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Fire Dynamics & Computer Fire Modeling

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  1. Fire Dynamics & Computer Fire Modeling Fire-Dynamics.Com Joe M. Ellington - IAAI – C.F.I., NAFI – C.F.E.I., ACFE – C.F.E. jmellington@fire-dynamics.com (713) 621-3550 (Office) (832) 767-9120 (Cell)

  2. Fire Dynamics & Computer Fire Modeling NFPA 921 - Guide for Fire & Explosion Investigations identifies fire modeling as a tool requiring specialexpertise that is available to assist the investigator in the analysis of a fire.

  3. Fire Dynamics & Computer Fire Modeling Fire Investigators may • Have limited knowledge about computer fire models and be skeptical of their practicality and usefulness. • Not have the time to invest or the willingness to make the effort necessary to use computer fire models efficiently and effectively.

  4. Fire Dynamics & Computer Fire Modeling • In addition to a requisite knowledge base of ignition properties, heat release rates of materials, and fire dynamics. • A considerable investment in time and effort is required to learn the hardware and software environments to run fire models.

  5. Fire Dynamics & Computer Fire Modeling The value and appropriateness of fire modeling as an investigative tool depends on: • Purpose of the investigation. • Scope of the investigator’s assignment. • Complexity and issues answered or raised by the investigation. • Availability and approval of time, money and resources.

  6. Fire Dynamics & Computer Fire Modeling • Even if an individual fire investigator never uses fire modeling, there is an ever increasing probability his opinions and conclusions will eventually be tested against a modeling analysis. • For this reason, the fire investigator should at least have a working knowledge of their capability and limitations.

  7. Fire Dynamics & Computer Fire Modeling • Fire investigation requires an understanding of materials and processes that take place within an environment of numerous, complex, and difficult to predict variables. • The ability of the computer to manipulate extremely large quantities of complex data with great accuracy and speed make it an excellent candidate as a tool for resolving these issues.

  8. Fire Dynamics & Computer Fire Modeling • Fire dynamics analysis and computer modeling has been influenced by the proliferation of personal computers that are economical, powerful and faster than those previously available.

  9. Fire Dynamics & Computer Fire Modeling Appropriate uses include: • Testing hypothesis • Validating or explaining post-fire indicators • Estimating time lines • Evaluating human factors and fire/smoke conditions • Predicting detector and sprinkler response times.

  10. Fire Dynamics & Computer Fire Modeling Fire modeling: • Is a tool for testing hypothesis regarding issues that are material and relevant to a complete fire analysis and may be useful to help fill in the details of a fire investigation.

  11. Fire Dynamics & Computer Fire Modeling • Fire modeling is intended to simulate or predict real-world phenomena using scientific principles and empirical data. • Is not a fix for a poor or inadequate fire investigation or proving causation.

  12. Fire Dynamics & Computer Fire Modeling Traditionally, fire investigation was a field where art and science merged but whose final outcome largely depended on the perceived skill and experience of the investigator rather than a scientific approach.

  13. Fire Dynamics & Computer Fire Modeling Traditionally, fire investigation was a field where art and science merged but whose final outcome largely depended on the perceived skill and experience of the investigator rather than a scientific approach.

  14. Fire Dynamics & Computer Fire Modeling • In the past, principles relied upon were often routinely accepted without challenge or scientific merit. • For example, evidence indicates that crazing of glass or spalling of concrete is observed as frequently in accidental fires as incendiary ones despite previously widely asserted opinion it was evidence of a fire accelerated with flammable liquids.

  15. Fire Dynamics & Computer Fire Modeling • Unless the investigator can support his opinions and conclusions with demonstrable facts and reproducible data, he runs the risk of a fate similar to that of Prometheus. Prometheus being punished for stealing fire from the Gods and giving it to mankind!

  16. Fire Dynamics & Computer Fire Modeling • The emergence of fire modeling as a legitimate investigative tool continues to evolve with demands for a scientific rather than intuitive approach to investigations.

