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 no evolves 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 (thermostatics):
• non equilibrium:
• quasi-static
• non quasi-static
• infinite time
• real processes of near equilibrium
• reversible/irreversible equi-
• librium processes
• 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 macroscopically point of view are completely characterized by

the extensive variables, U, V and n_k. Those states are reached because the sys-

stem interacts with the surroundings only by the exchange of mechanical

work and by thermal and chemical interaction. A particular type of equilibrum

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 sys-

tem 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 thinked as quasi-static because they occur in short time in the limit. Because of that, processes are ideal processes that no evolves 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).

1^st postulate says that for a system there exist particular states, equilibrium states, that, from a macroscopically point of view are completely characterized by the extensive variables V, V and n_k. Those states are reached because the sys- stem 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 sys- surrounded by walls, those walls can be:

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

26/02/22

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 1^st p