The experiment was performed on the test loop HRTL-5, which simulates the geometry and system design of the 5-MW nuclear heating reactor developed by the Institute of Nuclear Energy Technol-ogy, Tsinghua University. The flow behovior for a wied range of ilet subcoolings, in which the flow ex-perience varies from single-to two-phase, is described in a natural circulation system at different pressures (P=0.1, 0.24, and 1.5 Mpa). Several kinds of flow instability are investigated, including geysering, flashing-related flow instability. And high-frequency flow oscillation at P=0.1 and 0.24Mpa, as well as low steam quality density wave oscillation at P=1.5Mpa. The mechanisms of geysering, which has new features, and flashing-related flow instability. Which has never been studied well enough in this field, are particu-larly interpreted. The experimental results show the following: First. For a low-pressure natural circula-non system. The two-phase flow is unstable in most inlet subcooling conditions, and the two-phase stable flow can be reached only with very low inet subcoolings. Second, at high inlet subcoolings, the flow instability is dominated by subcooling boiling in the heated section, and at intermediate inlet subcoolings, it is dominated by void flashing in the adiabatic long riser. Third. In the two-phase stable flow region, the conditions for boiling out of the core. Namely. Single-phase flow in the heated section and two-phase flow in the riser due to vapor flashing, can be realized. The experimental results are of significance for the degign and accident analysis of vessel and swimming pool-type natural circulation nuclear hearting reactors.

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 occount. 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 5MW nuclear heating reactor operating at low steam quality and low system pressure. Analysis shows: (1) bifurcation phemomenon 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 phemomenon; (3) the bifurcation phenomenon of the natural circulation delay and disappear with increasing system pressure and decreasing heat flux.

This Paper describes expedmenm|and numerical investrga. tlons of sub-cooled boiling, condensadon, 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 govenffng equations was used, in which sub-cooled boiling, eondensalion nnd void flashing hove been taken into account. Based on the mathemadcal model, a program has been developed for analyzing the namral cir-culation system. As parameters, inlet sub-cooling, system pres-sure, and heat flux are varied. For comparison, some simpli-fied modeis, which are designed to reveal the importance of sub-cooled boding, condensation, flashlog in the HRTL-5 test loop. are adopted in the program. The results show: (1) sub-cooled boiling. condensatian, and void flashing may have great influence on the distributlon oF the void fracdon and morP intense at low system pressure; (2) the calculaiion of them is correlatire and interactive other than independent; (3) for a system with short heated sealon. long riser, and low pressure, it is possible to reach "boiling out of the core", where is almosl no void in the heated seelion bul much in the riser.

The experiment was carried out on fhe test loop HRTL-5, which simulates the geometry and system design of a 5 MW nuclear heating reactor The analysiswasbased on a one-dimensfonal two-phase flow drift model with conservation equations for mass, steam, energy and mo-mentum. Clausius-Clapeyron equation was used for the calculation offlashing front in the riser. A set ofordinary eguations, which describes the behavior oftwo phase flow in the natural circulation system. was derived through Integrafion of the above conservation equations for the subcooled boiling region, bulk boiling region in the heated section and for the riser. The method of time-domain was used for the talc uIation Both static and dynamic results are presented System pressure, inlet subcooling and heat flux are varied as input parameters. The results show that subcooled boiling in the heated section and void flashing In the riser have significant jnfluence oN the distribution of the void fraction, mass flow rate and flow instability ofthe system, especially at low pressure The response of nlass flow rate. after a small disturbance in the heat flux is shown, and based on it the instability map ofthe system is given through experiment and calculation. There exists three regions in the instability map of the investigated natural circulation system, namely, the stable two-phase flow region, the unstable bulk and subcooled boiling flow region and the stable subcooled boiling and single phase flow region. The mechanism of two-phase flow osciHation is interpreted.

The etperiment Was performed pn the test loop (HRTL-5). which sinmlates the geometry and system design of a 5MW nuclear beating rcator In a Wide range of inlel subcoolings different flow features. such as single phase stable flow, sub-Cooled boiling stable flow, subcooled boiling staticpow excur-sion, density wave oscillalion and stable two phase flow in the mlnlral circulalion Svstem have been described The pheno menon and mecchanisnt of static flow excursion. which has never been smdied well in this field is especialIv interpreted. The erperimental results show that static flow excursion exists under certain inlet sttbcooling condilion in which only sub-COOled boiling occurs in the heated section In the process of flow excursion the mass flow rate and the inlef temperanlre decreases. while the exit temperature increases smoothly. As tbe process of excursion contimles for about 1 hour, bubbles enter in the flow. channel resulling in shorl period dynamic flow oscillalion which can only be seen in the process of this static flow excursion and has also never be studied well. These static arid dynamic flo instabilities combine ror about 2 hour. Then a point iS reached. at which the stalic, flow excursion stops, but the dynamic flow oscillation continues. The Study of flow excursion ond its concerned dynamic flow oscillation is of great significance for the delelopment of muclear heating reactor under natural circubttion.