To facilitate the analysis, 𝑈 0 and 𝑈 1 are considered constant, while 𝑈 2 varies according to the following conditions on the outer surface of the outermost tube: natural convection, light air of 1.3 m/s and breeze of 7.9 m/s blowing perpendicular to the TTHE. Since the flow in the outer annular region is laminar, the calculation of ℎ 2 is performed using the Hausen correlation , while ℎ 𝑎 is calculated by the Morgan correlation for natural (free) convection, or by the Žukauskas correlation for forced (pure) convection.