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Abstract: In the last years, there has been a rapid growth of research and development activities on high temperature molten salts for nuclear and non-nuclear applications. The study of their heat transfer characteristics is a key issue in the current development of the molten salt reactor (MSR) that is one of the innovative nuclear reactors proposed by the Generation IV International Forum. The MSR is a sort of circulating fuel reactor (CFR), which adopts a molten halide salt mixture containing the fissile material and playing the distinctive role of both fuel and coolant. In a thermal-neutron-spectrum MSR, the reactor core is composed by a graphite matrix (neutron moderator), through which the liquid nuclear fuel flows and leads to a strong and intrinsic coupling between thermo-hydrodynamics and neutronics. This peculiar feature requires a suitable and qualified multi-physics simulation environment for a proper description of the system (fuel/coolant+graphite) behaviour. With reference to such complex and non-linear system, the present work is intended to give two different but linked contributions, in the perspective of a multi-physics modelling able to accurately describe the synergy of the involved different physical phenomena (e.g., by means of software). [(I)] On one hand, this work is aimed at establishing a useful validation framework for the assessment of computational fluid-dynamics (CFD) analyses of liquids with internal heat generation. For this purpose, an analytic approach for both fluid velocity and temperature fields in a circular pipe surrounded by a solid region has been developed. It is an extension of the approach elaborated for pipe flow in (Di Marcello, V., Cammi, A., Luzzi, L., 2010. A generalized approach to heat transfer in pipe flow with internal heat generation. Chemical Engineering Science 65, 1301−1310), in order to take into account the heat conduction in the solid domain (represented by the graphite matrix in the specific case of interest). [(II)] On the other hand, the paper presents a preliminary investigation of the thermal-hydraulic behaviour occurring in a typical graphite-moderated MSR core channel. The developed analytic approach has been conveniently applied to this case study: (i) by testing the capabilities of to evaluate the heat transfer characteristics and the hydrodynamic behaviour of such system; and (ii) by investigating the applicability of correlations for the Nusselt number to fluids with internal heat generation. For a deeper insight into the numerical solutions provided by COMSOL, a code-to-code comparison has been also carried out, adopting a dedicated CFD finite volume software (i.e., FLUENT®). As a result, a satisfactory agreement has been found between the analytic solution and the numerical computations provided by COMSOL and FLUENT for a wide range of Reynolds numbers. As concerns the Nusselt number evaluation, the correlations usually adopted for molten salts have been proved as unsuitable for the analysed system, whereas the correlation advanced in the above mentioned paper (that explicitly takes account of the internal heat source) appears more appropriate. The results of the present work are thought to be useful because they provide: (i) a “generalized” analytic approach to the heat transfer that is applicable in a more general context; (ii) an insight into the heat transfer characteristics of the considered graphite-moderated MSR core channel; and (iii) a preliminary assessment of computations that is essential in view of the adoption of multi-physics tools (like COMSOL) for more complex and representative simulations of the dynamic behaviour of the molten salt reactor, and more in general of other kinds of CFRs. [Copyright &y& Elsevier]

Abstract: Randomly packed bed reactors are widely used in chemical process industries, because of their low cost and ease of use compared to other packing methods. However, the pressure drops in such packed beds are usually much higher than those in other packed beds, and the overall heat transfer performances may be greatly lowered. In order to reduce the pressure drops and improve the overall heat transfer performances of packed beds, structured packed beds are considered to be promising choices. In this paper, the flow and heat transfer inside small pores of some novel structured packed beds are numerically studied, where the packed beds with ellipsoidal or non-uniform spherical particles are investigated for the first time and some new transport phenomena are obtained. Three-dimensional Navier–Stokes equations and RNG k–ε turbulence model with scalable wall function are adopted for present computations. The effects of packing form and particle shape are studied in detail and the flow and heat transfer performances in uniform and non-uniform packed beds are also compared with each other. Firstly, it is found that, with proper selection of packing form and particle shape, the pressure drops in structured packed beds can be greatly reduced and the overall heat transfer performances will be improved. The traditional correlations of flow and heat transfer extracted from randomly packings are found to overpredict the pressure drops and Nusselt number for all these structured packings, and new correlations of flow and heat transfer are obtained. Secondly, it is also revealed that, both the effects of packing form and particle shape are significant on the flow and heat transfer in structured packed beds. With the same particle shape (sphere), the overall heat transfer efficiency of simple cubic (SC) packing is the highest. With the same packing form, such as face center cubic (FCC) packing, the overall heat transfer performance of long ellipsoidal particle model is the best. Furthermore, with the same particle shape and packing form, such as body center cubic (BCC) packing with spheres, the overall heat transfer performance of uniform packing model is higher than that of non-uniform packing model. The models and results presented in this paper would be useful for the optimum design of packed bed reactors. [Copyright &y& Elsevier]

Two-phase flows are regularly involved in the heat and mass transfer in industrial processes. To ensure the safety and efficiency of such processes, an accurate prediction of the flow field and phase distribution by means of Computational Fluid Dynamics (CFD) is required. Nowadays, Direct Numerical Simulations (DNS) of large-scale two-phase flow problems are not feasible due to the computational costs involved. Therefore, the Euler–Euler framework is often employed for large-scale simulations, which involves macro-scale modelling of turbulence, mass and momentum transfer. The research activities at Helmholtz–Zentrum Dresden–Rossendorf (HZDR) focus on general closure models for multiphase flows that are closer to physics and include less empiricism. As part of this effort, an Algebraic Interfacial Area Density model (AIAD) is developed for the morphology detection in the two-fluid Euler–Euler approach. Drag models for free surface flows are often based on experimental correlations, their applicability thus being limited to certain flow regimes. In this paper, a modified free-surface drag model based on local shear stress is investigated that avoids this limitation. For this purpose, the algebraic morphology detection mechanism of the AIAD model is revised. In DNS of free surface flow a dampening of the gas side turbulent fluctuations in the near surface region was found by previous investigators. This effect has also been accounted for in Euler–Euler simulations by means of dampening functions. In this work, the significance of turbulence dampening, in case of free surface flows, is examined quantitatively for the k – ω turbulence model. Model validation is performed with the commercial CFD code ANSYS CFX by means of experimental data of countercurrent, supercritical stratified air–water flow. The revised morphology detection mechanism is seen as an improvement with respect to the detection of sharp interfaces. Satisfactory quantitative agreement is achieved for the modified free surface drag model based on experimental pressure difference, liquid levels and interfacial shear stress. Furthermore, it is demonstrated that turbulence dampening has to be accounted for in the k – ω model to qualitatively reproduce the mean flow and turbulence quantities from the experiment. More CFD grade experimental data is required for further model validation. [ABSTRACT FROM AUTHOR]

Abstract: With the study object of an 100kt/a SL-II ethylene cracking furnace, this paper used Computational Fluid Dynamics (CFD) method to carry out coupled simulation studies on the flow, combustion, radiative heat transfer and thermal cracking reaction processes in the cracking furnace. The standard k–ε two-equation model was applied to turbulence simulation. The finite-rate/eddy-dissipation model was used for modeling of non-premixed combustion of the bottom burners and premixed combustion of the sidewall burners. The Discrete Ordinates (DO) model was applied to the simulation of radiative heat transfer of furnace. The simulation results show the detailed information about velocity, temperature and concentration fields in the furnace and heat flux distribution on the reactor tubes skin. This work will provide a theoretical basis for the optimization of the geometrical structure and operational parameters of the cracking furnace. [Copyright &y& Elsevier]