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# How to include Mass Transfer Coefficient into MSX

Hi Everyone,

We're currently working on developing an MSX model for Chlorine Decay that can account for wall decay as a factor of Biofilm growth (thus removing the need to specify wall decay for specific pipes throughout the network).

Unfortunately the equation for this relies on the mass transfer coefficient, which is dependent on the type of flow, as explained here: https://docs.bentley.com/LiveContent/web/Bentley%20WaterGEMS%20SS6-v1/en/GUID-D7D70292-50F6-4066-A30E-3B98BBB2BF7B.html.

This is built into the WaterGEMS standard pipe wall reaction calculations, however I need to know whether we can reference this coefficient in MSX, does anyone know whether this is possible? My understanding is that EPANET is capable of this so I assume WaterGEMS is capable as well, I just don’t know how.

For anyone wondering, the equations for Bulk decay are very simple. Below is a copy of what we use currently (no wall decay): The Wall Decay equations are derived as follows (A&B are constants to be determined by trial and error, kw is what we need from WaterGEMS) Thank you,

Ryan

• Hello Ryan,

You would need access to the WaterObjects.NET functionality to use the "Mass Transfer Coefficient" used in the first order wall reaction rate.

Another approach would be to use Formula Based User Defined Extensions. From the formula provided develop an equation to calculate the mass transfer coefficient from the equations given. However, that would first require classification of flow based on Reynold's number.

You can develop an formula based UDX for Reynold's number and classify the flow as laminar or turbulent; then develop separate equations (using formula based UDX) for both the type of flows for the Sherwood number. It is best in this case that you create two separate selection sets for both the type of flows and work on them individually.

Once you have the Sherwood number values for both the flow use the equation to calculate mass transfer coefficient again using the formula based UDX.

This procedure is a bit complicated, but can be achieved.

Let me know if this helps.

Regards,

Yashodhan Joshi

• • Hi Ryan,

It seems that this is possible to do with MSX using the "TERMS" feature.

Terms can be defined to make writing the various water quality equations easier, by breaking equations up into manageable pieces.

One of the benefits of this feature is that one is able to use various pre-defined variables such as D for pipe diameter, Q for flow and Re for Reynolds number.

The MSX user guide provides the following example for calculating a mass transfer coefficient.

[TERMS]

Kf    1.2e-4*Re^0.88/D

You can then use this value Kf directly in your reaction formulae.

I'm not sure if the example above is directly portable to your case (i.e., without alteration), however, you should be able to do what you need. I encourage you to grab a copy of the EPANET MSX user manual (v1.1) if you don't already have it. WaterGEMS is running a version of MSX that is pretty close to the original US EPA version and thus the vast majority of the help content there should be directly applicable to WaterGEMS also.

I hope this helps.

Regards,

Wayne.  • • Yashodhan, if you could explain what you mean by 'selections sets' and how I can get the MSX code to differentiate between turbulent and laminar flow I should be onto a winner! • Hi Ryan,

Did you see my earlier response?

Regards,

Wayne.  • 