is the main concern along with global thermal properties like, for example, adiabatic flame temperature of a fuel and oxidizer and energy content of a species.

Thermal Event

Fuel

Products

Large Time Scale

Time Scale Difference

If Chemistry fast:Fuel -> Products directly

Tabulation Method:Just temperature evolution(not detail about chemistry)

When a combustion process is viewed as a thermal event, then all the chemistry that occurred to produce that thermal energy is of secondary importance.

In many combustion problems, the chemical process is very fast compared to the other processes, for example, molecular di!usion. In this case, the chemistry can be viewed as a before, the fuel and oxidizer, and after, the complete products. What happens in between is to fast to distinguish.

For example, one form of tabulation used in engine simulations is that of simply the ignition delay time and the temperature profile. There is no information about the detailed chemical process.

Chemical Event

Auto and Plant Emissions

Understanding the Chemical Processes of PollutantsHowever, there are examples when just knowing the thermal characteristics is not enough.

This is especially true when, for example, the concern is for more e!cient combustion leading to less emissions. In order to truly understand the origins of the emission species, detailed chemical models involving the emission species and their precursors are needed.

Another example is the HCCI engine whose combustion is steered by the chemistry of combustion and operates under ‘low’ temperature conditions where the consequences of the negative temperature coe!cient predominate. Traditional methods using very simple chemistry often does not give enough chemical detail to adequately model the chemical sources and consequences.

Chemical Event

CH3

CH4

CH2O

CO2

CH3OH

CH2

CH2

CH

(S)CH

2OH

Reverse

HCO

2O

H AbstractionH,OH

H,OHH Abstraction

CH3

OThird Body

H Loss

C2H

4COCH

2Abstractions

Addition

3CH

When combustion is viewed as a chemical event, then the complex network of individual reactions and intermediate species have to be considered. Understanding the complete chemistry give a better picture of, for example, the rate of the combustion process, the source of the thermal energy and origins of the harmful emissions that could arise.

Tracking the behavior of individual species under a variety of conditions and understanding these processes can give insight on how to improve e!ciency and determined, for example, under what conditions (and additives) can the harmful emissions be diminished.

Kinetic Modeling

Bottom Line

(Kinetic) Modelingis

the art of finding out what you can ignore

FromChemical Events to Thermal Events

And everything in between

Kinetic models within combustion can range from complex chemical models to simple thermal models.

The key to modeling is figuring out the balance between getting as much of the right information, meaning the properties you are interested in, out of a calculation with the computational power you have available to you.

Combustion modeling, as with all forms of modeling is essentially the art of figuring out what you can ignore. But to make intelligent decisions of what is not important, the process has to be understood.

In combustion kinetic modeling, the days are over where one can view kinetics as a simple fast event. This means that learning about kinetics is not just a theoretical academic exercise. It is an industrially relevant necessity.

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