10 Adiabatic Flames, dissociation and equilbirum

Question 1

What is an adiabatic flame?

Question 2

What is AFT?

Answer 2

Adiabatic Flame Temperature

The max (ideal) temperature that a system could reach for given reactants

The temperature that results from a complete combustion process that occurs without any work, HT, or changes in Ek or Ep

Question 3

Difference between constant pressure AFT and constant volume AFT

Answer 3

AFT at constant pressure has a lower temperature than that for a constant volume process.

Because during constant pressure process, some of the energy is used to change the volume of the system (ie generate work)

Question 4

Why will AFT never be reached in practice?

Answer 4

• non-adiabatic factors reduce the peak combustion temp, below AFT

Question 5

non-adiabatic factors limiting AFT from being reached

Answer 5

• HT through combustor walls, flame stabiliser, radiative and convection losses
• incomplete combustion
• incomplete mixing of air and fuel
• dissociation

Question 6

when is dissociation likely to occur and what is the significance of its occurrence?

Answer 6

At temps > 1500ºC

is a non-adiabatic factor limiting AFT from being reached

As dissociation reduces flame temps and smears the flame. The dissociated compounds then re-react in lower temp zones

Question 6

when is dissociation likely to occur and what is the significance of its occurrence?

Answer 6

At temps > 1500ºC

is a non-adiabatic factor limiting AFT from being reached

As dissociation reduces flame temps and smears the flame. The dissociated compounds then re-react in lower temp zones

Question 7

What is dissociation?

Answer 7

the process where molecules separate into smaller molecules/ions/radicals.

Usually reversible

occurs at high flame temps

sometimes deliberately caused to initiate the breaking of molecules within the products which will then re-react with each other to form the reactants. This will continue to happen until the reaction stabilises.

Question 8

How is reversibility common in hot flame reactions?

Answer 8

dissociation can occur with products within the hottest part of the flame. The vibrations of the recently-formed molecules causes many reactions to become reversible.

Question 9

when do flames reach equilibrium?

Answer 9

dissociation will occur between molecules in a reaction until the formation rate of the products reaches the same rate of the dissociation.

This then results in a stable (but not static) equilibrium. (but only <1500ºC!!!!)

Question 10

units of K

Answer 10

atmospheres (usually)