Multiple Species Chapter 7

Training Manual May 15, 2001 Inventory # Objectives The FLOTRAN multiple species capabilities. Dilute mixture option. Composite mixture option. Composite gas option. The steps for a multiple species transport analysis. Solve for the mixture of 3 gases.

Training Manual May 15, 2001 Inventory # Overview The multiple species transport allows you to track up to 6 different fluids at once. A single momentum equation is solved for the flow field. Transport equations are solved for each species mass fraction. The mass fractions must add up to 1.0. Advection and diffusion effects are modeled.

Training Manual May 15, 2001 Inventory # Overview (continued) The multiple species equations (in indicial notation): In these equations, Yi is the mass fraction for the ith species and Di is the mass diffusion coefficient (with units of length2/time) for the ith specie in relation to the bulk mixture. Note: To ensure the sum of N specie mass fractions at any location is 1, only N-1 equations are actually solved.

Training Manual May 15, 2001 Inventory # Overview (continued) The properties used in the momentum equation can be any of the following: –Dilute mixture: a “main” or “bulk” fluid (specified in the Fluid Properties menu) –A combination of component fluids: Composite mixture Composite gas

Training Manual May 15, 2001 Inventory # Overview (continued) Limitations include: –No chemical reactions –No combustion –No “settling out” of a fluid or deposition of particles

Training Manual May 15, 2001 Inventory # Dilute Mixture Analysis Small mass fractions of species fluids are tracked in a flow field. Species properties do not significantly influence the flow field. Transport equations use the bulk density and velocities from the bulk fluid analysis.

Training Manual May 15, 2001 Inventory # Dilute Mixture Analysis (continued) If the analysis is isothermal, then converge the flow field prior to activating the species transport option. Species input and boundary conditions can be entered at any time prior to activating the species transport option.

Training Manual May 15, 2001 Inventory # Dilute Mixture Analysis (continued) To activate multiple species transport, use one of the following: –Command: FLDA, SOLU, SPEC, T –Main Menu > Preprocessor > FLOTRAN Setup > Solution Options –Main Menu > Solution > FLOTRAN Setup > Solution Options

Training Manual May 15, 2001 Inventory # (Note: the MSNOMF command has no GUI menu equivalent.) Composite Mixture Analysis Properties (density, viscosity and/or conductivity) are calculated from a linear combination of the species, weighted by mass fraction as a function of space. The momentum and transport equations are strongly coupled. Initialize the species mass fraction using either the IC or MSNOMF commands.

Training Manual May 15, 2001 Inventory # Composite Mixture Analysis (continued) Activate the multiple species transport (see above). To activate the composite mixture option for an individual property, use one of the following: –Command: FLDA, PROT, label, CMIX –Main Menu > Preprocessor > FLOTRAN Setup > Fluid Properties –Main Menu > Solution > FLOTRAN Setup > Fluid Properties

Training Manual May 15, 2001 Inventory # R = the Universal Gas Constant Mi = molecular weight of the ith specie N = total number of species defined P = pressure T = absolute temperature Note: the composite gas option applies only for density. Composite Gas Analysis The fluid density is calculated at each node as a function of the mass fractions and molecular weights of the gases:

Training Manual May 15, 2001 Inventory # Composite Gas Analysis (continued) Activate the multiple species transport (see above). To activate the composite gas option for the bulk fluid, use one of the following: –Command: FLDA, PROT, DENS, CGAS –Main Menu > Preprocessor > FLOTRAN Setup > Fluid Properties –Main Menu > Solution > FLOTRAN Setup > Fluid Properties

Training Manual May 15, 2001 Inventory # The procedure for a multiple species analysis has eight steps. Note that the Multiple Species menus are only available in the Preprocessor. 1. Establish the number of species and assign species names. Default names for species are SP01, SP02, …, SP06. Species names are required for assigning boundary conditions, postprocessing, etc. Performing a Multiple Species Analysis

Training Manual May 15, 2001 Inventory # Performing a Multiple Species Analysis (continued) 2. Designate an algebraic specie. The transport equation for the algebraic specie is not solved. The mass fraction as a function of space is calculated by subtracting the mass fraction of the other N-1 species from Adjust output format if desired. You can set the format of the convergence monitor to list columns of values for all the degrees of freedom for a single iteration.

Training Manual May 15, 2001 Inventory # Performing a Multiple Species Analysis (continued) The example below used the command: FLDA, CONV, OUTP, LAND

Training Manual May 15, 2001 Inventory # Define species properties. –Available property options include, CONSTANT, LIQUID, GAS, CMIX, CGAS. –Enter the mass diffusion coefficient for each specie. where D mi = mass diffusion coefficient (L 2 /T) Y i = mass fraction of the specie –Sometimes the mass diffusion is expressed as a function of the Schmidt number. Performing a Multiple Species Analysis (continued)

Training Manual May 15, 2001 Inventory # Performing a Multiple Species Analysis (continued) Note: You must input the mass diffusion coefficient - NOT the Schmidt number.

Training Manual May 15, 2001 Inventory # If the flow field is turbulent, then the turbulent Schmidt number must be input. –The turbulent Schmidt number Scti is input. –The default value, Scti=1.0, is generally sufficient. Performing a Multiple Species Analysis (continued)

Training Manual May 15, 2001 Inventory # Specify boundary conditions. –The default boundary condition is –The mass fraction may be specified at a boundary; for example, at the inlet D, SP03, all, 0.25 –Remember that the mass fractions must be less than or equal to one, and that the sum of all specie mass fractions at any point in the problem domain must equal 1.0. Performing a Multiple Species Analysis (continued)

Training Manual May 15, 2001 Inventory # Set relaxation and solution parameters. –The default relaxation factors are 0.5. –Default values are usually fine. –Change solvers as (only if) required. –Turn capping on, 0 <= Y i <= Run the analysis. 8.Postprocess the results. –Note: specie names are not available in the GUI. –Use PLNS, specie name to plot Y i –Use PRNS, specie name to list Y i Performing a Multiple Species Analysis (continued)

Training Manual May 15, 2001 Inventory # Run this problem three different ways: 1. As a dilute mixture (use AIR-SI). 2. As a composite mixture. 3. As a composite gas. Are there any significant differences in the results? Why or why not? Input File: species Exercise Oxygen, Hydrogen and Nitrogen enter an insulated mixing chamber (shown below) with the specified temperatures and velocities. What is the concentration of each gas at the chamber outlet?

Training Manual May 15, 2001 Inventory # Exercise (continued)