Study On Hydraulic Characteristics Of Natural And Forced Circulation Loops Under Supercritical Pressure

Supercritical water cooled reactor (SCWR) is one of the six Generation IV reactors recommended by Generation IV International Forum (GIF), which has advantages such as high thermal efficiency, small size, simplified system, high safety, better economy and so on. Under supercritical pressure, thermo-physical properties of water experience abrupt variation around the pseudo-critical point, which couples nonlinearly with heat transfer and flow and makes heat transfer and flow features rather complex. At present, there is still little study on static characteristics of circulation loops (both for natural circulation and forced circulation), so the systematic understanding of the static flow characteristics of the circulation loop has not yet formed.In order to investigate the hydraulic characteristics of natural and forced circulation loops under supercritical pressure, two corresponding static models on natural and forced circulation loops are established in the thesis and the static models are calculated by applying appropriate nonlinear numerical computation based on continuation concept, with the hydraulic characteristics of the loops analyzed from the calculated results.The thesis is mainly involved in three aspects:(1) With the simplified theoretical modeling of both natural and forced circulation loops, as well as numerical analysis by applying appropriate nonlinear numerical computation based on continuation concept, the code of solving the static characteristics is developed for investigation.(2) Based on the typical natural circulation loop model under supercritical pressure, the hydraulic characteristic, namely, static flow characteristics, capacity of natural circulation and the maximum heat transmission limit are investigated. Parametric affects on static loop flow features from different factors are discussed. It is observed that, for typical natural circulation loop under supercritical, circulation flow of the loop firstly increases to its maximum value (maximum natural circulation capacity) with heating power increasing, with the corresponding heat power is the maximum heat transmission limit. And then the flow rate drops sharply, which means the natural circulation capacity decreases sharply. Before and near the region of the maximum circulation flow rate, it is favorable for heat transfer, while when the flow rate comes to the abrupt decreasing region, it is adverse to heat transfer. Also, the static flow instability phenomenon (flow excursion) is found in the sharp flow dropping region for certain low inlet temperature conditions.In the natural circulation loop under supercritical pressure, comparative analysis shows that: (i) system pressure has almost little effect on the maximum natural circulation capacity and the maximum heat transmission limit. While system pressure increases, flow excursion occurs for higher heating power, and the instability region becomes more narrow; (ii) With the increasing of loop diameters and height between heating and cooling centers, it tends to increasing natural circulation capacity and the maximum heat transmission limit, as well as the heating power for static instability onset, with the effect of diameter variation more obvious; (iii) Both the natural circulation capacity and the maximum heat transmission limit increase with inlet local resistance decreasing. Besides, heating power shape has some effects on natural circulation and heat transmission of the loop, but quite slight.(3) For the force circulation loop with identical geometry as the natural one, static flow characteristics and related parametric effects comparatively discussed. It is observed through analysis that the circulation flow rate - heating power relation seems quite different from that of natural circulation loop. With the increase of heat input in a certain range, the flow variation seems rather flat, while with the heat input increases to a threshold value, beyond which the system flow runs out of the region that pump characteristics dominant, circulation flow begins to decrease, which means that it goes into the adverse heat transfer region. In general, the pump plays a very important role in forced circulation, which makes the circulation flow rate and the maximum heat transmission limit much higher than those in natural circulation loop. Moreover, static flow instability (flow excursion) is also observed to occur for some conditions of low inlet temperature. Further, effects of pump characteristics on static flow feature of forced circulation are also analyzed in the thesis.