Magnetic field associated with an internal fire whirl : a simple model

Abstract

A fire whirl can be generated by a pool fire in vertical shafts in tall buildings under certain ventilation conditions. Internal fire whirls are much more dangerous and destructive than non-whirling fires in the shaft and deserve more attention. As fires or flames consist of ions in motion, the characteristics of the magnetic field generated by a fire whirl would provide important information. The present study aims at measuring the magnetic field generated by a fire whirl and at building a simple model to explain the generated magnetic field. The physical origin of the magnetic field associated with a fire whirl is proposed, which consists in the interaction of the moving ions with the Earth's magnetic field. It is shown that as far as the magnetic field around a fire whirl is concerned, the fire whirl is equivalent to a solenoid carrying a current Is, which is related to characteristics pertinent to the fire whirl. The vertical component of the magnetic field obtained from this model (Bzm) is compared with experimental results (Bze) acquired in fire whirls in a shaft model. The two sets of values (Bzm and Bze) are well correlated, with deviations which are reasonably acceptable. The results of the present work could be of value as a diagnostic tool in monitoring internal fire whirl and in studying various aspects of the whirl.