ME 567: Fire Safety Engineering
Estimated study time: 10 minutes
Table of contents
Sources and References
- Drysdale, An Introduction to Fire Dynamics, 3rd ed., Wiley.
- Karlsson and Quintiere, Enclosure Fire Dynamics, 2nd ed., CRC Press.
- Quintiere, Fundamentals of Fire Phenomena, Wiley.
- SFPE, Handbook of Fire Protection Engineering, 5th ed., Springer.
- Cote and Bugbee, Principles of Fire Protection, NFPA.
- NFPA codes including NFPA 101 (Life Safety Code) and NFPA 13 (Standard for the Installation of Sprinkler Systems).
Chapter 1: Combustion, Fuels, and Flammability
Fire-safety engineering treats fire as a controlled subject: a combustion process with predictable heat-release, spread, and species-generation rates. The designer’s task is to keep unwanted fires from starting, to limit their growth, to allow occupants to escape, and to preserve the building’s structural integrity long enough for rescue.
1.1 The Fire Triangle and Tetrahedron
Sustained combustion requires fuel, oxidizer, and heat — the classical fire triangle — together with an uninhibited chain-reaction, giving the tetrahedron. Removing any face extinguishes the fire. Control strategies thus map naturally to suppression agents: water removes heat, CO₂ displaces oxidizer, clean agents interrupt the chain reaction, and fuel cut-off removes the fuel.
1.2 Fuels
Solid, liquid, and gaseous fuels burn differently. Solids pyrolyze: heating releases combustible volatiles that mix with air above the surface and burn in a diffusion flame. The mass-loss rate per unit area is governed by the energy balance at the surface,
\[ \dot{m}'' = \frac{\dot{q}''_{net}}{\Delta h_v}, \]with \( \Delta h_v \) the effective heat of gasification. Liquids evaporate at their boiling points; pool fires are characterized by mass burning rate correlations such as Babrauskas’s expression in terms of diameter. Gaseous fuels premix before ignition (leading to deflagration or detonation) or burn as turbulent jet diffusion flames.
1.3 Flammability Limits and Ignition
Premixed flames propagate only within a composition band bounded by lower and upper flammability limits (LFL, UFL). Below LFL the mixture is too lean; above UFL too rich. Autoignition temperature is the minimum temperature at which a homogeneous mixture ignites without an external spark. Minimum ignition energy for a spark is tabulated for each fuel–oxidizer pair and governs the specification of intrinsically safe electrical equipment in hazardous areas.
Chapter 2: Heat Transfer in Fires
2.1 Conduction, Convection, Radiation
All three heat-transfer modes act in fires, but radiation dominates beyond the immediate flame zone because it scales as \( T^4 \). Plume entrainment is a convective phenomenon; target heating at range is radiative; structural fire response is a conduction problem driven by convective and radiative surface fluxes.
Radiation from a flame to a target is
\[ \dot{q}''_r = \phi \varepsilon_f \sigma T_f^4, \]with configuration factor \( \phi \), flame emissivity \( \varepsilon_f \), and flame temperature \( T_f \). Sooty flames (hydrocarbons, plastics) are nearly blackbody; clean flames (methanol, hydrogen) are transparent and dominated by gaseous bands.
2.2 Fire Plumes
A buoyant plume rising from a fire source of heat release \( \dot{Q} \) has centreline velocity and excess temperature governed by Heskestad’s correlations:
\[ T - T_\infty = 9.1 \left(\frac{T_\infty}{g c_p^2 \rho_\infty^2}\right)^{1/3} \dot{Q}_c^{2/3} (z - z_0)^{-5/3}, \]where \( \dot{Q}_c \) is the convective portion of heat release and \( z_0 \) the virtual origin. Plume entrainment sets the ceiling jet that triggers detectors and sprinklers.
