Flame temperatures – why actual flame temperature is below adiabatic Why is the measured (actual) flame temperature in furnaces and burners always lower than the calculated adiabatic flame temperature?

Introduction / Context:
The adiabatic flame temperature is the theoretical maximum temperature achieved when a fuel burns completely with no heat loss to the surroundings. In practice, measured flame temperatures are lower. Understanding the gap between ideal and real conditions is key to burner and furnace design.

Given Data / Assumptions:

• Adiabatic calculation assumes: complete combustion, no heat loss, reactants at reference conditions, and no dissociation constraints.
• Real flames experience heat transfer to walls, radiation to surroundings, and incomplete mixing.
• High-temperature dissociation and finite-rate chemistry can also reduce temperature.

Concept / Approach:
Actual systems depart from adiabatic assumptions in two primary ways: unavoidable heat losses and incomplete combustion (from mixing limitations, residence-time constraints, or local quenching). Additionally, endothermic dissociation at high temperatures reduces equilibrium temperature. These mechanisms ensure the real flame temperature is always below the adiabatic value.

Step-by-Step Solution:

Verification / Alternative check:
Measured flue gas containing CO or unburnt hydrocarbons indicates incomplete combustion; wall heat flux measurements confirm non-adiabatic operation.

Why Other Options Are Wrong:

• (a) alone is insufficient; complete combustion is achievable in many systems but not universally sustained.
• (b) alone is true but the combined explanation (c) is fuller and therefore best.
• (d) contradicts practical observations.

Common Pitfalls:
Ignoring high-temperature dissociation and finite-rate effects; assuming stoichiometric firing guarantees adiabatic temperatures in practice.

Final Answer:
Both (a) and (b)