Problem with if statement

Hi in this program I take data from an excel file, interpolate them with a spline, this part of the code doesn't appear because it works. The problem here is that a certain point we should enter in the if cycle with the last value of i and start another calculation, until the index j arrives to Nt, but the program never enter in the if cycle, even if the if statement is true, I don't know why?. maybe i should choose a comlpletely different structure using while, but how can I index the variables with while ?

kind regards

Francesco Renzulli

P.S I have the latest version of matlab

for i=1:Nt-1
    
    H_t_e(i) = ppval(H_ts,T(i));% ppval per valutare il polinomio 
    dQ(i)=m_vap(i)*H_t_e(i);
    
    Cp_liq_e(i)=ppval(Cp_liqs,T(i));
    dT(i)=(-dQ(i))/(m_liq(i)*Cp_liq_e(i));
    T(i+1)=T(i)+dT(i);
    rho_liq_e(i)=ppval(rho_liqs,T(i+1));
  
    
    rho_vap_e(i)=ppval(rho_vaps,T(i+1));
    P_tank(i)=ppval(P_vaps,T(i+1));
    P_cham(i)=0.4*P_tank(i);
    m_dot_liq(i)=N*A_orifice*sqrt((2*rho_liq_e(i)*(P_tank(i)-P_cham(i)))/2.0);
    m_tot(i+1)=m_tot(i)-m_dot_liq(i)*dt(i);
    m_liq(i+1)=m_liq(i)-m_dot_liq(i)*dt(i);
    m_vap(i+1)=+m_liq(i+1)-((V_tank-(m_tot(i)/rho_vap_e(i)))/((1/rho_liq_e(i))-(1/rho_vap_e(i))));
  if m_liq(i+1)>m_liq(i)
                for j=i:Nt
                P_tank(j)=ppval(P_vaps,T(j));
                C_liq_eg(j)=ppval(Cp_liqs,T(j));
                rho_vap_e(j)=ppval(rho_vaps,T(j));
                Z(j)=ppval(Zs,T(j));
                m_dot_vap(j)=N*A_orifice*sqrt((2*rho_vap_e(j)*(P_tank(j)-P_cham))/C_liq_eg(j));
                m_vap(j+1)=m_vap(j)-m_dot_vap(j)*dt;
                m_tot(j+1)=m_tot(j)-m_dot_vap(j)*dt;

                Z1(j+1)=0.9*Z(j);
                T1(j+1)=T(j)*(((Z1(j+1)*m_vap(j+1))/Z(j)*m_vap(j))^(gamma-1));
                Z2(j+1)=(Z(j)*m_vap(j)*((T(j+1)-T(j))^(1/(gamma-1))))/m_vap(j+1);
                T2(j+1)=T(j)*(((Z2(j+1)*m_vap(j+1))/Z(j)*m_vap(j))^(gamma-1));
                Z3(j+1)=(Z(j)*((T(j+1)-T(j))^(1/(gamma-1))))/m_vap(j+1);
                T(j+1)=T(j)*(((Z3(j+1)*m_vap(j+1))/Z(j)*m_vap(j))^(gamma-1));
                end
                
            end  
       
          
end 

[madhan ravi]

Just a guess , since I can't run your code it seems to me that m_liq(i+1) keeps decreasing in each iteration whereas you assume it to be the other way around.

[Francesco Renzulli]

yeah beacuase it is a simulation of a tank that has vapour and liquid inside, so at a certain iteration m_liquid would become negative and so the statement shold be verified

and when it is verified, it never enters the if cycle

Answer 1

[Gaurav Garg]

Hi Francesco,

The best workaround in such situations is to place a breakpoint where you feel the problem is and check all the variables and their values there.

For example, in your case, you should place a breakpoint at if m_liq(i+1)>m_liq(i) and then check the values of m_liq(i+1) and m_liq(i).

You can find out how to set breakpoints here.