  17. Fire Dynamics & Computer Fire Modeling • The concept of ‘modeling’ is neither new to the study of fire science or the field of fire investigation. • Fundamentally, while working a fire scene a fire investigator is consciously constructing a model in his mind applying a body of knowledge and principles accumulated through education, training, and experience.

  18. Fire Dynamics & Computer Fire Modeling • Unlike computer fire modeling, the fire investigator’s model is ‘conceptual’ and does not involve actual quantitative analysis or direct mathematical calculations. • The conceptual model is the sum of what the investigator has internalized (i.e. learned, been taught, or has come to believe) concerning what is normal or abnormal about structures, materials, and fire dynamics regardless of its validity.

  19. Fire Dynamics & Computer Fire Modeling • The investigator compares his observations about the fire scene against the conceptual model in his mind noting similarities and differences.

  20. Fire Dynamics & Computer Fire Modeling • The accuracy of the investigator’s conclusions is influenced by the thoroughness of his procedures, powers of observation, and depth of analytical reasoning. • The validity of the investigator’s internal conceptual model; however, ultimately determines whether the conclusions are right or wrong.

  21. Fire Dynamics & Computer Fire Modeling • If the knowledge, training and experience that underlies the investigator’s conceptual model is valid, the methodology sound, and attention is paid to detail; the conclusions will be accurate.

  22. Fire Dynamics & Computer Fire Modeling • If not, the conclusions will begin to unravel when subjected to closer examination.

  23. Fire Dynamics & Computer Fire Modeling • Good judgment comes from experience. • Experience comes from bad judgment!

  24. Fire Dynamics & Computer Fire Modeling • Crucial to the understanding of fire models and how they function is a fundamental and general understanding of what a model is.

  25. Fire Dynamics & Computer Fire Modeling • A model is really an idealized version of a physical system too complex to analyze easily in full without simplification.

  26. Fire Dynamics & Computer Fire Modeling • If we analyze, for example, the motion of a baseball thrown through the air, we find it quite complicated.

  27. Fire Dynamics & Computer Fire Modeling • The ball is neither spherical or perfectly rigid. It has raised seams and spins as it moves through the air. • Wind and air resistance influence its motions and the earth rotates beneath it. • The ball’s weight varies a little as its distance from the center of the earth changes and so on.

  28. Fire Dynamics & Computer Fire Modeling • If we try to include all of these small things the analysis gets pretty tangled. • Fortunately, “Intense simplicity emerge out of intense complexity!” (Winston Churchill) • Instead we invent a simplified version of the problem.

  29. Fire Dynamics & Computer Fire Modeling • We neglect the size and shape of the ball representing it as a point. • We neglect air resistance and make the ball move in a vacuum. • We also ignore the earth’s rotation and make the weight exactly constant.

  30. Fire Dynamics & Computer Fire Modeling • Now we have a problem simple enough to deal with. • The ball is in essence a particle moving along a parabolic path. • With these simplifications we can still make meaningful and reasonably accurate predictions about the balls’ path once it is thrown. • In fact, we can even apply these principles to artillery shells and nuclear warheads.

  31. Fire Dynamics & Computer Fire Modeling • In a similar vein, despite the almost infinite possible variations in which a fire can progress between ignition and extinguishment, a large majority of fires follow a somewhat probabilistic and predictable path from ignition, growth, decay, and extinguishment. • Further, most tend to have an identifiable and predetermined effect on structures, materials and contents.

  32. Fire Dynamics & Computer Fire Modeling • If this were not true, fire investigation would certainly be an ‘art’ rather than a ‘science’ and the fire investigator would operate in a subjective world with no frame of reference other than chance and intuition.

  33. Fire Dynamics & Computer Fire Modeling • Like the motion of a baseball, despite its perceived simplicity, the fire environment of a room is quite complex and requires simplification before the problem can be reasonably approached. • Similar to the baseball problem, the goal of fire models is to uncover laws governing the behavior of fire and to reduce and express them in mathematical terms.

  34. Fire Dynamics & Computer Fire Modeling The result can be: • A single equation model • A procedure or group of equations. • A full-scale model used to predict the behavior of real fires.

