Understanding of Fire Dynamics and its CFD Modelling

Understanding of Fire Dynamics 

  • Fire dynamics is the study of how fires start, grow, and spread, as well as how they can be controlled and extinguished
  • Understanding the science behind fire dynamics is essential for firefighters, fire safety professionals, and anyone interested in fire safety

Importance of Fire Dynamics 

it is an essential area of knowledge for firefighters, fire investigators, and other professionals who deal with fire safety. Understanding fire dynamics is important for several reasons:

Fire Prevention

  • Knowledge of fire dynamics can help prevent fires from occurring in the first place
  • By understanding the conditions that can lead to a fire, such as high temperatures, combustible materials, and an ignition source, people can take steps to eliminate or reduce these risks.

Firefighting

  • Understanding fire dynamics is crucial for firefighters, who need to know how to control and extinguish fires safely and efficiently
  • Firefighters must understand how fires behave, including how they spread and how they react to different firefighting techniques.

Fire investigation

  • Fire investigators use their knowledge of fire dynamics to determine the cause of a fire
  • By analyzing the pattern of fire damage and the behavior of the fire, investigators can often determine where the fire started and how it spread.

Building Design

  • Fire dynamics is important in the design and construction of buildings, particularly in terms of fire safety
  • By understanding how fires behave, architects and engineers can design buildings that are more resistant to fire and that provide safe means of egress for occupants.

Fire Rates for building

  • Overall, an understanding of fire dynamics is essential for preventing fires, fighting fires safely and effectively, investigating the causes of fires, and designing safe buildings.

Components fo Fire

  • The three components of fire are fuel, heat, and oxygen. These components are often referred to as the fire triangle. When these three components are combined in the right proportions, a fire can occur. For example, if a flammable liquid is heated to its ignition point in the presence of oxygen, it will catch fire.

Burning Material for Fuel

  • Without a fuel source, a fire would have no fuel materials to burn.
  • The firing rate depends on the type of fuels
    • Solid Fuel: paper, wood, plastic
    • Liquids Fuel: Diesel, gasoline
    • Gaseous Fuel: LPG, CNG
  • Example: Consider a gas stove, the amount of gas released from burners decides the size of the flames. If there is no gas means there is no fire.

Air for Oxygen

  • Fire is one of the fast oxidation reactions. Fire needs oxygen to thrive
  • If oxygen levels around the fire drop, the fire will go out
  • Example:  covering a candle with an upside-down glass cup
    • After doing this, you can see the fire on the wick shrink until all that remains is a wisp of smoke filling the cup
    • Since fire consumes oxygen and gives off other gases such as carbon dioxide
    • space within the cup loses its oxygen levels, and the candle’s fire is unable to continue.

Ignition Heat Source

  • Fire also needs heat to stay lit. Many people are familiar with the act of blowing out a candle
  • This action removes heat from the candle, making the flame disappear
  • Dumping water on a campfire has a similar effect of removing heat from the fuel source, disrupting the chemical reaction
  • A fire’s heat will increase from ignition into the growth stage
  • From there, it reaches its highest heat level as a fully developed fire. Its heat will decrease as it consumes all its fuel and begins to decay.

Chemical Reaction Rate

  • One of the crucial parameters of a fire is an unbroken chemical reaction
  • By reducing both temperature and amount of fuel rate, the reaction rate can be reduced. We can suppress fire easily.
  • Most fire extinguishing or suppressing agents work by interrupting the chain reaction of fire.

Heat Transfer During Fire

  • However, the fire triangle is not enough to fully explain the complex behavior of fires. Fire dynamics also considers the role of heat transfer and the ways in which heat can be transferred from one object to another. There are three types of heat transfer that are relevant to fire dynamics: conduction, convection, and radiation.
  • Conduction occurs when heat is transferred through a solid object, such as a metal pipe or a wooden beam
  • Convection occurs when heat is transferred through a fluid, such as air or water. Radiation occurs when heat is transferred through electromagnetic waves, such as the heat that is felt when standing near a fire
  • Another important aspect of fire dynamics is the concept of fire plumes. A fire plume is a column of hot gases that rises from a fire
  • The shape and size of the plume depend on the size and intensity of the fire, as well as the surrounding environment. Understanding fire plumes is important for predicting how a fire will spread and where it will produce the most heat and smoke
  • Fire dynamics also consider the ways in which fires can be controlled and extinguished. For example, fire suppression systems, such as sprinklers, work by removing one of the components of the fire triangle – either the heat or the oxygen. Firefighters use a combination of water, foam, and other agents to extinguish fires by cooling the fuel and reducing the amount of oxygen available.

