Search

This paper addresses the numerical description of lifted flames in axisymmetric laminar coflow jets. The analysis considers moderately large values of the Reynolds number, when the boundary-layer approximation can be used to describe the slender mixing region that extends between the jet exit and the flame, providing the profiles of velocity and mixture fraction that exist immediately upstream from the flame front region. The description of the nonslender flame front region, which provides the front propagation velocity as an eigenvalue, requires integration of the Navier-Stokes Equations with account taken of the reaction and thermal expansion effects. The limiting formulations corresponding to cases of practical interest, including large values of the air-to-fuel stoichiometric ratio, are briefly discussed. Illustrative numerical results are given for flames lifted or propagating at distances small compared with the jet development length, where the mixing layer is nearly planar. [ABSTRACT FROM AUTHOR]

The starting jet produced by the impulsively started discharge of a submerged gas stream of constant velocity through a circular orifice in a plane wall is investigated by integrating numerically the axisymmetric Navier–Stokes equations for moderately large values of the jet Reynolds number. The analysis is restricted to low-Mach-number jets, for which the jet-to-ambient temperature ratio γ=Tj/To emerges as the most relevant parameter. It is seen that the leading vortex approaches a quasisteady structure propagating at an almost constant velocity, which is larger for smaller values of γ. The action of the baroclinic torque in regions of nonuniform temperature leads to significant vorticity production, with a constant overall rate equal to that of an inviscid starting jet. [ABSTRACT FROM AUTHOR]

We present new correlations for the pressure loss coefficients in two-dimensional Tee joints and 90 ∘ sharp elbows as a function of the geometric and hydrodynamic governing parameters. These are the Reynolds number of the flow (defined in the straight duct with common flow for the Tee joints), the width ratio of the perpendicular ducts and, only for the Tee joints, the mass split ratio. The analysis assumes low-to-moderate values of the Reynolds number, for which the flow remains laminar and steady. This regime is of relevance for the cooling oil flow in oil-immersed power transformer windings operated in the natural cooling mode. The new correlations have been obtained by performing a multiple regression analysis on the numerical results for combining and dividing Tee joints presented in a companion paper, and on new simulations of the flow in 90 ∘ sharp elbows following the same methodology used for the joints. As main new feature, the correlations for the pressure drop in the elements considered include an explicit dependence on the width ratio between the straight duct and the lateral branch, which is found to have a great influence on the static pressure losses and is not explicitly available from previous studies. When used for the thermal-hydraulic modeling of oil-immersed power transformer windings, the new correlations are expected to give better estimates than those currently available in the literature. • New correlations for the static pressure loss coefficients in planar T-joints and 90°elbows are obtained. • Combining and dividing flows in planar Tee joints are considered. • Correlations are obtained for moderate to low Reynolds numbers. • Explicit dependence on the width ratio is included unlike in previous studies. • Satisfactory application to thermal-hydraulic models of oil-immersed power transformer windings. [ABSTRACT FROM AUTHOR]

This paper presents a numerical investigation of the laminar flow and pressure drop characteristics of planar Tee joints, a canonical flow of interest for the thermal-hydraulic design of oil-immersed power transformer windings. After formulating the problem in nondimensional form, the steady, constant property flow in planar 90 ∘ Tee joints is computed numerically by integrating the Navier–Stokes equations with fully developed upstream and downstream boundary conditions. The analysis assumes a straight-through configuration in which the straight duct holds flow in the same direction before and after the junction, whereas the flow in the side branch can either divide from the incoming flow or combine with it. The analysis starts with the description of the flow patterns that emerge in the dividing and combining flow cases for all mass split ratios, 0 ≤ m ̇ 1 ≤ 1 , and a wide range of straight duct to side branch width ratios, 1 ≤ α ≤ 3 , and Reynolds numbers of the common branch, 0 ¡ Re 3 ≤ 200, representative of the cooling oil flow in oil-immersed transformer winding. Flow maps for planar Tee joints are then presented, showing the existence of different regions in the (Re 3 , m ̇ 1)-plane that exhibit different number and location of recirculation zones. Pressure distributions and secondary loss coefficients are then computed and analyzed, providing a numerical database that is used to develop new local pressure loss correlations for planar Tee joints in an accompanying paper. • A numerical study of laminar flow and pressure drop in planar Tee joints is presented. • First description including both dividing and combining flows. • Flow maps showing different recirculation zones are obtained. • A numerical database useful for the development of pressure loss correlations is generated. • Application to the thermal-hydraulic design of oil-immersed power transformer windings. [ABSTRACT FROM AUTHOR]

