Answer:
By leaving the air valve almost close, so the flow of oxygen is as low as the combustion is not carried out properly, therefore, the flame will not rise beyond the middle of the ideally whole flame.
Explanation:
Hello,
On the attached picture, you will find a scheme showing the flame behavior of a bunsen burner in which it is seen that the middle of the flame matches with the hottest temperature due to the complete combustion of the fuel. It is possible for the match to remain unlit in the middle of the whole flame if the airflow is as low as the combustion is not carried out properly as shown below the middle of the flame; it implies that the feed of oxygen is low so its valve remains almost closed.
Best regards.
