Section 7.1: Setting Physical Properties. Section 7.4: Thermal Conductivity. Section 7.5: Specific Heat Capacity

Transcription

1 Chapter 7. Physical Properties This chapter describes the physical equations used to compute material properties and the procedures you can use for each property input. Each property is described in detail in the following sections. If you are using one of the general multiphase models (VOF, mixture, or Eulerian), see Section for information about how to define the individual phases, including their material properties. Section 7.1: Setting Physical Properties Section 7.2: Density Section 7.3: Viscosity Section 7.4: Thermal Conductivity Section 7.5: Specific Heat Capacity Section 7.6: Radiation Properties Section 7.7: Mass Diffusion Coefficients Section 7.8: Standard State Enthalpies Section 7.9: Standard State Entropies Section 7.10: Molecular Heat Transfer Coefficient Section 7.11: Kinetic Theory Parameters Section 7.12: Operating Pressure Section 7.13: Reference Pressure Location Section 7.14: Real Gas Model c Fluent Inc. November 28,

2 Physical Properties 7.1 Setting Physical Properties An important step in the setup of your model is the definition of the physical properties of the material. Material properties are defined in the Materials panel, which allows you to input values for the properties that are relevant to the problem scope you have defined in the Models panels. These properties may include the following: Density and/or molecular weights Viscosity Heat capacity Thermal conductivity Mass diffusion coefficients Standard state enthalpies Kinetic theory parameters Properties may be temperature- and/or composition-dependent, with temperature dependence based on a polynomial, piecewise-linear, or piecewise-polynomial function and individual component properties either defined by you or computed via kinetic theory. The Materials panel will show the properties that need to be defined for the active physical models. Note that, if any property you define requires the energy equation to be solved (e.g., ideal gas law for density, temperature-dependent profile for viscosity), FLUENT will automatically activate the energy equation for you. You will then need to define thermal boundary conditions and other parameters yourself. Physical Properties for Solid Materials For solid materials, only density, thermal conductivity, and heat capacity are defined (unless you are modeling semi-transparent media, in which case radiation properties are also defined). You can specify a constant value, a temperature-dependent function, or a user-defined function for 7-2 c Fluent Inc. November 28, 2001

3 7.1 Setting Physical Properties thermal conductivity; a constant value or temperature-dependent function for heat capacity; and a constant value for density. If you are using the segregated solver, density and heat capacity for a solid material are not required unless you are modeling unsteady flow or moving solid zones. Heat capacity will appear in the list of solid properties for steady flows as well, but the value will be used just for postprocessing enthalpy; it will not be used in the calculation Material Types In FLUENT, physical properties of fluids and solids are associated with named materials, and these materials are then assigned as boundary conditions for zones. When you model species transport, you will define a mixture material, consisting of the various species involved in the problem. Properties will be defined for the mixture, as well as for the constituent species, which are fluid materials. (The mixture material concept is discussed in detail in Section ) Additional material types are available for the discrete-phase model, as described in Section ! Materials can be defined from scratch, or they can be downloaded from a global (site-wide) database and edited. See Section for information about modifying your site-wide material property database. All the materials that exist in your local materials list will be saved in the case file (when you write one). Your materials will be available to you if you read this case file into a new solver session Using the Materials Panel The Materials panel (Figure 7.1.1) allows you to create new materials, copy materials from the global database, and modify material properties. Define Materials... These generic functions are described here in this section, and the inputs for temperature-dependent properties are explained in Section The specific inputs for each material property are discussed in the remaining sections of this chapter. c Fluent Inc. November 28,

4 Physical Properties Figure 7.1.1: The Materials Panel 7-4 c Fluent Inc. November 28, 2001

5 7.1 Setting Physical Properties By default, your local materials list (i.e., in the solver session) will include a single fluid material (air) and a single solid material (aluminum). If the fluid involved in your problem is air, you may use the default properties for air or modify the properties. If the fluid in your problem is water, for example, you can either copy water from the global database or create a new water material from scratch. If you copy water from the database, you can still make modifications to the properties of your local copy of water. Mixture materials will not exist in your local list unless you have enabled species transport (see Chapter 13). Similarly, inert, droplet, and combusting particle materials will not be available unless you have created a discrete phase injection of these particle types (see Chapter 19). When a mixture material is copied from the database, all of its constituent fluid materials (species) will automatically be copied over as well. Modifying Properties of an Existing Material Probably the most common operation you will perform with the Materials panel is the modification of properties for an existing material. The steps for this procedure are as follows: 1. Select the type of material (fluid, solid, etc.) in the Material Type drop-down list. 2. Choose the material for which you want to modify properties in the Fluid Materials drop-down list, Solid Materials list, or other similarly-named list. (The list s name will be the same as the material type you selected in step 1.) 3. Make the desired changes to the properties contained in the Properties area. If there are many properties listed, you may use the scroll bar to the right of the Properties area to scroll through the listed items. 4. Click on the Change/Create button to change the properties of the selected material to your new property settings. c Fluent Inc. November 28,

