Move the constants out of the loop. Then, see what variables depend on Tj and put them in the loop. Assuming you want to find the value of renergyfromreactor for each value of Tj, put the evaluation statement as well in the loop. Declare the result as an array of zeros. Keep a loop counter, say k to make entries into the result array. Each time in the loop, evaluate for that Tj and write into the output array. Hope this helps!
function [renergyfrom reactor, list_of_Tjs] = Radiant_Energy_From_Reactor_Surface(Ai, Ei, Viewfactorij, Ti, Tj);
Ri = 0.22;
Rj = 0.1;
H = 0.197;
Ai = pi*Ri^2;
Aj = pi*Rj^2;
Ti = 900;
Tj = 600;
Zz = Ti^4;
Ei = 0.8;
sigma = 5.6704e-8;
Viewfactorij = 0.1088;
renergyfromreactor = zeros(1, length(10:10:70));
list_of_Tjs = zeros(1, length(10:10:70));
k = 1;
for i=10:10:70
Yy = Tj^4;
ZY = Zz-Yy;
renergyfromreactor(k) = sigma*Ai*Ei*Viewfactorij*ZY;
list_of_Tjs(k) = Tj
Tj = Tj + i;
k=k+1;
end
end
