Enclosure Fire Dynamics

1. Enclosure Fire Dynamics • Chapter 1: Introduction • Chapter 2: Qualitative description of enclosure fires • Chapter 3: Energy release rates, Design fires • Chapter 4: Plumes and flames • Chapter 5: Pressure and vent flows • Chapter 6: Gas temperatures • (Chapter 7: Heat transfer) • Chapter 8: Smoke filling • (Chapter 9: Products of combustion) • Chapter 10: Computer modeling

2. Overview • Pre-flashover fires (~ first 0-20 minutes of fire) • Goal: Save human lives • Energy balance • Heat transfer coefficients • Calculate gas temp Tg (t) by MQH method • Post-flashover fires (0,5 – 3 hours of fire) • Goal: Structural fire protection • Energy balance • Calculate Tg by method of Magnusson et al • Calculate Tg by Eurocode method

3. Concern is damage to people

4. Concern now becomes damage to building

5. Importance of knowing compartment temperature • Life Safety • Structural fire protection • Results in vent mass flows • Spread of smoke away from fire • Heating of fuel • Activation of detection systems • Impact on suppression • By fire department or sprinklers

6. Look at two cases: • Pre-flashover • Conditions changing with time (short) • Derive energy balance based on design fire • Two zone model • Post-flashover • Much longer time frame • Conditions generally more steady • One zone model • Few calculation methods apply to both

7. Pre-flashover compartment temperature with the MQH method • Method of McCaffrey, Quintiere and Harkleroad (MQH) • Conservation of energy relation (balance) for a ventilated compartment • Temperature is a function of dimensionless variables • Experiments used to derive constants • Allows simple solution without a computer

8. Start by looking at an energy balance for a compartment

9. Energy lost through openings (1st term on right hand side) • We know: • From Chapter 5, calculate vent mass flow: • Ideal gas law: • But HN is not known, so we write

10. Energy loss to compartment(2nd term on right hand side) • Heat is lost to boundaries by convection and radiation • This energy is then conducted into the solid walls, ceiling and floor • Define an effective heat conduction factor, hk,to represent all boundaries • AT is the boundary surface area without openings

11. Effective heat conduction (transfer) coefficient, hk • Calculations based on thermal penetration time, tp

12. Two cases

13. Put the two parts together • Solving for temperature difference is possible, but difficult in this form • Express temperature rise with dimensionless variables, then correlate with experiment

14. Dimensionless temperature • Dimensionless from dividing by Ta • Substitute in function for mass flow rate

15. Finding function for temperature change • Represent function using a power law • Values for C, N and M from experiment • 100+ tests with different fuels and linings • Varied size of room, openings

16. Correlation of data • C = 1.63 • N = 2/3 • M = -1/3

17. Result of correlation • Assuming standard properties:

18. Limitations on method • Transient and steady fire growth with temperature rise of 20 – 600 oC • Method predicts average temperature • Mass flows through ventilation openings • Two way flow established (after filling period) • Well mixed upper layer with uniform temp • Compartment not too large or too long (such as a corridor) • Test: Height < 2.7 m Area < 12 m2 • Fuel controlled burning • Heat released inside compartment • HRR not growing too fast

19. Predicting HRR for flashover • Set Tg = ~ 500 °C • => solve for QFlashover • MQH method

20. Overview • Pre-flashover fires (~ first 0-20 minutes of fire) • Goal: Save human lives • Energy balance • Heat transfer coefficients • Calculate gas temp Tg (t) by MQH method • Post-flashover fires (0,5 – 3 hours of fire) • Goal: Structural fire protection • Energy balance • Calculate Tg by method of Magnusson et al • Calculate Tg by Eurocode method

21. Now look at post-flashover fires • Structural design for fire • How will we know what the thermal exposure will be over the life of the building?

22. International time-temperature curves Curves are intended to represent maximum exposure that reasonably will not be exceeded over the life of the building

23. Time-temperature curves

24. Measured compartment temperatures as a function of fuel load density

25. Standard time-temperature curve (ISO 834) • Intended to represent maximum temperatures observed during complete burnout of compartment contents • Furnace testing of structural elements follows time-temperature curve • Failure also includes allowing fire spread • Is this curve conservative? • What does it not take into account?

26. Options for time-temperature curves • Use standard time-temperature curve • ISO 834 • Calculate a design specific time-temperature curve • This will be the majority of what remains of this chapter • Magnusson et al method • Babrauskas method • Eurocode method

27. Background material • Enclosure surface area, At [m2] • Now includes openings • Fire load, Q [MJ] • Energy released from all fuels in compartment • Fire load density, Q”t [MJ/m2] • Q”t = fire load density based on total enclosure surface area, At

28. Opening factor and multiple openings

29. Result, Magnusson method • Time-temperature curve is a function of: • Fuel load density, Q”t • Opening factor, • Thermal properties of compartment • Solutions given for one type of construction, corrected by Kf factor

30. Method of Magnusson and Thelandersson • Collected many experimentally measured temperatures • Numerically solved conservation of energy equation

31. Final form of Magnussontime-temperature curves

32. Method of Babrauskas (See page 3-140 in SPFE [2nd]) • Factors account for physical phenomenon

33. Eurocode parametric fire exposure method Heating phase Cooling phase

34. Decay period results (400 MJ/m2)

35. Use of models • Due to difficulties in estimating compartment fire temperatures, computer models are often used to solve conservation of energy equations • The equations given in this unit are good for making initial estimates • A lot of money can be saved if temperature is calculated instead of using ISO 834 curve

36. Any questions?Next unit – Conservation equations