| Fuel rods are widely used in nuclear reactor cores and the coolant flow channel between the fuel rods is rod bundle channel.Different from the land-based nuclear power plant,the marine nuclear power plant rolls during operation.Rolling motion influences coolant heat transfer characteristics,which are significant to the safety operation of nuclear cores.The IAEA conferences noted that to improve the inherent safety of nuclear reactors,the passive safety system should be adopted in new designed nuclear reactors.Natural circulation system,which has high inherent safety,is one of the typical passive safety systems.The heat in the core can be removed by the natural circulation flow when the coolant systems lose power,preventing the core from burning out.Limited by the marine containment,the effective height difference between the reactor core and the steam generator is not high enough and the low driven head of the natural circulation system can only provide a low flow rate.Rolling motion changes the effective height difference and induces periodic additional inertial force as well,which can both result in oscillations in heat transfer characteristics.In this paper,the mechanism of rolling motion affecting heat transfer characteristics in a rod bundle has been investigated.Experimental research on convective heat transfer in a vertical rod bundle was performed under both natural circulation and forced circulation conditions.The differences in heat transfer characteristics between natural circulation and forced circulation flow have been studied and analyzed.Meanwhile,the differences in heat transfer characteristics for different heat transfer temperatnre differences were also studied under forced circulation conditions to investigate the mechanism of circulation mode affecting heat transfer under low flow rate conditions.The experimental results indicate that the convection in the rod bundle is mixed convection under low flow rate conditions.Natural convection has an non-negligible effect on heat transfer.For mixed convection regime,the heat transfer coefficient increased with the increasing heat transfer temperature difference.Based on the experimental data and theoretical analysis,a new correlation for mixed convection heat transfer in the rod bundle has been proposed,which is applicable to both natural circulation and forced circulation flow.Experimental research on inclination affecting convective heat transfer in a rod bundle was conducted and the experimental results shows that inclination can enhance convective heat transfer.To investigate the mechanism of this phenomenon,flow visualization experiments in an asymmetrically heated narrow rectangular channel which has simpler geometry were conducted.The experimental results indicate that transverse secondary flow induced by buoyance in the inclined ehannel i5 the key factor that enhances heat transfer.A theoretical model has been built to predict the thermal instability in the inclined asymmetrically heated narrow rectangular channel.Convective heat transfer characteristics under rolling conditions were experimental investigated for both natural circulation flow and forced circulation flow.The mechanism of rolling affecting transient heat transfer was studied by comparing the heat transfer characteristics between the two kinds of circulation modes.A correlation to predict time-average heat transfer coefficients for natural circulation flow in a rod bundle under rolling conditions was proposed.Flow boiling heat transfer characteristics,including subcooled boiling and saturated boiling,in a vertical and inclined rod bundle were experimentally studied.The flow boiling heat transfer eorrelations that are suitable for eonventional channels were benchmarked in the rod bundle.Thom correlation and modified Shah correlation can accurately predicted the subcooled boiling heat transfer coefficient of natural circulation flow in the rod bundle.Over 95%of the predictions have a deviation within 30%.Liu&Winterton correlation can accurately predicted the saturated boiling heat transfer coefficients with a deviation under 30%.The experimental result under inclined conditions shows that inclination can enhance subcooled flow boiling heat transfer and decrease saturated flow boiling heat transfer in the rod bundle.Flow boiling heat transfer characteristics under rolling conditions were experimentally investigated for both natural circulation flow and forced circulation flow.The mechanism of rolling affecting transient flow boiling heat transfer characteristics was also studied.Rolling motion was found having little effect on the time-average flow boiling heat transfer. |
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