Currently working on a project and trying to find historical maximum surface temperature on a facade. I use Ladybug to find solar radiation value for given date and time. I then switch to CFD and apply “heat flux” boundary condition in [W/m2]. Numerical value being solar radiation from Ladybug, times emissivity of material.

QUESTION: is resulting radiation value from Ladybug directly translatable into “Heat Flux” for CFD models? In other words, is the result from Ladybug basically just heat?

I tend to think that Ladybug’s radiation result includes integrated energy of complete sunlight spectrum, while I’m interested only in that part, that generates heat = IR part of spectrum. If that’s right, I should be able to factor Ladybug’s radiation result down by more than just emissivity of material, before I go and apply [W/m2] as boundary condition to a CFD model.

Mostapha, thanks for reply.
Well I probably could use something else. I’m not familiar with environment of E+.
The question though is: Is the resulting solar radiation from Ladybug basically just heat, or is there more radiation components included? I’m aware that weather files for the US are from DOE and E+ uses the same.
Thank you, regards

The solar radiation from Ladybug represents multiple components (IR, short-wave) that can be modeled separately, or cumulatively, depending on which option you choose in the GH component.

I’m not really sure what the CFD “Heat Flux” input requires, but I can tell you that if you are looking for total surface heat flux, it would not be accurate to just use the IR part of the spectrum. since the heat flux at your surface consists of the sum of your convection, and long-wave/short-wave radiation.

Where -kdT(L)/dx is your heat flux on exterior, which equals to the sum of convective, long-wave radiation, and your short-wave radiation.

As Mostapha said, it’s trivial to get surface heat flux from OpenStudio/EnergyPlus. If you’re looking for surface temperature, you can obtain that directly from EnergyPlus as well. Just make sure to specify these variables in the EPOutputs component.

thank you both for the answer. I am somewhat getting aware of complexity of the matter. I deal with lightweight facade. It is “kind of” back-ventilated. It is not air-tight, the cavity is small, so it’s really something between “ventilated” and “closed cavity”. That brought me to simply use CFD, where the convection takes care of itself on the on the outside AND in the cavity of cladding panel as natural convection develops. So the remaining piece in the puzzle is to correctly define surface heat flux.

What I was not able to find anywhere either is, how (if at all) sky cover influences the radiation value. Could you please comment on that?

Yes it will. I was a little unclear in the previous post: specifically the sky will contribute (short-wave) diffuse radiation from reflected solar, and (long-wave) infrared radiation due to the sky temperature and emissivity.

Don’t you need the the surface heat flux from the other side of your cavity as well, for your CFD. Even assuming you can obtain the heat flux from solar and infra, what about the heat transferred from the inside of the building to your cavity?

Is this a single-point in time analysis or an annual analysis?

If it’s a single point in time, you can model a steady-state condition with THERM. THERM has materials in its library to represent slightly-ventilated air cavities. You can also model the radiant environment in THERM (although I haven’t done this before) which I believe is fairly straightforward. Plus you get accurate thermal bridging.

Would you let me know the image credit? Because I am calculating the heat transfer on the exterior wall surface, I would like to know where I can find the image and formula above.

It’s from “Heat and Mass Transfer: Fundamentals & Applications.” by Yunus A. Cengel, and Afshin J. Ghajar, pg 91 of the 5th edition. You should be able to find a pdf of the entire textbook online pretty easily.

I would start with chapter 1, section 4-5, which goes over the theoretical foundations of surface heat balances and the three heat transfer modes. Then chapter 2, section 4 goes over the specific equations for each mode at the boundary of a mass, and will guide you to the specific example of steady-state surface heat flux of an exterior wall.