6 Physical Properties To change the properties of an additional material, repeat the process described above. Remember to click the Change/Create button after making the changes to each material s properties. Renaming an Existing Material Each material is identified by a name and a chemical formula (if one exists). You can change a material s name, but you should not change its chemical formula (unless you are creating a new material, as described later in this section). The steps for changing a material s name are as follows: 1. Select the type of material (fluid, solid, etc.) in the Material Type drop-down list. 2. Choose the material for which you want to modify properties in the Fluid Materials drop-down list, Solid Materials list, or other similarly-named list. (The list s name will be the same as the material type you selected in step 1.) 3. Enter the new name in the Name field at the top of the panel. 4. Click on the Change/Create button. A Question dialog box will appear, asking you if the original material should be overwritten. Since you are simply renaming the original material, you can click Yes to overwrite it. (If you were creating a new material, as described later in this section, you would click on No to retain the original material.) To rename another material, repeat the process described above. Remember to click the Change/Create button after changing each material s name. Copying a Material from the Database The global (site-wide) materials database contains many commonly used fluid, solid, and mixture materials, with property data from several different sources [151, 195, 263]. If you wish to use one of these materials 7-6 c Fluent Inc. November 28, 2001

7 7.1 Setting Physical Properties in your problem, you can simply copy it from the database to your local materials list. The procedure for copying a material is detailed below: 1. Click on the Database... buttoninthematerials panel to open the Database Materials panel (Figure 7.1.2). Figure 7.1.2: The Database Materials Panel 2. Select the type of material (fluid, solid, etc.) in the Material Type drop-down list. c Fluent Inc. November 28,

8 Physical Properties 3. In the Fluid Materials list, Solid Materials list, or other similarlynamed list (the list s name will be the same as the material type you selected in step 2), choose the material you wish to copy. Its properties will be displayed in the Properties area. 4. If you want to check the material s properties, you can use the scroll bar to the right of the Properties area to scroll through the listed items. For some properties, temperature-dependent functions are available in addition to the constant values. You can select one of the function types in the drop-down list to the right of the property and the relevant parameters will be displayed. You cannot edit these values, but the panels in which they are displayed function in the same way as those that you use for setting temperaturedependent property functions, as described in Section Click on the Copy button. The properties will be downloaded from the database into your local list, and your own copy of the properties will now be displayed in the Materials panel. 6. Repeat the process to copy another material, or close the Database Materials panel. Once you have copied a material from the database, you may modify its properties or change its name, as described earlier in this section. The original material in the database will not be affected by any changes you make to your local copy of the material. Creating a New Material If the material you want to use is not available in the database, you can easily create a new material for your local list. The steps for creating a new material are listed below: 1. Select the new material s type (fluid, solid, etc.) in the Material Type drop-down list. (It does not matter which material is selected in the Fluid Materials, Solid Materials, or other similarly-named list.) 2. Enter the new material s name in the Name field. 7-8 c Fluent Inc. November 28, 2001

9 7.1 Setting Physical Properties 3. Set the material s properties in the Properties area. If there are many properties listed, you may use the scroll bar to the right of the Properties area to scroll through the listed items. 4. Click on the Change/Create button. A Question dialog box will appear, asking you if the original material should be overwritten. Click on No to retain the original material and add your new material to the list. A panel will appear asking you to enter the chemical formula of your new material. Enter the formula if it is known, and click on OK; otherwise, leave the formula blank and click on OK. TheMaterials panel will be updated to show the new material name and chemical formula in the Fluid Materials list (or Solid Materials or other similarly-named list). Saving Materials and Properties All the materials and properties in your local list are saved in the case file when it is written. If you read this case file into a new solver session, all of your materials and properties will be available for use in the new session. Deleting a Material If there are materials in your local materials list that you no longer need, you can easily delete them by following these steps: 1. Select the type of material (fluid, solid, etc.) in the Material Type drop-down list. 2. Choose the material to be deleted in the Fluid Materials drop-down list, Solid Materials list, or other similarly-named list. (The list s name will be the same as the material type you selected in step 1.) 3. Click on the Delete button. Deleting materials from your local list will have no effect on the materials contained in the global database. c Fluent Inc. November 28,

10 Physical Properties Changing the Order of the Materials List By default, the materials in your local list and those in the database are listed alphabetically by name (e.g., air, atomic-oxygen (o), carbon-dioxide (co2)). If you prefer to list them alphabetically by chemical formula, select the Chemical Formula option under Order Materials By. The example materials listed above will now be ordered as follows: air, co2 (carbondioxide), o (atomic-oxygen). To change back to the alphabetical listing by name, choose the Name option under Order Materials By. Note that you may specify the ordering method separately for the Materials and Database Materials panels. For example, you can order the database materials by chemical formula and the local materials list by name. Each panel has its own Order Materials By options Defining Properties Using Temperature-Dependent Functions Material properties can be defined as functions of temperature. For most properties, you can define a polynomial, piecewise-linear, or piecewisepolynomial function of temperature: polynomial: piecewise-linear: φ(t )=A 1 + A 2 T + A 3 T (7.1-1) φ(t )=φ n + φ n+1 φ n T n+1 T n (T T n ) (7.1-2) where 1 n N and N is the number of segments piecewise-polynomial: for T min,1 <T <T max,1 : φ(t )=A 1 + A 2 T + A 3 T for T min,2 <T <T max,2 : φ(t )=B 1 + B 2 T + B 3 T (7.1-3) 7-10 c Fluent Inc. November 28, 2001