Chapter 3: Enclosure Fire Dynamics
3.1 Growth Stages
An enclosure fire grows from incipient through flashover to fully developed and finally decay. Flashover — the rapid transition from a local fire to one involving all combustible surfaces — typically occurs when upper-layer temperatures exceed about 500–600 °C and incident radiation to unburned surfaces exceeds 20 kW/m². After flashover the fire is ventilation- rather than fuel-limited in most compartments.
3.2 Two-Zone Models
Early enclosure-fire models divide the room into a hot upper layer and cool lower layer, with a well-defined interface. Mass and energy balances on each layer give ordinary differential equations for layer temperature, thickness, and species concentrations. Computer codes CFAST and FDS extend the approach — CFAST in the two-zone framework and FDS by solving the LES equations.
3.3 Ventilation Effects
A compartment fire ventilated through a window of width \( w \) and height \( h \) has peak heat release rate
\[ \dot{Q} \approx 1500\, A \sqrt{h}\,\ \mathrm{(kW)} \]with \( A \) the opening area in m² — the classical ventilation-controlled limit. Under-ventilated fires produce large quantities of CO, unburned hydrocarbons, and soot, often more hazardous than the heat itself. Ventilation design (natural or mechanical smoke control) is therefore integral to life safety.
Chapter 4: Fire Modelling
4.1 Design Fires
A design fire specifies heat-release rate versus time for the scenario under study. The \( t^2 \) growth model
\[ \dot{Q} = \alpha t^2 \]with \( \alpha \) from slow to ultra-fast classes is a standard parametrization. Fuel packages are characterized by peak heat release, total heat released, and radiant fraction from calorimeter testing (cone calorimeter, room corner test).
4.2 Zone Models and CFD
Zone models (CFAST) are fast enough for parametric studies and have decades of validation against compartment fire tests. CFD models (FDS) resolve the geometry and can capture complex plumes, smoke spread through ducted systems, and mechanical ventilation. The user chooses the coarsest model that answers the engineering question; a CFD simulation that delivers only compartment average temperatures has squandered resources.
4.3 Egress Modelling
Occupant egress is modelled by hydraulic (network-flow) methods at the simplest level and by microsimulation (agent-based) at the detailed level. Available safe egress time (ASET) — the interval before untenable conditions develop — must exceed required safe egress time (RSET). Tenability criteria include temperature, smoke obscuration, CO exposure, and radiant flux.
Chapter 5: Fire Safety in Practice
5.1 Building Fire Safety Strategy
A layered strategy prevents ignition, limits fire growth, detects early, controls smoke, protects egress, maintains structural integrity, and supports rescue. Each layer has designated subsystems:
- Prevention: housekeeping, hot-work permits, electrical design, fuel limits.
- Passive protection: compartmentation, fire-rated walls and doors, fire-resistive construction.
- Active protection: detection (smoke, heat, flame), alarm, suppression (sprinklers, clean agents), smoke management.
- Egress: layout, occupant counts, travel distances, exit capacity, signage.
- Emergency response: fire department access, water supply, command interfaces.
5.2 Sprinkler Design Principles
Wet-pipe sprinklers are specified by occupancy hazard category (NFPA 13). Density–area curves set the required water application rate over a demand area; piping is hydraulically sized so that the most remote head delivers its required flow at the minimum design pressure. Quick-response sprinklers, early suppression fast response (ESFR), and specific-application systems cover special occupancies.
5.3 Structural Response to Fire
Structural elements lose stiffness and strength at elevated temperature. Steel retains about half its room-temperature yield strength at 550 °C; concrete loses strength through dehydration and spalling; timber chars at approximately 0.6 mm/min, losing effective cross-section. Fire-resistance ratings follow standard time–temperature curves (ASTM E119, ISO 834) or, increasingly, performance-based analyses using real design-fire temperature histories.
5.4 Industrial Hazards
Industrial settings add process-specific hazards: flammable-liquid storage, combustible dust, reactive chemicals, lithium-ion batteries. Each demands targeted protection — deluge water spray, dust-collection design with suppression, inerting, specialized suppression agents — integrated with explosion-protection engineering and emergency-response planning.
