Computational Fluid Dynamics (CFD)
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Modeling — Computational Fluid Dynamics (CFD)
In engineering, it is desirable to use the most direct method in order to solve any given problem. Analytic solutions, tabulated data, and/or codified procedures can offer expedient resolutions. Computational fluid dynamics (CFD) modeling is applied when the problem is complex enough to the point where there is no available information on past testing. CFD is used to address problems with non-uniformly coupled transport phenomena, involved geometric and boundary features, and dynamic and distributed sources.
In CFD, the domain of interest is divided into small control volumes. A discrete version of the governing transport equations dictates how adjoining cells communicate with each other. Solution of the resulting network yields data for desired physical quantities such as velocity and temperature at each cell and time step. Other scalars may be tracked. At each time-step, Lagrangian particles may be transported to represent water sprays. Appropriate equations are solved for the interaction of particle- and grid-based variables. Hughes Associates, Inc (HAI) routinely applies CFD technology towards design and analysis problems. HAI has capabilities to perform detailed simulations of fire dynamics, sprinkler dynamics, fire/smoke vent actuation, and smoke spread. We use the three-dimensional, large eddy simulation computational fluid dynamics (CFD) code known as the Fire Dynamics Simulator (FDS) which originates from the National Institute of Standards and Technology (NIST) Building and Fire Research Laboratory (BFRL) to perform these studies.
HAI has substantial computer resources upon which to perform these numerical studies. Larger simulations can be performed on a 9 node cluster of Linux computers comprised of Pentium IV processors with 4 GB of memory each and 700 GB of storage, a Linux workstation with two Pentium IV Xeon processors with 4 GB of memory and 120 GB of storage, and an sgi (formerly Silicon Graphics Inc.) Octane2 graphics workstation with V6® graphics, the two R12,000A processors, 2 GB of RAM memory, and in excess of 136 GB of storage. More modest runs can be done on several Pentium IV PCs with up to 2 GB of memory and in excess of 70 GB of storage disk space. HAI has several applications for two- and three-dimensional, quantitative analysis and animation of data. HAI has used CFD to analyze a number of fire-hazard problems, ranging from atria to high-tech manufacturing facilities.
Selected Project Examples
- Smoke Control in Clean Room Environment
- Smoke Control System for a Telecommunications Facility
- Smoke Control for Large Mall
- Sprinkler Design Area for Clean Rooms
- Stairwell Tenability Conditions
- Heat Detector Activation in Industrial Plants
- Contaminants in Ventilation Ductwork
- Smoke Control in Atrium Enclosures
- The Role of Beams in Heat Detector Activation
- Warehouse Sprinkler Systems
- Heat and Smoke Vents
- Shipboard Water Mist Effectiveness
- Tunnel Fire Hazards
- Covered Roadway
- Ventilation Doctrine for Hangar Bay
- Backlayering Studies
- Mission Bay Ventilation During Fueling Operations
- Passive Fire Protection for a Document Warehouse
- Corner Fire Tests
- Data Centers
- Compartmentalization for the Library of Congress
- Investigation of World Trade Center Fires
- Fire Hazards in National Historic Landmark
- Smoke Movement in the Rhode Island Statehouse
Smoke Control in Clean Room Environment
For clean room facilities, even modest levels of smoke lead to contamination of the equipment and the facility. HAI designed a custom smoke control system to minimize the impact of particulate from an accidental fire. CFD was used to investigate the smoke spread pattern and depth, the effect that the prevailing ventilation system has on particulate transport, the rate at which the exhaust system removed smoke from the facility, and the how different-sized fires and different compartments affected the outcome of the smoke evacuation. HAI was able to model the complex arrays of ventilation inlets and outlets used to recirculate air in these facilities. The results were used to identify the concentration envelopes for equipment damage and life safety. CFD made it possible to predict how long it would take to raise the smoke layer to a uniformly acceptable height. Finally, the effectiveness of smoke curtains and compartmentalization was investigated.