11 7.1 Setting Physical Properties! In the equations above, φ is the property. If you define a polynomial or piecewise-polynomial function of temperature, note that temperature in the function is always in units of Kelvin or Rankine. If you are using Celsius or Kelvin as your temperature unit, then polynomial coefficient values must be entered in terms of Kelvin; if you are using Fahrenheit or Rankine as the temperature unit, values must be entered in terms of Rankine. Some properties have additional functions available, and for some only a subset of these three functions can be used. See the section on the property in question to determine which temperature-dependent functions you can use. This section will describe the inputs required for definition of polynomial, piecewise-linear, and piecewise-polynomial functions. Inputs for Polynomial Functions To define a polynomial function of temperature for a material property, follow these steps: 1. In the Materials panel, choose polynomial in the drop-down list to the right of the property name (e.g., Density). The Polynomial Profile panel (Figure 7.1.3) will open automatically. (Since this is a modal panel, the solver will not allow you to do anything else until you perform the steps below.) 2. Specify the number of Coefficients. (Up to 8 coefficients are available.) The number of coefficients defines the order of the polynomial. The default of 1 defines a polynomial of order 0: the property will be constant and equal to the single coefficient A 1 ; an input of 2 defines a polynomial of order 1: the property will vary linearly with temperature; etc. 3. Define the coefficients. Coefficients 1, 2, 3,... correspond to A 1,A 2, A 3,... in Equation The panel in Figure shows the inputs for the following function: c Fluent Inc. November 28,

12 Physical Properties Figure 7.1.3: The Polynomial Profile Panel ρ(t ) = T (7.1-4)! Note the restriction on the units for temperature, as described above c Fluent Inc. November 28, 2001

13 7.1 Setting Physical Properties Inputs for Piecewise-Linear Functions To define a piecewise-linear function of temperature for a material property, follow these steps: 1. In the Materials panel, choose piecewise-linear in the drop-down list to the right of the property name (e.g., Viscosity). The Piecewise- Linear Profile panel (Figure 7.1.4) will open automatically. (Since this is a modal panel, the solver will not allow you to do anything else until you perform the steps below.) Figure 7.1.4: The Piecewise-Linear Profile Panel 2. Set the number of Points defining the piecewise distribution.! 3. Under Data Points, enter the data pairs for each point. First enter the independent and dependent variable values for Point 1, then increase the Point number and enter the appropriate values for each additional pair of variables. The pairs of points must be supplied in the proper order (by increasing value of temperature); the solver will not sort them for you. A maximum of 30 piecewise points can be defined for each property. The panel in Figure shows the final inputs for the profile depicted in Figure If the temperature exceeds the maximum Temperature (T max )you have specified for your profile, FLUENT will use the Value corresponding to T max. If the temperature falls below the minimum c Fluent Inc. November 28,

14 Physical Properties Temperature (T min ) you have specified for your profile, FLUENT will use the Value corresponding to T min. Viscosity, µ 10 (Pa-sec) (250, ) (300, ) (440, ) (360, ) Temperature, T (K) Figure 7.1.5: Piecewise-Linear Definition of Viscosity as µ(t ) Inputs for Piecewise-Polynomial Functions To define a piecewise-polynomial function of temperature for a material property, follow these steps: 1. In the Materials panel, choose piecewise-polynomial in the dropdown list to the right of the property name (e.g., Cp). The Piecewise Polynomial Profile panel (Figure 7.1.6) will open automatically. (Since this is a modal panel, the solver will not allow you to do anything else until you perform the steps below.) 2. Specify the number of Ranges. For the example of Equation 7.1-5, two ranges of temperatures are defined: for 300 <T <1000 : c p (T ) = T T T T 4 for 1000 <T <5000 : 7-14 c Fluent Inc. November 28, 2001

15 7.1 Setting Physical Properties Figure 7.1.6: The Piecewise Polynomial Profile Panel c p (T ) = T T T T 4 (7.1-5) You may define up to 3 ranges. The ranges must be supplied in the proper order (by increasing value of temperature); the solver will not sort them for you. 3. For the first range (Range = 1), specify the Minimum and Maximum temperatures, and the number of Coefficients. (Up to 8 coefficients are available.) The number of coefficients defines the order of the polynomial. The default of 1 defines a polynomial of order 0: the property will be constant and equal to the single coefficient A 1 ;an input of 2 defines a polynomial of order 1: the property will vary linearly with temperature; etc. 4. Define the coefficients. Coefficients 1, 2, 3,... correspond to A 1,A 2, A 3,... in Equation The panel in Figure shows the inputs for the first range of Equation c Fluent Inc. November 28,

16 Physical Properties 5. Increase the value of Range and enter the Minimum and Maximum temperatures, number of Coefficients, andthecoefficients (B 1,B 2, B 3,...) for the next range. Repeat if there is a third range.! Note the restriction on the units for temperature, as described above. Checking and Modifying Existing Profiles! If you want to check or change the coefficients, data pairs, or ranges for a previously-defined profile, click on the Edit... button to the right of the property name. The appropriate panel will open, and you can check or modify the inputs as desired. In the Database Materials panel, you cannot edit the profiles, but you can examine them by clicking on the View... button (instead of the Edit... button.) Customizing the Materials Database The materials database is located in the following file: path/fluent.inc/fluent6. x/cortex/lib/propdb.scm where path is the directory in which you have installed FLUENT and the variable x corresponds to your release version, e.g., 0 for fluent6.0. If you wish to enhance the materials database to include additional materials that you use frequently, you can follow the procedure below: 1. Copy the file propdb.scm from the directory listed above to your working directory. 2. Using a text editor, add the desired material(s) following the format for the other materials. You may want to copy an existing material that is similar to the one you want to add, and then change its name and properties. The entries for air and aluminum are shown below as examples: 7-16 c Fluent Inc. November 28, 2001

