
[Sponsors] 
February 28, 2008, 13:19 
URGENT HELP NEEDEDSPECIESPLEASE!!

#1 
Guest
Posts: n/a

Hi Let me define the problem first 
I have a 2D rectangular domain.One side I have Cathode, Opposite side I have Anode. Species Wall surface reaction at Cathode is X + e <> Y, and at Anode is Y <> X + e. Using Fickcian law of diffusion, I know concentration gradient at Cathode/Anode by the formula  > Conc Grad = i_current/(n_noofelec*F_faradayconst*area_ofelectrode*diffus ioncoeff) Hence knowing concentration gradient of the species at electrodes, and also I know the initial concentration of the species in the medium. I WANT TO FIND THE CONCENTRATION DISTRIBUTION of the species IN THE WHOLE DOMAIN. I tried to set it up in Fluent as follows, but I am getting the same concentration in the whole domain. Procedure followed: [1] I have defined 1 scalar with 'flux function = none'. [2] compiled and loaded the DEFINE_PROFILE udf to calculate the conc gradient at electrode surfaces. [3] Turned on Species panelwall surface reaction. [4] I have X and Y as 'selected site species', and the medium in the domain as 'selected species'. [5] I have set up 2 reactions in reactions panel  (with wall surface option) X to Y (for cathode) and Y to X (for anode). [6] I have defined the properties of species X and Y and the medium. [7] In BC panel  a> for UDS tab , I have UDS BC = 'specified flux' and corresponding UDS BV = 'udf'(that I wrote) , for both the cathode and anode wall condition. Questions [1] how do I give the initial conc of the species during Initialization , which will be a value like 'x mol/cubic meter'?? [2] PLEASE tell me all possible points where I am going wrong.. My udf is as follows  #include"udf.h" #define Faraday_const 96485 /* value is in coulomb/mole */ #define Diffusion_coeff 2.4e9 /* diffusion coefficient or diffusivity is in sq.meters/second */ DEFINE_PROFILE(cathode_conc_gradient,t,i) { face_t f; real curr_i = 0.05; /* current is in Amperes */ real area_a = 0.0002; /* area of cathode is in sq. meters */ real num_n = 1; /* it is the number of electrons taking part in the reaction : O + ne <> R */ /** Applying Fick's Law of Diffusion for the electrode reaction O + ne <> R using the relation : i/nFA = D[conc gradient @ electrode surface], where i = current F = Faraday Constant A = crosssectional area of the electrode D = diffusion coefficient or diffusvity here we are finding the concentration gradient @ the electrode*/ real value = curr_i/(num_n*Faraday_const*area_a*Diffusion_coeff); begin_f_loop(f,t) { F_PROFILE(f,t,i) = value; /* used to set a boundary condition value in memory for a given face and thread */ } end_f_loop(f,t) } DEFINE_PROFILE(anode_conc_gradient,t,i) { face_t f; real curr_i = 0.05; /* current is in Amperes */ real area_a = 0.002; /* area of cathode is in sq. meters */ real num_n = 1; /* it is the number of electrons taking part in the reaction : R <> O + ne */ /** Applying Fick's Law of Diffusion for the electrode reaction R <> O + ne using the relation : i/nFA = D[conc gradient @ electrode surface], where i = current F = Faraday Constant A = crosssectional area of the electrode D = diffusion coefficient or diffusvity here we are finding the concentration gradient @ the electrode*/ real value = curr_i/(num_n*Faraday_const*area_a*Diffusion_coeff); begin_f_loop(f,t) { F_PROFILE(f,t,i) = value; /* used to set a boundary condition value in memory for a given face and thread */ } end_f_loop(f,t) } THIS SHOULD BE A SIMPLE CASE WHERE KNOWING INITIAL CONCENTRATION IN THE MEDIUM AND THE CONCENTRATION GRADIENT AT THE WALLS, I SHOULD BE ABLE TO FIND THE CONCENTRATION DISTRIBUTION IN THE WHOLE DOMAIN......unfortunately I am failing to do so. PLEASE, PLEASE  I NEED GUIDANCE AND HELP. 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
urgent help needed (rhiechow interpolation problem)  Ardalan  Main CFD Forum  2  March 18, 2011 16:22 
species transport(need urgent help)  sree  FLUENT  1  December 18, 2006 03:11 
species transport(need urgent help)  sree  CFX  0  December 14, 2006 02:16 
species transport(need urgent help)  sree  Main CFD Forum  0  December 14, 2006 01:52 
Species Transfer !!! Top Top Urgent!!!  Mohammed Shamol  FLUENT  2  April 27, 2004 03:20 