# .Buildings.HeatTransfer.Convection.Exterior

## Information

This is a model for a convective heat transfer for exterior, outside-facing surfaces. The parameter `conMod` determines the model that is used to compute the heat transfer coefficient:

1. If ```conMod= Buildings.HeatTransfer.Types.ExteriorConvection.Fixed ```, then the convective heat transfer coefficient is set to the value specified by the parameter `hFixed`.

2. If ```conMod= Buildings.HeatTransfer.Types.ExteriorConvection.TemperatureWind ```, then the convective heat transfer coefficient is computed based on wind speed, wind direction and temperature difference.

The total convection coefficient ht is the sum of the temperature-driven free convection coefficient hn and the wind-driven forced convection coefficient hf,

ht = hn + hf

The free convection coefficient hn is computed in the same way as in Buildings.HeatTransfer.Convection.Interior. The forced convection coefficient hf is computed based on a correlation by Sparrow, Ramsey, and Mass (1979), which is

hf = 2.537 W R √( P v ⁄ A )

where W=1 for windward surfaces and W=0.5 for leeward surfaces, with leeward defined as greater than 100 degrees from normal incidence, R is a surface roughness multiplier, P is the perimeter of the surface and A is the area of the surface. This is the same equation as implemented in EnergyPlus 6.0.

We make the simplified assumption that the surface is square, and hence we set

hf = 2.537 W R √( 4 v ⁄ √(A) )

The surface roughness is specified by the parameter `surfaceRoughness` which has to be set to a type of Buildings.HeatTransfer.Types.SurfaceRoughness.The coefficients for the surface roughness are

Roughness index R Example material
VeryRough 2.17 Stucco
Rough 1.67 Brick
MediumRough 1.52 Concrete
MediumSmooth1.13 Clear pine
Smooth 1.11 Smooth plaster
VerySmooth 1.00 Glass

#### References

Sparrow, E. M., J. W. Ramsey, and E. A. Mass. 1979. Effect of Finite Width on Heat Transfer and Fluid Flow about an Inclined Rectangular Plate. Journal of Heat Transfer, Vol. 101, p. 204.

Walton, G. N. 1981. Passive Solar Extension of the Building Loads Analysis and System Thermodynamics (BLAST) Program, Technical Report, United States Army Construction Engineering Research Laboratory, Champaign, IL.

## Revisions

• February 11, 2022, by Michael Wetter:
Change parameter `isFloor` to `is_floor`, and `isCeiling` to `is_ceiling`, for consistency with naming convention.
• May 7, 2020, by Michael Wetter:
Set wind direction modifier to a constant as wind velocity approaches zero.
This is for #1923.
• September 17, 2016, by Michael Wetter:
Refactored model as part of enabling the pedantic model check in Dymola 2017 FD01 beta 2.
This is for issue 557.
• November 29, 2011, by Michael Wetter:
Fixed error in assignment of wind-based convection coefficient. The old implementation did not take into account the surface roughness. Bug fix is due to feedback from Tobias Klingbeil (Fraunhofer ISE).
• March 10 2010, by Michael Wetter:
First implementation.

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