Learning Forum

[Julio Guzman]

Hi Rob,

The UDF I have written is to calculate saturation temperature given partial pressure of water and so does not involve any phase transfer values. I have diagnosed each line of the code using the Message() function (during initialisation) and everything outputted seems sensible! I haven't tracked any of them with a UDM yet!

Please see below:

#include "udf.h"

 

#define MOLAR_MASS_WATER 18.01534 //g/mol

#define MOLAR_MASS_AIR 28.97 // g/mol

#define RHO_WV 0.5542 // kg/m3

#define RHO_AIR 1.225 // kg/m3

 

DEFINE_PROPERTY(saturation_temp, c, ct)

{

// ct: mixture cell thread

// c: cell variable

 

// Cell volume

real vol = C_VOLUME(c, ct);

 

Thread *pp_ct = THREAD_SUB_THREAD(ct, 0); // Primary phase cell thread, phase index 0 for primary phase

Thread *sp_ct = THREAD_SUB_THREAD(ct, 1); // Secondary phase cell thread, add 1 for other phases

 

// Get the volume fraction of both phases

real vf_sp = C_VOF(c, sp_ct);

real vf_pp = 1 - vf_sp;

 

// Get the pressure of the mixture 

real p_mix = C_P(c, ct);

 

// Get the operating pressure

real p_op = RP_Get_Real("operating-pressure");

 

// Primary phase density

real rho_pp = C_R(c, pp_ct);

 

// Get mass fractions in primary phase

real mf[2]; // Array to store mass fractions

 

Material *m = THREAD_MATERIAL(pp_ct);

Material *spe = NULL;

int i; // Species index - 0 for water vapor and 1 for air

 

mixture_species_loop(m, spe, i)

{

mf[i] = C_YI(c, pp_ct, i);

}

 

real p_wv; // h20 pressure for cell

real t_sat; // saturation temperature

 

// If secondary phase (liquid water) only

if (vf_sp == 1)

{

p_wv = p_mix + p_op; // Gauge pressure of water + operating pressure

 

// Inverted Tetens equation (see wikipedia)

t_sat = (4947.46 - 82.46 * log10(p_wv)) / (23.69 - 2.3 * log10(p_wv));

}

 

// If primary phase (moist) or mixture of phases

else 

{

// Find the partial pressure of water vapour

// partial pressure = cell pressure * water mole fraction

 

// mass of primary phase in cell

real m_pp = rho_pp * vol * vf_pp;

 

// mass of water vapour and air in cell

real m_wv = mf[0] * m_pp;

real m_air = m_pp - m_wv;

 

// No of moles in water vapour and air

real N_wv = m_wv / MOLAR_MASS_WATER;

real N_air = m_air / MOLAR_MASS_AIR;

 

real N_total = N_wv + N_air;

 

 

// Water vapour partial pressure

p_wv = (C_P(c, ct) + p_op) * (N_wv / N_total);

 

 

if (mf[0] < 0.01)

{

// Match Tetens to linear interpolation from NIST data to account for low partial water pressure behaviour

t_sat = 8.64e-3 * p_wv + 273.15;

}

 

else

{

// Inverted Tetens equation (see wikipedia)

t_sat = (4947.46 - 82.46 * log10(p_wv)) / (23.69 - 2.3 * log10(p_wv));

}

 

}

 

return t_sat;

}

 

Regarding the high temperatures, I don't need to operate eventually at such high temperatures: I just wanted to check that, given solver settings etc, the simulation runs appropriately for the t_sat=const model (which it does), implying that nothing is wrong on the Solution Controls side of things.

 

There are two fluid domains: one for the water (so that I could patch), and one for the humid air mixture. The settings for each are:

In grey are the wall boundaries and in green the interal boundary between the two fluid domains:

 

Does this all look sensible to you?

 

Thanks for all your help so far!

 

Best,
Nick