{"id":160534,"date":"2021-07-20T15:37:54","date_gmt":"2021-07-20T15:37:54","guid":{"rendered":"\/forum\/forums\/topic\/saturation-temperature-udf\/"},"modified":"2021-07-22T10:06:48","modified_gmt":"2021-07-22T10:06:48","slug":"saturation-temperature-udf","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/saturation-temperature-udf\/","title":{"rendered":"Saturation temperature UDF"},"content":{"rendered":"
\n
\n

Hello all,<\/p>\n

<\/p>\n<\/p>\n

I have been trying to model evaporation at temperatures far well below 100C. This is primarily driven by the vapor content of air at the water-air interface.<\/p>\n

I found this discussion on the problem where a user had developed a UDF to solve this problem (see a version with a few adjustments below) : \/forum\/discussion\/26620\/saturation-temperature-udf-for-evaporation-condensation<\/a><\/p>\n

The setup this is used with has 2 phases, one a mixture of air and water vapor and the other is water. <\/p>\n

The results of using the UDF seem wrong intuitively. On a test where I ran warm air over a pool of water, the drier air had a higher saturation temperature and the lower air had a lower saturation temperature. Additionally, the fluctuation of saturation pressures across the domain is of the order 1e-1 which is much too low.<\/p>\n

I think the calculation of partial pressure on line 75 is wrong:<\/p>\n

(C_P(c, t)+ p_op)* (N_wv \/ N_total)<\/p>\n

I'm not sure if it should be this, or:<\/p>\n

(C_P(c, t))* (N_wv \/ N_total) + p_op<\/p>\n

or:<\/p>\n

(C_P(c, t))* (N_wv \/ N_total)<\/p>\n

or something else entirely. To me the final option makes sense, but this could lead to an error as there would be log_10(0) on line 81.<\/p>\n

Any advice would be appreciated.<\/p>\n

Cheers<\/p>\n

<\/p>\n

<\/p>\n

<\/p>\n<\/p>\n

#include "udf.h"<\/p>\n

<\/p>\n<\/p>\n

#define MOLAR_MASS_WATER 18.01534 \/\/g\/mol<\/p>\n

#define MOLAR_MASS_AIR 28.97 \/\/ g\/mol<\/p>\n

#define RHO_WV 0.5542 \/\/ kg\/m3<\/p>\n

#define RHO_AIR 1.225 \/\/ kg\/m3<\/p>\n

<\/p>\n

<\/p>\n<\/p>\n

DEFINE_PROPERTY(saturation_temp, c, t)<\/p>\n

{<\/p>\n

\/\/ t: mixture thread<\/p>\n

\/\/ c: cell variable<\/p>\n

<\/p>\n<\/p>\n

\/\/ Cell volume<\/p>\n

real vol = C_VOLUME(c, t);<\/p>\n

<\/p>\n

<\/p>\n<\/p>\n

Thread *pt = THREAD_SUB_THREAD(t, 0); \/\/ Primary phase thread<\/p>\n

Thread *st = THREAD_SUB_THREAD(t, 1); \/\/ Secondary phase thread<\/p>\n

<\/p>\n<\/p>\n

\/\/ Get the volume fraction of both phases<\/p>\n

real vf_s = C_VOF(c, st);<\/p>\n

real vf_p = 1 – vf_s;<\/p>\n

<\/p>\n<\/p>\n

\/\/ Get the pressure of the mixture <\/p>\n

real p_mix = C_P(c, t);<\/p>\n

<\/p>\n<\/p>\n

\/\/ Get the operating pressure<\/p>\n

real p_op = RP_Get_Real("operating-pressure");<\/p>\n

<\/p>\n<\/p>\n

\/\/ Primary phase density<\/p>\n

real rho_p = C_R(c, pt);<\/p>\n

<\/p>\n

<\/p>\n<\/p>\n

\/\/ Get mass fractions in primary phase<\/p>\n

real mf[2]; \/\/ to store mass fractions<\/p>\n

<\/p>\n<\/p>\n

Material *m = THREAD_MATERIAL(pt);<\/p>\n

Material *sp = NULL;<\/p>\n

int i; \/\/ Species index – 0 for water vapor and 1 for air<\/p>\n

<\/p>\n<\/p>\n

mixture_species_loop(m, sp, i)<\/p>\n

{<\/p>\n

mf[i] = C_YI(c, pt, i);<\/p>\n

}<\/p>\n

<\/p>\n<\/p>\n

real p_w; \/\/ h20 pressure for cell<\/p>\n

<\/p>\n<\/p>\n

\/\/ If secondary phase only<\/p>\n

if (vf_s == 1)<\/p>\n

{<\/p>\n

p_w = p_mix + p_op;<\/p>\n

}<\/p>\n

\/\/ If primary phase or mixture of phases<\/p>\n

else <\/p>\n

{<\/p>\n

\/\/ Find the partial pressure of water vapour<\/p>\n

\/\/ partial pressure = cell pressure * water mole fraction<\/p>\n

<\/p>\n<\/p>\n

\/\/ mass of primary phase in cell<\/p>\n

real m_prim = rho_p * vol * vf_p;<\/p>\n

<\/p>\n<\/p>\n

\/\/ mass of water vapour and air in cell<\/p>\n

real m_wv = mf[0] * m_prim;<\/p>\n

real m_air = m_prim – m_wv;<\/p>\n

<\/p>\n<\/p>\n

\/\/ No of moles in water vapour and air<\/p>\n

real N_wv = m_wv \/ MOLAR_MASS_WATER;<\/p>\n

real N_air = m_air \/ MOLAR_MASS_AIR;<\/p>\n

<\/p>\n<\/p>\n

real N_total = N_wv + N_air;<\/p>\n

<\/p>\n

<\/p>\n<\/p>\n

\/\/ water vapour partial pressure<\/p>\n

p_w = (C_P(c, t)+ p_op)* (N_wv \/ N_total);<\/p>\n

}<\/p>\n

<\/p>\n<\/p>\n

\/\/ Calculate saturation temperature<\/p>\n

real t_sat;<\/p>\n

<\/p>\n<\/p>\n

t_sat = (1730.63 \/ (10.196 – log10(p_w))) + 39.724;<\/p>\n

<\/p>\n<\/p>\n

return t_sat;<\/p>\n

<\/p>\n<\/p>\n

}<\/p>\n","protected":false},"template":"","class_list":["post-160534","topic","type-topic","status-closed","hentry","topic-tag-cfd","topic-tag-evaporation-condensation","topic-tag-fluent","topic-tag-udf-fluent"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t