 

  • Fire is considered the visible part of combustion which involves heat and mass transfer

Stages of Fire Development 

There are four stages of fire development: ignition, growth, fully developed, and decay

Ignition Stage

  • The ignition stage is when all four elements of the fire tetrahedron coalesce, the fuels reach their ignition temperature, and the fire begins.
Growth Stage
  • During the growth stage, the fire increases fuel consumption and creates more heat and smoke

Full Developed Stage

  • This is a free-burning stage
  • It is observed when the maximum amount of fuel and oxidizers are consumed
  • The highest heat release (burning) rate is produced.  The temperature decreases and intensity can be reduced depending on amount of matter and heating value

Decay Stage

  • The decay stage happens when the available fuel has been consumed.

 

Key Subjects For Fire Dynamics

The following subjects are important for understanding and CFD modeling of fire dynamics

Basic of Combustion

  • Understanding basic combustion is important to control fire
  • Fire is defined as unwanted combustion which occurs in uncontrolled  manners
  • These subjects will help to understand how to control firing by controlling firing rat

Turbulent Flow

  • All fired due to the burning of solid, gas or liquid fuel are turbulent flow
  • Flames generated from fires are highly turbulent and chaotic

Heat and Mass Transfer

  • Heat transfer plays role in combustion

Multiphase Flow

  • Smokes due to solid or liquid carbon materials are examples of multiphase reactive flows
  • Burning of high-carbon solid materials takes a long time for pyrolysis or devolatilization
  • For liquid droplets, heat is absorbed for phase (evaporation of droplets)  change before fires commence

Chemical Kinetics

  • Chemical kinetics will help to understand species and rate of reactions during the fires
  • Radicals formed in firing can be uncontrolled

CFD Modeling of Reactive Flow

  • Computational fluid dynamics (CFD) modeling is based on the principles of fluid mechanics, heat transfer, and chemical kinetics 
  • After utilizing suitable numerical methods and algorithms we can numerically  solve problems of fire dynamics
  •  Models can integrate chemical reactions for combustion processes with fluid flows
  • CFD modeling provides a three-dimensional understanding of fire dynamics

 

Fire Dynamics Simulation

  • Fire dynamics simulation is the process of using computational modeling techniques to study the behavior of fires in various environments. This involves simulating the physics of fire, including the combustion process, heat transfer, fluid dynamics, and the interactions between the fire and the surrounding environment
  • Fire dynamics simulations can provide detailed information on how a fire will grow, spread, and interact with its surroundings under different conditions, which can be valuable for fire safety analysis and design

  • Fire dynamics simulations typically use mathematical models to represent the various physical phenomena involved in a fire
  • These models may be based on simplified physical laws, such as the Navier-Stokes equations for fluid dynamics, or they may use more complex models that incorporate chemical reactions, heat transfer, and other factors that influence fire behavior
  • The models are typically solved numerically using computational methods, and the results can be visualized using computer graphics to help interpret the data.
  • Fire dynamics simulations have a wide range of applications, including fire safety analysis for buildings, vehicles, and other structures, fire investigation and reconstruction, and the development of fire suppression strategies
  • They can also be used to study the behavior of wildland fires and other large-scale fires, as well as the effects of different types of fuels and fire suppression methods.

 

Fire dynamics Simulations

Download and Installation

  • FDS (Fire Dynamics Simulator) and Smokeview are free and open-source  simulations software
  • Fire Dynamics Simulation (FDS) software can be downloaded and installed from the GIT hub
  • Download and install

Tutorials of FDS

  • Tutorials of FDS can be downloaded from the website of the National Institute of Standards and Technology (NST) of the United States Department of Commerce.

Conclusion

  • In conclusion, fire dynamics is a complex and important field of study that helps us understand the behavior of fires and how to control and extinguish them
  • By understanding the science behind fires, we can better protect ourselves, our buildings, and our communities from the devastating effects of fire.
  • To predict risk,  fire dynamics simulation (FDS) is an important tool for understanding and mitigating the risks associated with fires, and it is a growing area of research and development in the field of fire safety engineering.

Reference

  1. K. McGrattan, S.imo Hostikka, J. Floyd, R.  McDermott,  M. Vanella, Fire Dynamics Simulator Technical Reference Guide Volume 1: Mathematical Model, NIST, US Energy Department
  2. Download and Installation: FDS and Smoke View GIT Hub
  3. Tutorials on FDS: How to learn

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