• Energy Research

Abstract Turbulent thermal mixing of fluids at different temperature in T-junctions represents a major concern for the safety of nuclear reactors. This is due to the significant thermal fluctuations that can arise under such circumstances, consequently leading to cyclic thermal stress and thermal fatigue within the pipe wall. Computational Fluid Dynamics (CFD) can be employed in order to obtain useful insights on the characteristics of the transient behaviour of the turbulent heat transfer in T-junctions. Although LES has been found to be the most accurate approach to turbulence modelling for this type of flow, its application at high Reynolds numbers is limited by its considerable computational costs. In this respect, the Unsteady Reynolds-Averaged Navier-Stokes (URANS) approach can be considered as a less computationally demanding option. In the present work, different URANS models are applied for the simulation of the thermal mixing in a T-junction at high Reynolds numbers. The numerical results are thoroughly assessed against the available experimental data. It is shown that, despite its limitations, a proper use of the URANS approach can give reasonable results for the considered flow configuration; in particular, a good prediction of the temperature fluctuations near the wall has been obtained, which is important for the evaluation of the cyclic thermal stress induced within the pipe wall. Therefore, it is concluded that URANS models can be regarded a pragmatic approach for the evaluation of temperature fluctuations in T-junction pipes. Finally, general guidelines for the application of the URANS approach for the simulation of such configurations are given.

Abstract The numerical simulation of turbulent heat transfer in complex industrial flows represents a major challenge for RANS models. The impinging jet configuration is a popular test case for turbulence models, due to both the challenging nature of the flow field for the RANS approach and its widespread use in industrial applications. As a consequence, a large number of studies in the literature is dedicated to the assessment of a broad variety of turbulence models in this configuration. In particular, a significant effort has been put into the assessment of different closures for the turbulent momentum flux. In contrast, relatively little attention has been paid to the modelling of the turbulent heat flux term, and the classical Reynolds analogy is almost universally employed for this purpose despite its well-known limitations. Nevertheless, recently there has been a growing interest towards the development and assessment of more advanced closures for the turbulent heat flux. In this context, in the present work six turbulence models relying on different closures for both the turbulent momentum and heat fluxes have been used to simulate a planar impinging jet at unity, moderate and low Prandtl number and thoroughly assessed against a recently published reference DNS database. It is shown that the use of an advanced differential closure for the Reynolds stresses can result in an improvement in the prediction of the flow field. This, in turn, directly results in a more accurate prediction of the mean temperature at unity Prandtl number when the Reynolds analogy is employed for the closure of the turbulent heat flux. On the other hand, the inadequacy of the latter approach appears evident at low Prandtl values, and the necessity to account at least for the variability of the turbulent Prandtl number is demonstrated. It is then inferred that the most promising approach for such complex cases is represented by the combined use of anisotropic models for both the turbulent flow and the thermal fields.

Abstract The aim of this paper is to numerically investigate the effects of CO2 dilution on the operation of an industrial micro gas turbine combustor in order to assess the possible application of exhaust gas recirculation (EGR) for post-combustion CO2 capture. A complete 3D model of the combustion chamber has been developed, taking into account the conjugate heat transfer (CHT) and radiation effects, and a detailed chemical mechanism has been employed in the framework of the Flamelet Generated Manifolds approach to model the combustion process. The importance of including the effects of conjugate heat transfer in the model has been demonstrated for both air-fired and EGR conditions. Also, combustion with EGR resulted in lower temperature levels with respect to the air-fired case and thus in reduced NOx production. Further, the increased presence of carbon dioxide has been observed to have an impact on both the flame speed and the flame stabilization mechanism. According to the numerical results, EGR can be a viable way to increase the CO2 content in the flue gas of dry low-emissions (DLE) combustors, and therefore enhance the efficiency of post-combustion carbon separation. At the same time, due to the reduced temperature levels within the combustion chamber, it is possible to attain lower NOx emissions without compromising the combustion efficiency under the considered EGR levels.

Abstract Turbulent heat transfer represents a considerably challenging phenomenon from the modelling point of view. In the RANS framework, the classical Reynolds analogy provides a simple and robust approach which is widely employed for the closure of the turbulent heat flux term in a broad range of applications. At the same time, there is an ever growing interest in the development and assessment of advanced models which would allow, at least to some extent, for the relaxation of the simplifying assumptions underlying the Reynolds analogy. In this respect, the use of algebraic closures for the turbulent heat flux has been proposed in the literature by different authors as a viable approach. One of these algebraic closures has been extended for its application to low Prandtl number fluids in various flow regimes, by means of calibration and assessment of the model against some basic test cases, in what is known as the AHFM-NRG+ model. In the present work the AHFM-NRG+ is applied for the first time to a relatively complex configuration, i.e. a backward facing step in both forced and mixed convection regimes with a low Prandtl working fluid, and assessed against reference DNS data. The obtained results suggest that the AHFM-NRG+ is able to provide more accurate predictions for the thermal field within the domain and for the heat transfer at the wall in comparison to the Reynolds analogy assumption. These encouraging results indicate that the AHFM-NRG+ can be considered as a promising model to improve the accuracy in the simulation of the turbulent heat transfer in industrial applications involving low Prandtl fluids.

