Category: Computational Fluid Dynamics

An Introduction to Fire Simulation

In real world, it is never acceptable to set fire inside iconic and large commercial buildings or facilities. But if you are working at Mechartés, you will come across buildings & facilities that are set on fire routinely to simulate real and potential fire scenarios on computers to assess their impact and methods of controlling these fires.

The destruction caused by fire in urban establishments is known to human civilization since the time in antiquity when Rome burned for six days continuously. But in modern times, fire engineering practices have reduced causalities & infrastructure damage exponentially. Today, the scope of fire safety is not only limited to extinguishing fire but covers wide variety of disciplines like early fire detection, smoke management, provision for escape facilities; even during building space & layout planning and last but definitely not least, proactive use of dynamic fire modeling to assess risks to life and property and to develop effective fire protection & evacuation strategies for scenarios that cannot be predicted by conventional wisdom.

Even with modern engineering and safety practices, the threat of accidental fire cannot be negated in modern building and industrial infrastructures. This threat brings the necessity of fire modeling to the forefront of large building and industrial facility design & development. With advances made in computer hardware performance and numerical algorithms, Computational Fluid Dynamics (CFD) software are routinely used by engineers to come up with ingenious & effective solutions to minimize damage to life and property.

Fire Dynamics Simulator (FDS) is one such CFD software actively developed by National Institute of Standards and Technology (NIST), to simulate fire driven fluid flow in situations of varying complexities. The hydrodynamic model, combustion model, thermal radiation model, sprinkler model etc, that are incorporated in FDS are continuously verified and validated by the active community of academic, scientific and professional interest group. This community effort has developed FDS into a robust and reliable software to model and simulate fire & smoke propagation inside buildings and facilities. Apart from dynamic fire modeling, numerical simulation methods are also actively used to simulate evacuation and predict indoor air quality inside buildings. Modern evacuation modeling software have reached the capability to incorporate psychological responses of human beings to fire inside enclosed spaces. With software like CONTAM, important information about exposure of occupants to airborne contaminants can be modeled and simulated for eventual risk assessment.

Modern design practices for fire safety are increasingly adopting numerical simulations to study the behavior of a fire protection system without building it. The results are accurate in general, compared to analytical model, especially when applied in complex situations. These simulations are also helping fire engineers to find unexpected phenomenon, behavior of the system and easily perform “What-If” analyses. Mechartés has been enabling our customers to address the following design & safety concerns:

  • What locations inside a building can be described as the “worst” location for a fire incident?
  • What is the minimum response time required during evacuation in a given “worst” condition?
  • With the proposed fire safety & evacuation methodology, what changes can reduce loss of life and property?
  • What is the effectiveness of smoke extraction fans and equipments?

Finally, do take out 5 minutes to view our presentation on how fire simulation services can be used to engineer your buildings and facilities for fire safety.

Ventilation Design verification of Car Park basement – CFD Analysis

Objective: The objective of this project was to verify the  design of a ventilation system for the basement car parking area using CFD. The specific aim is to find the optimal locations, number & configuration of the induction fans required for the ventilation of the car parking lot, for the given positions of inlets and exhaust, and given configuration of the Supply air fans & exhaust fans, such that it meets requirements of CO exposure limit set by health & safety regulations and also to check whether it meets the sufficient visibility & temperatures in case of fire.

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CO PPM Without Jet fans at 1.7 m

Technical Challenge: The acceptance contaminant level criterion for parking garages is specified in ASHRAE 2007 handbook. According to ASHRAE, the main criterion for car parks is that the carbon monoxide levels should remain below a designated peak value.

 

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CO PPM with Jet fans at 1.7 m

Engineering Solution: The mathematical modelling using CFD can be developed which would simulate the actual behavior of the system. At first the analysis was done without placing any jet fans. This provides an indication of the stagnant areas in the car park. The subsequent analysis is done with jet fans operating so as to remove the stagnant areas , to reduce the concentrations of CO and to remove smoke in case of fire.

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Velocity Vectors at 1.7 m without Jet fans

 

Results and Conclusion: 

  • The CO concentrations from the CFD simulations, without using induction fans has been obtained as 167.5 ppm which is higher than the safe CO limit for human beings.
  • After conducting CFD analysis by placing and optimizing the jet fans in the car park area, maximum CO level found to be as 27.8 ppm (33.36 mg/m3), which is with in safe CO limit for humans.
  • Ventilation System in the car park area has been designed optimally to reduce the CO level within the acceptable limits.

    Velocity_vector_1.7m_Withjet1.png
    Velocity Vectors with Jet fans at 1.7 m

Heat Transfer Analysis in Hot Water Generator

Objective: The objective of the project was to verify the hot water generator design by ensuring that the operation conditions are met based on actual experimental results of combustion, heat transfer and fluid flow through the combustion chamber and pipes. CFD Analysis was done to simulate  the combustion and flow of hot flue gases through the combustion chamber, hot water generator and economizer till the exhaust and to calculate the temperature of water passing through the heat exchanger pipes as it comes out of the water outlet. Suggestions were given based on the simulation result to enhance turbulence in the fluid flowing inside the tube.

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Model of Hot Combustion Chamber

Modeling and Engineering Solutions:  The proposed design uses the Lamont type hot water generator. The hot water generator has a rectangular box type construction which has an economizer over the top and it finally connects to the exhaust draft tube. The meshing scheme is used to distribute the mesh uniformly in the outer region and  a very fine mesh in the central core region is used. The figure shows the geometry created and the meshing scheme used for this geometry.

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Fig. Showing the Meshing scheme used for the Generator

Conclusion: After the analysis was done it was verified that the operating conditions of the hot water generator was similar to the actual experimental results of combustion, heat transfer and fluid flow through the combustion chamber and pipes. The total volume flow rate of water entering the hot water generator is 900 GPM. The economizer is designed to ensure that the water enters the inlet drums located at the bottom of the hot water generator at 160 F – 180 F. The temperature in the core combusting region is high and progressively reduces through the heat exchange region as it is carried away by the water and the flue gases. The positions of the baffles were clearly understood from the flow.

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Temperature contours inside the Generator