Bunsen burner flame gets colder with air hole fully open

[mcairtime]

Hello,

My GCSE class just did a practical investigation into how the width of a Bunsen's air hole affects the temperature change of a beaker of water. The entire class found as expected that the size of the temperature change increased as the hole was opened further, but they all found that with the air hole fully open the temperature change was smaller than the previous data point.

Any thoughts as to why?

Thanks very much

 

[willem2]

Eventually there's more are than needed to react with the fuel and the extra air still has to be heated.