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49 lines
1.5 KiB
C
49 lines
1.5 KiB
C
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/* gas-model.c */
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/* gas compressibility model */
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#include <stdio.h>
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#include <stdlib.h>
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#include "dive.h"
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/*
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* This gives an interative solution of hte Redlich-Kwong equation for the compressibility factor
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* according to https://en.wikipedia.org/wiki/Redlich–Kwong_equation_of_state
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* in terms of the reduced temperature T/T_crit and pressure p/p_crit.
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*
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* Iterate this three times for good results in our pressur range.
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*
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*/
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double redlich_kwong_equation(double t_red, double p_red, double z_init)
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{
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return (1.0/(1.0 - 0.08664*p_red/(t_red * z_init)) -
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0.42748/(sqrt(t_red * t_red * t_red) * ((t_red*z_init/p_red + 0.08664))));
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}
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/*
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* At high pressures air becomes less compressible, and
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* does not follow the ideal gas law any more.
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*/
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#define STANDARD_TEMPERATURE 293.0
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double gas_compressibility_factor(struct gasmix *gas, double bar)
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{
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/* Critical points according to https://en.wikipedia.org/wiki/Critical_point_(thermodynamics) */
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double tcn2 = 126.2;
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double tco2 = 154.6;
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double tche = 5.19;
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double pcn2 = 33.9;
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double pco2 = 50.5;
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double pche = 2.27;
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double tc, pc;
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tc = (tco2 * get_o2(gas) + tche * get_he(gas) + tcn2 * (1000 - get_o2(gas) - get_he(gas))) / 1000.0;
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pc = (pco2 * get_o2(gas) + pche * get_he(gas) + pcn2 * (1000 - get_o2(gas) - get_he(gas))) / 1000.0;
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return (redlich_kwong_equation(STANDARD_TEMPERATURE/tc, bar/pc,
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redlich_kwong_equation(STANDARD_TEMPERATURE/tc, bar/pc,
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redlich_kwong_equation(STANDARD_TEMPERATURE/tc, bar/pc,1.0))));
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}
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