## Vapor Surface transfer coefficients

marco.larcher
WUFI User
Posts: 2
Joined: Thu May 13, 2021 2:52 am -1100

### Vapor Surface transfer coefficients

Hello,
is it possible to set the value of the Vapor Transfer Coefficients in WUFI?
In know it is possible to add an additional Sd value to include the effects of coating but I guess that this adds to the equivalent vapour diffusion thickness of the "basic" boundary layer. I am personally interested in setting this basic value rather than in adding an additional Sd-value.

Marco

Thomas
Posts: 325
Joined: Sun Jun 19, 2005 10:33 pm -1100

### Re: Vapor Surface transfer coefficients

Hi Marco,

this feature is on the wish list for future program versions, but currently it is not possible in WUFI-Pro to set the natural "basic" vapor transfer coefficient for the surfaces to a user-defined value.

If you wish to increase the natural vapor transfer resistance (i.e. decrease the vapor transfer coefficient), you can add an appropriate user-defined sd-value, but it is currently not possible to decrease the transfer resistance below the value which is automatically derived from the convective heat transfer coefficient.

Kind regards,
Thomas

(Note to the programmers: Item #2519 on the wish list.)

marco.larcher
WUFI User
Posts: 2
Joined: Thu May 13, 2021 2:52 am -1100

### Re: Vapor Surface transfer coefficients

Would it be possible to know how it is derived from the convective heat transfer coefficient? Do you use the Lewis relation?
And how is the overall Heat Transfer Coefficient split in the convective part and the radiative part, for instance when one uses the User Defined option to specify interior and exterior Heat Transfer Coefficients?

Marco

Thomas
Posts: 325
Joined: Sun Jun 19, 2005 10:33 pm -1100

### Re: Vapor Surface transfer coefficients

Hi Marco,

yes, this is a simplified version of the Lewis relation. The vapor transfer coefficient $$\beta$$ is computed as

$$\beta = 7 \cdot 10^{-9} \ \cdot \ \alpha_c$$

where $$\alpha_c$$ is the convective component of the heat transfer coefficient.

WUFI subtracts the radiative component 6.5 W/(m²K) from the overall exterior heat transfer coefficient and 4.5 W/(m²K) from the interior heat transfer coefficient. If the result is less than 3.5 W/(m²K) it is set to 3.5 W/(m²K).

If the option "explicit radiation balance" is selected, the user can tell WUFI which radiative component (if any) to subtract from the exterior heat transfer coefficient.

See also the topic "Reference | Surface Transfer | Water Vapor Transfer Coefficients" in WUFI's help file.

The hourly values for $$\alpha$$ and $$\beta$$ used during the calculation are available via WUFI's ASCII export option.

Kind regards,
Thomas