The equivalence ratio plays a critical role in influencing the reaction rate and extinction velocity in combustion processes. Understanding these effects helps optimize combustion conditions for efficient and stable operation while minimizing emissions and combustion instabilities. Balancing the equivalence ratio is essential to achieve optimal combustion performance and meet environmental and operational requirements.

1. Effect of equivalence
ratio on reaction rate and
extinction velocity
BY
ER. T. AYISHA NAZIBA, DR. D. RAMESH, DR. S. PUGALENDHI

2. Equivalence ratio
• The equivalence ratio (ϕ) in combustion is a key parameter that describes the
ratio of actual fuel-to-air ratio to the stoichiometric fuel-to-air ratio required for
complete combustion. The effect of equivalence ratio on reaction rate and
extinction velocity in combustion processes is significant
• 𝜙 =
𝑆𝑡𝑜𝑖𝑐ℎ𝑖𝑜𝑚𝑒𝑡𝑟𝑖𝑐 𝐹𝑢𝑒𝑙−𝑡𝑜−𝐴𝑖𝑟 𝑅𝑎𝑡𝑖𝑜
𝐴𝑐𝑡𝑢𝑎𝑙 𝐹𝑢𝑒𝑙−𝑡𝑜−𝐴𝑖𝑟 𝑅𝑎𝑡𝑖𝑜
•

3. Effect of Equivalence Ratio on Reaction Rate:
• Generally, the reaction rate increases with increasing equivalence ratio up to a
certain point (slightly rich conditions).
• Beyond this point, the reaction rate decreases due to lack of sufficient oxidizer
(oxygen) for complete combustion.
• Stoichiometric Condition (ϕ=1):
• At ϕ=1, the mixture is exactly at the stoichiometric condition where all the fuel and
oxygen are consumed completely.

4. • Lean Mixture (ϕ<1):
• For ϕ<1 (lean mixture), there is an excess of air compared to the stoichiometric
requirement.
• In lean mixtures, the reaction rate tends to decrease because of insufficient fuel for
complete combustion.
• The slower reaction rate in lean mixtures is due to reduced fuel availability, resulting
in longer ignition delays and incomplete combustion.

5. • Rich Mixture (ϕ>1):
• For ϕ>1 (rich mixture), there is an excess of fuel compared to the stoichiometric
requirement.
• In rich mixtures, the reaction rate can increase initially due to the availability of
excess fuel.
• However, excessively rich mixtures can lead to inefficient combustion, increased
emissions, and potential carbon monoxide (CO) formation.

6. Effect of Equivalence Ratio on Extinction Velocity
• Extinction velocity (Sext​) is the minimum flow velocity of a reactant mixture at which
the flame can be sustained.
• Extinction velocity generally varies inversely with equivalence ratio.
• As equivalence ratio increases (leaner mixtures), the extinction velocity increases due
to higher oxygen demand for combustion.
• Conversely, as equivalence ratio decreases (richer mixtures), the extinction velocity
decreases because less oxygen is required to sustain combustion.

7. • Lean Mixtures (ϕ<1):
• In lean mixtures (ϕ<1), the extinction velocity tends to be higher.
• This is because lean mixtures require higher air (oxygen) flow rates to sustain
combustion and prevent flame extinguishment.

8. • Rich Mixtures (ϕ>1):
• In rich mixtures (ϕ>1), the extinction velocity tends to be lower.
• Excess fuel in rich mixtures can sustain combustion even at lower air flow rates,
reducing the extinction velocity.

9. THANK YOU