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

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 I_s, which is related to characteristics pertinent to the fire whirl. The vertical component of the magnetic field obtained from this model (B_zm) is compared with experimental results (B_ze) acquired in fire whirls in a shaft model. The two sets of values (B_zm and B_ze) 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.