Modeling of ceiling fires

Ceiling fires are examined both experimentally and theoretically, revealing their chemical and physical scaling laws. A novel experimental technique is used which simulates flammable liquid and solid materials with gaseous fuel-inert mixtures flowing through a sintered metal burner. This technique permits wide variation in the mass-transfer driving force B without affecting transport properties or thermodynamics. The experimental correlation of flame stand-off distance as well as heat and mass transfer holds for significant variations in burner diameter and fuel-mixture chemistry. An interesting cellular instability was observed which appears to be of a Rayleigh character with internal rotational flow of fuel vapor. The critical Rayleigh numbers are approximately 2000. A generalized, although approximate, theory is developed for a wide variety of simultaneous heat- and mass-transfer (free-convective) processes occurring beneath horizontal surfaces. This laminar theory, which uses no adjustable constants, is confirmed by the experiment. It reveals a 1/5 power (rather than 1/4 power) dependence on Grashof number.