This computational study addresses deflagration initiation of lean, stoichiometric, and moderately rich hydrogen–air mixtures by the sudden discharge of a hot planar jet of its adiabatic combustion products. The objective is to determine the minimum slot size required for ignition, a relevant quantity of interest for safety and technological applications concerning the accidental ignition of hydrogen. For sufficiently small jet velocities, the numerical solution of the problem requires integration of the two-dimensional Navier–Stokes equations for chemically reacting ideal-gas mixtures, supplemented by standard descriptions of the molecular transport terms and a reduced chemical–kinetic mechanism suitable for hydrogen–air combustion. The computations provide the variation of the critical slot size for hot-jet ignition with both the jet Reynolds number and the equivalence ratio of the mixture. In particular, it is seen that, while the Reynolds number exerts only a relatively weak effect on the ignition process, the influence of the equivalence ratio is much more pronounced, with the smallest slot widths found for stoichiometric or slightly-rich conditions. The numerical results show three different ignition modes, with the flame developing from a clearly identified ignition kernel located either at the core of the leading vortex pair (mode 1), at the symmetry plane near the leading edge of the starting jet (mode 2), or at the jet stem connecting the jet exit with the starting vortex (mode 3). [ABSTRACT FROM AUTHOR]

A novel numerical algorithm for the simulation of transient combustion problems at low Mach and moderately high Reynolds numbers is presented. These problems are often characterized by the existence of a large disparity of length and time scales, resulting in the development of directional flow features, such as slender jets, boundary layers, mixing layers, or flame fronts. This makes local anisotropic adaptive techniques quite advantageous computationally. In this work we propose a local anisotropic refinement algorithm using, for the spatial discretization, unstructured triangular elements in a finite element framework. For the time integration, the problem is formulated in the context of semi-Lagrangian schemes, introducing the semi-Lagrange–Galerkin (SLG) technique as a better alternative to the classical semi-Lagrangian (SL) interpolation. The good performance of the numerical algorithm is illustrated by solving a canonical laminar combustion problem: the flame/vortex interaction. First, a premixed methane–air flame/vortex interaction with simplified transport and chemistry description (Test I) is considered. Results are found to be in excellent agreement with those in the literature, proving the superior performance of the SLG scheme when compared with the classical SL technique, and the advantage of using anisotropic adaptation instead of uniform meshes or isotropic mesh refinement. As a more realistic example, we then conduct simulations of non-premixed hydrogen–air flame/vortex interactions (Test II) using a more complex combustion model which involves state-of-the-art transport and chemical kinetics. In addition to the analysis of the numerical features, this second example allows us to perform a satisfactory comparison with experimental visualizations taken from the literature. [ABSTRACT FROM AUTHOR]

This study investigates the impact of electrolyte mixing inside the tanks of Vanadium Flow Battery (VFB) on capacity degradation. Heterogeneous mixing inside the tanks may lead to a severe capacity drop due to the existence of stagnant electrolyte regions and asymmetries in the State of Charge (SoC) of the positive and negative electrolytes reaching the cell. The study is based on a preliminary order-of-magnitude analysis involving the dimensionless Richardson and Reynolds numbers. A subsequent experimental campaign is carried out in which cell voltage and current response are screened for different tank designs and operating conditions to identify different fluid-dynamics events in the tanks. The results highlight the two main competing phenomena affecting the electrolyte fluid dynamics: buoyancy effects induced by density variations, and the inertia of the inlet submerged jet. The interplay between these two forces determines the flow field and therefore the SoC of the electrolyte feed to the cell, which affects the usable capacity and battery performance. Inertia dominated flows induce homogeneous electrolyte mixing, leading to a higher usable capacity, whereas buoyancy dominated flows result in a lower one. Furthermore, various inner structures were tested within the tanks to optimize mixing. These structures were created using additive manufacturing techniques. The implementation of helicoidal geometries within the tanks, promoting longer electrolyte paths and ample cross-sectional areas for convective mixing, markedly enhanced battery capacity compared to traditional empty tanks. These findings call for further investigation into optimizing the geometry of large-scale (VFB) storage tanks. • Heterogeneous mixing within the tanks may lead to a severe capacity drop in VFBs. • The flow dynamics results from the conflicting action of buoyancy and inertia. • Various 3D printed inner structures were tested within the tanks to optimize mixing. • An order of magnitude analysis enabled to proper understand the experimental results. • These findings call for further investigation to optimize large-scale tanks. [ABSTRACT FROM AUTHOR]