# RE: [EnergyPlus_Support] a problem about the inside convection models

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The complete reference for the Window5 convection algorithm (ISO 15099) is:
ISO15099. Thermal performance of windows, doors and shading devices―detailed calculations. Draft, July 18, 2001.

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 rest of the zone heat balance is the same, e.g., transmitted solar, long-wave radiation between surfaces, etc.

For a vertical cavity, the correlation from ISO 15099 is:

Nu1 = 0.0673838Ra^(1/3), for 5e4 < Ra < 1e6
Nu1 = 0.028154*Ra^(0.4134) for 1e4 < Ra < 5e4
Nu1 = 1+1.7596678e-10*Ra^(2.2984755) for Ra <= 1e4

Nu2 = 0.242*(Ra/A)^0.272

Nu = Max(Nu1, Nu2)

Nu = Nusselt number, Ra = Rayleigh number (consult any heat transfer text book), A = aspect ratio of cavity

This is then used in EnergyPlus as follows:

Net convection coefficient from glazing to wall is:  hnet = k*Nu/L, k = conductivity of air, L = air gap thickness
Convection coefficient applied to each wall separately and actually used in the zone heat balance is:  hc = 2*hnet

Peter G. Ellis
National Renewable Energy Laboratory
Center for Buildings and Thermal Systems
1617 Cole Blvd
Golden, CO 80401

-----Original Message-----
From: mshirong [mailto:mshirong2003@xxxxxxxxxxxx]
Sent: Monday, November 22, 2004 7:03 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] a problem about the inside convection models

Thank you very much for your help!
but still I have some questions. In the E+ Reference manual, there is no description of the methematical model of trombe wall algorithm which I want to know.
In "Trombe Wall Algorithm" it says:
<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. >
After a long search, I'm also not sure the relation between the variable and the detail of the model. Where I can find it ?

Regards,
mshirong

Drury.Crawley@xxxxxxxxxx wrote:

Also note what it says in the Engineering Doc (beginning on page 418, page
456 of the PDF):

A passive Trombe wall is a passive solar wall designed for thermal storage
and delivery. It
consists of a thick wall (8" to 16") faced with a selective surface solar
absorber, air gap, and
high transmissivity glass pane.

In EnergyPlus, there is no Trombe wall object; rather, it is composed of
other existing
EnergyPlus objects in the input file.

To simulate the Trombe wall, a very narrow zone is coupled to the desired
surface via an
interzone partition. The depth of the zone corresponds to the size of the
air space [usually
?amp;quot; (18 mm) to 6" (150 mm)]. In most cases the Trombe zone will be a sealed
zone with no ventilation or
infiltration. The exterior wall of the Trombe zone contains a single or
double-pane window.
Optimally, the window covers nearly all of the wall area and has a very
high transmissivity to
allow the maximum amount of solar flux into the Trombe zone. The interior
wall is usually
constructed of very thick masonry materials with a solar absorber surface
as the innermost
layer of the wall. The absorber is a selective surface material with very
high absorptivity and
very low emissivity, e.g. copper with a special black surface treatment. It
is important to
make sure the Solar Distribution field in the BUILDING object is set to
FullInteriorAndExterior
so that the majority of the solar flux is directed on the absorber surface
and not just on the
very small area of the Trombe zone floor. The Zone Inside Convection
Algorithm for the
Trombe zone should also be set to TrombeWall to correctly model the air
space. As is the
case for all interzone partitions, the wall construction of the adjoining
zone must be the mirror
image of the wall construction in the Trombe zone. Finally, an overhang is
optionally
attached to the Trombe zone to control the amount of seasonal sun exposure.
Since all of
the Trombe wall parameters are selected by the user in the input file,
there is considerable
freedom to experiment with different materials, sizes, and configurations.

"Linda Lawrie"
<linda@xxxxxxxxxx>       To: EnergyPlus_Support@xxxxxxxxxxxxxxx
cc:
11/21/2004 10:48 AM      Subject:  Re: [EnergyPlus_Support] a problem about the inside
EnergyPlus_Support

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Check in the Engineering Reference manual:

page 87 -- trombe wall convection correlation

page 345 -- "Active Trombe Wall"

page 456 -- "Passive Trombe Wall"

Reminder -- the PDF documentation is "indexed"  press the "search" button

be able to search on any word)

Two example files for Trombe Wall are included.  Both have "trombe" in
their names.

Linda

EnergyPlus WebSite: http://www.energyplus.gov
Owner: EnergyPlus_Support list
Member: EnergyPlus Development Team

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