Modeling CO dispersion in parking lots

[javiercarlos]

I have a problem with planning carbon monoxide extraction in basements using the SDS (Safety Data Sheet).

In my facility, the standard practice is to fill the entire building to 50 ppm, and with cars emitting CO, the extraction system (jet fan + injectors and extractors) is activated, expecting the concentration to reduce to 35 ppm in less than one hour. However, this scenario seems unrealistic to me.

Therefore, I have tried to simulate cars emitting CO, and that the sensors, set to a 50 ppm setpoint, trigger the ventilation system.

The CO emitted per vehicle is considered to be 31.6 mg/s (hot) and 61 mg/s (cold), with an average of 46.25 mg/s.

The standard states that between 3% and 5% of vehicles should be considered to be emitting CO.

But if I simulate 5% or 10% of vehicles emitting 46.25 mg/s, I observe that the CO doesn't disperse well in the parking lot (even though I choose the option to emit it at a horizontal speed of 0.3 m/s). When the gas sensor (configured with a set point of 50 ppm) activates, many parts of the parking lot have already reached 300 ppm. Should I also assume a certain speed when entering via the ramps? How can I improve the CO dispersion? Or perhaps it's because I'm not using very fine mesh. The opening of my centrifugal jet fan is 0.1 m high, and the obstruction emitting the CO is 0.2 m x 0.2 m, while my overall mesh for the entire setup is 0.2 m x 0.2 m x 0.2 m. Or perhaps I should assign an outlet temperature to the CO, so that it disperses better; what values?

I look forward to your comments; they will be very helpful. Thank you very much.

before sensor detect :

after sensor detect : There is an excessive concentration of CO, exceeding 50 ppm everywhere. It seems to me that the CO has not dispersed well, which is why high concentrations are observed dispersed in the air when the fans are turned on.

[drjfloyd]

Again without an input file it is very difficult to provide concrete assistance; however, based on your description one observation is that cars do not emit pure CO from the exhaust.

Your CO rates seem high to me. US EPA fleet estimate over all driving conditions for grams CO per mile is on the order of 1 gram/mile. If you are 5 mph in a parking garage, then that is 5 gram/hour or 1.3 mg/s. Another way of looking at it is at 5 mph and 20 mpg you are burning about 190 mg/s of fuel. A 30 mg/s CO rate is a 15 % CO yield which is extremely high for a modern vehicle at operating temperature.

[javiercarlos]

Thanks for the clarification.

I'm attaching the FDS.
https://drive.google.com/file/d/1N8ouwRtPxOzVjCtxEELtKbWCMvH9XNoj/view?usp=sharing

I have tried testing the values in the 2019 ASHRAE Handbook HVAC Applications - Chapter 16 "Enclosed Veicular Facilities"

in simulation type = VLES (I don't know if that matters).
But I have the impression that if the emission rate were lower, good dispersion would still not be achieved. Perhaps I should use a finer mesh, I don't know.

Thank you in advance.

[drjfloyd]

That ASHRAE table does not agree with the referenced source. The current EPA data is Mobile 5 (chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P101CUB1.pdf) and it says that 1996 model year cars were between 1 and 2 mg per mile which at 8 km/h would be 7.5 to 12 g/hr or 2.1 to 3.5 mg/s. It almost looks like the ASHRAE table has misconstrued the mg/mi unit as mg/min. For 2010 model year (and old car now) the levels are 1/3 to 1/2 that.

Your mass fraction is 1000 ppm. Maybe for a cold vehicle at start. For a warmed up vehicle with working emissions controls the mass fraction should be a factor of 10 lower.

[javiercarlos]

ASHRAE has maintained these values since 1984, apparently to ensure a fairly conservative scenario. Many designers around the world use these tables for smoke extraction calculations (CFM). On the other hand, CO dispersion simulation seems to me to be a somewhat complex issue. I have seen several studies on research websites on this topic, but the only one that deals with dispersion in enclosed parking lots is this article: "Carbon Monoxide Dispersion in Enclosed Car Parks: Pollutant Source Modeling Methods" https://www.researchgate.net/publication/343151772_Carbon_Monoxide_Dispersion_in_Enclosed_Car_Parks_Pollutant_Source_Modelling_Methods

The problem is that they have worked with OpenFoam and other models that are not implemented in Pyrosim. They talk about the CALINE4 model, which simplifies the turbulence scenario of the car emitting CO and the vehicles passing behind it, as well as the thermal turbulence generated when the gases are emitted and the buoyancy of the exhaust emitted. They also had to customize the solver (something that is beyond my reach).

For now, I will have to settle for the method that everyone uses in my country.

[uporersith]

having worked on ASHRAE chapter 16: Enclosed Vehicular Facilities, I can confidently say that the emissions tables in the handbook have not been updated for many many years. A major challenge to updating the table is locating good sources of data. The most comprehensive data source is the US EPA MOVES (https://www.epa.gov/moves/latest-version-motor-vehicle-emission-simulator-moves) However it is complex and has a steep learning curve. Typically, you would run the model for stopped or slow moving traffic to generate the emissions rates for use in parking garages.

I would recommend this paper for some tips on modeling CO in FDS: https://www.femtc.com/events/2014/d2-10-boehmer/

[javiercarlos]

I have reviewed that article and read others. In my opinion, the best way I have found to simulate carbon monoxide extraction in parking lots is to emit CO from certain points (simulating cars) and have the extraction and injection system turned on from the start of the simulation. In this way, the CO is distributed throughout the environment and the system is tested when the gas sensor detects CO at 50 ppm, and the jet fans are turned on. This creates a scenario with CO dispersed throughout the parking garage and, at the same time, the entire system (injectors + extractors + jet fans) is tested by dispersing the concentration existing in the parking garage. Better dispersion is achieved by using the EPA's MOVE5 values than by using ASHRAE's mission values. However, this procedure can take many minutes while the CO disperses until the sensors detect 50 ppm and the jet fans are activated. I think that is why in my country, they use a “faster” method, assuming that the entire environment is already at 50 ppm, and since the system is “generally” oversized, the system can reduce that concentration to 35 ppm in approximately 15 to 20 minutes. This fast method takes 15 to 20 minutes (more economical).