CFPS Fire Dynamics and Behavior 3 — Questions and Answers
Question 1: Which mode of heat transfer is primarily responsible for the rapid spread of fire across the ceiling in a compartment during the pre-flashover stage?
- Conduction through the ceiling material
- Radiation from the hot gas layer to the ceiling
- Convection of hot gases along the ceiling surface (Correct answer)
- Direct flame impingement on the ceiling
Correct answer: Convection of hot gases along the ceiling surface
Convection carries hot combustion gases that travel along the ceiling, spreading heat and forming the characteristic ceiling jet that heats detectors and sprinklers.
Question 2: The 'heat release rate' (HRR) of a fire is measured in which units?
- BTU/ft² (kW/m²)
- Kilowatts (kW) (Correct answer)
- Degrees Fahrenheit per second (°F/s)
- Foot-candles (fc)
Correct answer: Kilowatts (kW)
Heat release rate is measured in kilowatts (or megawatts for large fires) and represents the rate at which energy is being released by a burning fuel.
Question 3: Which phenomenon describes a sudden, rapid increase in fire growth when flaming combustion propagates across a fuel surface faster than a person can walk?
- Flashover
- Rapid fire development (Correct answer)
- Deflagration
- Spread fire
Correct answer: Rapid fire development
Rapid fire development refers to an accelerated growth phase where fire spreads across surfaces quickly, sometimes approaching flashover without warning.
Question 4: In fire dynamics, 'pyrolysis' is best defined as:
- The exothermic oxidation reaction between fuel and oxygen
- The thermal decomposition of a solid or liquid fuel to produce combustible vapors (Correct answer)
- The spontaneous ignition of a material without an external ignition source
- The transfer of heat through direct molecular contact
Correct answer: The thermal decomposition of a solid or liquid fuel to produce combustible vapors
Pyrolysis is the thermal decomposition of fuel that generates flammable gases and vapors that mix with air and can be ignited.
Question 5: The Ignition Flux (critical heat flux for piloted ignition) for typical wood products is approximately:
- 1–2 kW/m²
- 12–20 kW/m² (Correct answer)
- 50–60 kW/m²
- 100 kW/m²
Correct answer: 12–20 kW/m²
Wood typically requires a radiant heat flux of approximately 12–20 kW/m² to ignite when a pilot flame or ignition source is present.
Question 6: What is the primary driving force behind the buoyant plume that rises above a fire?
- Wind currents created by the fire's convective zone
- Pressure differentials caused by building HVAC systems
- Density difference between hot combustion gases and the cooler surrounding air (Correct answer)
- Oxygen depletion near the base of the flame
Correct answer: Density difference between hot combustion gases and the cooler surrounding air
The buoyant fire plume rises because hot combustion gases are less dense than the surrounding cooler air, creating an upward buoyancy force.
Question 7: Which fire behavior concept states that doubling the diameter of a fire source increases its peak centerline flame height by approximately a factor of:
- 1.4 (square root of 2) (Correct answer)
- 2
- 4
- 8
Correct answer: 1.4 (square root of 2)
Flame height scales with the two-fifths power of HRR and is proportional to approximately the square root of the diameter change for similar fuel arrays.
CFPS Fire Dynamics and Behavior 3
Which mode of heat transfer is primarily responsible for the rapid spread of fire across the ceiling in a compartment during the pre-flashover stage?