Surface Convection

Construction tab in model data under Construction header

 

You can select the inside and outside surface convection algorithms in the Model data on the Construction tab under the Surface Convection header.  This allows you to make different selections in different parts of the building. Further details of the algorithms are provided in the EnergyPlus Engineering reference document.

Inside convection algorithm

You can select from 34 different EnergyPlus inside convection algorithms for calculating the convection between internal zone surfaces and the rest of the zone air in the simulation calculations. Unless you have good reason to do so you are advised to use the default adaptive convection algorithm:

 

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    • The 5-Cavity Inside convection algorithm is not available at the surface level.

     

    Paraphrased note from EnergyPlus developers on the Cavity option: "The Trombe wall convection coefficients only make sense for a zone.  They are specific coefficients calculated for a narrow enclosed space.  The two major walls of a Trombe wall zone are so close together that the convection patterns for the two walls actually interact. If they are close enough they can fight each other and totally stagnate the convection cell in the space.  This is not free-boundary convection such as that found in a typical room.  Therefore, it does not make sense to apply these coefficients to a single surface.  The algorithm analyses the zone to figure out which are the two major surfaces and then sets the coefficients on those surfaces.  The other minor surfaces receive negligible convection."

     

    The following inside convection options are also available for advanced users:

     

     

    To avoid discontinuities in surface heat transfer rate calculations, all correlations are extrapolated beyond the lower limit of the data set (3 ACH) to a natural convection limit which is applied during the hours when the system is off. These models are explained in greater detail in the EnergyPlus Engineering Reference Document.

    Outside convection algorithm

    Substantial research has gone into the formulation of models for estimating the exterior convection coefficient. Since the 1930's there have been many different methods published for calculating this coefficient, with much disparity between them (Cole and Sturrock 1977; Yazdanian and Klems 1994). You can select from 7 different outside convection algorithms:

     

    Note: selecting the SimpleCombined option at building level on the Constructions tab under Surface Convection or on the Calculation options or Model options dialogs has a special meaning. In this case the outside convective selection covers both convection and radiation combined. This means for example that any hard-set outside convective heat transfer coefficients set on the Surface properties tab of the Construction dialog will actually be used as a combined outside convective plus radiation coefficient.

     

    The following outside convection options are also available for more advanced users:

     

    Note: When the surface is wet (i.e. it is raining and the surface is exposed to wind) then the convection coefficient appears in results as a very large number (1000) and the surface is exposed to the Outdoor Wet Bulb Temperature rather than the Outdoor Dry Bulb Temperature.

     

    As well as being able to define convection options on the Model data on Construction tab under the Surface Convection header as described above, building level inside and outside convection algorithm settings can be made on:

     

     

    You cannot make changes to individual surface convection settings where the surface is an internal partition.

     

    All of the above algorithms are described fully in the EnergyPlus Engineering Reference.

     

    Note: the equivalent inside and outside surface convection options in the Calculation and Model options dialogs control building default values (equivalent of making a setting in Model data at building level) and any changes made at block, zone or surface levels in the Model data will override these default settings.