to answer the question What is Fire we have to understand the following

During the initial phase of a fire, radiant energy travels as light waves. This radiant energy can heat other objects that it reaches. Radiant energy fire detectors can be used to detect the radiation produced by fires. A plume of hot gases and flame rises from the fire and mixes with the cooler air within the room. This process is called “entrainment.”

entrainment causes the hot gases to be slightly cooler than those directly above the fire, and it increases the volume of the gases, causing the plume to be wider at the top than it is at the base. This inverted cone has an angle of about 22 degrees in total or 1 1 degrees on each side of the axis. This equates to a diameter equal to about 40 percent of the height from the base.

As the fire plume hits the A ceiling and turns horizontally into a ceiling jet, it can form a distinct layer with a depth of about 10 percent of the height from the base of the fire. Fire detection devices, such as smoke detectors, heat detectors, radiant energy detectors, and gas detectors, detect changes in the atmosphere resulting from a fire. These changes are known as “fire signatures.“

Combustion of a fuel can take place as an open flaming fire, or as any solid phase combustion called “smoldering.” Fire ignition requires three factors: heat, fuel, and oxygen. Once combustion begins. how the fire develops is largely dependent on the characteristics and configuration of the fuel involved and the available oxygen

So, unburned fuels rise above the fire in a plume. Energy carried away by the hot gases is called convected energy. If the rising plume hits a horizontal surface such as a ceiling, it turns and flows horizontally as a ceiling jet. The hot fire gases in the plume and ceiling jet can be detected by heat detectors. The solid and liquid particles in the plume in the ceiling jet can be detected by smoke detectors

An important characteristic of open flaming fires, as compared to smoldering fires, is that the combustion process is more efficient than the solid phase smoldering mode. For this reason, the open flaming fire produces many small particles and can affect your choice of smoke detector I operating principle