Scalar (heat or mass) transfer plays an important role in many industrial and engineering applications. Difficulties in experimental measurements means that there is limited detailed information available, especially in the near-wall region. Prediction in simple flows is well documented and the basis for development of many Computational Fluid Dynamics (CFD) models. This is, however, not the case for scalar transfer, especially when the Prandtl (Pr) or Schmidt number (Sc) is much greater than unity. In complex flows that involve separation and reattachment, the scalar transfer coefficient is significantly different to that of fully developed turbulent flow. The purpose of this Thesis is to investigate high Prandtl number (Pr ≥ 10) scalar transfer in fully developed (pipe) and disturbed (sudden pipe expansion) turbulent flow using CFD. Direct Numerical Simulation (DNS) is the most straight-forward approach to the solution of turbulent flows with scalar transfer. However, this technique is computationally intensive because all turbulent scales need to be resolved by the simulation. Large eddy simulation (LES) is a compromise compared to DNS. Instead of resolving all spatial scales, LES resolves only the large-scales with the small-scales being accounted for by a subgrid-scale model. Chapter 2 details the mathematical, numerical and computational details of LES with scalar transfer. From this, an optimized and highly scalable parallel LES solver was developed based on state-of-the-art LES subgrid-scale models and numerical techniques. Chapter 3 provides a verification of the LES solver for fully developed turbulent pipe flow. Reynolds numbers between Re = 180 and 1050 were simulated with a single Prandtl number of Pr = 0.71. Detailed turbulent statistics are provided for Re = 180, 395 and 590 with varying grid resolution for each Reynolds number. The results from these simulations were compared to established experimental and numerical databases of fully developed turbulent pipe and channel flows. The LES solver was shown to be in good agreement with the prior work with most discrepancies being accounted for by only reporting the resolved (large-scale) component directly reported from the LES results. For a Prandtl number close to unity, the mechanisms of turbulent transport and scalar transfer are similar. The near-wall region was shown to be dominated by large-scale sweeping structures that bring high momentum and scalar concentrations to the near-wall region. These are convected parallel to the wall as diffusion mechanisms act to transfer this to the wall where dissipation takes effect. An ejection structure then acts to transport the resultant low momentum, scalar depleted fluid back to the bulk to be replenished and continue the cycle. As the Prandtl number increases, molecular diffusivity decreases relative to viscosity, and the mechanisms of scalar transfer differ to those at Pr = 0.71. This is investigated in Chapter 4 using simulations at Re = 180, 395 and 590, with detailed statistics at Re = 395 for Pr = 0.71, 5, 10, 100 and 200. Where possible the results are compared to other numerical work and the LES solver was shown to accurately resolve the higher Prandtl number flows. There are marked variations in the scalar transfer with increasing Prandtl number as the turbulent scalar transfer becomes concentrated closer to the wall and dominated by large-scale turbulent structures. Sweeping structures are still responsible for bringing the high scalar concentrations towards the wall, however, high Prandtl number scalars are unable to completely diffuse to the wall in the time that the structure is convected parallel to the wall adjacent to the diffusive sublayer. Therefore, most of the high Prandtl number scalar is returned to the bulk via the ejection structure rather than being dissipated at the wall. Chapter 5 uses the sudden pipe expansion (SPE) to investigate disturbed turbulent flow for an inlet Reynolds numbers of Reb = 15600 and a diameter ratio of E = 1.6. These simulation parameters were chosen to match the experimental LDA measurements of Stieglmeier et al. (1989). The LES results for a range of grid resolutions were shown to be in very good agreement with the experimental work. From the LES results it was determined that the fluctuations in the wall shear stress are important in the near-wall turbulent transport. These are the result of eddies originating from the free shear layer down-washing and impinging upon the wall. This is a more effective sweeping mechanism than that observed for the fully developed turbulent pipe flow. Despite the down-wash structures impinging upon the wall, a viscous sublayer still exists in the reattachment region, albeit much thinner than the fully developed turbulent pipe flow further downstream. Using the same Reynolds number and diameter ratio, scalar transfer simulations were also undertaken in the SPE with Prandtl numbers of Pr = 0.71, 5, 10, 100 and 200. An applied scalar flux was used to heat the expanded pipe wall. The LES results are in agreement with experimental Nusselt numbers from Baughn et al. (1984) for Pr = 0.71. The disturbed turbulent flow enhances the scalar transfer and this is the result of down wash events transporting low (cold) scalar from the inlet pipe to the near-wall of the expanded pipe. This cools the heated wall and enhances localized scalar transfer downstream of the expansion. A diffusive sublayer still exists in the reattachment region within the viscous sublayer for Prandtl numbers greater than unity. As the Prandtl number increases the diffusivity decreases relative to viscosity and near-wall scalar transfer enhancement decreases as the diffusion time-scales increase.

Une analyse critique de la modélisation des échanges thermiques des câbles d’énergie en galerie est réalisée, avec l’étude de facteurs d’influences propres aux modes de pose des câbles dans ces ouvrages. Les câbles, de plusieurs kilomètres, chauffent du fait du courant transmis dans leurs âmes. Ils sont alors refroidis par une convection forcée turbulente, dans l’axe des câbles, et échangent par rayonnement avec les parois. Pour étudier ce refroidissement, un banc d’essais expérimental a été conçu, en similitude de Reynolds, modélisant différents câbles soumis à un écoulement turbulent développé. Les résultats sont comparés et étayés par des simulations numériques 3D, développées sous l’outil OpenFOAM. Les études réalisées ont permis de mettre en évidence la mésestimation importante de l’échange convectif par la norme existante. Celle-ci propose un refroidissement qui se compose en réalité pour un tiers de l’effet de supports maintenant les câbles et d’un autre tiers d’un écoulement encore en début d’établissement. Le dernier tiers étant le refroidissement effectif des câbles. L’impact des groupements des câbles est aussi étudié, avec un résultat étonnant où un groupe de deux câbles en nappe est trouvé mieux refroidis qu’un seul, lorsque ceux-ci sont placés en proche paroi de la galerie. A l’issue des travaux, une unique loi de refroidissement conservative est proposée, permettant de modéliser tous les cas traités. Cette loi a un impact important sur l’intensité maximale admissible des câbles, qui est dégradée de 5% à 9% par rapport à précédemment pour un cas idéalisé. / A critical analysis of the thermal rating of power cables in tunnels is carried out, with the study of factors of influence specific to cable laying constrains. The heat generation in the cables, due to the transiting current, is dissipated by an axial turbulent forced convection and by radiation with the tunnel walls. To characterize this cooling, an experimental mock-up tunnel has been designed, modelling various cables in a fully-developed turbulent flow. The results are compared and bolstered by 3D numerical simulations, developed under the OpenFOAM code. The studies have highlighted an important underestimation of the convective exchange by the existing standard. Their cooling law is actually a sum of the effects of racks supporting the cables, for a third, and the effect of a flow still underdeveloped for another third. The last third being the actual cooling of the cables. The impact of groups configurations is also studied, with an astonishing result for groups of two cables, which are better cooled than a single cable case, when installed close to a wall. Following the analysis, a single conservative cooling law is proposed to model all studied cases. This law has a significant impact on the permissible current in the cables, which is downgraded from 5% to 9% from previous ratings.