Study On The Flow And Heat Transfer Behaviors In Rod Bundles At Low Flow Rate

In order to improve the safety and the economy of nuclear systems,the passive safety technology and the fully natural circulation mode are of increasing importance in current research and development of nuclear systems.Under condition of a natural circulation phase,the coolant velocity in the core ismuch smaller compared with that of in normal forced circulation conditions of traditional PWRs or BWRs.Generally,investigations of the flow and heat transfer in fuel assemblies focus on high flow rate conditions.At low flow conditions,the effect of buoyancy force comes into appearance more easily and modifys the flow field in the rod bundle channel and influences the heat transfer behavior.However,very few researches focus on this topic and the knowledge of this phenomenon is limited.It is meaningful to carry out studies on flow and heat transfer in rod bundles at low flow rate.It is helpful to the improvement of designs of small modular reactors and the development of passive safety systems.In this thesis,the flow and heat transfer characteristics in a 5×5 rod bundles at low flow rate were experimentally and therotically studied,both for single and two phase heat transfer conditions.The flow field and heat transfer characteristics under fully developed conditons and spacer grids effects were both investigated.In order to analyse the mechanism of the buoyancy effect to the flow field,numerical and theoretical investigations were performed.The flow boiling heat transfer behaviors in rod bundles were also studied.The main contents of present work are as follows:?1?Characteristics of fully developed single phase heat transfer in a 5×5 rod bundle were investigated experimently.The detailed wall temperature distributions of the rod bundle were measured by a special designed sliding measuring instrument.The experiment was performed at 6 MPa.The mass flow rate ranged from 25 to 150 kg/m²s and heat flux was varying from 25 to 300 kW/m².The corresponding Reynolds number ranged from 1000 to 30000,and buoyancy parameter Bo*from 2×10^-7 to 3×10^-3.The experimental data in buoyancy dominant region,mixed convection region and forced convective dominant region were obtained.The heat transfer reduction phenomenon was observed in rod bundles in mixed convection regime.It was found that the onset of buoyancy effect in rod bundles is with higher buoyancy parameter compared with that of in circular or annular channels.A correlation was porposed to predict the mixed convection heat transfer coefficients in rod bundles.?2?The effects of the spacer grid on heat transfer wereinvestigated in the 5×5 rod bundle.The experimental results indicate that,when the buoyancy parameter is high,the heat transfer downstream of the spacer grid decreases with oscillations.Furthormore,the development of heat transfer downstream of spacers could be well correlated by buoyancy parameter Bo*.A correlation used to predict the heat transfers coefficients downstream of spacers was proposed.An analytical method was also established to model the heat transfer development downstream of the spacer grid.?3?CFD method was used to investigate the flow and heat transfer behavior in rod bundles.The CFD results indicate that the buoyancy force has significant influence on the flow structure in the rod bundle channels.Due to the effect of the buoyancy force,the highest velocity in the sub-channel moves from center to the vicinity of heating wall.The turbulenct fluctuations could also be influenced by the buoyancy force,as well as the subchannel mixing in the rod bundles were much more complicated.?4?Research on the flow boiling heat transfer in the 5×5 rod bundle was also investigated.Boiling heat transfer experimental data was obtained with mass flow rate from 25 kg/m²s to 200 kg/m²s,heat flux from 25 to 75 kW/m²,and maximum quality was 0.4 The experimental data indicates that the effect of spacer grids on heat transfer can be ignored under boiling conditions.The experimental data about onset of nucleate boiling was obtained and compared with correlations reported in the literature.It's found that the existing correlations overestimated the wall temperature at ONB point.The deviations are more than 30%.For relative high flow rate conditions,the Chen's correlation predicts the boiling heat transfer coefficients well.However,owing to the effect of buoyancy force at low flow rates,the experimental boiling heat transfer coefficients are higher than Chen's correlation.