ML18213A076
| ML18213A076 | |
| Person / Time | |
|---|---|
| Issue date: | 07/31/2018 |
| From: | Tammie Rivera NRC/RES/DRA/FRB, Electric Power Research Institute |
| To: | |
| Shared Package | |
| ML18213A072 | List: |
| References | |
| Download: ML18213A076 (18) | |
Text
Module III - Fire Analysis Fire Fundamentals: Fires in the Open and Fully Ventilated Fires Joint EPRI/NRC-RES Fire PRA Workshop August 6-10, 2018 A Collaboration of the Electric Power Research Institute (EPRI) & U.S. NRC Office of Nuclear Regulatory Research (RES)
Recall: Fuel limited fires A fire where the fuel burning rate is limited only by the surface burning rate of the material.
Sufficient air is always available for the fire (plenty of oxygen to support burning)
Fire generates hot gases (convective fraction) and emits radiative heat (radiative fraction)
Generally applies to fires in the open or fires in large compartments
- A nuclear power plant has lots of large compartments 2
Heat Release Rate (HRR)
For a simple fire, the HRR can be estimated using the following equation:
Q&= m & A H c
- m & is the burning mass flux (kg/sm2)
- Hc is the net* heat of combustion (kJ/kg)
- A is the burning area (m2)
So HRR ends up as kJ/s or kW
- net heat of combustion implies that a burn efficiency has been included - fuels dont burn at 100% efficiency in real fires Energy Released Rate m&
Fuel q&
3
Heat Release Rate HRR can be estimated experimentally using oxygen consumption calorimetry Q&= m&O2 H c (kJ / kg O2 )
where:
H c ~ 13.1 MJ/kgO2 for many common fuels 4
Flames Laminar - very small fires Turbulent - most real fires Fuel Oxygen Reaction Zone 5
Ignition of Gases With a spark or small flame (pilot) present, ignition is based on whether the gaseous fuel concentration is between the upper (rich) and lower (lean) flammability limits.
- The fuel-air (oxidizer) mixture is said to be flammable if a flame will propagate in this mixture.
With no pilot present, a gaseous fuel in air can still ignite if the mixture is at or above the auto-ignition temperature.
- The auto-ignition temperature is usually measured for a stoichiometric mixture - just the right mix so that no fuel or oxygen remains after the reaction.
6
Ignition of Liquids For a liquid to ignite, it must first evaporate sufficiently to form a flammable mixture of gaseous fuel and oxygen
- This occurs at a liquid temperature called a flash-point temperature.
Evaporating
- In general, this temperature can be called the piloted ignition temperature and the same term carries over to fuel Spark solids.
- The flash-point is the temperature at which the amount of liquid evaporated from the surface achieves the lower flammable limit.
If no pilot is present, the mixture must be heated to the auto-ignition temperature in order to ignite.
The auto-ignition temperature of a gas will be higher than the boiling point of the liquid.
Liquids 7
Ignition of Solids Solids do not evaporate like liquids when heated. Solids form gaseous decomposition compounds, generally leaving behind char, in a process called pyrolysis.
Hot Surface At some point, the gases reach the lower flammability limit and may ignite by piloted ignition or, if hot enough, auto-ignition.
Pyrolysis Typically, piloted ignition temperatures for solids range from 250°C (~480°F) to 450°C(~840°F). products Auto-ignition temperatures can exceed 500°C (~930°F).
- For a given material, these temperatures are not constants and can change with the nature of heating.
- Spark Radiant Heat
- For practical purposes, a (piloted) ignition temperature (Tig) may be treated as a property of a combustible solid.
We shall consider thin (less than ~1 mm) and thick solids to have different time responses to ignition when exposed to impinging heat flux Solids 8
Flame Spread Motion of vaporization front at the ignition temperature for solids and liquids Cable tray
- The surface is heated by the existing flames
- More material pyrolyzes (or zf evaporates) ahead of the flame front
- The existing flame acts as the pilot
- The flame (fire) spreads xp Fire 9
Typical Flame Spread Rates It is very difficult to compute flame spread rates because formulas are not completely available, rates may not be steady, and fundamental fuel properties are not generally available.
Nevertheless, we can estimate approximate magnitudes for spread rates for various cases.
Spread case Spread Rate (cm/s)
Smoldering solids 0.001 to 0.01 Lateral or downward spread on thick solids 0.1 Upward spread on thick solids 1.0 to 100. (0.022 to 2.2 mph)
Horizontal spread on liquids 1.0 to 100.
Premixed flames (gaseous) 10. to 100.(laminar) 105 (detonations) 10
Zone of Influence Regions near the fire where damage or fire propagation is expected.
For fires in the open we consider:
Target
- Flame Radiation
- Convection, especially inside the q& Target fire plume x
11
Buoyant Flow Temperature rise causes a decrease in gas density Potential energy converted into kinetic energy - gasses flow upwards Buoyant plume V
Unit volume at plume gas at density and temperature T Z Unit volume of air at density a and temperature Ta D
12
Turbulent Entrainment Entrainment is air drawn into the fire plume by upward movement of the buoyant plume
- Engulfing air from the surroundings into the fire plume Eddies: fluctuating and rotating balls of fluid, large scale rolling fluid motion on the edge of the plume.
Buoyant force Eddies Flame 13
Turbulent Fire Plume Very low initial fuel velocity Entrainment and flame height controlled by buoyancy 14
Fire Plume Temperature Along the Centerline 15
Example Case - Zone-of-Influence Calculation Flame Height and Plume Temperature
( )
53 k Q& (1 ) 2 5 T pl = Tamb + 25 f f r Heskdestad's Flame Height Correlation ((H p Fe ) zo )
where:
& 2 L = 0.235Q f 5 1.02 D z o = 0.083Q&f 5 1.02 D 2
Input D - Fire diameter [m] 0.6 Heskestad's Plume Temperature Qf - HRR [kW] 250 Correlation Input Result Tamb - Ambient temp. [C] 20 L - Flame height [m] 1.5 Qf - HRR [kW] 250 Fe - Fire elevation [m] 0 Hp - Target Elevation [m] 3.7 D - Fire Diameter [m] 1 kf - Location factor 1 (2 or 4)
Xr - Radiative Fraction 0.4 Result Tpl - Plume Temp [C] 328 16
Example Case - Zone-of-Influence Calculation Radiation Heat Flux Flame Radiation: Point Source Model Q&f r
=
q&irr 4R 2 Input Parameters:
Qf: Fire heat release rate (kW)
R: Distance from flames (m)
Xr: Radiative fraction (FIVE recommends 0.4)
D: Fire diameter (m) 17
Example Case - Zone-of-Influence Calculation Radiation Heat Flux Q&f r
=
q&irr 4R 2 Point Source Flame Radiation Model Inputs Fire heat release rate [kW] 317 Radiation fraction 0.40 Distance from flames [m] 1.5 Results Heat flux [kW/m2] 4.5 18