which flames in the bunsun burner is called a safety flame?

Yellow flame is often called a safety flame because yellow flame is luminous and thus it can be easily seen by human eyes.

In contrast, blue flame is non-luminous and difficult to see. This gives rise to higher chance to cause heat burn by a blue flame.

The term "safety flame" is based on the ease of observation, but NOT the flame temperature.

yellow flame is called a safety flame at around 1,000 °C (1,800 °F).