[dxm2008]

Hello everybody

I have written a UDF for the Unal bubble departure diameter model, but it is still some problems. could you help me with that. the code is below:

#include "udf.h"
#include "stdio.h"
#define pi 3.1416

#define g 9.81
#define U0 0.61

DEFINE_BOILING_PROPERTY(bubble_depart_dia,f,t,c0,t 0,from_index,from_species_index,to_index,to_specie s_index)
{
int liq_phase = from_index;
int vap_phase = to_index;
Thread **pt0 = THREAD_SUB_THREADS(t0);
FILE *fp;
double a;
double b;
double phi;
double pre;
double dw;
double pre_op;
double rho_g;
double h_lg = 1.5038425e+6;
double rho_s;
double cp_s;
double k_s;
double T_wall;
double T_sat;

double T_l;
double rho_l;
double delta_Tsub;

double Ub;

T_sat = C_STORAGE_R(c0,t0,SV_SAT_TEMPERATURE);
T_wall = F_T(f, t);

rho_g = C_R(c0, pt0[vap_phase]);
rho_l = C_R(c0, pt0[liq_phase]);

rho_s = 8440.0;
cp_s = -5.0e-06*T_wall*T_wall+0.13123*T_wall+416.33;
k_s = -2.0e-05*T_wall*T_wall+0.052926*T_wall+1.6355;


a=(T_wall-T_sat)/(2.0*rho_g*h_lg)*sqrt(rho_s*cp_s*k_s/pi);

T_l = C_T(c0, pt0[liq_phase]);
delta_Tsub = T_sat-T_l;

if (delta_Tsub<=3.0)
b = delta_Tsub/(2.0*(1-rho_g/rho_l))*exp(delta_Tsub/3.0-1);
else
b = delta_Tsub/(2.0*(1-rho_g/rho_l));

Ub = C_U(c0,t0);
phi = MAX(pow(Ub/U0,0.47),1.0);

pre = C_P(c0, pt0[liq_phase]);
pre_op = RP_Get_Real("operating-pressure");
pre = (pre+pre_op);
if (b>0.0)
dw = 2.4145e-05*pow(pre,0.709)*a/sqrt(b*phi);
else
dw = 3.3216e-05*pow(pre,0.709)*a/sqrt(-b*phi);
return dw;
}

the calculated bubble departure diameter is too large and i don't know why. please give me some suggestion, thank you very much.

[AlexanderZ]

DEFINE_BOILING_PROPERTY(bubble_depart_dia,f,t,c0,t 0,from_index,from_species_index,to_index,to_specie s_index)

here you have space between t and 0, and between to_specie s_index

there should be no spaces, check it in your code
if this is not the reason of your problem, than check your equations,
because UDF code seems to be correct

best regards

[dxm2008]

Originally Posted by AlexanderZ

DEFINE_BOILING_PROPERTY(bubble_depart_dia,f,t,c0,t 0,from_index,from_species_index,to_index,to_specie s_index)

here you have space between t and 0, and between to_specie s_index

there should be no spaces, check it in your code
if this is not the reason of your problem, than check your equations,
because UDF code seems to be correct

best regards

thank you for your reply, it may be the copy problem. In the UDF code, no space exist between t and 0, specie and s.

I want to know if the fluent has some modificaiton when using the Unal bubble departure diameter. or if I have some problem about unit?

[dxm2008]

Originally Posted by dxm2008

Hello everybody

I have written a UDF for the Unal bubble departure diameter model, but it is still some problems. could you help me with that. the code is below:

#include "udf.h"
#include "stdio.h"
#define pi 3.1416

#define g 9.81
#define U0 0.61

DEFINE_BOILING_PROPERTY(bubble_depart_dia,f,t,c0,t 0,from_index,from_species_index,to_index,to_specie s_index)
{
int liq_phase = from_index;
int vap_phase = to_index;
Thread **pt0 = THREAD_SUB_THREADS(t0);
FILE *fp;
double a;
double b;
double phi;
double pre;
double dw;
double pre_op;
double rho_g;
double h_lg = 1.5038425e+6;
double rho_s;
double cp_s;
double k_s;
double T_wall;
double T_sat;

double T_l;
double rho_l;
double delta_Tsub;

double Ub;

T_sat = C_STORAGE_R(c0,t0,SV_SAT_TEMPERATURE);
T_wall = F_T(f, t);

rho_g = C_R(c0, pt0[vap_phase]);
rho_l = C_R(c0, pt0[liq_phase]);

rho_s = 8440.0;
cp_s = -5.0e-06*T_wall*T_wall+0.13123*T_wall+416.33;
k_s = -2.0e-05*T_wall*T_wall+0.052926*T_wall+1.6355;


a=(T_wall-T_sat)/(2.0*rho_g*h_lg)*sqrt(rho_s*cp_s*k_s/pi);

T_l = C_T(c0, pt0[liq_phase]);
delta_Tsub = T_sat-T_l;

if (delta_Tsub<=3.0)
b = delta_Tsub/(2.0*(1-rho_g/rho_l))*exp(delta_Tsub/3.0-1);
else
b = delta_Tsub/(2.0*(1-rho_g/rho_l));

Ub = C_U(c0,t0);
phi = MAX(pow(Ub/U0,0.47),1.0);

pre = C_P(c0, pt0[liq_phase]);
pre_op = RP_Get_Real("operating-pressure");
pre = (pre+pre_op);
if (b>0.0)
dw = 2.4145e-05*pow(pre,0.709)*a/sqrt(b*phi);
else
dw = 3.3216e-05*pow(pre,0.709)*a/sqrt(-b*phi);
return dw;
}

the calculated bubble departure diameter is too large and i don't know why. please give me some suggestion, thank you very much.

Hello everybody

the reason maybe these lines

if (delta_Tsub<=3.0)
b = delta_Tsub/(2.0*(1-rho_g/rho_l))*exp(delta_Tsub/3.0-1);
else
b = delta_Tsub/(2.0*(1-rho_g/rho_l));

in the Unal's original paper, the subcooling is requested to be larger than 3K. in the Fluent, a modificaiton is used in the calculation of b. so when delta_Tsub is close to 0, b suffer a large fluctuation which lead to a large bubble departure diameter.

but if anyone knows how is dealed with in the fluent calculation without UDF?