UNIT 2: INTRODUCTION TO FIRE MODELING

1. UNIT 2:INTRODUCTION TO FIRE MODELING

2. TERMINAL OBJECTIVE • Given two types of fire effects models, give an example of each, describe each model’s capabilities and limitations, and explain their roles in building or fire code performance-based design to include alternative materials and methods.

3. ENABLING OBJECTIVES The student will: • Describe the field fire model. • Describe the zone fire model.

4. INTRODUCTION TO COMPUTER FIRE MODELS • Use of a computer to recreate or “model” the fire environment is another tool for the investigator to use in fire scene analysis. • The recreation of the fire environment is accomplished through the use of mathematical equations. • The concept of computer fire modeling has been in existence since the 1960s.

5. INTRODUCTION TO COMPUTER FIRE MODELS (con.) • The need to understand and explain scientific principles relevant to fire growth, development, and flashover • Focus of fire researchers at the National Institute of Standards and Technology (NIST) since the 1970s

6. THE IMPACT OF COMPUTER MODELING ON FIRE RESEARCH • Used in performance-based designs because of the “ease” in which the model can replicate the fire environment • Cost and labor of the traditional methods of testing has been greatly reduced

7. OVERALL DEGREE OF ACCURACY • No model should be expected to be more accurate than the physical world it attempts to represent • The following slide show a comparison the results of an study conducted by the Nuclear Regulatory Commission. • Studies compared the range of predictions between the different types of models.

8. ACCURACY OF FIRE MODELS • NUREG 1824 Analysis

9. ACCURACY OF FIRE MODELS (con.) • NUREG 1824 Analysis

10. MODEL SENSITIVITY • A sensitivity analysis demonstrates the relative magnitude of change in output that can be expected by changing an input parameter.   • Some parameter changes will result in small changes in model predictions, while others may result in large changes in the predicted values.

11. VARIABILITY AND UNCERTAINTY OF FIRE TEST DATA • Even under idealized test conditions, replicate tests will not provide exactly the same results. • There is also uncertainty in obtaining fire test data as a result of measurement errors and other factors.

12. SCIENCE BEHIND THE MODELS • Navier-Stokes – equations that mathematically describe the transport of mass, momentum,and energy in fluids • Models continually solve for these equations in the “fire environment”

13. NAVIER-STOKES FORMULAS

14. FIRE MODEL TYPES • Zone: • FPETool • ASET-B • DETACT-QS • CFAST • Computational Fluid Dynamics (CFD aka Field): • Fire Dynamics Simulator (FDS)

15. ZONE MODELS • Zone models work on the concept that the compartment is divided into two distinctive zones: upper (hot layer) and lower (cool) layer.

16. ZONE MODEL FEATURES • Assume uniform temperature and gas in a zone

17. OVERVIEW OF ZONE MODELS • ASET-B (Available Safe Egress Time-BASIC) • DETACT-QS (DETector ACTuation-Quasi Steady) • DETACT-T2 (DETector ACTuation-Time squared) • FASTLite • FIRST (FIRe Simulation Technique) • FPETool • CFAST (Consolidated Fire and Smoke Transport)

18. FIRE MODELING - INPUT • Domain size • Grid resolution (some models) • Computational time • Physical geometry • Material properties • Initial conditions • Boundary conditions • Instruments • Output

19. ZONE MODEL LIMITATIONS • Interior finish flame spread not included • Ignitability from smoldering combustion not modeled • HRR is an input • Fire suppression not fully modeled (first sprinkler only, affects only the HRR)

20. SOME LIMITATIONS • Compartments must have smooth, flat ceilings (defined in NFPA 72) • Maximum size of compartments is limited • Smoke detector (DETACT) is based on temperature rise over ambient (41ºF, 5ºC) • Location of slice files is fixed

21. SOME LIMITATIONS (con.) • Location of detectors and sprinklers does not visualize in Smokeview • Vent status (open, closed) can only be changed one time during a simulation • Slice files only show temperature • Radiant heat transfer is limited by a default amount

22. OTHER ISSUES • Walls thicknesses are not a data point • Concern is with surface material properties • Any openings to the exterior are considered open to the world

23. SMOKEVIEW • Loads along with CFAST • Provides a visualization of the simulation • 3-D visual output of the “fire”

24. FIRE DYNAMICS SIMULATOR AKA FDS • Current version being used is 5.4 • Version 6 is under development • Version 4 and 5 are not directly compatible • Also includes Smokeview

25. View of CFAST in SmokeView

26. CFD (FIELD) MODELS • Divide space into small rectangular cells • Compute: • Density • Velocity • Temperature • Pressure

27. CFD MODELS • Compute gas species concentrations in each cell • Compute gas movement based on laws of mass, momentum, and energy • Fire suppression (sprinklers) can be modeled • Smoke detector (ion and photo) operation can be modeled

28. SMOKEVIEW Smokeview

29. Smokeview SMOKEVIEW

31. FIRE MODELING - INPUT • Domain size • Physical space in which you are going to model (x,y,z)

32. COORDINATE SYSTEM USED • Location of “obstructions” is entered using the x,y,z coordinates in a sextuplet format • x1,x2,y1,y2,z1,z2 or • xmin,xmax,ymin,ymax,zmin, zmaz • All are referenced from 0,0,0 • Must be entered in metric (meters)