17 7.1 Setting Physical Properties (air fluid (chemical-formula. #f) (density (constant ) (premixed-combustion )) (specific-heat (constant )) (thermal-conductivity (constant )) (viscosity (constant e-05) (sutherland e ) (power-law e )) (molecular-weight (constant )) ) (aluminum (solid) (chemical-formula. al) (density (constant. 2719)) (specific-heat (constant. 871)) (thermal-conductivity (constant )) (formation-entropy (constant )) ) When you next load the materials database in a FLUENT session started in the working directory, FLUENT will load your modified propdb.scm, rather than the original database, and your custom materials will be available in the Database Materials panel. If you want to make the modified database available to others at your site, you can put your customized propdb.scm file in the cortex/lib directory, replacing the default database. Before doing so, you should save the original propdb.scm file provided by Fluent Inc. to a different name (e.g., propdb.scm.orig) in case you need it at a later time. c Fluent Inc. November 28,

18 Physical Properties 7.2 Density FLUENT provides several options for definition of the fluid density: constant density temperature- and/or composition-dependent density Each of these input options and the governing physical models are detailed in this section. In all cases, you will define the Density in the Materials panel. Define Materials Defining Density for Various Flow Regimes The selection of density in FLUENT is very important. You should set the density relationship based on your flow regime. For compressible flows, the ideal gas law is the appropriate density relationship. For incompressible flows, you may choose one of the following methods: Constant density should be used if you do not want density to be a function of temperature. The incompressible ideal gas law should be used when pressure variations are small enough that the flow is fully incompressible but you wish to use the ideal gas law to express the relationship between density and temperature (e.g., for a natural convection problem). Density as a polynomial, piecewise-linear, or piecewise-polynomial function of temperature is appropriate when the density is a function of temperature only, as in a natural convection problem. The Boussinesq model can be used for natural convection problems involving small changes in temperature c Fluent Inc. November 28, 2001

19 7.2 Density Mixing Density Relationships in Multiple-Zone Models If your model has multiple fluid zones that use different materials, you should be aware of the following: For calculations with the segregated solver that do not use one of the general multiphase models, the compressible ideal gas law cannot be mixed with any other density methods. This means that if the compressible ideal gas law is used for one material, it must be used for all materials. This restriction does not apply to the coupled solvers. There is only one specified operating pressure and one specified operating temperature. This means that if you are using the ideal gas law for more than one material, they will share the same operating pressure; and if you are using the Boussinesq model for more than one material, they will share the same operating temperature Input of Constant Density If you want to define the density of your fluid as a constant, check that constant is selected in the drop-down list to the right of Density in the Materials panel, and enter the value of density for the material. For the default fluid (air), the density is kg/m Inputs for the Boussinesq Approximation To enable the Boussinesq approximation for density, choose boussinesq from the drop-down list to the right of Density in the Materials panel and specify a constant value for Density. You will also need to set the Thermal Expansion Coefficient, as well as relevant operating conditions, as described in Section Density as a Profile Function of Temperature If you are modeling a problem that involves heat transfer, you can define the density as a function of temperature. Three types of functions are available: c Fluent Inc. November 28,

20 Physical Properties piecewise-linear: piecewise-polynomial: ρ(t )=ρ n + ρ n+1 ρ n T n+1 T n (T T n ) (7.2-1) for T min,1 <T <T max,1 : ρ(t )=A 1 + A 2 T + A 3 T (7.2-2) for T min,2 <T <T max,2 : ρ(t )=B 1 + B 2 T + B 3 T (7.2-3) polynomial: ρ(t )=A 1 + A 2 T + A 3 T (7.2-4) For one of the these methods, select piecewise-linear, piecewise-polynomial, or polynomial in the drop-down list to the right of Density. You can input the data pairs (T n,ρ n ), ranges and coefficients, or coefficients that describe these functions using the Materials panel, as described in Section Incompressible Ideal Gas Law In FLUENT, if you choose to define the density using the ideal gas law for an incompressible flow, the solver will compute the density as ρ = p op R M w T (7.2-5) where R is the universal gas constant, M w is the molecular weight of the gas, and p op is defined by you as the Operating Pressure in the Operating Conditions panel. In this form, the density depends only on the operating pressure and not on the local relative pressure field c Fluent Inc. November 28, 2001

21 7.2 Density Density Inputs for the Incompressible Ideal Gas Law Inputs for the incompressible ideal gas law are as follows: 1. Enable the ideal gas law for an incompressible fluid by choosing incompressible-ideal-gas from the drop-down list to the right of Density in the Materials panel. You must specify the incompressible ideal gas law individually for each material that you want to use it for. See Section for information on specifying the incompressible ideal gas law for mixtures.! 2. Set the operating pressure by defining the Operating Pressure in the Operating Conditions panel. Define Operating Conditions... The input of the operating pressure is of great importance when you are computing density with the ideal gas law. See Section 7.12 for recommendations on setting appropriate values for the operating pressure. Operating pressure is, by default, set to Pa. 3. Set the molecular weight of the homogeneous or single-component fluid (if no chemical species transport equations are to be solved), or the molecular weights of each fluid material (species) in a multicomponent mixture. For each fluid material, enter the value of the Molecular Weight in the Materials panel Ideal Gas Law for Compressible Flows For compressible flows, the gas law has the following form: ρ = p op + p R M w T (7.2-6) where p is the local relative (or gauge) pressure predicted by FLUENT and p op is defined by you as the Operating Pressure in the Operating Conditions panel. c Fluent Inc. November 28,

