Try this:
function main
% Defining constants
a=5.9e5; % J/(min^0.5dCm^1.5)
b=0.5;
Cp=4187; % J/kgdC
rho=1000; % kg/m^3
F=0.085; % m^3/min
Tc0=25; % dC
V=2.1; % m^3
T0s=150; % dC
Tsps=85; % dC
dFc=0.05; % m^3/min
Fcs=0.51775; % m^3/min obtained from Script32
tspan=[0,10];
Tstart=[T0s,T0s,T0s,T0s];
options=odeset('RelTol', 1e-6, 'AbsTol', 1e-6);
[T,Y]=ode45(@(t,y) f33trial(t,y,a,b,Cp,rho,F,Tc0,V,T0s,Tsps,dFc,Fcs),tspan,Tstart,options);
plot(T,Y)
function output = f33trial(t,T,a,b,Cp,rho,F,Tc0,V,T0s,Tsps,dFc,Fcs)
output=zeros(4,1);
Fc=Fcs+dFc*heaviside(t-5);
output(1)=F/0.7/V*(T0s-T(1))-Fc/0.1/V*(T(1)-Tc0)*(1-exp(-a/rho/Cp*Fc^(b-1)));
output(2)=F/0.1/V*(T(1)-T(2));
output(3)=F/0.1/V*(T(2)-T(3));
output(4)=F/0.1/V*(T(3)-T(4));
end
Best wishes
Torsten.
