Why CFD to simulate basement car parks

Basement car parks are where natural ventilation is not possible, for the reason a mechanically extract system should be used. For mechanically ventilated car parks, it should be meet certain objectives.

Firstly, when the car park is in general use, it is important that the exhaust air produced by the vehicles is effectively removed from the basement without any accumulation of smoke in stagnant areas.

Secondly, in an event of a fire accident the ventilation system should be designed in such a way to clear smoke from the basement and provide the means of escape from the car park during and after fire.

Computational Fluid Dynamics (CFD) is used as a design tool for car park ventilation systems. The main advantage of CFD analysis is that the design strategy can be optimized prior to the installation of ventilation system, providing confidence to the contactor early in the project.

CFD use case – Explained

The car park basement which is being analysed has two basements. Basement-I is of three different heights, i.e., 3.05m, 4.2m and 3.15m. 188 cars can be parked in basement-I. Basement-I have two fresh air fan rooms through which fresh air enters and two exhaust fan rooms through which polluted air/smoke exits out of the car park.

Basement-II has an average height of 3 meters, 200 cars can be parked in basement-II. This Basement has one ramp which goes to the Basement-I. Basement-II have two fresh air fan rooms through which fresh air enters and two exhaust fan rooms through which polluted air exits out of the car park.

Following are the objectives identified for the CFD simulation in the car park ventilation:

No Jet Fan: In this simulation, only fresh air and exhaust fans are ON and no jet fans are placed. This simulation will help in understanding the worst case scenario, i.e., no jet fan working. Based on these results the number of jet fans and their optimized location will be determined.

Pollution Mode: Study the performance of the car parking system with a default fan arrangement and rearrange the fans if the default arrangement does not satisfy the acceptance criteria.

Fire Mode: Study the smoke and fire propagation w.r.t time and find the evacuation time and best possible evacuation path at 30 ACPH (Air changes per hour).

Ventilation Scenario Descriptions               

Ventilation Scenario (Pollution Mode):

• Without Jet Fans
• With Optimized Jet Fan locations
• The number of fans and fan locations are optimized based upon following post-processed data
• Velocity profile in the car park area to determine dead regions.
• CO concentration in the car parking area.

Fire Scenario:

• Car is set to fire in the car parking area and visibility profile is studied.

• In fire mode simulation, a fire size of 4MW was chosen to validate the design; the standard fire size and profile.

CFD findings

The basement 1 & 2 was first analysed without the jet fans. The results for velocities and CO ppm at various locations were obtained and with the help of these plots the fan positions were optimized.

Pollution Mode – Basement I & II:

Without Jet Fans: There were lot of dead regions where the air is stagnant, and CO limit is above the acceptable limits of 50 PPM.

Optimized Position: The jet fans suck air from the fresh air shafts and smoothly distribute into the entire car park area and divert it to the exhausts, thus air moves freely in the entire car park and flushes CO smoothly from the basement. CO concentration plot of the various basements are shown in their respective appendixes.

Fire mode:

Fire is set to a car which is in the centre and as well as in a corner of the car parking area; this is the worst case scenario which will hinder in fire escape. The fire simulation is carried out for 30min and visibility is plotted at an interval of 5min. It is observed that at 10 min when the fire intensity is 4MW, most of the region has visibility less than 1m, but as the intensity of fire decreases the visibility improves and at 30 min most of the regions has visibility above 30m as the smoke is cleared by the jet fans

Visibility index (0-30m) and CO₂ concentration in Basement I:

Conclusion

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