33. DATA INPUT • Lines of “code” written in notepad format • Commercially produced software from Pyrosym • Excel spreadsheets developed by the engineering staff of Montgomery County (MD) Fire Rescue Services

34. JUST A LITTLE “CODE” &OBST XB=0.00, 11.58, 0.00, 0.1524, 0.00, 2.44 SURF_ID='GYPSUM BOARD' PERMIT_HOLE=.TRUE. SAWTOOTH=.TRUE. THICKEN=.FALSE./ Wall &OBST XB=0.00, 11.58, 7.62, 7.77, 0.00, 2.44 SURF_ID='GYPSUM BOARD' PERMIT_HOLE=.TRUE. SAWTOOTH=.TRUE. THICKEN=.FALSE./ Wall &OBST XB=11.58, 11.73, 0.00, 7.77, 0.00, 2.44 SURF_ID='GYPSUM BOARD' PERMIT_HOLE=.TRUE. SAWTOOTH=.TRUE. THICKEN=.FALSE./ Wall &OBST XB=0.00, 0.1524, 0.1524, 7.62, 0.00, 2.44 SURF_ID='GYPSUM BOARD' PERMIT_HOLE=.TRUE. SAWTOOTH=.TRUE. THICKEN=.FALSE./ Wall &OBST XB=3.96, 4.11, 4.11, 7.62, 0.00, 2.44 SURF_ID='GYPSUM BOARD' PERMIT_HOLE=.TRUE. SAWTOOTH=.TRUE. THICKEN=.FALSE./ Wall &OBST XB=3.96, 7.32, 4.11, 4.27, 0.00, 2.44 SURF_ID='GYPSUM BOARD' PERMIT_HOLE=.TRUE. SAWTOOTH=.TRUE. THICKEN=.FALSE./ Wall &OBST XB=5.79, 5.94, 0.1524, 3.35, 0.00, 2.44 SURF_ID='GYPSUM BOARD' PERMIT_HOLE=.TRUE. SAWTOOTH=.TRUE. THICKEN=.FALSE./ Wall &OBST XB=8.08, 11.58, 4.11, 4.27, 0.00, 2.44 SURF_ID='GYPSUM BOARD' PERMIT_HOLE=.TRUE. SAWTOOTH=.TRUE. THICKEN=.FALSE./ Wall

35. FIRE MODELING - INPUT • Grid resolution • Determines how often within the domain it will calculate variables, such as, temperature, pressure, and velocity • Defines the number of grid cells

36. THE EFFECT OF CELL, GRID, OR MESH SIZE

37. FIRE MODELING - INPUT (con.)

38. FIRE MODELING - INPUT • Computational time • How much time do you want to simulate? • This is not the amount of time that it will take for the computer to complete the simulation!

39. FIRE MODELING – INPUT (con.) • Material properties • The surface of the physical geometryjust defined • Initial conditions • Define what is going on at time = 0 • Temperature, velocity, density, conductivity, specific heat, pressure, and other conditions when simulation starts

40. FIRE MODELING – INPUT (con.) • Boundary conditions • Regulates conditionsat domain edges • Allows air to move in and out freely basedon physics

41. DEMONSTRATION: CHOICE OF BOUNDARY • Consider an experiment • 3x3x3m box inside a laboratory • Box has “hot” vertical wall to create natural convection • Measured temperature and airflow within the box

42. DEMONSTRATION: CHOICE OF BOUNDARY (con.) • Now, simulate the 3x3x3m box with FDS • Choice of boundary outside of the • Building containing the laboratory • Laboratory • Box • Two boundaries selected for simulation • Set computational boundaryat box exterior • Extend computational boundary so itis larger than the box exterior and includes someof the lab space

43. BOUNDARY AT BOX EXTERIOR

44. EXTENDED BOUNDARY

45. BOUNDARY DIFFERENCES

46. FIRE MODELING - OUTPUT • Instruments and output • Instruments, such as, thermocouples, smoke detectors, heat detectors • “Thermocouples” are used for various purposes in FDS • Necessary to model what values to output and in what format

47. Fire Scenarios • A specific set of conditions that define the development of fire, the spread of combustion products throughout a building or portion of a building, the reactions of people to fire, and the effects of combustion products.

48. Design Fires • Fire scenarios are those that are used for the evaluation of a proposed design. • All stakeholders in a performance-based design must agree on the fire and design fire scenarios. • Fire models are used to evaluate the effects of the design fires on fire and life safety issues

49. SUMMARY • The concept of computer fire modeling has been in existence since the 1960s. • NIST. • Classifications of enclosure fire models: probabilistic models and deterministic models. • Deterministic models can be divided into two separate categories: field models and zone models.

50. SUMMARY (con.) • Zone models work on the concept that the compartment is divided into two distinctive zones: upper (hot layer) and lower (cool) layer. • Field models are the newest and most sophisticated of all deterministic models. They rely upon computational fluid dynamics.