Investigation Of Pressure Drop And Flow Instability Characteristics Of Natural Circulation Flow Boiling In Narrow Rectangular Channel

Heat transfer of flow boiling in narrow channels has become a hot issue in the fieldof two phase flow in recent years. The narrow channel is widely used in thermalengineering, nuclear engineering, and other engineering applications due to itscompactness in structure. Heat transfer of flow boiling is a complex process, which iseven more complex in narrow channels, therefore, it is significant to study thecharacteristics of flow boiling in narrow channels. Moreover, flow instability may occurin the process of flow boiling under natural circulation, which may have serious impacton the stable operation of nuclear safety and thermal equipment. Therefore, it isnecessary to study the instability characteristics, in order to avoid running the systemunder unstable region and ensure its security and economy.Pressure drop characteristics of flow boiling under natural circulation and forcedcirculation were carried out in this paper. It is found that the mass flow rate had animportant effect on the pressure drop characteristics, and increasing of mass flow ratelead to obvious raise in pressure drop, while pressure drop and heat flux did not showmonotonic relationship under the same mass flow rate. The effect of inlet subcooling onpressure drop increased with the increase of heat flux. With high heat flux, tiny decreaseof inlet subcooling would cause a significant increase on pressure drop.Under the condition of the natural circulation, the average flow has a high degreeof coupling with the pressure decrease. With the increase in heat flux, the capacity ofthe circulation get increased which result the decrease of the flow pressure drop. Whenthe flow of the system reached the max limit value, the pressure decrease will drop to itsminimum correspondingly. On this occasion, increasing the heat flux further, the naturalcycle flow will decreased while the drop of the pressure will strongly increased.In this paper, the FDB point is used as the starting point of the two phase sectionwhich calculated by the adjusted Saha-Zuber model. A pressure drop prediction modelis proposed. It turned out that the result calculated based this new model is under the15%error range compared with the experimental data.The experiment research on flow instability characteristics of natural circulationflow boiling was conducted in this paper. The results made clear that, the density waveflow instability occurred and flow rate would fluctuate widely at the same amplitudewhen heat flux increases at the flow instability boundary. The effect of pressure and inlet subcooling on flow instability was also investigated in the narrow channel, and thesimilar conclusion was reached in the round tube. Meanwhile, the flow instabilityboundary map was drawn from the experimental results.The semi-empirical equation to predict the density wave flow instability wasobtained by dimensionless treatment on the two phase governing equation of naturalcirculation, whose error was found within±15%.