Smoke Control System for a Telecommunications Facility
Hughes has used CFD as a design tool for a smoke control system in a telecommunications facility. CFD was the natural choice for this existing facility since it stocked with functioning equipment. The project looked at life safety and property protection issues. The results showed that the placement and the exhaust rate of the vents have widely varying effects on the performance of the system.
This four-level mall consists of walkways connecting four domed atria. The image below shows how the smoke control system prevents the smoke from spreading into the mall walkways.
Sprinkler Design Area for Clean Rooms
The downward flow caused by the action of ceiling mounted filters in clean rooms works against the timely activation of sprinklers. Observe how the temperature contours deviate from the line of the ceiling. HAI has used CFD to help industries that employ clean rooms install effective sprinkler systems.
Stairwell Tenability Conditions
Concern was raised about the proximity of garages to stairwells in a large luxury apartment complex. The investigation looked at various wind directions on the hottest days with the strongest winds. The cooling effects of the sprinklers were also incorporated into the simulations.
Heat Detector Activation in Industrial Plants
FDS was used to evaluate the effectiveness of different heat detector systems in an industrial plant large enough to accommodate the fabrication of double-wide mobile homes. CFD modeling was the only way in which to include the influence of the slanted roof, of support beams of nonuniform cross sections, and gaps caused by the purlins.
Contaminants in Ventilation Ductwork
FDS was used to model the transport of contaminants introduced into ventilation ductwork. The goal was to determine the concentration at any given exit point along the duct network.
Smoke Control in Atrium Enclosures
Atrium-style architecture is used in public areas such as malls, performing arts centers, and resort hotels. HAI used CFD to analyze the smoke-hazard potential of a performing arts complex with a barrel ceiling which encompassed two auditoriums. The goal was to determine how long it would take for hazardous smoke to descend to ground level.
The Role of Beams in Heat Detector Activation
Hughes Associates has performed CFD modeling for the Navy in order to investigate the impact of support beams upon heat detector activation. HAI first looked at the temperature predicted within the compartment. A discrepancy was found with the experimental data. It was eventually determined that the experimental estimates of the heat release rate were flawed. A refinement of the technique resulted in good agreement.
HAI has used CFD to investigate the effectiveness of different sprinkler systems and configurations for warehouses with high ceilings. The typical scenario consists of a fire spreading through rack-mounted commodities. Along with the standard information that CFD can yield (such as velocity and temperature fields), FDS predicted the time at which each sprinkler actuated, when water first reached the surface, the average droplet size at each surface cell, the water concentration pattern, and the time at which a burning cell was extinguished. CFD modeling was able to determine the ability of a sprinkler system to control spreading fires of varying intensities.
HAI used CFD to determine ways in which to improve the effectiveness of heat and smoke vents. CFD was employed to narrow the parameter space and to elucidate the effects that vents, curtains, and sprinklers have on each other. It was shown that venting provides a profound improvement in visibility which is not hampered by the action of the sprinklers.
Shipboard Water Mist Effectiveness
HAI has also used CFD to predict the effectiveness of water mist in Navy Compartments. The results for different system configurations were compared against experimental data (such as temperature and extinguishment time). Data collected from these runs was used to improve the FDS model.
HAI has used FDS to determine the impact of car and truck fires in tunnels and underground facilities. In this fashion, the effectiveness of ventilation designs can be established. HAI has also employed CFD to determine the results of water delivery systems and to track smoke as it spreads along the tunnel complexes. These analyses determined how much additional egress time was afforded to inhabitants by various fire protection systems.
Due to confusion about the meaning and intent of the fire codes for tunnels, a client was being required to install a tunnel ventilation system in a covered roadway. HAI was able to show, using CFD, that without a ventilation system, the conditions remained tenable within the roadway, even when a truck was the source of the fire.