Abstract Thermal-hydraulics is recognized as a key safety challenge in the development of liquid metal cooled reactors. At nominal operating conditions, the Prandtl number of liquid metals which are used as primary coolants, such as lead and sodium, is very low: typically of the order of 0.025–0.001. Obtaining an accurate prediction of the turbulent heat transfer at such a low Prandtl number is not an easy task for the standard turbulence models and has challenged the modellers over several decades. In the framework of the EU SESAME project, an effort has been put forward to assess and/or further develop/calibrate different turbulent heat flux closures. In this regard, the present article reports an assessment of four different turbulent heat flux closures for applications involving low-Prandtl fluids. These closures include: (i) the Reynolds analogy based on a constant turbulent Prandtl number (ii) a four-equation explicit algebraic heat flux model (AHFM) (ii) a three-equation implicit AHFM called AHFM-NRG and (iv) a non-linear second-order heat flux model called Turbulence Model for Buoyant Flows (TMBF). The performance of these turbulence models has been assessed in three different test cases against high-fidelity numerical reference data been generated within the SESAME project. The three test cases are: a natural Rayleigh-Benard convection flow, a mixed convection planar channel flow and a forced convection impinging jet flow. The shortcomings of the classical Reynolds analogy approach for low-Prandtl fluids in all flow regimes are highlighted; hence, more advanced and well-calibrated closures are recommended.

Abstract Large Eddy Simulation (LES) has become an attractive option for the modelling of turbulent combustion, and a significant effort has been put into the development of suitable models for combustion at the subgrid-scale (SGS). Comparatively, little attention has been paid to the impact of SGS stress models in the simulation of complex reactive flows. In the present work, a partially-premixed case from the Sydney swirl-stabilised flames series has been investigated using LES and the Flamelet Generated Manifold (FGM) approach, employing four different SGS stress models on three numerical grids with different levels of refinement, quantified with an a priori mesh quality criterion. It was found that the use of advanced SGS models can improve the accuracy of the results significantly in both the highly turbulent region close to the burner and in the low-turbulence region further downstream. Also, a physically sound formulation for SGS stresses was found to reduce the sensitivity of the results to mesh refinement, giving acceptable results on relatively coarse grids. This can be a significant advantage in the modelling of complex geometries where there is a need to find a compromise between mesh refinement and computational costs.

Abstract Considering the importance of safety features in the development of Generation IV nuclear reactors, an innovative and passive decay heat removal system (DHRS) has been proposed for liquid metal cooled reactors. The attention is here focused on the direct heat exchanger (DHX) of the system constituted by a bayonet tube that allows to remove the decay heat from the primary coolant; both primary and secondary fluids flow in natural circulation. Since each bayonet tube is equipped with a vacuum gap, the most important heat transfer mechanism characterizing the DHX is radiation. Furthermore, the presence of the vacuum gap guarantees a physical separation and a complete decoupling between primary and secondary fluids, enhancing the safety features of the whole system. Several CFD analyses have been carried out in order to obtain a characterization of the DHX both for sodium and lead cooled fast reactors, in order to optimize the DHX geometry on the basis of the specific application, and the results are discussed in the paper.

Abstract Turbulent heat transfer is a complex phenomenon, which is ubiquitous in engineering applications and has challenged turbulence modellers for several decades. In an attempt to simplify the problem it is often assumed that turbulent heat transfer can be inferred from the knowledge of the turbulent momentum transport, in what is known as the Reynolds analogy. This approach presents well-known drawbacks that limit its applicability to low-Prandtl fluids such as liquid metals. In an effort to overcome such limitations, an implicit Algebraic Heat Flux Model named AHFM-NRG has been recently proposed by the Nuclear Research and Consultancy Group (NRG). In the framework of the EU THINS project, this model was initially tested for a limited number of academic test cases in all three flow regimes (i.e.: natural, mixed and forced convection) and showed encouraging results. Further assessment and development of this or any other turbulent heat flux model with low-Prandtl fluids was hampered by the lack of accurate and reliable reference data. Thanks to the extensive reference database generated within the subsequent EU SESAME and MYRTE projects, the AHFM-NRG formulation has been further tested and developed. This article reports the development of the AHFM-NRG approach and its assessment against some representative test cases in all three flow regimes. It is shown that the AHFM-NRG formulations result in significant improvement with respect to the classical Reynolds analogy in all considered flow configurations.