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
Each course unit represents breaking down the problem into individual pieces

2. Overview Post-flashover fires (0,5 – 3 hours of fire)
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
Calculate Tg by method of Magnusson et al
Calculate Tg by Eurocode method

3. Concern is damage to people
This is often viewed by fire fighters and they assume it is flashover
As the fuel needs to flow longer to mix with sufficient oxygen for combustion, the flames appear to elongate as it travels to find sufficient oxygen to burn

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 Post-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
Other terms that we are missing (assume very small)
Radiation through opening
Energy accumulation in hot gas layer
We will use the energy balance again in more detail in Chapter 8

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
Hn is a function of Tg, Q, Ao and Ho. This function can also take up the constants
Also, W Ho^3/2 is the same as Ao Ho^1/2

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
AT is the boundary surface area

11. Effective heat conduction (transfer) coefficient, hk
Calculations based on thermal penetration time, tp
tp is the time for the unexposed temp to increase to 15% of exposed side temp

12. Two cases Thermally thin and thermally thick behavior
Thermally thin case represents steady heating
Thick case represents storing heat
Methods in EFD for combining different thickness boundaries and for different materials
Thus the effective heat transfer coef is a function of fire duration and material properties and thickness

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
Temperature change now given as a function of two groups

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
Calculation starts to over predict temperature above 500 C

17. Result of correlation Assuming standard properties: LHS Terms
First: Ratio of energy released to energy convected
Second: Ratio of energy lost to energy convected

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
If fuel controlled, are flames allowed to extend outside compartment?
If flames extend outside compartment, it is necessary to modify the HRR to represent burning inside the compartment

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

20. Overview Post-flashover fires (0,5 – 3 hours of fire)
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
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?
This should be familiar from passive fire protection course
What does dashed line represent?
HRR changes when fuels move or when ventilation openings are altered, for example

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
NPD=Norwegian petroleum directorate – hydrocarbon fire curve
ISO 834 curve on right

24. Measured compartment temperatures as a function of fuel load density
Number in ( ) is fraction of one wall open for ventilation

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?
Due to rapid rise in temperature, it represents a very severe exposure. It may not be conservative for some occupancies
Standard time-temp curve does not take into account
Decay
Compartment geometry
Ventilation openings
Fuels
Thermal properties

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] Fire load, Q [MJ]
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
Surface area used for pre-flashover fires did not include openings

28. Opening factor and multiple openings
Also a function of fuel load and interior surface area
Ao = area of opening Ho = height of opening
More on this in units 5 and 6

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 Magnusson time-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
Advantage of natural log function is that you can take the derivative of them easily

34. Decay period results (400 MJ/m2)
Curves of Magnusson, ISO 834, Eurocode and Pettersson
Eurocode drops temp down too fast
Less agreement during decay period of fires due to complexity of prediction (involves many factors)

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
Modeling makes transient calculations much easier.

36. Any questions? Next unit – Conservation equations