When a fire plume strikes a ceiling it turns and races outward as a shallow, fast, horizontal flow called the ceiling jet, typically occupying only the bottom five to twelve percent of the ceiling height. Its temperature and velocity are highest directly over the fire and fall with radial distance, and the peak of both sits within about one percent of the ceiling height below the surface, which is precisely why heat and smoke detectors and sprinkler heads are mounted right up against the ceiling rather than a comfortable few centimetres below it. Standard correlations, first published by Alpert, predict jet temperature and velocity from the heat release rate, the ceiling height and the radial distance, so a designer can estimate when a device at a given spacing will see enough heat or smoke to respond. The confusion this resolves is treating a detector's response as depending only on the fire's size; distance from the plume axis, ceiling height and mounting depth all change the answer. After this Concept you can explain why detector spacing and mounting position change how fast a device responds.
Alpert's correlation is an empirical model in fire protection engineering (compartment fire dynamics) that estimates the gas temperature and velocity within an unconfined ceiling jet produced by a fi…