22 Physical Properties Density Inputs for the Ideal Gas Law for Compressible Flows Inputs for the ideal gas law are as follows: 1. Enable the ideal gas law for a compressible fluid by choosing idealgas from the drop-down list to the right of Density in the Materials panel. You must specify the ideal gas law individually for each material that you want to use it for. See Section for information on specifying the ideal gas law for mixtures.! 2. Set the operating pressure by defining the Operating Pressure in the Operating Conditions panel. Define Operating Conditions... The input of the operating pressure is of great importance when you are computing density with the ideal gas law. Equation notes that the operating pressure is added to the relative pressure field computed by the solver, yielding the absolute static pressure. See Section 7.12 for recommendations on setting appropriate values for the operating pressure. Operating pressure is, by default, set to Pa. 3. Set the molecular weight of the homogeneous or single-component fluid (if no chemical species transport equations are to be solved), or the molecular weights of each fluid material (species) in a multicomponent mixture. For each fluid material, enter the value of the Molecular Weight in the Materials panel Composition-Dependent Density for Multicomponent Mixtures If you are solving species transport equations, you will need to set properties for the mixture material and for the constituent fluids (species), as described in detail in Section To define a composition-dependent density for a mixture, follow these steps: 1. Select the density method: 7-22 c Fluent Inc. November 28, 2001

23 7.2 Density For non-ideal-gas mixtures, select the volume-weighted-mixinglaw method for the mixture material in the drop-down list to the right of Density in the Materials panel. If you are modeling compressible flow, select ideal-gas for the mixture material in the drop-down list to the right of Density in the Materials panel. If you are modeling incompressible flow using the ideal gas law, select incompressible-ideal-gas for the mixture material in the drop-down list to the right of Density in the Materials panel. 2. Click on Change/Create. 3. If you selected volume-weighted-mixing-law, define the density for each of the fluid materials that comprise the mixture. You may define constant or (if applicable) temperature-dependent densities for the individual species. If you are modeling a non-ideal-gas mixture, FLUENT will compute the mixture density as ρ = 1 i Y i ρ i (7.2-7) where Y i is the mass fraction and ρ i is the density of species i. For compressible flows, the gas law has the following form: ρ = p op + p RT Y i (7.2-8) i M w,i where p is the local relative (or gauge) pressure predicted by FLUENT, R is the universal gas constant, Y i is the mass fraction of species i, M w,i is the molecular weight of species i, andp op is defined by you as the Operating Pressure in the Operating Conditions panel. In FLUENT, if you choose to define the density using the ideal gas law for an incompressible flow, the solver will compute the density as c Fluent Inc. November 28,

24 Physical Properties ρ = p op RT i Y i (7.2-9) M w,i where R is the universal gas constant, Y i is the mass fraction of species i, M w,i is the molecular weight of species i, andp op is defined by you as the Operating Pressure in the Operating Conditions panel. In this form, the density depends only on the operating pressure and not on the local relative pressure field. 7.3 Viscosity FLUENT provides several options for definition of the fluid viscosity: Constant viscosity Temperature- and/or composition-dependent viscosity Kinetic theory Non-Newtonian viscosity User-defined function Each of these input options and the governing physical models are detailed in this section. (User-defined functions are described in the separate UDF Manual.) In all cases, you will define the Viscosity in the Materials panel. Define Materials... Viscosities are input as dynamic viscosity (µ) in units of kg/m-s in SI units or lb m /ft-s in British units. FLUENT does not ask for input of the kinematic viscosity (ν) Input of Constant Viscosity If you want to define the viscosity of your fluid as a constant, check that constant is selected in the drop-down list to the right of Viscosity in the Materials panel, and enter the value of viscosity for the fluid. For the default fluid (air), the viscosity is kg/m-s c Fluent Inc. November 28, 2001

25 7.3 Viscosity Viscosity as a Function of Temperature If you are modeling a problem that involves heat transfer, you can define the viscosity as a function of temperature. Five types of functions are available: piecewise-linear: piecewise-polynomial: µ(t )=µ n + µ n+1 µ n T n+1 T n (T T n ) (7.3-1) for T min,1 <T <T max,1 : µ(t )=A 1 + A 2 T + A 3 T (7.3-2) for T min,2 <T <T max,2 : µ(t )=B 1 + B 2 T + B 3 T (7.3-3) polynomial: Sutherland s law (described below) Power law (described below) µ(t )=A 1 + A 2 T + A 3 T (7.3-4)! Note that the power law described below is different from the non- Newtonian power law described in Section For one of the first three, select piecewise-linear, piecewise-polynomial, polynomial in the drop-down list to the right of Viscosity, and then input the data pairs (T n,µ n ), ranges and coefficients, or coefficients that describe these functions using the Materials panel, as described in Section For Sutherland s law or the power law, choose sutherland or power-law in the drop-down list and enter the parameters as described below. c Fluent Inc. November 28,

