Why does solve function return an empty value in a for cycle?

for i=1:N
    syms Tout
    sol=solve (energy_balance,Tout,'Real',true, 'IgnoreAnalyticConstraints',true);
    T_gp_out(i)=double(sol);
end

Solve function returns an empty value at the second iteration of the for cycle which i am using.

When the solve function is run separately outside the for cycle it returns a real value.

I can't understand what is going wrong when all inputs and syntax are correct.

Should I define a syms vector function outside the for cycle?

Can someone help me?

Thanks in advance.

Giorgio Ceresoli

[darova]

Show energy_balance function

[Giorgio Ceresoli]

energy_balance= Q_rad(i)+Q_conv(i)+n_CO2_in*h_CO2_in+n_CaO_in*h_CaO_in+n_CaCO3_in*h_CaCO3_in == n_CO2_out*(h_f_CO2+a_CO2*(Tout-T_ref)+b_CO2/2*(Tout^2-T_ref^2)+c_CO2/3*(Tout^3-T_ref^3))+n_CaO_out*(h_f_CaO+a_CaO*(Tout-T_ref)+b_CaO/2*(Tout^2-T_ref^2)+c_CaO/3*(Tout^3-T_ref^3))+n_CaCO3_out*(h_f_CaCO3+a_CaCO3*(Tout-T_ref)+b_CaCO3/2*(Tout^2-T_ref^2)+c_CaCO3/3*(Tout^3-T_ref^3));

[Giorgio Ceresoli]

all variables defined in energy balance equation are changed at every iteration.

during the second iteration the inputs values are saved in the workspace.

the problem is at output of solve, it seems like that there is no real solution.

Answer 1

[darova]

You have cubic equation

It means that you have 3 roots

try

T_gp_out(i)=double(sol(1));     % choose first root

[Giorgio Ceresoli]

thanks!

[Giorgio Ceresoli]

I put 'Real', true in the solve input.

it should return just one value being a third grade equation