Pressure Scaling of Fire Dynamics

Abstract

Fundamental to fire behavior are convection/diffusion phenomena which are not quantitatively captured by subscale experiments in an ordinary atmosphere. It has been known for decades that combining pressurization with length scale reduction, via the L³P² preservation rule, offers the possibility of rigorous convection/diffusion scaling. But the potential usefulness of pressure scaling of fires is still not delineated. Other phenomena are not encompassed by pressure scaling theory and thus present scaling deficiencies. These must be evaluated and, as necessary, dealt with by compensation in experimental procedure or interpretation of results. Thermal radiation presents the most critical potential scaling deficiencies; others stem from difficulties with rate-dependent processes, such as pyrolysis. Pressure scaling theory is reviewed. For context, focus is on prototype length scale L from 3 to 10 m and length scaling ratios of from 2 to 5. Pressure scaling still holds out potential for cost savings in testing fire protection designs. Scaling of fire spread in enclosures appears feasible but only up to points in time where product recirculation or gas layer influence on fire dynamics becomes important. For optically thick fires with burning rates of both model and prototype controlled by energy feedback, accounting for radiation will force compromise modification of the L³P² rule itself.