2.11 User-Accessible Functions

As noted in Section  1.6, you can directly incorporate your own formulations and data for the properties of the fuel cell membrane using the pem_user.c source code file.

The following listing represents a description of the contents of the pem_user.c source code file:

• real Get_P_sat(real T): Returns the value of the water vapor saturation pressure as a function of temperature (Equation  1.6-11).

• real Water_Activity(real P, real T, cell_t c, Thread *t): Returns the value of water activity (Equation  1.6-9).

• real Water_Content (real act): Returns the value of the membrane water content at the membrane catalyst interface (Equation  1.6-8).

• real Osmotic_Drag_Coefficient(real P, real T, cell_t c, Thread *t): Returns the value of the osmotic drag coefficient (Equation  1.6-5).

• real Membrane_Conductivity(real lam, cell_t c, Thread *t): Returns the value of the membrane's protonic conductivity (Equation  1.6-4).

• real Electrolyte_Conductivity(cell_t c, Thread *t): Returns the value of the ionic conductivity in the electrolyte (Equation  1.6-3). (SOFC and Electrolysis only)

• real Water_Content_Diffusivity(real lam, real T, real mem_mol_density, cell_t c, Thread *t): Returns the value of the water content diffusivity in the membrane (Equation  1.6-7).

• real Gas_Diffusivity(cell_t c, Thread *t, int j_spe): Returns the value of the gaseous species diffusivities in the channels, gas diffusion layers and catalysts (Equation  1.6-1).

• real MCD_Gas_Diffusivity(cell_t c, Thread *t, int i): Returns the tortuosity-corrected value of the gas species diffusion coefficients computed with the multicomponent diffusion option (Equation  1.6-2).

• real Saturation_Diffusivity(real sat, real cos_theta, real porosity,
  cell_t c, Thread *t): Returns the value of diffusivity of the liquid saturation. It comprises the term

  
  

  
  

  from Equation  1.5-1.

• real Anode_AV_Ratio(cell_t c, Thread *t): Returns the value of the specific active surface area ( $\zeta_{\rm an}$ in Equation  1.2-3) for the anode catalyst.

• real Cathode_AV_Ratio(cell_t c, Thread *t): Returns the value of the specific active surface area ( $\zeta_{\rm cat}$ in Equation  1.2-4) for the cathode catalyst.

• real Anode_J_TransCoef(cell_t c, Thread *t): Returns the value of the anode reaction reference current density $\frac{\zeta_{\rm an} j_{\rm an}^{ref}}{([H_2]_{\rm ref})^{\gamma_{\rm an}}}$ used in Equation  1.2-3.

• real Cathode_J_TransCoef(cell_t c, Thread *t): Returns the value of the cathode reaction reference current density $\frac{\zeta_{\rm cat} j_{\rm cat}^{ref}}{([O_2]_{\rm ref})^{\gamma_{\rm cat}}}$ used in Equation  1.2-4.

• real Open_Cell_Voltage(cell_t c, Thread *t): Returns the value of the open-circuit voltage $V_{\rm oc}$ used in Equation  1.2-8.

• real Leakage_Current (cell_t c, Thread *t): Returns the value of the specific leakage current ( $I_{\rm leak}$ in Equations  1.8-1 through 1.8-6).

• void Set_UDS_Names(char uds[n_uds_required][STRING_SIZE]): Used to rename user defined scalars (UDSs). Note that the units of the user defined scalars cannot be changed.

  void Set_UDS_Names(char uds[n_uds_required][STRING_SIZE])
  {
      strncpy(uds[0], "Electric Potential", STRING_SIZE-1);
      strncpy(uds[1], "Protonic Potential", STRING_SIZE-1);
      strncpy(uds[2], "Water Saturation",   STRING_SIZE-1);
      strncpy(uds[3], "Water Content",      STRING_SIZE-1);
  }
  

  If you want to change the names of UDSs, change the second argument of the strncpy functions, recompile and link the module as with any modification to pem_user.c. Note that STRING_SIZE is fixed in pem.h and should not be changed.

