Heat transfer coefficients for internal surfaces
Convective heat transfer coefficients for internal surfaces
The convective heat transfer coefficient (W/(m2K)) is calculated with an external Fortran subroutine U_FILM. It is described by the system parameter ConvType[1:9], which can be set in Outline (available only in Expert edition of IDA ICE). This setting is for all zones. At advanced level it can also be set for individual zones.
The convective heat transfer coefficient model is set by ConvType[1]. See below for the description of the different models. By default, ConvType[1] = 3. ConvType[2:6] are the coefficients used if the Simple natural convection (SNC) model is chosen. They are given in the same order as in the model description below. In the same way, ConvType[7:9] are the coefficients used if User Defined Values is chosen. The latter is only available for the Energy zone model.
Model: ConvType[1]:
BRIS model -1
Simple Natural Convection (SNC) model 0
Detailed Natural Convection (DNC) model 1
Ceiling Diffuser (CD) model 2
Max (BRIS Model, CD model) 3
Max (DNC Model, CD model) 4
User Defined Values* 5
*Only available in the Energy zone model
BRIS model (Brown, 1963)
The coefficient is a function of temperature difference between the air and the surface and the slope of the surface. The data is given in terms of a table where intermediate values are evaluated using linear interpolation as shown in the figure below. The x-axis is the temperature difference between air and surface. In the floor case, the temperature difference is between surface and air.
Figure: BRIS model convective heat transfer coefficient
Simple Natural Convection (SNC) model (Walton, 1983)
The simple natural convection model uses constant coefficients for each of the heat transfer configurations. Criteria to calculate reduced and enhanced convection conditions are similar to those used in the detailed model. The convective heat transfer coefficient, h, is computed as follows:
h = 0.948 if (ΔT > 0 and |α| < 10 ) or (ΔT < 0 and |α-180| < 10 )
horizontal surface with reduced convection
h = 4.040 else if (ΔT < 0 and |α| < 10 ) or (ΔT > 0 and |α-180| < 10 )
horizontal surface with enhanced convection
h = 3.076 else if |α-90| < 10
vertical surface
h = 2.281 else if (ΔT > 0 and α < 90 ) or (ΔT < 0 and α > 90 )
tilted surface with reduced convection
h = 3.870 else if (ΔT < 0 and α < 90 ) or (ΔT > 0 and α > 90 )
tilted surface with enhanced convection
where ΔT = Tair - Tsurf ,
α = angle relative horizontal floor.
The coefficients given here are default values but can be changed by the user.
Detailed Natural Convection (DNC) model (Walton, 1983)
The detailed convection model is based on plate experiments and correlates the convective heat transfer coefficient to the surface orientation and the difference between the surface and zone air temperatures. The following algorithm is used:
if (ΔT < 0 and α < 90 ) or (ΔT > 0 and α > 90 )
upward and downward facing surface with enhanced convection
otherwise
upward and downward facing surface with reduced convection and vertical surfaces
where ΔT = Tair - Tsurf ,
α = angle relative horizontal floor.
Ceiling Diffuser (CD) model (Fisher and Pedersen, 1997)
The ceiling diffuser model is based on a room outlet temperature reference. The heat transfer coefficient is expressed as a function of Air Change per Hour (ACH) as follows:
if α < 60 floor
else if α > 120 ceiling
otherwise wall
where ACH = air change per hour,
α = angle relative horizontal floor,
r = min(5, ACH)/5.
User Defined Values (an option only in the Energy zone model)
Constant values can be set by the user for the floor, ceiling and wall. Default, the values are set to the same values as for the SNC model, for the case when all surfaces are colder than the room air. The values may be changed by the user.
h = 0.948 if α < 60 floor
h = 4.040 else if α > 120 ceiling
h = 3.076 otherwise wall
where α = angle relative horizontal floor.
References
Brown, G. 1963 ”Metod för datamaskinberäkning av värme- och ljusstrålning i rum” (in Swedish), in: VVS, 34 (10), 1963.
Fisher, D.E. and C.O. Pedersen. 1997. "Convective Heat Transfer in Building Energy and Thermal Load Calculations", ASHRAE Transactions, Vol. 103, Pt. 2.
Walton, G. N. 1983. Thermal Analysis Research Program Reference Manual. NBSSIR 83-2655. National Bureau of Standards (now NIST). This is documentation for “TARP.”.
Here is a link to the pdf document "Convective heat transfer coefficients for internal surfaces".