You have to initilaize the variables inside the loop. Read about initliaing an array in aloop.
r1=18e-2; % inner radius of sphere, m
r2=24e-2; % outer radius of sphere, m
k=0.034; % thermal conductivity of sphere wall, W /(m C)
bc1=40; % temperature at inner sphere wall, deg C
bc2=-81; % temperature at outer sphere wall, deg C
n=100*2*2*2*2; % number of unknown temperatures between the two radii
R2 = [24e-2,30e-2,36e-2] ;
for j = 1:length(R2)
r2 = R2(j) ;
m=(r2-r1)/(n+1); % step size: difference between two consecutive difference pointa, m
%%%%%%%%%% GENERATION OF MATRIX A5
for i=1:n
r(i)=r1+i*m;
a(i,i)=-2*r(i)^2/m^2;
if(i==1)
a(i,i+1)=(r(i)^2/m^2 +r(i)/(m));
end
if((i>1)&&(i<n))
a(i,i-1)=(r(i)^2/m^2 -r(i)/(m));
a(i,i+1)=(r(i)^2/m^2 +r(i)/(m));
end
if(i==n)
a(i,i-1)=(r(i)^2/m^2 -r(i)/(m));
end
end
%%%% BOUNDARY CONDITION MATRIX GENERATION OF MATRIX B5
b(1,1)=-(r(1)^2/m^2 -r(i)/(m))*bc1;
b(n,1)=-(r(n)^2/m^2 +r(i)/(m))*bc2;
t=inv(a)*b;
temp(1,1)=bc1;
temp(2:n+1,1)=t(1:n);
temp(n+2,1)=bc2;
radius(1,1)=r1;
radius(2:n+1,1)=r(1:n);
radius(n+2,1)=r2;
R(:,j) = radius' ;
T(:,j) = temp' ;
end
figure(1)
plot(R,T)
xlabel('Radius, m','fontsize',20,'fontweight','b')
ylabel('Temperature, (^{o}C)','fontsize',20,'fontweight','b')
