Using X1T to model O2 transport; why does porosity matter when flow is in cm/day?

[O2 transport]

Hi Jia or Brian,

My class has been running X1T to model the transport of O2 created by the reaction of CaO2 (oxygen release compound).  I have created a PPT file to illustrate our observations.  There are two questions.  1)  How does the O2 concentration reach much greater than background concentrations at > double the distance  than the flow would allow ?  2) Why does the porosity alter the distance the O2 is transported when the discharge is given in cm/day?  Why should porosity matter when these are the discharge units?

Regards,

Rob

X1T-Model output.pptx

[Jia Wang]

Hello Rob,

X1t carries the flow rate in terms of specific discharge (i.e. volumetric flux) rather than the average linear velocity of fluid flow. The volumetric flux is written in units of volume of fluid / area of your domain cross section/ time. Because the units for volume is length cubed (cm^3) and area is length square (e.g. cm^2) , the simplification reduces the unit to length per time (e.g. cm/day). To attain the fluid velocity, you have to divide the specific discharge by the porosity of your domain.  So if you decrease the porosity of your domain, then you are increase the fluid flow velocity.  You can actually plot the fluid velocity and the specific discharge in Xtplot to check that they are being calculated correctly in your simulation. Double-click on the plot in Xtplot and under the Y Axis type, choose Variable type "Physical parameters". You can plot both specific discharge and fluid velocity for your simulation. For more details and examples, please refer to section 3.2 Setting flow rate in the Reactive Transport Modeling User Guide.

Hope this helps,

Jia Wang