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[Objective] Soil respiration occupies an important proportion in the annual total soil respiration during freeze-thaw process. The effects of water content, freezing temperature and freeze-thaw cycles on soil carbon mineralization dynamics during soil freeze-thaw process were studied. [Methods] The study focuses on chernozem soil from the Nenjiang County Heshan Farm Jiusan Soil and Water Experimental Station in Heilongjiang Province, China was used as the research object to carry out an indoor freeze-thaw degree simulation test. Seven freeze-thaw cycles were carried out, and three soil moisture content were set 100% field water holding capacity (100% WHC), 60% field water holding capacity (60% WHC), and 30% field water holding capacity (30% WHC). Additionally, three environmental temperatures were used a constant 10 °C (control), -5 °C (mild freezing) and -15 °C (severe freezing). [Results] The number of freeze-thaw cycles, moisture content and freezing temperature had significant effects on CO2 emissions and the impact degrees were -0.63, 0.21 and 0.14, respectively. The thawing process notably increased soil carbon mineralization. The soil carbon mineralization of 60% WHC in the first three freeze-thaw cycles increased 33.0% and 35.2%, respectively, compared with 100% WHC and 30% WHC in the last 4 freeze-thaw cycles. In the case of severe freezing, the carbon mineralization of 100% WHC soil in the first 2 freeze-thaw cycles increased by 25.2% and 68.0% respectively, compared with 60% WHC and 30% WHC soil in the last 5 freeze-thaw cycles. [Conclusion] The number of freeze-thaw cycles had the greatest effect on soil CO2 emission, followed by moisture content, and the least freezing temperature. The freeze-thaw effect increased the cumulative CO2 emissions of soil with low water content. The cumulative CO2 emission of soil with high water content was reduced. For soils with medium moisture content, mild freezing increased CO2 cumulative emissions, while severe freezing decreased them. The first-order kinetic equation fitted the CO2 emissions from freeze-thaw soil well (R2 > 0.997), both moisture content and freezing temperature significantly influenced the potential for organic carbon mineralization (C0 value).

The annular fuel has two cooling surfaces inside and outside, which can fully take away the heat of the fuel assemblies. It is urgent to conduct research on sodium-cooled fast reactor annular fuel assemblies. The Prandtl number Pr of liquid sodium is much less than 1 (Pr ˂˂ 1). The problem is that using the traditional constant turbulent Prandtl number Prt of 0.85 ~ 0.9, which will greatly affect the accuracy of the numerical prediction of the thermal-hydraulic properties of liquid sodium. In order to address this issue, the turbulent heat transfer characteristics of the bare and wire spacers 7-pin annular fuel assembly for a sodium-cooled fast reactor are investigated by a SST k-ω-kθ-εθ model. The open-source CFD software OpenFOAM is used for this calculation. In order to verify the validity of the present calculation method, some classical liquid metal heat transfer correlations are used to compare and analyze the results of the present calculation method (kθ-εθ model), the results of the Reynolds analogy hypothesis (Prt = 0.85 model), and the results of the turbulent Prandtl number correlation (Prt = kays model). It is shown that the SST k-ω-Prt = 0.85 model overestimates the Nusselt number Nu of the annular fuel assembly. The SST k-ω-kθ-εθ model agrees well with the SST k-ω-Prt = Kays model of the bare and wire spacers 7-pin annular fuel assembly. It suggests that the numerical calculation results of the SST k-ω-kθ-εθ model are reliable in both the simple and complex fuel assembly models. In brief, the SST k-ω-kθ-εθ model is more capable of predicting the turbulent heat transfer characteristics of the bare and wire spacers 7-pin annular fuel assembly in sodium-cooled fast reactors. In addition, the thermal-hydraulic characteristics of annular fuel assemblies with wire spacers for sodium-cooled fast reactors have been investigated. This research can provide a reference for the study of turbulent heat transfer characteristics of annular fuel assemblies in sodium-cooled fast reactor. [ABSTRACT FROM AUTHOR]

Compared with the constant turbulent Prandtl number (Prt) model, a four-equation model considers the difference between the velocity and temperature boundary layer of liquid metals by introducing the turbulence time-scale, which is expected to improve the numerical heat transfer accuracy of liquid metals with low Prandtl number (Pr) heat transfer characteristics. However, the transport form of a four-equation model is limited by its complex turbulent boundary conditions. In order to simplify the boundary conditions of turbulent variables and improve numerical stability for a four-equation model, an isotropic four-equation model that can use natural near-wall boundary conditions was considered in the present work. So, based on Taylor series expansion and near-wall turbulence analysis method, an isotropic four-equation kkθεθ model was established and the kkθεθ model coefficients and key damping functions for liquid metals were obtained. Based on the opensource computational fluid dynamics program OpenFOAM and the kkθεθ model solver, the fullydeveloped flow and heat transfer processes of liquid metals (Pr＝0.01) in triangular rod bundles with different Peclet numbers (Pe＝2504 000) and different pitchdiameter ratios (P/D＝125146) were numerically calculated. The numerical Nusselt number results of the isotropic fourequation kkθεθ model, Prt＝0.85 model and Kays model were compared with the available experimental and derivation correlations. The results show that Prt＝0.85 model and Friedland correlation overestimate the Nusselt number of liquid metals, and the heat transfer results of the Kays model and the kkθεθ model are almost between the experimental correlation. The prediction Nusselt number results of the Kays model and the kkθεθ model are similar at low Peclet numbers, but the isotropic kkθεθ model is conservative compared with Kays model at high Peclet numbers. With the increase of P/D, the kkθεθ model gradually intersects with the Subbotin correlation at a certain Peclet number. If P/D is constant, the Nusselt number increases with the increase of the Peclet number. While the Peclet number is not changed, the Nusselt number increases with the increase of P/D. Then detailed local heat transfer phenomena for liquid metals in triangular rod bundles were analyzed, including dimensionless temperature, dimensionless temperature fluctuation, dimensionless thermal diffusion coefficient and dimensionless turbulent Prandtl number. Due to the large thermal conductivity and thick thermal boundary layer of liquid metals, the molecular heat conduction nearwall is stronger than the turbulent heat diffusion. When the Peclet number reaches a certain degree, the inflection point of turbulent heat diffusion greater than molecular heat conduction begins to appear. With the increase of Peclet number, the mean turbulent Prandtl number decreases. With the increase of P/D, the average turbulent Prandtl number increases. Based on the isotropic fourequation kkθεθ model, simple turbulent boundary conditions can be used and more references can be provided for the calculation of liquid metal flow and heat transfer.