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July 4, 2011, 16:48 
Fluent UDF urgent! Variable reaction rate

#1 
Member
Nathan
Join Date: Aug 2010
Posts: 64
Rep Power: 8 
Hi
I've written a UDF in Fluent for a volumetric reaction rate. I want to know how I can make the reaction rate variable along my reactor due to variation in catalyst activity. Any idea? Example: r= p* k C^2 ( p is a function of distance into the reactor) How do you implement this, I'm new to UDf Thanks 

July 6, 2011, 05:24 

#2 
New Member
Kristian Etienne Einarsrud
Join Date: Oct 2010
Location: Trondheim
Posts: 29
Rep Power: 7 
Hi,
I guess what you need is a function which gives you the position of the each cell center, which is C_CENTROID(x,c,ct). Here, x is an array which holds the x,y,z coordinates, while c and ct are the cell and thread pointers which are passed from the solver through an appropriate macro. Assuming you are using a DEFINE_ADJUST macro to update your reaction rate, and that your reaction rate is r=x*k*C^2 (x being the xcoordinate), I would use  DEFINE_ADJUST(adjustRXR,domain) { Thread *ct; cell_t c; real x[ND_ND]; real k, C,r,pos; k=0.25; /* just an example*/ C=0.1; /* just an example*/ r=0.0; /* initialization to be sure*/ pos=0.0; /* initialization to be sure*/ thread_loop_c(ct,domain) /*Loop over the threads in the domain*/ { begin_c_loop(c,ct) /*Loop over all cells in the thread*/ { C_CENTROID(x,c,ct); /*Find position of cell and store in x*/ pos = x[0]; /*Store the xcoordinate as pos. ycoordinate is x[1], zcoordinate is x[2]*/ r=pos*k*C*C; /*Store reaction rate in local variable r*/ C_UDMI(c,ct,0) = r; /*Store reaction rate in user defined memory position 0 so that other functions can access it*/ }end_c_loop(c,ct) } }  You will need one UDM to store the reaction rate if you do it in this way. Remember to hook the DEFINE_ADJUST function before running your simulations. For a more complete example, see section 2.3.22.3 in the UDFguide. Good luck! KE 

July 6, 2011, 16:36 

#3 
Member
Nathan
Join Date: Aug 2010
Posts: 64
Rep Power: 8 
Thanks A lot for the comment
The problem is my reaction is defined through a volumetric reaction rate DEFINE_VR_RATE , the reaction needs the temperature and mass fraction of each node (300000 nodes) at each iteration. Apparently inside the ADJUST udf we don't ahve access to node values. I get error. Below is my original UDf for VR_Rate, if you like take a look at it. #include"udf.h" DEFINE_VR_RATE(proprate,c,t,r,mw,yi,rr,rr_t) { /*mass fraction of species i */ real y_c3h8=yi[3]; real y_h2o=yi[4]; real y_h2=yi[0]; real y_co=yi[1]; real y_co2=yi[2]; real y_n2=yi[5]; real Nsum, R, Keq1, T_w, r1,r2; /*calculate species i in the unit of kgmol i/kg mix*/ y_c3h8 *= 1/mw[3]; y_h2o *= 1/mw[4]; y_h2 *= 1/mw[0]; y_co *= 1/mw[1]; y_co2 *= 1/mw[2]; y_n2 *= 1/mw[5]; Nsum = y_c3h8+ y_h2o + y_h2 + y_co + y_co2 + y_n2; y_c3h8 *= 1/Nsum; y_h2o *= 1/Nsum; y_h2 *= 1/Nsum; y_co *= 1/Nsum; y_co2 *= 1/Nsum; y_n2 *= 1/Nsum; R=8.314; T_w=C_T(c,t); Keq1=pow(10,(2073/T_w)2.029); if(STREQ(r>name, "reaction1")){ /*unit for rr, kg/m3s*/ r1= 44*3e8*exp(105000/(R*T_w))*(y_c3h8*y_h2o)/(1+105*exp(3000/(R*T_w))*pow(y_c3h8,2)); *rr = r1; } elseif (STREQ(r>name, "reaction2")){ r2= 28*3.4e9*exp(130000/(R*T_w))*(y_co*y_h2o(y_h2*y_co2)/Keq1) ; *rr = r2; } } 

