External Film Resistance
The External Film Resistance subnode is available from the context menu (right-click the Wall Heat Transfer parent node) or from the Physics toolbar, Attributes menu. Use it to define the external film resistance features including the material, thermal conductivity, density, external velocity, and pressure of the external fluid. You can select between forced and natural convection.
Specification
Select an External film heat transfer model — External forced convection (the default), External natural convection, or User defined.
External Forced Convection and External Natural Convection
For External forced convection or External natural convection the Surrounding fluid uses the Domain material by default.
Be careful to select the right material in the Settings window for External Film Resistance. Many times this should be another material than the Domain material which is the internal fluid.
The default Heat capacity at constant pressure Cp (SI unit: J/(kg·K)), Thermal conductivity k (SI unit: W/(m·K)), Density ρ (SI unit: kg/m3), and Dynamic viscosity μ (SI unit: Pa·s), all use values From material. For User defined enter different values or expressions than the default, which is 0 for all options.
The External velocity uext (SI unit: m/s) is User defined by default. This is the velocity of the cooling or heating fluid outside the pipe.
For External natural convection only, also enter a value or expression for the External pressure pext (SI unit: Pa). The default is 1 atm.
The natural convection correlations (see External Film Resistance under The Heat Transfer Equation in the section Theory for the Heat Transfer in Pipes Interface) require a temperature dependent density, or else the coefficient of thermal expansion β evaluates to 0 (Equation 3-26), which might generate unexpected results or even unstable models. Also, make sure that the density correlation ρ(T) is smooth throughout the temperature interval used, or else β is discontinuous and can cause numerical instability.
Figure 3-2: Before you use the Natural Convection option, it is good practice to select Density in the used material, and click Plot in the Settings window. That way you can inspect the behavior and make sure it is smooth within the temperature interval you plan to use it.
User Defined
For User defined:
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Enter the Nusselt number (dimensionless). The default is 3.66.
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The Surrounding fluid uses the Domain material by default.
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The Thermal conductivity k (SI unit: W/(m·K)) takes its value From material. For User defined enter a different value or expression. The default is 0 W/(m·K).
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Enter a value or expression for the External pressure pext (SI unit: Pa). The default is 1 atm).