  When you load older Fuel Cell and Electrolysis cases into ANSYS FLUENT, and you are monitoring a UDS using volume or surface monitors, make sure you re-visit the corresponding monitors dialog (e.g., the Volume Monitor or the Surface Monitor dialog) to make sure that the correct UDS name is used for the appropriate monitor.

• void Set_UDM_Names(char udm[n_udm_required][STRING_SIZE]): Used to rename user defined memory (UDMs). Note that the units of user defined memory cannot be changed.

  void Set_UDM_Names(char udm[n_udm_required][STRING_SIZE])
  {
     strncpy(udm[ 0], "X Current Flux Density",         STRING_SIZE-1);
     strncpy(udm[ 1], "Y Current Flux Density",         STRING_SIZE-1);
     strncpy(udm[ 2], "Z Current Flux Density",         STRING_SIZE-1);
     strncpy(udm[ 3], "Current Flux Density Magnitude", STRING_SIZE-1);
     strncpy(udm[ 4], "Ohmic Heat Source",              STRING_SIZE-1);
     strncpy(udm[ 5], "Reaction Heat Source",           STRING_SIZE-1);
     strncpy(udm[ 6], "Overpotential",                  STRING_SIZE-1);
     strncpy(udm[ 7], "Phase Change Source (PEM)",      STRING_SIZE-1);
     strncpy(udm[ 8], "Osmotic Drag Coefficient (PEM)", STRING_SIZE-1);
     strncpy(udm[ 9], "Liquid Water Activity (PEM)",    STRING_SIZE-1);
     strncpy(udm[10], "Membrane Water Content (PEM)",   STRING_SIZE-1);
     strncpy(udm[11], "Protonic Conductivity",          STRING_SIZE-1);
     strncpy(udm[12], "Back Diffusion Source (PEM)",    STRING_SIZE-1);
     strncpy(udm[13], "Transfer Current",               STRING_SIZE-1);
     strncpy(udm[14], "Osmotic Drag Source (PEM)",      STRING_SIZE-1);
  }
  

  If you want to change the names of UDMs, change the second argument of the strncpy functions, recompile and link the module as with any modification to pem_user.c. Note that STRING_SIZE is fixed in pem.h and should not be changed.

  When you load older Fuel Cell and Electrolysis cases into ANSYS FLUENT, and you are monitoring a UDM using volume or surface monitors, make sure you re-visit the corresponding monitors dialog (e.g., the Volume Monitor or the Surface Monitor dialog) to make sure that the correct UDM name is used for the appropriate monitor.

• real Transfer_Current(real i_ref, real gamma, int species_i,
  real alpha_a, real alpha_c, real *dRade, real *dRcde, Thread *t,
  cell_t c): Computes the transfer current (A/m $^3$), corresponding to $R_{\rm an}$ in Equation  1.2-3 and $R_{\rm cat}$ in Equation  1.2-4.

  Inputs for this function include:

  
  

  i_ref	effective transfer current coefficient, computed by Cathode_J_TransCoef(c,t) or Anode_J_TransCoef(c,t)
  gamma	cathode or anode concentration exponent
  species_i	species index used in fuelcells (e.g. i_o2, i_h2, i_h2o)
  alpha_a	product of anode exchange coefficient and $\frac{F}{RT}$
  alpha_c	product of cathode exchange coefficient and $\frac{F}{RT}$
  t	current thread
  c	current cell

  
  

  Outputs for this function include:

  
  

  source	anode or cathode volumetric transfer current ( $R_{\rm an}$ in Equation  1.2-3 or $R_{\rm cat}$ in Equation  1.2-4)
  *dRade	partial derivative of $R_{\rm an}$ with respect to activation loss
  *dRcde	partial derivative of $R_{\rm cat}$ with respect to activation loss