26 Physical Properties Sutherland Viscosity Law Sutherland s viscosity law resulted from a kinetic theory by Sutherland (1893) using an idealized intermolecular-force potential. The formula is specified using two or three coefficients. Sutherland s law with two coefficients has the form µ = C 1T 3/2 T + C 2 (7.3-5) where µ is the viscosity in kg/m-s, T is the static temperature in K, and C 1 and C 2 are the coefficients. For air at moderate temperatures and pressures, C 1 = kg/m-s-k 1/2,andC 2 = K. Sutherland s law with three coefficients has the form µ = µ 0 ( T T 0 ) 3/2 T 0 + S T + S (7.3-6) where µ is the viscosity in kg/m-s, T is the static temperature in K, µ 0 is a reference value in kg/m-s, T 0 is a reference temperature in K, and S is an effective temperature in K, called the Sutherland constant, which is characteristic of the gas. For air at moderate temperatures and pressures, µ 0 = kg/m-s, T 0 = K, and S = K. Inputs for Sutherland s Law To use Sutherland s law, choose sutherland in the drop-down list to the right of Viscosity. TheSutherland Law panel will open, and you can enter the coefficients as follows:! 1. Select the Two Coefficient Method or the Three Coefficient Method. Note that you must use SI units if you choose the two-coefficient method. 2. For the Two Coefficient Method, set C1 and C2. For the Three Coefficient Method, set the Reference Viscosity µ 0, the Reference Temperature T 0,andtheEffective Temperature S c Fluent Inc. November 28, 2001

27 7.3 Viscosity Power-Law Viscosity Law Another common approximation for the viscosity of dilute gases is the power-law form. For dilute gases at moderate temperatures, this form is considered to be slightly less accurate than Sutherland s law. A power-law viscosity law with two coefficients has the form µ = BT n (7.3-7) where µ is the viscosity in kg/m-s, T is the static temperature in K, and B is a dimensional coefficient. For air at moderate temperatures and pressures, B = ,andn =2/3. A power-law viscosity law with three coefficients has the form µ = µ 0 ( T T 0 ) n (7.3-8)! where µ is the viscosity in kg/m-s, T is the static temperature in K, T 0 is a reference value in K, µ 0 is a reference value in kg/m-s. For air at moderate temperatures and pressures, µ 0 = kg/m-s, T 0 = 273 K, and n =2/3. The non-newtonian power law for viscosity is described in Section Inputs for the Power Law To use the power law, choose power-law in the drop-down list to the right of Viscosity. ThePower Law panel will open, and you can enter the coefficients as follows:! 1. Select the Two Coefficient Method or the Three Coefficient Method. Note that you must use SI units if you choose the two-coefficient method. 2. For the Two Coefficient Method, set B and the Temperature Exponent n. FortheThree Coefficient Method, set the Reference Viscosity c Fluent Inc. November 28,

28 Physical Properties µ 0,theReference Temperature T 0,andtheTemperature Exponent n Defining the Viscosity Using Kinetic Theory If you are using the gas law (as described in Section 7.2), you have the option to define the fluid viscosity using kinetic theory as µ = Mw T σ 2 Ω µ (7.3-9) where µ is in units of kg/m-s, T is in units of Kelvin, σ is in units of Angstroms, and Ω µ =Ω µ (T )where T = T (ɛ/k B ) (7.3-10) The Lennard-Jones parameters, σ and ɛ/k B, are inputs to the kinetic theory calculation that you supply by selecting kinetic-theory from the drop-down list to the right of Viscosity in the Materials panel. The solver will use these kinetic theory inputs in Equation to compute the fluid viscosity. See Section 7.11 for details about these inputs Composition-Dependent Viscosity for Multicomponent Mixtures If you are modeling a flow that includes more than one chemical species (multicomponent flow), you have the option to define a compositiondependent viscosity. (Note that you can also define the viscosity of the mixture as a constant value or a function of temperature.) To define a composition-dependent viscosity for a mixture, follow these steps: 1. For the mixture material, choose mass-weighted-mixing-law or, if you are using the ideal gas law for density, ideal-gas-mixing-law in the drop-down list to the right of Viscosity c Fluent Inc. November 28, 2001

29 7.3 Viscosity 2. Click Change/Create. 3. Define the viscosity for each of the fluid materials that comprise the mixture. You may define constant or (if applicable) temperaturedependent viscosities for the individual species. You may also use kinetic theory for the individual viscosities, or specify a non- Newtonian viscosity, if applicable. If you are using the ideal gas law, the solver will compute the mixture viscosity based on kinetic theory as µ = i X i µ i j X iφ ij (7.3-11) where φ ij = [ ) 1+( µi 1/2 ( ) ] Mw,j 1/4 2 µ j M w,i [ ( 8 1+ M )] 1/2 (7.3-12) w,i M w,j and X i is the mole fraction of species i. For non-ideal gas mixtures, the mixture viscosity is computed based on a simple mass fraction average of the pure species viscosities: µ = i Y i µ i (7.3-13) c Fluent Inc. November 28,

30 Physical Properties Viscosity for Non-Newtonian Fluids For incompressible Newtonian fluids, the shear stress is proportional to the rate-of-deformation tensor D: where D is defined by τ = µd (7.3-14) D = ( uj + u ) i x i x j (7.3-15) and µ is the viscosity, which is independent of D. For some non-newtonian fluids, the shear stress can similarly be written in terms of a non-newtonian viscosity η: ( ) τ = η D D (7.3-16) In general, η is a function of all three invariants of the rate-of-deformation tensor D. However, in the non-newtonian models available in FLUENT, η is considered to be a function of the shear rate γ only. γ is related to the second invariant of D andisdefinedas γ = D : D (7.3-17) FLUENT provides four options for modeling non-newtonian flows: Power law Carreau model for pseudo-plastics Cross model Herschel-Bulkley model for Bingham plastics 7-30 c Fluent Inc. November 28, 2001

