What defines a premixed flame?

A premixed flame is defined as a flame that occurs when the fuel and oxidizer are combined thoroughly before the combustion process takes place. This means that the molecular mixture of fuel and air (or another oxidizer) occurs, allowing for a uniform reaction when ignition occurs. In this type of flame, as both the fuel and oxidizer are already mixed, the combustion happens rapidly and uniformly, leading to distinct characteristics often seen in applications like gas burners and certain industrial processes.

The other options don't accurately convey this definition. For instance, a flame that occurs with fuel only suggests a situation lacking an oxidizing agent, which is not applicable to any combustion processes, as combustion requires both fuel and an oxidizer. A flame that does not propagate indicates an inability to sustain itself, which does not describe a premixed flame either, as such flames can propagate if conditions allow. Lastly, a flame that burns without producing heat is contradictory, as combustion inherently produces heat energy as a byproduct of the chemical reaction between the fuel and oxidizer. Thus, option B accurately captures the essence of what a premixed flame is.