In addition, the conservation of mass also governs this situation. If no additional accumulation of the mass of air is assumed in the space above the timber ceiling, the mass flow out of this space must equal the mass inflow into the space. Assuming that the space above the timber ceiling is unable to create a vaccum (a reasonable assumption with leakage areas and vents to the outside atmosphere), expansion of the products of combustion and bouyancy will drive flows through vents. Any vent outflow will have to have a equal inflow by mass. Thus, in general, bouyancy may dominate the driving force with respect to thermal expansion and pressure differences and the bulk flow will tend to be in a vertical direction through any vents at the upper portions of the space. The "new vents" in the timber ceiling will allow "fresh" air to flow upwards into the space to satisfy the law of conservation of mass.

Also:

1) Smoke gets cooler as it is transported away fron the fire due to air entrainment. This air entrainment also dilutes the concentration, which has a bearing on the activation of the smoke detector. Cooler smoke, also reduces the velocity of the bulk flow, which also has a bearing on the activation of the smoke detector.

2) In most residential situations, I would doubt the smoke would sufficently cool to create no bouyancy.

3) Smoke "filtering down" still requires a driving force. Diffusion is certainly one driving force, but is it generally small for "smoke" when compared to the bouyancy force. Carbon monoxide is one product of combustion that does diffuse relatively rapidly through air.

4) Also, a significant point is that, in this case, there is not one scenario or set of conditions that dominates the entire event, but a number of scenarios that can happen at different points in time (i.e. transient).

5) If a detailed analysis is required to adress your specific issue, then with some reasonable assumptions, you could evaluate use an appropriate CFD code to help determine the transient flow conditions and the local conditions around a smoke detector. This would not be a trivial exercise and would require considerable time and money, but it could be done. You may be able to adress your issue with a simplier analysis.


As an aside to the question(s) asked:

This is a good example of how fundamental knowledge can be applied to everyday hypothesis testing associated with the investigation of fire and explosion events. If we are going to significantly advance our profession in the coming decades, such fundamental knowledge must be obtained by every investigator. Such knowledge is not being provided in the training given to the general fire investigation community.

In my experience, the lack of understanding of the transient nature of fires has been a contributor to "bad arson calls" where only one set of conditions are assumed through the entire event. One of the problems is that a single compartment fire with a door opening has been mostly used for training purposes. The majority of fatal fire occur in the early morning hours in the residential setting. In general, the houses in these cases has all significant openings (i.e. doors and windows) closed at the start ofthe fire. When the fire starts, it reaches an unventilated condition relatively quickly. Only after a change in ventilation occurs (e.g. window breakage or the opening of doors or windows by occupants), does the behavior of the fire changes significantly. Unfortunately, the baseline of knowledge applied to this scenario is from compartment fires with open doors to an infinite supply of ventilation from the outside. This has resulted in the analysis of the wrong fire scenario.

Douglas J. Carpenter, MScFPE, CFEI, PE, FSFPE
Vice President & Principal Engineer
Combustion Science & Engineering, Inc.
8940 Old Annapolis Road, Suite L
Columbia, MD 21045
(410) 884-3266
(410) 884-3267 (fax)
www.csefire.com