SCALAR MIXING - CONCENTRATION PPM

[AWIL125]

Good morning,

I have a query about a scalar variable simulation. 

I am running a simulation hoping to determine the concentrations in parts-per-million (ppm) of Carbon Monoxide in a car park.  I have set up my model as per the guidelines in wiki help and it is running and converging nicely. 

When I view the scalar variable results as a plane for example, I get a nice plane showing the scalar variable between 0 (blue) and 1 (red).  Is this showing where there is 100% CO (red) and 100% air (blue)?? 

How do I convert this scalar plane result to a concentration in ppm??

Any advice is greatly appreciated.

Regards,

Andy.

[srhusain]

PPM-This is a way of expressing very dilute concentrations of substances. Just as per cent means out of a hundred, so parts per million or ppm means out of a million. Usually describes the concentration of something in water or soil. One ppm is equivalent to 1 milligram of something per liter of water (mg/l) or 1 milligram of something per kilogram soil (mg/kg).

So, roughly, you would multiply the scalar fraction by 1e6 to obtain PPM

[AWIL125]

Thanks,

So for the image above, where Scalar variable is red (1), the ppm result is 1*1e6 = 1,000,000 pppm ?

Is there a way i can extract the average across the whole plane?

Regards,

[heath.houghton · Accepted solution]

Andy,

You are correct in your assessment of 0 and 1.  0 is 100% one fluid and 1 is 100% the other.  I imagine that your concentration of CO is very small for the majority of the model (scale of 1e-6).  I make sure to call out scalar of 0 as my dominant fluid, so I can look at ppm scale numbers for the dilute material.  If you assign using this method, then you can change the legend properties to values that appropriately display ppm concentrations.  Have the legend range from 0 to 200 e-6 would give you a gradient from 0 ppm to 200 ppm. 

Hope that helps,

Heath

[AWIL125]

Also could i please confirm that if i wanted to have 1 L/s of CO entering the model from a modelled vent, then i would set up the following:

• Define internal air volume as a material with the density modified as a function of scalar, i.e.

                  Air: scalar = 0, (with the appropriate density)

                  CO: scalar = 1 (with the appropriate density)

• make sure internal air volume is set as variable
• For the CO 'vent', set 2 boundary conditions:

                  1. Volume flow = 1 L/s

                  2. Scalar = 0

• In the solve dialog ensire General Scalar is enabled and set the appropriate diffusivity coefficient.

[heath.houghton · Accepted solution]

Without seeing the setup of that model it is hard to determine if scalar 0 equates to 100% air or 100% CO.  if 0 scalar is pure air, then yes, scalar 1 is 1million ppm CO.  You can pull out the average on a plane using bulk properties, but most people like to export the bulk results to excel to do further data manipulation.  There is an article about one of those methods in the help system http://help.autodesk.com/view/SCDSE/2015/ENU//?caas=caas/sfdcarticles/sfdcarticles/How-to-calculate-... .  This particular one is about coefficient of variance, but it details out how to export the results to excel as well.

[AWIL125]

Thanks, much appreciated.

[heath.houghton · Accepted solution]

You are mostly correct in your setup description except that if scalar 1 is pure CO, then any pure CO inlet needs to be scalar 1.  In my experience, though if you are simulating actual CO entering the space from a car, etc...the car doesn't exhaust pure CO, so either you need a very small flow rate that is equivalent to the amount of pure CO, or use the total flow rate from the car and put in a scalar quantity more applicable to the actual air quality of the car output i.e. 1L/s flow rate and scalar that is not much above 0.  For reference on vehicle emissions:

U.S. Environmental Protection Agency estimates of average passenger car emissions in the United States for July 2000^[27]

Component	Emission Rate	Annual pollution emitted
Hydrocarbons	2.80 grams/mile (1.75 g/Km)	77.1 pounds (35.0 kg)
Carbon Monoxide	20.9 grams/mile (13.06 g/Km)	575 pounds (261 kg)
NOx	1.39 grams/mile (0.87 g/Km)	38.2 pounds (17.3 kg)
Carbon Dioxide - Green house gas	0.916 pounds per mile (258 g/km)	11,450 pounds (5,190 kg)

Using that mixture above, the scalar value should be more on the scale of 0.02.

[rpadovani]

Andrew

Have you tried to setup this model with a scalar of 0.02 for the CO inlet and a higher volumetric flow rate for its corresponding boundary condition?

Did you manage to obtain meaningful scalar results compatible with the CO mass intake?

Regards

Roberto

[jack.hansen4NXVP]

Hello there,

I am about to try and undertake a similar car park study, I was just wondering where these materials are found?