Time step help within a while loop
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Ok I have the following matlab code I have written I am trying to find the time,mass, and final temperature of a tank being filled by a supply line, however I need help defining my time step in matlab. or I need help with the while loop itself the results I am supposed to be getting are Final Time= about 15 seconds Final Temp= about 700 R Final Mass= about 20 lbs %%%%%%%%%%%%%% My program is getting these answers but it is not converging at each time step so my plots are all linear when the should not be. Please any suggestions will be greatly appreciated!!
P1=75; % in psi
T1=(80+459.67); %in R
A1=(pi*1)/(4*144); % Area in ft^2
D1=1; % Diameter in inch
P2=(14.7); % in psi
T2=T1; %in R
V2=70; % in ft^3
Cp=0.24; % in Btu/lb-R
Cv=0.17; % in Btu/lb-R
Cd=0.6; % is unitless
R=53.33; % in ft-lb/lb/R
rho1=(P1*144)/(R*T1); % density in lb/ft^3
rho2=(P2*144)/(R*T2); % density in lb/ft^3
mass2=(rho2*V2); % mass in lb
g=32.174; % in ft/sec^2
dt=0.01; % change in time intervals
t=0;
n=1;
k=1.4;
m(1)=mass2;
P(1)=P2;
T(1)=T2;
Pcrit=(2/(k+1))^(k/(k-1));
Prat= (P2/P1);
while P2<P1-.01
t==0+dt;
error=1;
n=n+1;
while error>0.001
%Equations
mdot=A1*sqrt(((2*k)/(k-1))*P1*18*g*rho1*Prat^(2/k)*(1-(Prat))^(k-1/k));
if Prat <= Pcrit;
Prat= Pcrit;
end
mass2new=mass2+mdot*dt;
u=(mass2*Cv*T2+mdot*Cp*T1*dt)/mass2new; %energy equation
T2=u/Cv;
P2new=mass2new*R*T2/(V2*144);
error=abs(P2new-P2)/P2;
P2=P2new;
end
mass2=mass2new;
T(n)=T2;
P(n)=P2;
m(n)=mass2new;
t(n)=n*dt;
end
%%Output
fprintf('The final temperature = %7.3f R\n',T2)
fprintf('The mass of air in tank = %7.3f lb\n',mass2new)
fprintf('The time required to pressurize the tank = %7.3f s\n',t(n))
figure(1)
plot(t,T,'g','Linewidth',2)
grid
ylabel('Temperature (R)')
xlabel('Time (s)')
title('Time Vs Temperature')
figure(2)
plot(t,P,'b','Linewidth',2)
grid
xlabel('Time (s)')
ylabel('Pressure (psi)')
title('Time Vs Pressure')
figure(3)
plot(t,m,'r','Linewidth',2)
grid
xlabel('Time (s)')
ylabel('Mass (lb)')
title('Time Vs Mass')
I am getting the correct answers, but my plots are wrong because my while loop is not converging at each time step. Please help!
采纳的回答
the cyclist
2011-3-25
I wonder if the location of your assignment
Prat= (P2/P1);
is incorrect. Should that ratio stayed fixed for the entire program?
It was also not clear to me if your P2=Pnew statement is in the right place.
3 个评论
John
2011-3-25
the cyclist
2011-3-25
I have to admit that I have not spent the time to understand your problem deeply. I don't really know exactly when Prat should be changing.
John
2011-3-25
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