  35. Fire Dynamics & Computer Fire Modeling The choice of individual procedures or models depends upon: • The issues to be examined. • The degree of preciseness required. • The level of detail desired. • Capability of the person applying these procedures.

  36. Fire Dynamics & Computer Fire Modeling • Scientifically sound equations and other empirically derived engineering relationships exist that permit reasonably quantitative approximation of the development of hazardous conditions (e.g. temperature, smoke, toxic products) from fire in a single room or several rooms.

  37. Fire Dynamics & Computer Fire Modeling Single Equation Models are Specialized Fire Dynamics Routines • Are simplified procedures designed to solve a single, narrowly focused question. • May answer questions relating to a fire without the use of a full scale model • Typically require much less data to run than a full-scale fire model.

  38. Fire Dynamics & Computer Fire Modeling FPETOOL (Fire Protection Engineering Tools for Hazard Estimation) • Computerized package of relatively simple engineering equations, procedures and models. • Useful in estimating potential fire hazard and the response of space and fire protection systems in single compartment fires.

  39. Fire Dynamics & Computer Fire Modeling • The heart of FPETOOL is ASET-B, a room compartment fire model that calculates the impact on a room environment of a time varying rate of heat release. • Designed to approximate, rather than exactly predict, fire conditions. • The procedures used are based on sound physics or established correlations. • Simplicity, applicability, and computation speed have been emphasized with some sacrifice of mathematical rigor.

  40. Fire Dynamics & Computer Fire Modeling FPETOOL Attempts to predict at user defined intervals over the course of a single compartment enclosure fire: • Temperature and height of the upper layer. • O2, CO and CO2 concentrations • Rate of smoke production and flow.

  41. Fire Dynamics & Computer Fire Modeling • Energy (KW) venting to both the outside or another room through the HVAC system via user defined vents (e.g. doors, windows, etc.) • Visibility • Activation time of smoke detectors, heat detectors, and automatic sprinklers. • Estimated time to flashover.

  42. Fire Dynamics & Computer Fire Modeling FPETOOL is: • A good choice for ‘quick and dirty’ calculations that do not require a high level of accuracy and detail. • Suited for investigators with limited experience with computers in general and fire modeling in specific.

  43. Fire Dynamics & Computer Fire Modeling Fire models may be: • Physical • Mathematical

  44. Fire Dynamics & Computer Fire Modeling Physical models: • Are not mathematical. • May be either full or reduced scale. • Reduced scale models do not replicate all aspects of full-scale fire behavior. • Their limitations are safety, cost, time, and feasibility.

  45. Fire Dynamics & Computer Fire Modeling Mathematical models • Primarily use quantitative data or arithmetic expressions manipulated by the computer to describe the processes that go on during a fire. • Mathematical fire models may be Zone or Field Models (described later.)

  46. Fire Dynamics & Computer Fire Modeling Mathematical models • The underlying basis for most current computer fire models is the room or enclosure fire that, typically, fire investigators have the most experience and familiarity with.

  47. Fire Dynamics & Computer Fire Modeling • There are important and complex, interdependent relationships between the mass burning rate, the rate of heat release, and the available air (i.e. oxygen) on the course of an enclosure fire.

  48. Fire Dynamics & Computer Fire Modeling Hot Upper Layer • Zone models simplify an enclosure fire by idealizing the compartment as consisting of two regions. An upper layer filled with hot combustion gases and a lower layer filled with essentially cool air. • The simplification is based on experimental observation that during an enclosure fire, room conditions stratify into two distinct layers and, although measurable variations do occur within each layer, they are generally small in comparison to differences between the layers. Cooler Bottom Layer

  49. Fire Dynamics & Computer Fire Modeling • Each layer is assumed to have uniform temperatures. The gas concentrations and the interface dividing the layers moves vertically during a fire descending downward as the fire develops.

  50. Fire Dynamics & Computer Fire Modeling • An enclosure fire may be viewed simply as a pump that releases heat energy into an enclosure. • At ignition the enclosure is basically a volume of clean air at ambient temperature and pressure.