Experimental Study of Two-Phase Flow Oscillation in Natural Circulation

Experimental and numerical investigation of sub-cooled boiling, condensation, and void flashing in nuclear heating reactor test loop

This paper describes experimental and numerical investigations of sub-cooled boiling, condensation, and void flashing in the HRTL-5 test loop, which simulates the primary loop of a 5 MW nuclear heating reactor. A drift-flow model of two-phase with four governing equations was used, in which sub-cooled boiling, condensation, and void flashing have been taken into account. Based on the mathematical model, a program has been developed for analyzing the natural circulation system. As parameters, inlet sub-cooling, system pressure, and heat flux are varied. For comparison, some simplified models, which are designed to reveal the importance of sub-cooled boiling, condensation, flashing in the HRTL-5 test loop, are adopted in the program. The results show: (1) sub-cooled boiling, condensation, and void flashing may have great influence on the distribution of the void fraction and more intense at low system pressure; (2) the calculation of them is correlative and interactive other than independent; (3) for a system with short heated section, long riser, and low pressure, it is possible to reach “boiling out of the core”, where there is almost no void in the heated section, but much in the riser.

A mathematic model of two-phase flow taking account of sub-cooled boiling, condensation, and void flashing

Based on a physical model of the test loop HRTL-5 simulating the 5 MW nuclear heating reactor, a drift-flux model of two-phase flow has been derived, in which the sub-cooled boiling and saturated boiling in the heated section, condensation and void flashing in the adiabatic riser etc, are taken into account. It is suitable to investigate the natural circulation two-phase flow, especially at heating reactor conditions. By introducing the concept of condensation boundary layer, a condensation formula has been derived and complemented into the drift-flux model, which was used to investigate the behavior of the HRTL-5 in our previous research works. Based on the mathematical model, a computer program has been developed. At first, through comparing the calculations with the experimental results, the parameters in the two-phase drift-flux model have been determined. Then calculations on the flow characteristics of HRTL-5 have been performed, including the simulation of the mass flow rate and the distribution of various physical variables. At last, the characteristic curves of natural circulation have been simulated; thereby, also useful experiences have been acquired.

Investigation on bifurcation characteristics of a natural circulation system with low steam quality and low pressur

The two-phase flow is a kind of dissipative structure in multiphase hydrodynamics. A drift-flux model with four equations is adopted to investigate the natural circulation bifurcation, in which the sub-cooled boiling, condensation, and void fraction etc. have been taken into account. The bifurcation spectrum of a natural circulation system with low steam quality and low system pressure is presented and the physical mechanisms of bifurcation are described in detail. Bifurcation characteristics of the two-phase natural circulation indicate typical static instability observed in HRTL-5 simulating the geometry and system design of the 5 MW nuclear heating reactor operating at low steam quality and low system pressure. Analysis shows: (1) bifurcation phenomenon can occur in a two-phase natural circulation under certain thermal and geometrical condition; (2) with changing, namely increasing and decreasing, control parameters, the tracks of the work points of the natural circulation system are not identical, which shows the basic characteristic of the bifurcation phenomenon; (3) the bifurcation phenomenon of the natural circulation delay and disappear with increasing system pressure and decreasing heat flux.