The understanding of the heat transfer process for helium gas cooling in the fusion blanket is important for the development of fusion reactors. This research aims at clarifying the transient heat transfer characteristics of helium gas for turbulent flow in a narrow tube. A circular platinum tube with an inner diameter of 1.8 mm was heated by exponentially increasing heat inputs and cooled by helium gas. The lengths of the tube were 30 mm and 50 mm. The heat transfer coefficients gradually increased for the e-folding time shorter than about 1.5 s. This shows that the heat transfer process is in a transient state for this region. The heat transfer coefficients increased with the increase of flow velocity, and the effect of the flow velocity became weak in the transient state region. By comparing with the experimental results of tube with different lengths, it was obtained that the quasi-steady state heat transfer coefficients for the length of 30 mm were higher than those for the lengths of 50 mm and 90 mm. However, the heated length showed a weak influence for the lengths of 30 mm and 50 mm in the transient heat transfer with a relatively smaller e-folding time. The variation of transient Nusselt number with Fourier number at different Reynolds number describes well the trend for the variation of heat transfer coefficient with e-folding time at different velocities. Based on the experimental data, an empirical correlation was obtained for transient heat transfer at different lengths by using Fourier number.
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