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 provides you with the option of making different selections in different parts of the building.

Inside convection algorithm

You can select from 5 different EnergyPlus inside convection algorithms for calculating the convection between internal zone surfaces and the rest of the zone air in the simulation calculations:

1-Detailed - The detailed inside convection algorithm adjusts the convective heat transfer coefficient based on the surface orientation and the difference between the surface and zone air temperatures. The algorithm is taken directly from Walton (1983). Walton derived his algorithm from the ASHRAE Handbook (2001), Table 5 on p. 3.12, which gives equations for natural convection heat transfer coefficients in the turbulent range for large, vertical plates and for large, horizontal plates facing upward when heated (or downward when cooled). A note in the text also gives an approximation for large, horizontal plates facing downward when heated (or upward when cooled) recommending that it should be half of the facing upward value. Walton adds a curve fit as a function of the cosine of the tilt angle to provide intermediate values between vertical and horizontal. The curve fit values at the extremes match the ASHRAE values very well.
2-Simple - The simple convection model uses constant coefficients for different heat transfer configurations, using the same criteria as the detailed model to determine reduced and enhanced convection. The coefficients are also taken directly from Walton (1983). Walton derived his coefficients from the surface conductances for ε=0.90 found in the ASHRAE Handbook (1985) in Table 1 on p. 23.2. The radiative heat transfer component was estimated at 1.02 * 0.9 = 0.918 BTU/h-ft2-F and then subtracted off. Finally the coefficients were converted to SI units to yield the values below. For a vertical surface: h = 3.076 For a horizontal surface with reduced convection: h = 0.948 For a horizontal surface with enhanced convection: h = 4.040 For a tilted surface with reduced convection: h = 2.281 For a tilted surface with enhanced convection: h = 3.870.
3-CIBSE - applies constant heat transfer coefficient derived from traditional CIBSE values.
4-Ceiling diffuser - a mixed and forced convection model for ceiling diffuser configurations. The model correlates the heat transfer coefficient to the air change rate for ceilings, walls and floors. The ceiling diffuser algorithm is based on empirical correlations developed by Fisher and Pedersen (1997). The correlation was reformulated to use the room outlet temperature as the reference temperature. The correlations are shown below. For Floors: h = 3.873 + 0.082 x ACH ^ 0.98, For ceilings: h = 2.234 + 4.099 x ACH ^ 0.503 and for Walls: h = 1.208 + 1.012∗ACH ^ 0.604
5-Cavity - The Trombe wall algorithm is used to model convection in a "Trombe wall zone", i.e. the air space between the storage wall surface and the exterior glazing. (See the later sections on Passive and Active Trombe Walls below for more information about Trombe walls.) The algorithm is identical to the convection model (based on ISO 15099) used in Window5 for convection between glazing layers in multi-pane window systems. The use of the algorithm for modelling an unvented Trombe wall has been validated against experimental data by Ellis (2003).This algorithm gives the convection coefficients for air in a narrow vertical cavity that is sealed and not ventilated. This applies both to the air gap in between panes of a window or to the air gap between the Trombe wall glazing and the inner surface (often a selective surface). These convection coefficients are really the only difference between a normal zone and a Trombe zone. The 5-Cavity Inside convection algorithm is not available at the surface level.

Paraphrased note from EnergyPlus developers: "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."

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:

1-Detailed - The Detailed, BLAST, and TARP convection models are very similar. In all three models, convection is split into forced and natural components (Walton 1981). The forced convection component is based on a correlation by Sparrow, Ramsey, and Mass (1979. The natural convection component is calculated in the same way as the Detailed Inside convection algorithm (see above).
2-Simple - applies heat transfer coefficients depending on the roughness and wind speed. This is a combined heat transfer coefficient that includes radiation to sky, ground, and air. The correlation is based on Figure 1, Page 25.1 (Thermal and Water Vapor Transmission Data), 2001 ASHRAE Handbook of Fundamentals.
3-CIBSE - applies constant heat transfer coefficients depending on orientation, derived from traditional CIBSE values.
4-BLAST - Identical to the Detailed option
5-TARP - Identical to the Detailed option
6-DOE-2 - The DOE-2 convection model is a combination of the MoWiTT and BLAST Detailed convection models (LBL 1994).
7-MoWiTT - The MoWiTT model is based on measurements taken at the Mobile Window Thermal Test (MoWiTT) facility (Yazdanian and Klems 1994). The correlation applies to very smooth, vertical surfaces (e.g. window glass) in low-rise buildings. The MoWiTT algorithm may not be appropriate for rough surfaces, high-rise surfaces, or surfaces that employ movable insulation.

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

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.

Inside and Outside convection algorithm settings can be made in various places.

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

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 changes at block, zone or surface levels in the Model data will override these settings.