Compartment Flashover Heat Release RateMQH, Babrauskas, and Thomas correlations

What this calculates

This sheet is used to find the heat release rate (Q) at which a compartment is likely to reach flashover, using the dimensions of the compartment and the ventilation source (door, window, etc.). Three different equations have been provided for the same general answer to show the range of values that different techniques will predict. One of the calculations (MQH) takes into account the wall lining material of the room, the other two do now.

How to use it

Enter the compartment and vent dimensions (in feet) in the yellow cells.

Choose a wall lining material from the blue dropdown box; that will automatically reference a thermal conductivity taken from literature data.

Look at all three answers given by the three equations in the results section. These numbers represent the heat release rate at which flashover is likely to occur in that compartment. If your fire size is larger than that number, then flashover would be expected to occur.

Variables

Q
Heat release rate at flashover [kW]
A_T
Total area of compartment surfaces [m²]
A_v
Area of vent opening [m²]
H_v
Height of vent opening [m]
h_k
Effective heat transfer coefficient = k / δ [kW/m²·K]
k
Thermal conductivity of lining material [kW/m·K]
δ
Thickness of interior lining [m]

Equations

$$ Q_{MQH} = 610 \, \sqrt{ h_k \, A_T \, A_v \, \sqrt{H_v} } $$Method of McCaffrey, Quintiere & Harkleroad (MQH)
$$ Q_{Bab} = 750 \, A_v \, \sqrt{H_v} $$Method of Babrauskas
$$ Q_{Thomas} = 7.8 \, A_T + 378 \, A_v \, \sqrt{H_v} $$Method of Thomas

Discussion

All three of these correlations will provide fairly similar answers when looking at compartments lined with gypsum board. The MQH equation will diverge from the others for small room sizes and if different types of wall lining materials are chosen. When compared to experimental data, all three tend to slightly under-predict the heat release rate required for flashover in some cases.

If one correlation provides an answer vastly different than the others, it might be best to consider why that may be, and if that application is outside the valid range of that equation.

If you have a situation where there is more than one vent opening (for example a door and a window, you will combine the areas into a single vent, estimating an appropriate width assuming the height of the tallest vent.

Worked example

Example

A compartment is 8’ x 10’ with a ceiling height of 7’. The vent opening is a single door 3’ wide by 6’ tall. The room is lined with ½” gypsum board.

Entering these variables should yield the following answers:

MQH = 642 kW

Babrauskas = 1697 kW

Thomas = 1141 kW

Flashover is likely to occur somewhere in the range of 650 to 1700 kW for that space. As this is a fairly small compartment, I would put less weight on the MQH correlation (see above ‘discussion”), and would expect a fire of closer to 1000 to 1700 kW to result in flashover.

References

  1. Thermal conductivities of wall lining materials taken from NUREG 1805, Chapter 13.
  2. MQH equation: McCaffrey, B. J., Quintiere, J. G., and Harkleroad, M. F. — see SFPE Handbook of Fire Protection Engineering, 3rd ed. (2002), p. 3-185, eq. 58.
  3. Babrauskas equation: SFPE Handbook, 3rd ed. (2002), p. 3-185, eq. 56.
  4. Thomas equation: SFPE Handbook, 3rd ed. (2002), p. 3-186, eq. 62.