31 7.3 Viscosity! Note that the non-newtonian power law described below is different from the power law described in Section Appropriate values for the input parameters for these models can be found in the literature (e.g., [238]). Power Law for Non-Newtonian Viscosity If you choose non-newtonian-power-law in the drop-down list to the right of Viscosity, non-newtonian flow will be modeled according to the following power law for the non-newtonian viscosity: η = k γ n 1 e T 0/T (7.3-18) FLUENT allows you to place upper and lower limits on the power law function, yielding the following equation: η min <η= k γ n 1 e T 0/T <η max (7.3-19) where k, n, T 0, η min,andη max are input parameters. k is a measure of the average viscosity of the fluid (the consistency index); n is a measure of the deviation of the fluid from Newtonian (the power-law index), as described below; T 0 is the reference temperature; and η min and η max are, respectively, the lower and upper limits of the power law. If the viscosity computed from the power law is less than η min,thevalueofη min will be used instead. Similarly, if the computed viscosity is greater than η max, the value of η max will be used instead. Figure shows how viscosity is limited by η min and η max at low and high shear rates. The value of n determines the class of the fluid: n =1 Newtonian fluid n>1 shear-thickening (dilatant fluids) n<1 shear-thinning (pseudo-plastics) Inputs for the Non-Newtonian Power Law To use the non-newtonian power law, choose non-newtonian-power-law in the drop-down list to the right of Viscosity. TheNon-Newtonian Power c Fluent Inc. November 28,

32 Physical Properties η max log η η min. log γ Figure 7.3.1: Variation of Viscosity with Shear Rate According to the Non-Newtonian Power Law Law panel will open, and you can enter the Consistency Index k, Power- Law Index n, Reference Temperature T 0, Minimum Viscosity Limit η min,and Maximum Viscosity Limit η max. For temperature-independent viscosity, the value of T 0 should be set to zero. If the energy equation is not being solved, FLUENT uses a default value of T =273 K in Equation The Carreau Model for Pseudo-Plastics The power law model described in Equation results in a fluid viscosity that varies with shear rate. For γ 0, η η 0,andfor γ, η η,whereη 0 and η are, respectively, the upper and lower limiting values of the fluid viscosity. The Carreau model attempts to describe a wide range of fluids by the establishment of a curve-fit to piece together functions for both Newtonian and shear-thinning (n <1) non-newtonian laws. In the Carreau model, the viscosity is η = η +(η 0 η )[1 + ( γλe T 0/T ) 2 ] (n 1)/2 (7.3-20) where the parameters n, λ, T 0, η 0, and η are dependent upon the fluid. λ is the time constant, n is the power-law index (as described 7-32 c Fluent Inc. November 28, 2001

33 7.3 Viscosity above for the non-newtonian power law), T 0 is the reference temperature, and η 0 and η are, respectively, the zero- and infinite-shear viscosities. Figure shows how viscosity is limited by η 0 and η at low and high shear rates. η 0 log η. log γ η Figure 7.3.2: Variation of Viscosity with Shear Rate According to the Carreau Model Inputs for the Carreau Model To use the Carreau model, choose carreau in the drop-down list to the right of Viscosity. TheCarreau Model panel will open, and you can enter the Time Constant λ, Power-Law Index n, Reference Temperature T 0, Zero Shear Viscosity η 0, and Infinite Shear Viscosity η. For temperatureindependent viscosity, the value of T 0 should be set to zero. If the energy equation is not being solved, FLUENT uses a default value of T =273 K in Equation Cross Model The Cross model for viscosity is η = η 0 1+(λ γ) 1 n (7.3-21) c Fluent Inc. November 28,

34 Physical Properties where η 0 = zero-shear-rate viscosity λ = natural time (i.e., inverse of the shear rate at which the fluid changes from Newtonian to power-law behavior) n = power-law index The Cross model is commonly used when it is necessary to describe the low-shear-rate behavior of the viscosity. Inputs for the Cross Model To use the Cross model, choose cross in the drop-down list to the right of Viscosity. TheCross Model panel will open, and you can enter the Time Constant λ, Power-Law Index n, andzero Shear Viscosity η 0. Herschel-Bulkley Model for Bingham Plastics The power law model described above is valid for fluids for which the shear stress is zero when the strain rate is zero. Bingham plastics are characterized by a non-zero shear stress when the strain rate is zero: where τ 0 is the yield stress: τ = τ 0 + ηd (7.3-22) For τ<τ 0, the material remains rigid. For τ>τ 0, the material flows as a power-law fluid. The Herschel-Bulkley model combines the effects of Bingham and powerlaw behavior in a fluid. For low strain rates ( γ <τ 0 /µ 0 ), the rigid material acts like a very viscous fluid with viscosity µ 0. As the strain rate increases and the yield stress threshold, τ 0, is passed, the fluid behavior is described by a power law. η = τ 0 + k[ γ n (τ 0 /µ 0 ) n ] γ (7.3-23) 7-34 c Fluent Inc. November 28, 2001

