In thermohydraulic analysis, unsteady subcooled flow boiling is of vital importance to both natural circulation systems where flow instabilities are frequently encountered and offshore nuclear power systems which operate under the influence of ocean waves. An experimental study was conducted here to investigate how an imposed periodic flow oscillation affects the subcooled flow boiling heat transfer of water in a vertical tube. The average heat transfer characteristics and variations of the transient parameters are investigated. The result shows that there is a wall temperature overshoot as a consequence of boiling onset and the wall temperature downstream of boiling front could even drop below the saturation temperature under the high inlet subcooling of 75 °C. Under flow pulsation, intermittent flow boiling appears, when the imposed heat flux, q, is close to the boiling onset heat flux of steady flow, qs,onset. As a result of intermittent flow boiling, the average wall temperature of pulsating flow is lower than the wall temperature in steady flow when the q<qs,onset and the average wall temperature of pulsating flow is higher than the wall temperature in steady flow when the q>qs,onset. Moreover, during intermittent flow boiling, the boiling induced a decrease of the total pressure drop and can cause a large pressure fluctuation. In addition, two types variation of outlet fluid temperature fluctuations were observed and the wall temperatures present an extra local maximum and minimum values beside the extrema corresponding to the extremal mass flux.

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