Numerical framework for transcritical real-fluid reacting flow simulations using the flamelet progress variable approach

An extension to the classical FPV model is developed for transcritical real-fluid combustion simulations in the context of finite volume, fully compressible, explicit solvers. A double-flux model is developed for transcritical flows to eliminate the spurious pressure oscillations. A hybrid scheme with entropy-stable flux correction is formulated to robustly represent large density ratios. The thermodynamics for ideal-gas values is modeled by a linearized specific heat ratio model. Parameters needed for the cubic EoS are pre-tabulated for the evaluation of departure functions and a quadratic expression is used to recover the attraction parameter. The novelty of the proposed approach lies in the ability to account for pressure and temperature variations from the baseline table. Cryogenic LOX/GH2 mixing and reacting cases are performed to demonstrate the capability of the proposed approach in multidimensional simulations. The proposed combustion model and numerical schemes are directly applicable for LES simulations of real applications under transcritical conditions.Comment: 55th AIAA Aerospace Sciences Meeting, Dallas, T

Adsorptionsuntersuchungen an Organokieselsäurepolymeren

BET-Oberflächen und Isothermen von n-Hexan, Benzen, Wasser und Stickstoff wurden an vier Organokieselsäurepolymeren vermessen, die sich im Vernetzungsgrad der Doppelvierring-Kieselsäureeinheiten Si8O20 und in der Struktur der Brücken unterscheiden. Ein Polymer ist mikroporös, die anderen sind makroporös. Alle Polymere verhalten sich hydrophob. Neben der Adsorption tritt Quellung bei der Einwirkung organischer Adsorptive auf. Die Zusammenhänge zwischen der Länge der Brücken und der Porosität werden diskutiert

Numerical generation of multidimensional flamelet databases using an adaptive wavelet method

The Wavelet Adaptive Multiresolution Representation (WAMR) code is used for the numerical time integration of the one-dimensional laminar diffusion flames equations in trans-critical and supercritical conditions, where the thermodynamic and transport properties exhibit large changes. These steep gradients are efficiently captured by the WAMR algorithm with an a-priori defined accuracy and an associated large reduction of the number of degrees of freedom, allowing a highly efficient flamelet database generation critical conditions

Combustion waves in a model with chain branching reaction and their stability

In this paper the travelling wave solutions in the adiabatic model with two-step chain branching reaction mechanism are investigated both numerically and analytically in the limit of equal diffusivity of reactant, radicals and heat. The properties of these solutions and their stability are investigated in detail. The behaviour of combustion waves are demonstrated to have similarities with the properties of nonadiabatic one-step combustion waves in that there is a residual amount of fuel left behind the travelling waves and the solutions can exhibit extinction. The difference between the nonadiabatic one-step and adiabatic two-step models is found in the behaviour of the combustion waves near the extinction condition. It is shown that the flame velocity drops down to zero and a standing combustion wave is formed as the extinction condition is reached. Prospects of further work are also discussed.Comment: pages 32, figures 2

Unconventional carrier-mediated ferromagnetism above room temperature in ion-implanted (Ga, Mn)P:C

Ion implantation of Mn ions into hole-doped GaP has been used to induce ferromagnetic behavior above room temperature for optimized Mn concentrations near 3 at.%. The magnetism is suppressed when the Mn dose is increased or decreased away from the 3 at.% value, or when n-type GaP substrates are used. At low temperatures the saturated moment is on the order of one Bohr magneton, and the spin wave stiffness inferred from the Bloch-law T^3/2 dependence of the magnetization provides an estimate Tc = 385K of the Curie temperature that exceeds the experimental value, Tc = 270K. The presence of ferromagnetic clusters and hysteresis to temperatures of at least 330K is attributed to disorder and proximity to a metal-insulating transition.Comment: 4 pages, 4 figures (RevTex4

Using LES to Study Reacting Flows and Instabilities in Annular Combustion Chambers

