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!

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


<Pre\>

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:

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)

Sorry, I am an idiot.

Pascale was right. It is a source of 1.0 for volumetric sources and density for specific. Sorry.

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

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

Thanks a lot, Pascale,

I will try as you suggested.

Jim