In this research, to obtain fundamental experimental data of transient heat transfer and to clarify the transient heat transfer process at wide experimental conditions for the safety assessment of very high temperature reactor (VHTR), forced convection transient heat transfer coefficients were measured for Helium, Carbon dioxide, Argon and Nitrogen gases flowing over a horizontal plate due to exponentially increasing heat input. The platinum ribbon with a thickness of 0.1 mm and a width of 4.0 mm was used as the test heater and heated by electric current. The heat generation rate was controlled and measured by a heat input control system, it was exponentially increased with a function of Q0exp(t/τ). The periods (e-fold times) of heat generation rate, τ, ranged from 46 ms to 17 s, the gas flow velocities ranged from 1 to 10 m/s, the pressures ranged from 400 kPa to 800 kPa, and the gas temperatures ranged from 290 to 353 K. It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period longer than about 1 s, and it becomes higher for the period shorter than around 1 s. The heat transfer coefficient increases with the increases in pressure and velocity, and it shows some dependence on temperature at the experimental range of this research. The dependence of transient heat transfer on the gas flow velocity becomes weaker when the period becomes very shorter. Effect of gas thermal physical properties on heat transfer was investigated, and helium gas shows higher heat transfer coefficients than those of other gases due to its higher thermal conductivity. Empirical correlations for quasi-steady-state heat transfer and transient one for various gases were obtained based on the experimental data.

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