ode45 returning Nan values

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Nishant Gupta
Nishant Gupta 2020-8-1
评论： Nishant Gupta 2020-8-4
Iam trying to solve a system of differential equations using the ode45 command. However I am only getting Nan values as the output. Here's the code I have so far-
clc
global rho Cp MW dH
tspan = 60*[1 300000];
y0 = [1 333.15];
[t, y] = ode45(@myODE, tspan, y0)
plot(t/60, y(:,2)-273.15)
function dydt = myODE(t, y)
rho = 906;
Cp = 0.4365;
MW = 104.15;
dH = -17800;
y2 = 1-y(1);
A0 = 1.964*(10^5)*exp(-10040/y(2));
A1 = 2.57-5.05*y(2)*(10^(-3));
A2 = 9.56-1.76*y(2)*(10^(-2));
A3 = -3.03+7.85*y(2)*(10^(-3));
A = A0*exp(A1*(y2) + A2*(y2^2) + A3*(y2^3));
dydt = zeros(2,1);
dydt(1) = -A*((rho*y(1)/MW)^(1.5))*y(1);
dydt(2) = (dH/(Cp*MW))*(-A*((rho*y(1)/MW)^(1.5))*y(1));
end
This is the output I get-
y =
1.0e+02 *
0.0100 + 0.0000i 3.3315 + 0.0000i
NaN + NaNi NaN + NaNi
NaN + NaNi NaN + NaNi
NaN + NaNi NaN + NaNi
I am not sure where I am going wrong here. Is there a problem with the equations? Any help is appreciated!

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回答（1 个）

Alan Stevens
Alan Stevens 2020-8-1
Have you basically got the wrong sign in dydt(2)?
tspan = 60*[1 300000];
y0 = [1 333.15];
[t, y] = ode45(@myODE, tspan, y0);
plot(t/60, y(:,2)-273.15)
function dydt = myODE(~, y)
rho = 906;
Cp = 0.4365;
MW = 104.15;
dH = -17800;
y2 = 1-y(1);
A0 = 1.964*(10^5)*exp(-10040/y(2));
A1 = 2.57-5.05*y(2)*(10^(-3));
A2 = 9.56-1.76*y(2)*(10^(-2));
A3 = -3.03+7.85*y(2)*(10^(-3));
A = A0*exp(A1*(y2) + A2*(y2^2) + A3*(y2^3));
dydt = zeros(2,1);
dydt(1) = -A*((rho*y(1)/MW)^(1.5))*y(1);
dydt(2) = (dH/(Cp*MW))*(A*((rho*y(1)/MW)^(1.5))*y(1)); % Changed the sign from -A to +A
end
  3 个评论
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Alan Stevens
Alan Stevens 2020-8-2
编辑：Alan Stevens 2020-8-2
Ok. What about in dydt(1)?
Do you expect the temperatures to increase or decrease?
Nishant Gupta
Nishant Gupta 2020-8-4
I expect the temperature to increase. As dH is negative, heat will be evolved as the reaction proceeds. y(1) is the mole fraction and y(2) is the temp.

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