Flame extension characteristics and the temperature profile below the tunnel ceiling with various source-ceiling heights

Flame behavior of impinging on ceiling and spreading are fundamental phenomenon in tunnel fire accidents, it involves fundamental scientific issues such as heat and mass transfer, flow, and combustion (fire). The flame boundary extension along the tunnel ceiling is a crucial parameter of ceiling jet, directly impacting temperatures, heat fluxes, and heat radiations below the tunnel ceiling. Currently, there is no systematic analysis of the evolution of ceiling flame extension behavior and temperature profile during fire growth for various source-ceiling heights. In this paper, the impinging flame extension behavior and its evolution with heat release rate below the tunnel ceiling was explored by using the computational fluid dynamics (CFD) simulation. A series of simulation tests were conducted, varying burner dimensions, source-ceiling heights, and heat release rates. The results show that, for a given source-ceiling height, a larger fire heat release rate results in a more extensive flame spread. For a given lower fire heat release rate, the flame extension length or higher temperature profile decreases with an increase in source-ceiling height. Notably, for higher fire heat release rates, the flame extension length remains relatively constant regardless of the source-ceiling height. A non-dimensional overall function has been developed to describe the evolution of ceiling flame extension length during fire growth, that takes into account both tunnel width and source-ceiling height. These novel findings, along with the proposed model provide an important basis for understanding tunnel fires.