The study of steady and transient boiling critical heat flux caused by heat inputs exponentially increasing with time is important to the safety evaluation of reactivity accidents in water cooled nuclear reactors. Steady and transient critical heat fluxes were measured on 1.0 mm-diameter horizontal cylinders in a pool of Freon-113 and ethanol under saturated conditions for exponentially increasing heat inputs. The periods of exponentially increasing heat inputs ranged from about 6 ms to 20 s, and the pressures ranged from atmospheric up to 690 kPa. A high-speed video camera system with a speed of 1000 frames/s was used to observe the transient boiling behavior from the start of boiling to film boiling region through maximum heat flux point. Following results were obtained. (1) Steady critical heat flux agreed with the value obtained by Kutateladze's correlation. (2)Transient critical heat fluxes can be classified into three regions depending on the period and pressure. (3) Empirical correlations for transient critical heat fluxes were obtained based on the experimental data.
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