Heat Transfer Coefficients

Hx is the length of the edge sides of the cubic cavity and Tci is the cold wall temperature.

The radiative Nusselt number used by Yucell et al. in 1989 and some other authors such as Behnia et al. in 1990 is given by:

qrad(Hx, y,z) is interior glazing surface the heat flux.

3.1 Solution Procedure

The conservation equations (1)-(5) and its boundary conditions (6)-(12) were solved using the well known finite-volume method described in detail by Patankar 1980. The SIMPLE algorithm was used in order to couple the velocity and pressure fields. A 21x21x21 no uniform grid was used and near the wall surfaces a finer grid was used. All equations were coupled and solved using a iterative procedure. The iterative solution procedure was terminated when the residuals of all grid points, normalized by suitable reference quantities, felt below 0.01 percent. Knowing the temperature distributions and the heat flux through the glazing, the solar heat gain coefficient can be calculated by [ASHRAE, 2001]:

where G is the solar radiation that strikes the exterior surface of the glazing, т is the transmissivity of the glazing and q, is the total thermal heat flux through the glazing.

2. Results

Table 1 shows the input parameters that were used in the numerical code from the mathematical model.

Table 1. Input parameters for the simulation.

Ambient conditions

Geometry of the cavity

Rayleigh number





Initial temperature

25 °C

Thickness of the glass

0.002 m

Normal incident radiation

1000 W/m2

Reference temperature T0

35.4 oC

Temperature of cold wall

25.0 oC

Temperature of hot wall

Texo(y, z)

Optical properties

Thermophysical properties at T0

Transmittance of glass


Thermal conductivity of glass

1.4 W/m K

Reflectance of glass


Specific heat of glass

750 J/kg K

Absorptance of glass


Density of glass

2500 kg/m3

Absorptance of the solar control coating


Thermal conductivity of air

0.0263 W/m K

Exterior emittance of wall 2, (glass)


Specific heat of air

1006 J/kg K

Interior emittance of wall 1,3, 4, 5 y 6


Density of air

1.22 kg/m3

Interior emittance of wall 2


Viscosity of air

0.0000155 m2/s

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