Radiation Properties, needed to be specified for solid materials in DO radiation model.
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While simulating a radiation problem with DO radiation model, Fluent asks for radiation properties of materials in terms of "Absorption co-efficient" , "Scattering co-efficient", "Refractive index" etc. Some of these properties are not available directly, but are required to be found out some how to simulate in Fluent. * Thermal Conductivity and Refractive Index are properties that should be directly (and easily) available from the glass manufacturer. In any case, there are several sources (basic and comprehensive) available on the web (besides ASHRAE) that are helpful here, <a target=_blank href="http://en.wikipedia.org/wiki/Thermal_conductivity">http://en.wikipedia.org/wiki/Thermal_conductivity</a>http://en.wikipedia.org/wiki/Thermal_conductivity <a target=_blank href="http://en.wikipedia.org/wiki/List_of_refractive_indices">http://en.wikipedia.org/wiki/List_of_refractive_indices</a>http://en.wikipedia.org/wiki/List_of_refractive_indices <a target=_blank href="http://www.glassproperties.com/">http://www.glassproperties.com/</a>http://www.glassproperties.com/ <a target=_blank href="http://www.robinwood.com/Catalog/Technical/Gen3DTuts/Gen3DPages/RefractionIndexList.html">http://www.robinwood.com/Catalog/Technical/Gen3DTuts/Gen3DPages/RefractionIndexList.html</a>http://www.robinwood.com/Catalog/Technical/Gen3DTuts/Gen3DPages/RefractionIndexList.html <a target=_blank href="http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html">http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html</a>http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html * The transmissivity and reflectivity of the radiation is a function of refractive index of the materials which is set in materials panel. Fluent will automatically calculate the reflectivity at an interface between two materials based on their refractive index (material properties again) and their surface finish (specular/ diffuse). One can override the calculated reflectivity by using reflectivity UDFs (DEFINE_DOM_DIFFUSE_REFLECTIVITY for the diffuse component and DOM_SPECULAR_REFLECTIVITY for the specular component). For more information about the above UDFs please refer the following documentation link: <a target=_blank href="http://www.fluentusers.com/fluent/doc/ori/html/udf/node68.htm#sec-define-dom-diffuse-reflectivity">http://www.fluentusers.com/fluent/doc/ori/html/udf/node68.htm#sec-define-dom-diffuse-reflectivity</a>http://www.fluentusers.com/fluent/doc/ori/html/udf/node68.htm#sec-define-dom-diffuse-reflectivity <a target=_blank href="http://www.fluentusers.com/fluent/doc/ori/html/udf/node70.htm#sec-define-dom-specular-reflectivity">http://www.fluentusers.com/fluent/doc/ori/html/udf/node70.htm#sec-define-dom-specular-reflectivity</a>http://www.fluentusers.com/fluent/doc/ori/html/udf/node70.htm#sec-define-dom-specular-reflectivity * Absorption Coefficient is the property of a medium that describes the amount of absorption of thermal radiation per unit path length within the medium. It can be interpreted as the inverse of the mean free path that a photon will travel before being absorbed (if the absorption coefficient does not vary along the path). The unit quantity for Absorption Coefficient is length-inverse. If we have radiation incident on a glass (Ii) with thickness L, some will transmit (It), some would reflect (Ir) from the surface, and some would absorb (Ia). Based on Lambert's Law, It=Ii*exp(-k*L) where k is absorption coefficient. Absorptivity (a) = Ia/Ii Transmisivity(tu) = It/Ii = exp(-k*L) Therefore, k = -ln(tu)/L. Some handbooks give values of "tu" for a given glass thickness. But it may be hard to find absorption coefficient directly. For example, in introduction of Heat Transfer (2nd edition) by Incropera and Dewitt (page 694). tu for 6mm fused quartz is about 0.9 from 0.2 to 3 um, so k = 17.56 1/m. So a large value of absorption coefficient would mean that this distance is small i.e. the material is optically denser, so for clean air, the absorption coefficient will be less than that for polluted air. Glass can have absorption coefficient in the range of 0.29 to 4000/cm. Likewise, Scattering Coefficient is the property of a medium that describes the amount of scattering of thermal radiation per unit path length for propagation in the medium. It can be interpreted as the inverse of the mean free path that a photon will travel before undergoing scattering (if the scattering coefficient does not vary along the path). The unit quantity for