35 7.3 Viscosity where k is the consistency factor, and n is the power-law index. Figure shows how shear stress (τ) varies with shear rate ( γ) forthe Herschel-Bulkley model. n > 1 Bingham (n=1) 0 < n < 1 τ τ 0.. γ 0 Figure 7.3.3: Variation of Shear Stress with Shear Rate According to the Herschel-Bulkley Model γ If you choose the Herschel-Bulkley model for Bingham plastics, Equation will be used to determine the fluid viscosity. The Herschel-Bulkley model is commonly used to describe materials such as concrete, mud, dough, and toothpaste, for which a constant viscosity after a critical shear stress is a reasonable assumption. In addition to the transition behavior between a flow and no-flow regime, the Herschel- Bulkley model can also exhibit a shear-thinning or shear-thickening behavior depending on the value of n. Inputs for the Herschel-Bulkley Model To use the Herschel-Bulkley model, choose herschel-bulkley in the dropdown list to the right of Viscosity. TheHerschel-Bulkley panel will open, and you can enter the Consistency Index k, Power-Law Index n, Yield Stress Threshold τ 0,andYielding Viscosity µ 0. c Fluent Inc. November 28,

36 Physical Properties 7.4 Thermal Conductivity The thermal conductivity must be defined when heat transfer is active. You will need to define thermal conductivity when you are modeling energy and viscous flow. FLUENT provides several options for definition of the thermal conductivity: Constant thermal conductivity Temperature- and/or composition-dependent thermal conductivity Kinetic theory User-defined function Anisotropic/orthotropic (for solid materials only) Each of these input options and the governing physical models are detailed in this section. (User-defined functions are described in the separate UDF Manual.) In all cases, you will define the Thermal Conductivity in the Materials panel. Define Materials... Thermal conductivity is defined in units of W/m-K in SI units or BTU/hr-ft-R in British units Input of Constant Thermal Conductivity If you want to define the thermal conductivity as a constant, check that constant is selected in the drop-down list to the right of Thermal Conductivity in the Materials panel, and enter the value of thermal conductivity for the material. For the default fluid (air), the thermal conductivity is W/m-K c Fluent Inc. November 28, 2001

37 7.4 Thermal Conductivity Thermal Conductivity as a Function of Temperature You can also choose to define the thermal conductivity as a function of temperature. Three types of functions are available: piecewise-linear: piecewise-polynomial: k(t )=k n + k n+1 k n T n+1 T n (T T n ) (7.4-1) for T min,1 <T <T max,1 : k(t )=A 1 + A 2 T + A 3 T (7.4-2) for T min,2 <T <T max,2 : k(t )=B 1 + B 2 T + B 3 T (7.4-3) polynomial: k(t )=A 1 + A 2 T + A 3 T (7.4-4) You can input the data pairs (T n,k n ), ranges and coefficients A i and B i, or coefficients A i that describe these functions using the Materials panel, as described in Section Defining the Thermal Conductivity Using Kinetic Theory If you are using the gas law (as described in Section 7.2), you have the option to define the thermal conductivity using kinetic theory as k = 15 4 [ R 4 µ M w 15 c p M w R + 1 ] 3 (7.4-5) where R is the universal gas constant, M w is the molecular weight, µ is the material s specified or computed viscosity, and c p is the material s specified or computed specific heat capacity. c Fluent Inc. November 28,

38 Physical Properties To enable the use of this equation for calculating thermal conductivity, select kinetic-theory from the drop-down list to the right of Thermal Conductivity in the Materials panel. The solver will use Equation to compute the thermal conductivity Composition-Dependent Thermal Conductivity for Multicomponent Mixtures If you are modeling a flow that includes more than one chemical species (multicomponent flow), you have the option to define a compositiondependent thermal conductivity. (Note that you can also define the thermal conductivity of the mixture as a constant value or a function of temperature, or using kinetic theory.) To define a composition-dependent thermal conductivity for a mixture, follow these steps:! 1. For the mixture material, choose mass-weighted-mixing-law or, if you are using the ideal gas law, ideal-gas-mixing-law in the dropdown list to the right of Thermal Conductivity. If you use ideal-gas-mixing-law for the thermal conductivity of a mixture, you must use ideal-gas-mixing-law or mass-weighted-mixinglaw for viscosity, because these two viscosity specification methods are the only ones that allow specification of the component viscosities, which are used in the ideal gas law for thermal conductivity (Equation 7.4-6). 2. Click Change/Create. 3. Define the thermal conductivity for each of the fluid materials that comprise the mixture. You may define constant or (if applicable) temperature-dependent thermal conductivities for the individual species. You may also use kinetic theory for the individual thermal conductivities, if applicable. If you are using the ideal gas law, the solver will compute the mixture thermal conductivity based on kinetic theory as 7-38 c Fluent Inc. November 28, 2001

39 7.4 Thermal Conductivity k = i X i k i j X jφ ij (7.4-6) where φ ij = [ ) 1+( µi 1/2 ( ) ] Mw,j 1/4 2 µ j M w,i [ ( 8 1+ M )] w,i 1/2 (7.4-7) M w,j and X i is the mole fraction of species i. For non-ideal gases, the mixture thermal conductivity is computed based on a simple mass fraction average of the pure species conductivities: k = i Y i k i (7.4-8) Anisotropic Thermal Conductivity for Solids The anisotropic conductivity option in FLUENT solves the conduction equation in solids with the thermal conductivity specified as a matrix. The heat flux vector is written as q i = k ij T x j (7.4-9)! Two options are available: orthotropic and general anisotropic. Note that the anisotropic conductivity options are available only with the segregated solver; you cannot use them with the coupled solvers. Orthotropic Thermal Conductivity When the orthotropic thermal conductivity is used, the thermal conductivities (k ξ,k η,k ζ ) in the principal directions (ê ξ, ê η, ê ζ ) are specified. The conductivity matrix is then computed as c Fluent Inc. November 28,