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Step 6: Boundary Conditions

figure Boundary Conditions

figure

1.   Set the boundary conditions for the baffle.

figure Boundary Conditions figure figure baffle figure Edit...

figure

(a)   Click the Thermal tab and enter 0.1 for Internal Emissivity.

(b)   Click the Radiation tab and enter 0 for Diffuse Fraction.

(c)   Click OK to close the Wall dialog box.

2.   Set the boundary conditions for the baffle-shadow.

figure Boundary Conditions figure figure baffle-shadow figure Edit...

figure

(a)   Click the Thermal tab and enter 0.1 for Internal Emissivity.

(b)   Click the Radiation tab and enter 0 for Diffuse Fraction.

(c)   Click OK to close the Wall dialog box.

3.   Set the boundary conditions for the bulb-outer.

figure Boundary Conditions figure figure bulb-outer figure Edit...

figure

(a)   Click the Thermal tab and enter 150000  ${\rm W/m}^{2}$ for Heat Flux.

(b)   Retain the value of 1 for Internal Emissivity.

(c)   Click OK to close the Wall dialog box.

4.   Set the boundary conditions for the housing.

figure Boundary Conditions figure figure housing figure Edit...

figure

(a)   Click the Thermal tab and select Mixed in the Thermal Conditions group box.

(b)   Enter 10  ${\rm W/m}^{2}-K$ for Heat Transfer Coefficient.

(c)   Enter 20 C for Free Stream Temperature.

(d)   Retain the value of 1 for External Emissivity.

(e)   Enter 20 C for External Radiation Temperature.

(f)   Enter 0.5 for Internal Emissivity.

(g)   Click OK to close the Wall dialog box.

5.   Set the boundary conditions for the lens-inner.

figure Boundary Conditions figure figure lens-inner figure Edit...

   The inner and outer surface of the lens will be set to semi-transparent conditions. This allows radiation to be transmitted through the wall between the two adjacent participating cell zones. It also calculates the effects of reflection and refraction at the interface. These effects occur because of the change in refractive index (set through the material properties) and are a function of the incident angle of the radiation and the surface finish. In this case, the lens is assumed to have a very smooth surface so the diffuse fraction will be set to 0.

On the internal walls (wall/ wall-shadows) it is important to note the adjacent cell zone: this is the zone the surface points into and may influence the settings on diffuse fraction (these can be different on both sides of the wall).

figure

(a)   Click the Radiation tab.

(b)   Select semi-transparent from the BC Type drop-down list.

(c)   Enter 0 for Diffuse Fraction.

(d)   Click OK to close the Wall dialog box.

6.   Set the boundary conditions for the lens-inner-shadow.

figure Boundary Conditions figure figure lens-inner-shadow figure Edit...

(a)   Click the Radiation tab.

(b)   Retain the default selection of semi-transparent from the BC Type drop-down list.

(c)   Enter 0 for Diffuse Fraction.

(d)   Click OK to close the Wall dialog box.

7.   Set the boundary conditions for the lens-outer.

figure Boundary Conditions figure figure lens-outer figure Edit...

   The surface of the lamp cools mainly by natural convection to the surroundings. As the outer lens is transparent it must also lose radiation to the surroundings, while the surroundings will supply a small source of background radiation associated with the temperature. For the lens, a semi-transparent condition is used on the outside wall. A mixed thermal condition provides the source of background radiation as well as calculating the convective cooling on the outer lens wall. For a semi-transparent wall, the source of background radiation is added directly to the DO radiation rather than to the energy equation - an external emissivity of 1 is used, in keeping with the assumption of a small object in a large enclosure. As the background radiation is supplied from the thermal conditions, there is no need to supply this as a source of irradiation under the Radiation tab for the wall boundary condition. The only other setting required here is the surface finish of the outer surface of the lens - the diffuse fraction should be set to 0 as the lens is assumed to be smooth.

figure

(a)   Click the Thermal tab and select Mixed in the Thermal Conditions group box.

(b)   Enter 10  ${\rm W/m}^{2}-K$ for Heat Transfer Coefficient.

(c)   Enter 20 C for Free Stream Temperature.

(d)   Retain the value of 1 for External Emissivity.

   For a semi-transparent wall the internal emissivity has no effect as there is no absorption or emission on the surface. So the set value is irrelevant.

(e)   Enter 20 C for External Radiation Temperature.

(f)   Click the Radiation tab.

figure

(g)   Select semi-transparent from the BC Type drop-down list.

(h)   Enter 0 for Diffuse Fraction.

(i)   Click OK to close the Wall dialog box.

8.   Set the boundary conditions for the reflector.

figure Boundary Conditions figure figure reflector figure Edit...

  Like the baffles, the reflector is made of highly polished aluminum, giving it highly reflective surface property. About 90% of incident radiation reflects from this surface. Only 10% gets absorbed. Based on Kirchhoff's law, we can assume emissvity equals absorptivity. Therefore, we apply internal emissivity=0.1. We also assume a clean reflector (diffuse fraction = 0).

(a)   Click the Thermal tab and enter 0.1 for Internal Emissivity.

(b)   Click the Radiation tab and enter 0 for Diffuse Fraction.

(c)   Click OK to close the Wall dialog box.

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Release 12.0 © ANSYS, Inc. 2009-02-09