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For loop doesn't work well.
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Giovanni Karahoxha
on 17 Nov 2023
Commented: Steven Lord
on 17 Nov 2023
I am trying to size a tube for a two-phase heat exchange problem.
I calculate the local pressure drop, I update the thermophysical properties via refrprop and then I calculate the heat exchange coefficient, that permits me to find finally the tube lenght.
The problem is that, a change on the steps number occurs in a change of results; it seems like the pressure drop is proportional to the numner of steps.
[SL: reformatted code as code not italicized text]
n = 500;
dx = (X_outlet_eva-X_inlet_eva)/(n);
x = zeros(n,1);
x(1) = X_inlet_eva;
P = zeros(n,1);
P(1) = Lowpressure;
for i = 2 : n+1
x(i) = x(i-1) + dx;
Viscosity_liquid(i) = refpropm('V','P',P(i-1),'Q',0,fluid); %[Pa s]
Viscosity_vapor(i) = refpropm('V','P',P(i-1),'Q',1,fluid); %[Pa s]
Density_liquid(i) = refpropm('D','P',P(i-1),'Q',0,fluid); %[kg/mc]
Density_vapor(i) = refpropm('D','P',P(i-1),'Q',1,fluid); %[kg/mc]
Enthalpy_liquid(i) = refpropm('H','P',P(i-1),'Q',0,fluid); %[J/kg]
Enthalpy_vapor(i) = refpropm('H','P',P(i-1),'Q',1,fluid); %[J/kg]
Latent(i) = (Enthalpy_vapor(i) - Enthalpy_liquid(i)); %[J/kg]
Prandtl_liquid(i) = refpropm('^','P',P(i-1),'Q',0,fluid); %[-]
Surfacetension_liquid(i) = refpropm('I','P',P(i-1),'Q',0,fluid); %[N/m]
Thermalconductivity_liquid(i) = refpropm('L','P',P(i-1),'Q',0,fluid); %[W/mK]
Specificheat_liquid(i) = refpropm('C','P',P(i-1),'Q',0,fluid); %[J/kgK]
Ki_tt(i) = (((1-x(i))/x(i))^(0.9))*((Density_vapor(i)/Density_liquid(i))^(0.5))*((Viscosity_liquid(i)/Viscosity_vapor(i))^(0.1));
Reynolds_liquid(i) = (G*(1-x(i))*Internalradius_eva*2)/Viscosity_liquid(i);
Moody(i) = 0.316/(Reynolds_liquid(i)^(0.25));
alfa_martinelli(i) = (1+0.28*Ki_tt(i)^0.71)^(-1);
phi_nonhomo(i) = (((1 - x(i))^2)/(1-alfa_martinelli(i)))+(Density_liquid(i)/Density_vapor(i))*((x(i)^2)/alfa_martinelli(i));
dPD_acc(i) = (((x(i)^2)*G^2)/(alfa_martinelli(i)*Density_vapor(i))) + (((1-x(i))^2)*G^2)/(((1-alfa_martinelli(i))*Density_liquid(i))); %[Pa]
dPD_fric(i) = phi_nonhomo(i)*Moody(i)*(L_tube(i-1)/(Internalradius_eva*2))*((G^2)/(2*Density_liquid(i))); %[Pa]
dPD_tot(i) = (dPD_acc(i)+dPD_fric(i))*10^(-3); %[kPa]
P(i) = P(i-1) - dPD_tot(i);
T(i) = refpropm('T','P',P(i),'Q',0,fluid);
% Convective heat exchangeo
h_cb(i) = 0.023*(Thermalconductivity_liquid(i)/(Internalradius_eva*2))*(Reynolds_liquid(i)^(0.8))*(Prandtl_liquid(i)^(0.4)); %[W/m^2K]
%Nucleate boiling heat transfer
Twall_eva(i) = (Convectiveexchange_air*Temperature_air_inlet_eva*Externalradius_eva + h_chen(i-1)*T(i)*Internalradius_eva)/(Convectiveexchange_air*Externalradius_eva + h_chen(i-1)*Internalradius_eva);
Psat_twall(i) = refpropm('P','T',Twall_eva(i),'Q',x(i),fluid);
A_NB(i) = 0.00122*(Thermalconductivity_liquid(i)^(0.79))*(Specificheat_liquid(i)^(0.45))*(Density_liquid(i)^(0.49));
B_NB(i) = (Twall_eva(i)-T(i))^(0.24);
C_NB(i) = ((Psat_twall(i)-refpropm('P','T',T(i),'Q',0,fluid))*10^3)^(0.75);
D_NB(i) = (Surfacetension_liquid(i)^(0.5))*(Viscosity_liquid(i)^(0.29))*(Latent(i)^(0.24))*(Density_vapor(i)^(0.24));
h_nb(i) = (A_NB(i)/D_NB(i))*B_NB(i)*C_NB(i); %[W/m^2K]
if (1/Ki_tt(i)) > 0.1
F_chen(i) = 2.35*((0.213+(1/Ki_tt(i)))^(0.736));
elseif (1/Ki_tt(i)) < 0.1
F_chen(i) = 1;
end
S_chen(i) = 1/(1+((2.53*10^(-6))*(Reynolds_liquid(i)*(F_chen(i)^(1.25))^(1.17))));
h_chen(i) = F_chen(i)*h_cb(i) + S_chen(i)*h_nb(i); %[W/m^2K]
deltaT(i) = (Temperature_air_inlet_eva-T(i));
U(i) = 1/((1/Convectiveexchange_air)+(1/h_chen(i)));
Qv(i) = (Massflow_R32*Latent(i)*(x(i)-x(i-1))); %[W]
L_tube(i) = Qv(i)/(U(i)*deltaT(i)*2*pi*Externalradius_eva);
1 Comment
Steven Lord
on 17 Nov 2023
What does "doesn't work well" mean in this context?
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