A New Point of View on the Onset of Significant Void in Subcooled Boiling

Introduction: In a low pressure flow boiling applications such as research nuclear reactors, emergency cooling systems in a nuclear power plants etc., the Onset of Significant Void (OSV) may lead the system to the Ledinegg instability [1]. Such a phenomenon is undesirable because it may generate mechanical vibrations and trigger the occurrence of a boiling crisis. Many researchers such as Whittle and Forgan [2] and others tried to evaluate experimentally the conditions for the Onset of Flow Instability while Saha and Zuber [3] and others correlated experimental results of the Onset of Significant Void.

The aim of the present paper is to present a new method for measurement of the OSV and new observation point on the behavior of the void fraction in a subcooled boiling and its influence on the channel instability.

Experimental Equipment: The test section was made of 16- mm ID, stainless steel tube with a heated length of 350 mm and a copper electrode was silver soldered to each end of the tube. It was heated electrically by a D.C power supplier. The water flew inside the tube and the inlet and outlet temperature of the water as well as the pressure at the inlet and at two points of the channel were measured. Since the water in the test loop was circulated in a close loop, the inlet temperature was increase continuously during the experiment while the pressure was measured. Each experiment was ended when the exit temperature reached the saturation temperature. The OSV was achieved when the pressure drop of the channel start to increase.

Results and discussions:During the experiments the point of OSV at the exit of the channel was observed when the measured pressure drop starts to increase but due to the subcooling of the water, the generated vapor was recondensed downstream the end of the heated zone. That behavior reducing the acceleration term of the entire channel pressure drop and delaying the occurrence of the flow instability.

Conclusions: The void in a subcooled boiling recondensed downstream at the unheated zone and delaying the occurrence of the flow instability.
When the exit subcooling became small enough, the recondensation phenomenon of the void became smaller and the pressure drop on the entire channel began to increase leading to flow instability of the channel.
REFERENCES
[1] Ledinegg, M., Instability of Flow during Natural and Forced Circulation, Warmer, Vol. 61, 8, (1938)

[2] R.H. Whittle, R. Forgan, A correlation for the minima in the pressure drop versus flow-rate curves for sub-cooled water flowing in narrow heated channels, Nucl. Eng. Des. 6 (1967) 89–99.

[3] Saha, P. and Zuber, N., 1974, Point of Net Vapor Generation and Vapor Void Fraction in Sub-Cooled Boiling, Proceedings of Fifth Int. Heat Transfer Conf., Tokyo, Vol IV, pp. 175-179