Experimental and theoretical considerations concerning the hydrodynamics of post dryout in two-phase downflow

Cris Steven Eberle, Purdue University

Abstract

A full extent, 100 cm long, flow visualization of the two-phase hydrodynamics of post dryout downflow was performed at the Thermalhydraulics and Reactor Safety Laboratory (TRSL) at Purdue University. The overall experimental portion of this research was divided into two parts which included a quench front propagation data series and an inverted annular break-up length data series. The quench front research was used to investigate post dryout mechanisms which can occur, and it was also used to develop new instrumentation techniques for extending the visualization experiment. The experimental results were also compared to upflow rewetting experiments. Mechanisms which influenced the transient rewetting front propagation included flooding, condensible volume effects and precursory cooling. The overall effect of these phenomena was to inhibit the propagation velocity as compared to upflow. The visualization technique was extended by simultaneously acquiring both visualization and flow field data. This allowed for quantitative comparison of the visualization data which eliminated the criticism of the visualization technique as being only a qualitative research tool. The inverted annular study is unique since it represents the only post dryout full extent visualization research for concurrent downflow with heat transfer. The results provide a mechanistic means for evaluating heat transfer influences on the break-up length as a function of fluid subcooling. The most significant result for the diabatic break-up length was that global system changes, such as pressure fluctuation, cause severe decreases in the break-up length. This causes transition from inverted annular to inverted slug flow sooner than previous studies predicted. In this research a new theoretical method was developed to improve the accuracy of void fraction measurements in multi-phase flow fields. The theoretical results of this research extends the previous literature in several ways. First, a system of dimensionless equations were derived from first principles for the general neutral particle transport problem. Also, a mechanistic approach was applied to the design requirements of the densitometer, which provided an optimization technique that was used to ensure system errors were minimized. Furthermore, an extensive experimental validation of the optimization method was performed. Direct in situ measurements of the void fraction using differential pressure, double sensor resistivity probe were compared to the optimized gamma densitometer and good agreement was demonstrated between the reference measurements and the optimized gamma densitometer measurements. The results spanned the bubbly, slug and churn turbulent flow regimes in a 25.4 mm tube. The optimization was also extended to diabatic flow measurements in post dryout film boiling.

Degree

Ph.D.

Advisors

Ishii, Purdue University.

Subject Area

Engineering|Nuclear physics|Mechanical engineering

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