Ceiling jet flows

Abstract

Most devices associated with measures to detect and suppress fires in the built environment (e.g., commercial or modern residential buildings) are located near ceiling surfaces. The gas flow induced by an accidental fire tends to form a shallow layer beneath the ceiling surface that carries heat and smoke to areasremote from the fire position. Such a flow, known as a Ceiling Jet, can activate fire detection and suppression devices that are properly positioned in the shallow layer but can also cause damage eventually by heating the ceiling surface or structure. In this chapter, important characteristics of the ceiling jet, such as layer thickness, gas temperature or velocity and heat transfer rate for unconfined and confined ceiling configurations and for both steady and transient fires are discussed. Algebraic formulas are presented in all cases discussed to allow for rapid and reasonably accurate calculation of ceiling jet characteristics that can be used to verify aspects of more detailed numerical models. These formulas have also been embedded in comprehensive zone fire models and in design standards or codes