Appunti di combustion

Chemical equilibrium

A new approach to thermodynamics is necessary and useful to put a box on it: we build the theory of contributions. To do that, the number of postulates that we need will be lower than usual, giving essentially a basic set.

For first, must be noted that thermodynamics is not a right name, in the sense that there is no dynamics and a word that suggests only the study of equilibrium states of macroscopic systems. This means that it would be better to think of thermodynamics of the processes should be thought as quasi-static because they contain the time in the limit. Because of that, processes are real processes that do not evolve in time and so time generates means physically a rate of charge.

Equilibrium process that doesn’t evolve in time but simply drive one system from an equilibrium state to another. There is a change but time is not taken into account.

By what we said thermodynamics doesn’t say anything about how a system change in time but only the direction and eventually some information on equilibrium processes.

Equilibrium and non equilibrium

Equilibrium (thermostatics):

• Quasi-static
• Infinite time
• Reversible/irreversible equilibrium processes

Non equilibrium:

• Non quasi-static
• Real processes of near equilibrium
• Or far from equilibrium
• Finite time

By the way, the basic set of postulates is for thermostatics.

1st postulate says that for a system there exist particular states, equilibrium states, 'i: the from a macroscopic point of view are completely characterized by the extensive variables, U, V and n_k. Those states are reached because the system interacts with the surroundings only by the exchange of mechanical work and by thermal and chemical interaction. A particular type of equilibrium state are the metastable states which are related to the presence of some constraints that don't allow the system to reach the equilibrium state (x O₂ and H₂ in room, and the temperature fixed to 20° then means O₂ and H₂ can react because - observe the temperature will increase). Because of that, if we consider a system surrounded by walls, those walls can be:

• Rigid or mobile, so that work can be exchanged or not.

Chemical equilibrium

A new approach to thermodynamics is necessary and useful to put a base on it to build the theory of contributions. To do that, the number of postulates that will be used will be lower than usual, giving essentially a basic set.

For first, must be noted that thermodynamics is not a right name, in the sense that there is no dynamics and a word that implies only the study of equilibrium states of macroscopic system. This means that it would be better to avoid thermodynamics and the processes should be thought as quasi-static because they occur in short time in the limit. Because of that, processes are ideal processes that do not evolve in time and so time derivative means physically a rate of change.

Equilibrium processes that doesn't evolve in time but simply drive one system from an equilibrium state to another. There is a change but time is not taken into account.

By what we said thermodynamics doesn't say anything about how a system changes in time but only the direction and eventually some information on equilibrium processes.

Thermodynamics

• Equilibrium (thermostatics)
• Quasi-static
• Infinite time
• Reversible/irreversible equilibrium processes

Non equilibrium

• Non quasi-static
• Real processes or near equilibrium or far from equilibrium
• Finite time

By the way, the basic set of postulates is (for thermostatics).

1st postulate says that for a system there exist particular states, equilibrium states, that, from a macroscopic point of view are completely characterized by the extensive variables V, V and n_k. Those states are reached because the system interacts with the surroundings only by the exchange of mechanical work and by thermal and chemical interaction. A particular type of equilibrium state are the metastable states which are related to the presence of some constraints that doesn't allow the system to reach the equilibrium state (ex. O₂ and H₂ in room and the temperature fixed to 20°, they can be reached because otherwise the temperature will increase). Because of that, if we consider a system surrounded by walls, those walls can be:

• Rigid or mobile, so that work can be exchanged or not;

26/02/22

Premixed combustion

Premixed combustion can be of two types:

• Deflagration if it is subsonic;
• Detonation if flame velocity is supersonic.

The shape of a flame is such that the normal velocity of the flame is exactly equal to the flame velocity.

• Irreversible or reversible which lead or not to exchange of matter and so a change of chemical composition;
• Adiabatic or diathermal allowing or not the exchange of heat.

Because of those, an equivalence between the conservation of energy ΔU=W and the 1st p