In the process of determining the location of a fire’s origin in a building, investigators often rely upontheir interpretation of burn patterns created during the fire. During a series of recent fire testsdesigned to further understand burn pattern development during ventilation-controlled, post-flashoverfires, unexpected findings emerged related to patterns commonly known as “clean-burn”.Experienced investigators who examined the compartments after each fire noted variances in cleanburnpattern generation despite each of the fires having been initiated in the same manner and at thesame locations. If analyzed solely based upon current knowledge of clean burn pattern development,these burn patterns could have led to widely varying conclusions.Most of the present-day definitions in fire investigation literature associated with the term “cleanburn”, relate to oxidative combustion of soot deposited on various surfaces. According to NFPA 921,“clean burn” is defined as, “… a phenomenon that appears on noncombustible surfaces when the sootand smoke condensate that would normally be found adhering to the surface is burned off. Thisproduces a clean area adjacent to areas darkened by products of combustion…”In three, single-room compartment fire tests conducted in 2008 at the U.S. Bureau of ATF’s FireResearch Laboratory in Maryland, each fire was started in the same manner and location. The mainvariable in each of the tests was the length of time the fires burned under ventilation-controlledconditions. While some similarities in the ensuing burn patterns were identified, there were alsovariances, particularly in clean burn indicators. Analysis of video recordings made during the tests,suggest that the clean burn patterns might not have been generated via the mechanism suggested bycommon definitions like that referenced above from NFPA 921. Further in two tests where cleanburn patterns were expected, they were not seen. The anticipated pattern areas were instead coveredover with soot deposits.Persistence of clean burn patterns is examined. The author’s initial hypothesis is that the mechanismfor clean-burn pattern development may be different than the popular notion of combustion ofpreviously deposited soot. The patterns may be due in part to high thermal gradients on surfacespreventing localized deposition from occurring. A better understanding of such mechanisms mayimprove the interpretation of such patterns, particularly in relation to identifying aspects related to thetiming of their creation.A review of the literature related to the deposition of soot on wall surfaces is conducted to identifythoughts on potential variables and mechanisms at play. Additionally, clean-burn patterndevelopment will be discussed in relation to ventilation flows.
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