The middle zone of a candle flame is also called the zone of ______ combustion.

The middle zone of a candle flame is also called the zone of luminous combustion. This part of the flame is where the temperature is high enough for the wax (or fuel) to vaporize and combust, producing a bright yellow light. The luminosity comes from the incandescence of tiny soot particles that are heated to a high temperature in this zone. This occurs due to incomplete combustion, where not all of the fuel burns completely, resulting in the production of soot. The luminous zone is surrounded by the outer zone, which is the hottest part where complete combustion occurs, and the inner zone, where the fuel begins to vaporize. Understanding the different zones of a candle flame can provide insights into combustion processes and the efficiency of fuel use.