This study is aimed to clarify transient heat transfer process between the surface of solid and the neighboring helium gas in Very High Temperature Reactor (VHTR) or intermediate heat exchanger (IHX). In this paper a series of platinum heaters with different widths under different pressures inside a circular channel have been tested for forced convection flow. The heat generation rate of the platinum heater was increased with a function of Q_0exp(t/τ) (where t is time and τ is period of heat generation rate or e-fold time). The heaters were platinum plates with a thickness of 0.1 mm and widths of 2 mm, 4 mm and 6 mm. In the present study, the heat flux, surface temperature, and transient heat transfer coefficients were measured for helium gas passing by horizontal plates under wide experimental conditions such as velocities, pressures and periods of heat generation rate. It was clarified that the heat transfer coefficient approaches the quasi-steady-state when the period is more than around 1 s and it becomes higher when the period shorter than around 1 s. Based on the experimental data, empirical correlations for both quasi-steady-state heat transfer and transient state one at various plate-widths were obtained. It was also found that the heat transfer coefficient becomes higher with the increases of gas pressure.
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