# Define decay constant of an additional variable

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 January 29, 2004, 20:15 Define decay constant of an additional variable #1 jim Guest   Posts: n/a Hi, Does anyone know how to define a decay constant for a additional variabe in cfx5.6? I am trying to simulate the transport of a chemical which can self-decay in the reactor(decay at a constant rate). I set this chemical as an additional variable, but do not know where to set the decay rate. Could you please give me some suggestions on how to start such a simulation? Thanks!

 January 30, 2004, 03:50 Re: Define decay constant of an additional variabl #2 Pascale Fonteijn Guest   Posts: n/a You should define this as a negative source term, which should be a function of the residence time (= age). Thus, first you have to define an additional additional variable called 'age' (see below). Then, you have to define a subdomain, in which you can define the source terms. The decay rate of zero order will be something like -constant*age. You should do this in Pre. Pascale LIBRARY: ADDITIONAL VARIABLE: age Option = Definition Units = [ s ] Variable Type = Volumetric END # ADDITIONAL VARIABLE: AGE END # LIBRARY: DOMAIN: Domain 1 FLUID MODELS: ADDITIONAL VARIABLE: AGE Option = Transport Equation END # ADDITIONAL VARIABLE: AGE END # FLUID MODELS: END # DOMAINomain 1 END # FLOW: FLOW: DOMAIN: Domain 1 SUBDOMAIN: Subdomain 1 Coord Frame = Coord 0 Location = Assembly 1 SOURCES: EQUATION SOURCE: AGE Option = Source Source = 1 [s s^-1] END # EQUATION SOURCE:AGE END # SOURCES: END # SUBDOMAIN:Subdomain 1 END # DOMAINomain 1 END # FLOW: FLOW: SOLVER CONTROL: EQUATION CLASS: av ADVECTION SCHEME: Option = High Resolution END # ADVECTION SCHEME: CONVERGENCE CONTROL: Physical Timescale = 100 [s] Timescale Control = Physical Timescale END # CONVERGENCE CONTROL: END # EQUATION CLASS:av END # SOLVER CONTROL: END # FLOW:

 January 30, 2004, 04:16 Re: Define decay constant of an additional variabl #3 jon Guest   Posts: n/a You could make the assumption that its a first order reaction and solve the ODE to get concentration in the form of: c(t)=c0*exp(-k*t) The constany k, calculated will be the constant in Pascale's above message. By the way, I think that the above source should be set to density of fluid for a volumetric AV or 1.0 for a specific source. Above you have 1.0 for a volumetric source (which is fine for fluid density=1.0)

 January 30, 2004, 07:52 Re: Define decay constant of an additional variabl #4 jon Guest   Posts: n/a

 January 30, 2004, 07:54 Re: Define decay constant of an additional variabl #5 jon Guest   Posts: n/a Sorry, I am an idiot. Pascale was right. It is a source of 1.0 for volumetric sources and density for specific. Sorry.

 February 2, 2004, 12:14 Re: Define decay constant of an additional variabl #6 jim Guest   Posts: n/a Hi,Pascale and jon Sorry for the late reply! Thank you so much for your comments. Those are very helpful for me. I will try those today. I have two questions regarding to your comments: 1. In Pascale's post,source term was defined as: Source = 1 [s s^-1] I am wondering if the unit should be s^-1? 2. Another question is: It seems that modelling the first order decay is much more easier that modeling other orders. However, some of my decay kinetics are second order or half order. Can I directly set input in cfx-pre? if no, how to do it? Thanks again! Best wishes! Jim

 February 5, 2004, 17:15 Re: Define decay constant of an additional variabl #7 Pascale Fonteijn Guest   Posts: n/a 1) I copied it from CFX and it works fine for me. 2) Sorry, I do not see why it is easier to use 1st order than 2nd order. In 1st order your source term contains k1*C, in 2nd order it contains k2*C^2. You have to change the units of your constants k1 or k2 to obtain kmol/m^3/s. Pascale

 February 8, 2004, 22:43 Re: Define decay constant of an additional variabl #8 jim Guest   Posts: n/a Thanks a lot, Pascale, I will try as you suggested. Jim

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