July 7, 2011, 03:59 

#4 
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Kristian Etienne Einarsrud
Join Date: Oct 2010
Location: Trondheim
Posts: 29
Rep Power: 7 
Hello again,
If I understand your original post correctly, your problem is that you want to include a position dependency in your DEFINE_VR_RATE? This can be done by the following modification (I have made my changes in bold text for clarity): #include"udf.h" DEFINE_VR_RATE(proprate,c,t,r,mw,yi,rr,rr_t) { /*mass fraction of species i */ real Nsum, R, Keq1, T_w, r1,r2,x[ND_ND],xpos,ypos,zpos; real y_c3h8=yi[3]; real y_h2o=yi[4]; real y_h2=yi[0]; real y_co=yi[1]; real y_co2=yi[2]; real y_n2=yi[5]; /* Find and store coordinates for each cell*/ C_CENTROID(x,c,t); /* store position in array x*/ xpos = x[0]; ypos = x[1]; zpos = 0.0; #if RP_3D zpos = x[2]; #endif /* The above #if statement ensures that you only can access the zposition (x[2]) if you are doing a 3Dsimulation */ /* The coordinates of each cell are now stored in xpos,ypos and zpos (if 3D) for future use*/ /*calculate species i in the unit of kgmol i/kg mix*/ y_c3h8 *= 1/mw[3]; y_h2o *= 1/mw[4]; y_h2 *= 1/mw[0]; y_co *= 1/mw[1]; y_co2 *= 1/mw[2]; y_n2 *= 1/mw[5]; Nsum = y_c3h8+ y_h2o + y_h2 + y_co + y_co2 + y_n2; y_c3h8 *= 1/Nsum; y_h2o *= 1/Nsum; y_h2 *= 1/Nsum; y_co *= 1/Nsum; y_co2 *= 1/Nsum; y_n2 *= 1/Nsum; R=8.314; T_w=C_T(c,t); Keq1=pow(10,(2073/T_w)2.029); if(STREQ(r>name, "reaction1")){ /*unit for rr, kg/m3s*/ r1= 44*3e8*exp(105000/(R*T_w))*(y_c3h8*y_h2o)/(1+105*exp(3000/(R*T_w))*pow(y_c3h8,2)); *rr = r1; } elseif (STREQ(r>name, "reaction2")){ r2= 28*3.4e9*exp(130000/(R*T_w))*(y_co*y_h2o(y_h2*y_co2)/Keq1) ; *rr = r2; } } Another point; note that the values you are using are *not* node values but *cell* values, as it is the cell pointer "c" which is passed from the macro DEFINE_VR_RATE. Happy simulating! KE 

March 14, 2016, 16:53 
Calcination Reaction

#5 
New Member
Diogo
Join Date: Mar 2016
Posts: 1
Rep Power: 0 
Hello all, first of all i would like to introduce myself as new on this field... I've read your posts and i have some doubts... How do you access the molecular weight of each component?? Have you defined it previously?? Because i coudn't realise where are these values on your program line...
Another doubt is about making an User Defined Function for this reaction: caco3=cao+co2 I'm simulating a rotary kiln used for calcination... I need to compilate this reactions in the Fluent but i have no idea about how to do this... Can anyone help me?? Thank you in advance 

July 25, 2016, 08:17 

#6 
New Member
Omar
Join Date: Jul 2016
Posts: 2
Rep Power: 0 
Hello
Can someone please help me write a FLUENT UDF the reaction below which is defined by reaction rate equation of (R) below CO+3H2=>CH4+H2O R=k exp (E/RT)*(Pco^m*Ph2^n) Where: R is the volumetric reaction rate k is Rate constant E is Activation energy for reaction R is Universal gas constant T is Temperature Pco is Partial pressure of carbon monoxide Ph2 is Partial pressure of hydrogen m is Power for CO partial pressure n is Power for H2 partial pressure UDF for volumetric reaction rateunnamed.png Thank you. 

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