Ventilation Doctrine for Hangar Bay
Hangar bays on modern naval combat ships contain many highly reactive fuel packages in the form of aviation fuel and ordnance. Any fire within a hangar bay needs to be brought under control as soon as possible to avoid reduction of combat effectiveness from damage caused by the fire. The project examined the benefits and risks of varying levels of hangar bay ventilation during a large, fast growing fire with the goal of maximizing tenability for fire fighting crews and minimize damage.
As part of the hangar bay ventilation program, the effect that wind flowing from different directions would have on the smoke exiting through a small opening was investigated. The goal was to study the complex flow patterns that could arise in the hangar bay which would result in backlayering of the smoke.
Mission Bay Ventilation During Fueling Operations
FDS was used to determine the envelope of ignitable vapors resulting from a large spill of gasoline onto the deck of the mission bay. The movement of the ensuing vapor cloud was determined under the impact of different wind speeds, different wind directions, and different ship speeds. Calm conditions were also considered. The results determined the effectiveness of various fan arrangements.
Passive Fire Protection for a Document Warehouse
A client wished to evaluate an existing set of rack storage for use in storing documents. The goal of the study was to determine the potential for radiant ignition of a rack unit by an adjacent burning unit as well as to determine if ignition could be avoided by allowing intervening empty shelving units. FDS computations were performed to evaluate the radiant heat flux from a burning rack storage unit to an adjacent unit as well as to units separated by one or more empty units.
A long desired goal of computational fire modeling is to aid in the reduction of costs associated with fire testing such as the ISO 9705 Room Corner Test. The ability to accurately represent the heat fluxes resulting from an ISO 9705 test is the first step in developing a computational methodology for doing numerical testing of wall coverings. While this may never replace fire testing of final products, the hope is to enable one to "test" many more candidates and reduce expenditures related to testing failures. A series of corner fire experiments performed by Hughes Associates, Inc. for the US Coast Guard are being used to benchmark FDS for corner fires.
HAI employed FDS to quantify the undesirable effects that a conventional sprinkler system can have on a computer data center. These include water damage to electronic equipment and continued smoke exposure to the facility in general from fires that are suppressed, not extinguished. FDS was then utilized to demonstrate the positive impact that a clean agent can have. The clean agent quickly put out the fire, leaving little residual effects.
Compartmentalization for Library Archive Storage
FDS was used as a tool to demonstrate the impact that different compartmentalization schemes would have on library archive storage rooms. The example below shows how building a wall around the work area effectively limits the spread of hot gases, allowing occupants more time to egress and firefighters more time to arrive before the records contained in the room would be exposed to high heat fluxes.
Investigation of World Trade Center Fires
This project examined the hypothetical scenario of a fire started in World Trade Center (WTC) 1 as a result of the collapse of WTC 2 on September 11, 2001. This calculation involved computing the building aerodynamics caused by the northerly wind and imposing upon this the haze resulting from WTC 2 debris. The result was a determination of the visibility of a smoke plume from a hypothetical fire start in WTC 1.
Fire Hazards in National Historic Landmark
This project examined the impact of hypothetical fire scenarios within the large public spaces of the Ahwahnee Lodge in Yosemite National Park. Results of the analysis were used to recommend fire detection and suppression systems. The examples below show (clockwise from upper left) a sofa fire in the Great Lounge, a table fire in the dining room, 0 m/s contour for wind against the back of the building, and a fire in a delivery vehicle parked in the service yard.
Smoke Movement in the Rhode Island Statehouse
Fire modeling of the Rhode Island Statehouse was performed using the NIST Fire Dynamics Simulator to evaluate the impact of fires within the Statehouse on the ability of its occupants to egress. The potential for mitigation measures to improve life safety was examined along with the need for changes to the Rotunda. The examples show (clockwise from upper left) visibility contours through the center of the Statehouse for a tree fire in the rotunda, smoke spilling into the rotunda from a fire in the State Library, visibility contours from a fire in the House Chamber, and a rendering of visibility due to a fire in an office located in a hallway adjacent to the Rotunda.
For more information contact:
Javier Trelles
jtrelles@haifire.com
410-737-8677 x212
