Why burning velocity decreases with increase in equivalence ratio?

Answer 1

When equivalence ratio increase, the actual air-fuel ratio decrease, which mean the mass flow rate of air is decrease. So, the volume flow rate of air is dercreasing and its mean the velocity is decreasing.

Answer 2

As the equivalence ratio increases, the fuel concentration increases relative to the amount of oxidizer available for combustion. This causes incomplete combustion and an increase in the amount of unburned fuel in the flame, leading to a slower burning velocity. Additionally, the higher fuel concentration reduces the flame temperature, which further contributes to the decrease in burning velocity.