Numerical Study On Turbulent Heat Transfer Of Liquid Lead Bismuth Based On An Isotropic Four-Equation Model

The wire-wrapped assembly cooled by liquid lead bismuth is a candidate type of fuel assembly for China initiative accelerator driven transmutation system(Ci ADS)subcritical reactor.The study on the thermal-hydraulic characteristics of liquid lead bismuth in wire-wrapped rod bundles is of great significance to the safety design of Ci ADS subcritical reactor.The four-equation model is expected to improve the numerical heat transfer accuracy of liquid lead bismuth by introducing dynamic and thermal turbulence time-scales to transport turbulent Prandtl number(Pr_t).However,the numerical stability of the model is limited by its complex wall boundary conditions and transport forms.To study the thermal-hydraulic characteristics of Ci ADS wire-wrapped rod bundles,the near-wall Taylor series expansion and derivation of turbulent heat transfer variables were carried out,and an isotropic four-equation model based on the transport of isotropic dissipation rates was optimized.Based on the open-source computational fluid dynamics program Open FOAM,the conservation equations of mass,momentum and energy and the isotropic four-equation model were compiled,and a numerical solver iso4eqn Foam suitable for the study of turbulent heat transfer of liquid lead bismuth was developed.Taking liquid lead bismuth as the research object,this paper mainly obtains the following research conclusions:(1)Based on iso4eqn Foam,the full-developed process of liquid lead bismuth in parallel plates and circular tubes with molecular Prandtl number(Pr)of 0.01～0.05 was calculated.The calculated results of turbulent heat transfer are in good agreement with the direct numerical simulation data and empirical relationship,which shows that iso4eqnfoam can better predict the flow and heat transfer characteristics of liquid lead bismuth.(2)Based on iso4eqn Foam,the full-developed process of liquid lead bismuth with Pr of0.01 in quadrilateral arrangement,triangular arrangement and bare 19 rod bundle channel was analyzed.The results show that the Nusselt number predicted by iso4eqn Foam is between the empirical relationship under low Peclet number(Pe);It is relatively conservative under high Pe.Under the same Pe,the greater the pitch diameter ratio,the greater the average Pr_t;Under the same pitch diameter ratio,the average Pr_t decreases with the increase of Pe.(3)Based on iso4eqn Foam,the heat transfer experiment of wire-wrapped 19 rod bundle cooled by liquid lead bismuth was simulated.The maximum relative errors between the calculated coolant temperature and wall temperature and KALLA experimental data are 2.69%and 3.63%respectively,indicating that iso4eqn Foam has the ability to simulate the turbulent heat transfer of liquid lead bismuth under the wire-wrapped geometry.(4)Based on iso4eqn Foam,the thermal-hydraulic characteristics of liquid lead bismuth in the wire-wrapped 19 rod bundle channel of Ci ADS visual hydraulic experiment loop(VHELP)were studied.The results show that the heat transfer of liquid lead bismuth is fully developed between the 6th and 7th pitches;The average temperature of coolant in the inner channel is higher than that in the side channel and corner channel;The larger the inlet flow,the more uniform the fluid temperature and wall temperature in each subchannel;The change of center rod heat flux has little effect on the overall distribution of temperature fluctuation and Pr_t.The relevant research results of this paper can provide reference for the study of the turbulent heat transfer mechanism of liquid lead bismuth,and provide data support and solution program for the design and safety evaluation of rod bundle assembly for wire winding and positioning cooled by liquid lead bismuth of Ci ADS.