Variation of cone angle with concentration of fuel-air mixture A stoichiometric fuel-air mixture flowing in a Bunsen burner forms a well-defined conical flame. The mixture is then made leaner. For the same flow velocity in the tube, how does the cone angle change?
The cone angle of the Bunsen flame is determined by the flame speed of the mixture and the average flow velocity. To first approximation one can write: $$ \sin \alpha = \frac{S}{U} $$
where $\alpha$ is the half cone angle, $S$ is flame speed and $U$ is flow velocity.

When the mixture is made fuel-lean, depending on how flame speed changes the cone angle will change accordingly.

Usually flame speed for fuel-lean mixture is less than that for stoichiometric mixture. Therefore we can expect the cone angle to decrease.