What is a primary characteristic of diffusion flames?

A primary characteristic of diffusion flames is that they consume most fuels. In diffusion flames, fuel and oxidizer (usually oxygen in the air) mix in the combustion zone as they diffuse into one another. This mixing process allows for a wide range of fuels to be burned effectively, making diffusion flames versatile in their fuel consumption.

This characteristic is significant in fire investigations, as the behavior of diffusion flames can help determine fuel sources involved in a fire. An example of diffusion flames includes those found in candles or gas flames where the fuel is mixed with air as it rises, leading to a stable and continuous burn.

The other options describe characteristics that are not typically associated with diffusion flames. For instance, while some diffusion flames can produce less soot, this is not a definitive feature, as it can vary significantly based on fuel type and combustion conditions. Likewise, the efficiency of diffusion flames can vary and they are not inherently known for their high efficiency compared to other flame types, particularly premixed flames which are specifically designed for complete combustion. Burning cooler is also not a defining trait; in fact, diffusion flames can reach considerable temperatures depending on the specific fuel and conditions in which combustion occurs.