Using the thermal resistance concept, heat flow through the composite is as follows: Q ˙ ˙ = T i − − T o R i + R 1 + R 2 + R o = T i − − T 1 R i = T i − − T 2 R i + R 1 = T i − − T 3 R i + R 1 + R 2 = T 1 − − T 2 R 1 = T 3 − − T o R 0 where R i = 1 h i A , R o = 1 h o A , R 1 = L 1 k 1 A , and R 2 = L 2 k 2 A