What type of flame is characterized by having not mixed fuel and oxidizer until combustion?

The term "diffused flame" refers to a type of flame where the fuel and oxidizer are not mixed prior to combustion. In a diffused flame, combustion occurs at the boundary layer where the fuel and oxygen intermingle, leading to a distinct burning pattern. This type of flame is common in situations where the combustion happens naturally, allowing the reactants to mix only at the moment of ignition. As a result, this flame tends to be less efficient and can produce a larger amount of smoke and unburnt hydrocarbons compared to a premixed flame, where fuel and oxidizer are combined before reaching the ignition point.

In contrast, a premixed flame involves fuel and oxidizer mixed together before combustion occurs, resulting in a more controlled and often more efficient combustion process. Backdraft, on the other hand, refers to a rapid influx of oxygen into a smoldering fire, leading to an explosive ignition, which is unrelated to the mixing of fuel and oxidizer. Lastly, a secondary flame typically describes the flames that ignite from combustion products of primary flames, rather than relating to the process of mixing fuel and oxidizer prior to ignition.