Scattering Coefficient is length-inverse. Note: The external web links in this solution are given for reference only. ANSYS will not be held responsible for correctness of the information in the web links. * Thermal Conductivity and Refractive Index are properties that should be directly (and easily) available from the glass manufacturer. In any case, there are several sources (basic and comprehensive) available on the web (besides ASHRAE) that are helpful here, <a target=_blank href="http://en.wikipedia.org/wiki/Thermal_conductivity">http://en.wikipedia.org/wiki/Thermal_conductivity</a>http://en.wikipedia.org/wiki/Thermal_conductivity <a target=_blank href="http://en.wikipedia.org/wiki/List_of_refractive_indices">http://en.wikipedia.org/wiki/List_of_refractive_indices</a>http://en.wikipedia.org/wiki/List_of_refractive_indices <a target=_blank href="http://www.glassproperties.com/">http://www.glassproperties.com/</a>http://www.glassproperties.com/ <a target=_blank href="http://www.robinwood.com/Catalog/Technical/Gen3DTuts/Gen3DPages/RefractionIndexList.html">http://www.robinwood.com/Catalog/Technical/Gen3DTuts/Gen3DPages/RefractionIndexList.html</a>http://www.robinwood.com/Catalog/Technical/Gen3DTuts/Gen3DPages/RefractionIndexList.html <a target=_blank href="http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html">http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html</a>http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html * The transmissivity and reflectivity of the radiation is a function of refractive index of the materials which is set in materials panel. Fluent will automatically calculate the reflectivity at an interface between two materials based on their refractive index (material properties again) and their surface finish (specular/ diffuse). One can override the calculated reflectivity by using reflectivity UDFs (DEFINE_DOM_DIFFUSE_REFLECTIVITY for the diffuse component and DOM_SPECULAR_REFLECTIVITY for the specular component). For more information about the above UDFs please refer the following documentation link: <a target=_blank href="http://www.fluentusers.com/fluent/doc/ori/html/udf/node68.htm#sec-define-dom-diffuse-reflectivity">http://www.fluentusers.com/fluent/doc/ori/html/udf/node68.htm#sec-define-dom-diffuse-reflectivity</a>http://www.fluentusers.com/fluent/doc/ori/html/udf/node68.htm#sec-define-dom-diffuse-reflectivity <a target=_blank href="http://www.fluentusers.com/fluent/doc/ori/html/udf/node70.htm#sec-define-dom-specular-reflectivity">http://www.fluentusers.com/fluent/doc/ori/html/udf/node70.htm#sec-define-dom-specular-reflectivity</a>http://www.fluentusers.com/fluent/doc/ori/html/udf/node70.htm#sec-define-dom-specular-reflectivity * Absorption Coefficient is the property of a medium that describes the amount of absorption of thermal radiation per unit path length within the medium. It can be interpreted as the inverse of the mean free path that a photon will travel before being absorbed (if the absorption coefficient does not vary along the path). The unit quantity for Absorption Coefficient is length-inverse. If we have radiation incident on a glass (Ii) with thickness L, some will transmit (It), some would reflect (Ir) from the surface, and some would absorb (Ia). Based on Lambert's Law, It=Ii*exp(-k*L) where k is absorption coefficient. Absorptivity (a) = Ia/Ii Transmisivity(tu) = It/Ii = exp(-k*L) Therefore, k = -ln(tu)/L. Some handbooks give values of "tu" for a given glass thickness. But it may be hard to find absorption coefficient directly. For example, in introduction of Heat Transfer (2nd edition) by Incropera and Dewitt (page 694). tu for 6mm fused quartz is about 0.9 from 0.2 to 3 um, so k = 17.56 1/m. So a large value of absorption coefficient would mean that this distance is small i.e. the material is optically denser, so for clean air, the absorption coefficient will be less than that for polluted air. Glass can have absorption coefficient in the range of 0.29 to 4000/cm. Likewise, Scattering Coefficient is the property of a medium that describes the amount of scattering of thermal radiation per unit path length for propagation in the medium. It can be interpreted as the inverse of the mean free path that a photon will travel before undergoing scattering (if the scattering coefficient does not vary along the path). The unit quantity for Scattering Coefficient is length-inverse. Note: The external web links in this solution are given for reference only. ANSYS will not be held responsible for correctness of the information in the web links. |
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