Which phase of combustion involves no flame being present?

The phase of combustion that involves no flame being present is the pre-ignition phase. This stage occurs before the actual ignition of the combustible materials and is characterized by heat being applied to the fuel. During pre-ignition, vapors may be produced from the fuel, and the materials may undergo thermal decomposition, but a flame has not yet developed.

In this phase, the conditions needed for ignition are being met, such as temperature and concentration of gases, but ignition has not yet occurred. This crucial phase is critical for understanding how fires start and evolve, as it sets the stage for the transition to the ignition phase when the necessary conditions bring about a visible flame.