Heat and moisture flux

[james bertram]

Good morning - I've exported ASCII data for heat flux and moisture flux (total, diffusion & capillary).

For heat flux (exterior and interior surfaces) I have summed the hourly data and then divided the total by 3600 - is this correct (outcomes matches the "status of last calc" report, so I presume it coverts per hour to per second)?

For moisture flux (exterior surface) I have summed the hourly data and then multipled by 3600 (converts per second to per hour) - is this correct?

For total flux, the outcome correlates with the "status of last calc" report. For diffusion and capillary the outcomes are significantly higher for "exterior surface", but match the "status of last calc" for the "interior surface". When totalled however diffusion flux (left) + capillary flux (left) = total flux (left).

For diffusion flux (left) and capillary flux (left), the difference between the total and the values shown on the "status of last calc" report seems to be the total of the "starting water content".

Your support will be greatly appreciated

[Thomas]

Good morning - I've exported ASCII data for heat flux and moisture flux (total, diffusion & capillary).

For heat flux (exterior and interior surfaces) I have summed the hourly data and then divided the total by 3600 - is this correct (outcomes matches the "status of last calc" report, so I presume it coverts per hour to per second)?

Hi James,

the heat flux data are flow rates (Watt per square meter). In order to get something that can be summed, you have to convert to amounts of energy. One option is to multiply by 3600, which gives you the Joules per square meter that have crossed the surface during the hour. Summing over all the hours gives you the total amount in Joules per square meter, and dividing by 1*10^6 converts to Megajoules per square meter.

Another option is to multiply the W/m² by 1 hour (i.e., do nothing) and interpret the numbers as the Watt-hours of energy which have crossed one square meter of the surface during one hour. Summing the numbers will then give you the total amount of energy in Watt-hours per square meter.

However, you will not be able to exactly reproduce the amounts of heat flux displayed by WUFI in the "Status of Last Calculation" dialog. Those numbers also include the latent heat flow associated with vapor diffusion across the surfaces, and they exclude the heat which has been released at the surface due to absorbed solar radiation.

For moisture flux (exterior surface) I have summed the hourly data and then multipled by 3600 (converts per second to per hour) - is this correct?

For total flux, the outcome correlates with the "status of last calc" report. For diffusion and capillary the outcomes are significantly higher for "exterior surface", but match the "status of last calc" for the "interior surface". When totalled however diffusion flux (left) + capillary flux (left) = total flux (left).

For diffusion flux (left) and capillary flux (left), the difference between the total and the values shown on the "status of last calc" report seems to be the total of the "starting water content".

I'm afraid I cannot completely follow you. In my tests for both the exterior and the interior surface, the sum of diffusive and capillary fluxes in the ASCII output is always identical with the total flux for that hour, and the sum over all the hours (multiplied by 3600) is identical to the moisture flux integrals displayed in the "Status" dialog, as it should be.

Kind regards,
Thomas

[james bertram]

Good morning Thomas - thank you for the quick reply, apologies if my explanation wasn't clear.

Yes, ASCII total flux,exterior = diffusion + capillary flux, exterior.

However when I multiply diffusion and capillary flux totals by 3600 they don't correlate with "Status of last calc - check for numerical quality - integral of fluxes, left side (kl, dl)" outcomes.

Using ASCII export data example:
Total flux, outside surface = -0.002516561 x 3600 = -9.059619257 = "status of last calc - time integral fluxes - moisture fluxes, left side = -9.06....ASCII and report correllate.

Diffuse flux, outside surface = -0.013551798 x 3600 = -48.78647123, and capillary flux, outside surface = 0.011035237 x 3600 = 39.72685192. Sum of -48.78647123 + 39.72685192 = -9.059619257 = total flux...ASCII and report totals correlate.

However "Status of last calc - check for numerical quality - integral of fluxes, left side (kl, dl) = 9.1 and -18.16 (9.1 - 18.16 = -9.06). Should ASCII total diffusion flux, exterior = integral of fluxes, left side (dl), and ASCII total capillary flux, exterior = integral of fluxes, left side (kl)?

Thank you for your support.

[Thomas]

Ah, but you should not compare the fluxes from the ASCII export with the integral of fluxes shown in the "Check for numerical quality" section of the "status" dialog. As explained in the on-line help topic for that dialog, the fluxes shown in this section are not the fluxes through the surfaces but the fluxes through the boundary between the first and the second (or between the second-to-last and the last) grid element.

The ASCII-exported fluxes can be compared with the "Moisture Fluxes" displayed in the section "Time integral of fluxes". You should find that these time integrals are identical with your summed ASCII-exported fluxes.

The reasons why the outermost grid elements are ignored for the purposes of the numerical quality test are explained in the online help. Since some of the flowing moisture may be stored in the outermost elements, the moisture flows through the surface and through the first (or last) interior grid element boundary may be slightly different at any moment.

Furthermore, the way in which the moisture flow is divided up into diffusive and capillary components may drastically change between the first and the second (or the second-to-last and the last) grid element. For example, water vapor arriving at the surface from the interior of the material may condense there and flow back into the material as capillary flow. In this case, you will have some diffusive and some capillary flow between the first and the second grid element. On the other hand, capillary water cannot leave the material, so the capillary flow from inside the material across the surface will always be zero, while the diffusive flow across the surface will be the difference between what arrived at the surface in the form of vapor and what flowed back into the material in the form of liquid water.

So it's no wonder that the summed ASCII-exported fluxes and the fluxes used for the numerical quality check do not agree: they are evaluated at slightly different locations of the building component.

Kind regards,
Thomas

[james bertram]

Thank you Thomas - makes sense, I have read the section on fluxes and remember the comment about first and second layer...this issue helps put a useful learning context around the terms and issues.

[zhanqsh]

Hello!
I am using WUFI2D 4.1 and WUFI Graph2.20.0 to study how diffusion and capillary flow transfer over cross a wall assembly under different surface/weather conditions. Is it possible for me or how:
1) to tell the flux direction of the diffusion/ capillary flux;
2) to export the summed flux over a defined cross section of a period?

Thank you very much!

Qiaosheng Zhan

[Christian Bludau]

Dear Qiaosheng,

please first update to the latest version of WUFI 2D, its free viewtopic.php?f=15&t=998.

Positive fluxes are directed to the right and top, negative to the left and bottom.

To export the fluxes first you have to ensure that you select them in the Computational Parameters section under "Result file contains". Flu. C are the capillary fluxes, D are diffusion fluxes and H are heat fluxes. You have to do that prior to the calculation. This is also described in the WUFI Graph program help.
After that use WUFI Graph to export your fluxes. To display them you have to draw a line between two crossings of the calculation grid. The result will be summed up over the cross section for each time step. See also WUFI Graph help in the "Area selection / Settings" section types flux and flux density.

Christian

[zhanqsh]

Dear Christian,

Thank you for your quick reply. It perfectly works with the new version.

Viel Grüße!