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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?

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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.

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