Learning Forum

[Zwernjayden]

I have attached the UDF at the end. I am trying to use the heat flux to update the mass flow rate at different x values on the inlet surface. I keep getting errors and the temp distribution is weird. On some faces it make sense and others its completely wrong

Temp of face 391: -nan(ind) K

Heat flux on face 392: -4.41347e+07 W/m^2

Temp of face 392: -nan(ind) K

Heat flux on face 393: -2.6595e+06 W/m^2

Temp of face 393: 99.0877 K

Heat flux on face 408: 6.20341e+06 W/m^2

Temp of face 408: 1249.73 K

Heat flux on face 409: 4.17052e+06 W/m^2

Temp of face 409: 921.292 K

Heat flux on face 410: 3.9361e+06 W/m^2

Temp of face 410: 900.957 K

pdf_initial_enthalpy: failed to converge for T = -nan(ind), fmean = 0.7, fvar = 0, error = -nan(ind)

pdf_initial_enthalpy: failed to converge for T = -nan(ind), fmean = 0.7, fvar = 0, error = -nan(ind)

pdf_initial_enthalpy: failed to converge for T = -nan(ind), fmean = 0.7, fvar = 0, error = -nan(ind)

 

#include "udf.h"

#include

#define rho 754 //kg/m^3

#define Hg 2033631 //J/Kg

#define Hf -310000 //J/Kg

#define T_ref 298 //T

#define A 0.01104 //m/s

#define Cp 1500 //J/(Kg*K)

#define E 20557.188 //J/mol

#define R_u 8.314 //J(m*K)

#define D_p 0.04064 //port diameter (m)

#define L 0.43815 //Length (m)

#define alpha .1

DEFINE_PROFILE(surface_temperature_profile, t, position)

{

    face_t f;

    real Ts, heat_flux, heat, a_face, Hv, r, Ts_old, Ts_new_prime, Ts_new, error, Area[ND_ND];

    int iter;

    begin_f_loop(f, t)

    {

        if (THREAD_ID(t) == 8)  // Adjust the surface thread ID as per your case

        {

            Ts = F_T(f, t);  // Current temperature at the face

   

            heat = BOUNDARY_HEAT_FLUX(f, t);  // Calculating the heat flux at face f in watts

            F_AREA(Area, f, t);

            a_face = NV_MAG(Area);

            heat_flux = heat / a_face; // Heat flux in w/m^2

            printf("Heat flux on face %d: %g W/m^2\n", f, heat_flux);

 

            // Iterative calculation for new surface temperature

            Ts_old = Ts;

            iter = 0;

            error = 1;

            while (error > 1e-6 && iter < 100)

            {

                Hv = Hg - Hf + Cp * (Ts_old - T_ref);  // Calculation heat of formation

                r = (fabs(heat_flux)) / (Hv * rho);  // Regression rate

                Ts_new = E / (R_u * (log(A) - log(r)));  // New surface temperature

                error = fabs(Ts_new - Ts_old);  // Error calculation

                Ts_old = Ts_new;  // Update Ts_old for next iteration

                iter++;  // Increment iteration count

 

            }

    Ts_new_prime = Ts+(alpha*(Ts_new-Ts));

    printf("Temp of face %d: %g K\n", f, Ts_new_prime);

            F_PROFILE(f, t, position) = Ts_new_prime;  // Update the face temperature profile

        }

    }

    end_f_loop(f, t)

}

DEFINE_PROFILE(mass_flow_rate_profile, t, position)

{

    face_t f;

    real heat_flux, heat, a_face, Hv, r, m_dot, Ts, Area[ND_ND];

    begin_f_loop(f, t)

    {

        if (THREAD_ID(t) == 8)  // Adjust the inlet thread ID as per your case

        {

            Ts = F_T(f, t);  // Get the current temperature at the face

            heat = BOUNDARY_HEAT_FLUX(f, t);  // Calculating the heat flux at face f

            F_AREA(Area, f, t);

            a_face = NV_MAG(Area);

            heat_flux = heat / a_face;

            Hv = Hg - Hf + Cp * (Ts - T_ref);  // Adjusted Hv calculation with temperature dependency

            r = (fabs(heat_flux)) / (Hv * rho);  // Regression rate calculation

            m_dot = rho * r * a_face;  // Mass flow rate calculation

    printf("m_dot fuel %d: %g kg/s\n", f, m_dot);

            F_PROFILE(f, t, position) = m_dot;  // Set the profile for the mass flow rate

        }

    }

    end_f_loop(f, t)

}

 

[Rob]

Why are you assigning a thread to the DEFINE_PROFILE? You write a profile and then manually attach it to a surface as per the example in the manual. 

[Zwernjayden]

Ya ok that makes sense I was just following this pseudo code that I have. I doubt this is my problem though right?

[Rob]

Single or multiphase? 

[Zwernjayden]

The oxidizer steam is n2o with a shower head injector so maybe 2 phase flow not sure. The fuel inlet described in the udf is a solid heating up and turning into a gas. Is this something I would specify or does Fluent solve it automatically

[Rob]

Did you set a multiphase model? Solid to gas? What inlet type did you set, and what materials? 

[Zwernjayden]

I tried setting the fuel inlet to both the composition of the solid fuel, as well as the gaseous monomer for HTPB. I used a mass flow inlet for the solid fuel and tried both a mass flow inlet and pressure inlet for the nitrous oxide oxizider. I didnt specifically set a multiphase model, im not sure how to do that.