Great prominence is put on the design of aeronautical gas turbines due to increasingly stringent regulations and the need to tackle rising fuel prices. This drive towards innovation has resulted sometimes in new concepts being prone to combustion instabilities. In the particular field of annular combustion chambers, these instabilities often take the form of azimuthal modes. To predict these modes, one must compute the full combustion chamber, which remained out of reach until very recently and the development of massively parallel computers. Since one of the most limiting factors in performing Large Eddy Simulation (LES) of real combustors is estimating the adequate grid, the effects of mesh resolution are investigated by computing full annular LES of a realistic helicopter combustion chamber on three grids, respectively made of 38, 93 and 336 million elements. Results are compared in terms of mean and fluctuating fields. LES captures self-established azimuthal modes. The presence and structure of the modes is discussed. This study therefore highlights the potential of LES for studying combustion instabilities in annular gas turbine combustors

Towards a common C0-C2 mechanism: a critical evaluation of rate constants for syngas combustion kinetics

Since the pioneering studies of Tsang and Hampson [1], and of Baulch and co-workers [2, 3], the knowledge of elementary combustion kinetics has increased, largely due to more accurate theories, advanced computing facilities and progresses in experimental measurements [4]. However, no effort has been devoted to the collection and reinterpretation of this knowledge after the early 2000s. Starting in February 2017, we have collected and interpreted a very large number of direct and indirect rate constant measurements from the literature, as well as every state of the art theo-retical calculation available for 50 elementary reaction steps involved in H2/CO pyrolysis and combustion. A strong need for reconciling rate constant measurements and theory has emerged from this analysis. A significant number of the indirect measurements of rate constants and theoretical determinations seem, in fact, to disagree beyond the expected accuracy of parame-ters in the syngas subset. This is mostly due to the need for reconciliation of data and theory and the reinterpretation of the raw signals of the measurements with more accurate and better constrained models according to a careful iterative procedure. The joint effort of SMARTCAT partners at Politecnico di Milano, NUI Galway, ELTE Buda-pest and Denmark Technical University together with RWTH Aachen University (DE) and Argonne National Laboratory (USA), aims to propose a fundamentally based state of the art mechanism for syngas combustion, to serve as a reference for the entire combustion kinetics community. Due to many different reasons, models for real fuels available in the literature rely on more or less different C0-C2 subsets. These differences often do not have substantial im-pacts on the overall performances as different rates in the core mechanism are often counter-balanced by different rates in the model subset relating to heavier fuels. This leads to very sim-ilar radical distributions and therefore in similar macroscopic behavior. However, the adoption of a fundamentally based common core mechanism will constitute a substantial thrust to in-crease the robustness of higher molecular weight fuel’s kinetics

Crystallographic structure of ultrathin Fe films on Cu(100)

We report bcc-like crystal structures in 2-4 ML Fe films grown on fcc Cu(100) using scanning tunneling microscopy. The local bcc structure provides a straightforward explanation for their frequently reported outstanding magnetic properties, i.e., ferromagnetic ordering in all layers with a Curie temperature above 300 K. The non-pseudomorphic structure, which becomes pseudomorphic above 4 ML film thickness is unexpected in terms of conventional rules of thin film growth and stresses the importance of finite thickness effects in ferromagnetic ultrathin films.Comment: 4 pages, 3 figures, RevTeX/LaTeX2.0

On the interaction of vortices with mixing layers

We describe the perturbations introduced by two counter-rotating vortices - in a two-dimensional configuration - or by a vortex ring - in an axisymmetric configuration - to the mixing layer between two counterflowing gaseous fuel and air streams of the same density. The analysis is confined to the near stagnation point region, where the strain rate of the unperturbed velocity field, A0, is uniform. We restrict our attention to cases where the typical distance 2r0 between the vortices - or the characteristic vortex ring radius r0 - is large compared to both the thickness, δv, of the vorticity core and the thickness, δm∼(ν/A0)1/2, of the mixing layer. In addition, we consider that the ratio, Γ/ν, of the vortex circulation, Γ, to the kinematic viscosity, ν, is large compared to unity. Then, during the interaction time, A0,-1, the viscous and diffusion effects are confined to the thin vorticity core and the thin mixing layer, which, when seen with the scale r0, appears as a passive interface between the two counterflowing streams when they have the same density. In this case, the analysis provides a simple procedure to describe the displacement and distortion of the interface, as well as the time evolution of the strain rate imposed on the mixing layer, which are needed to calculate the inner structure of the reacting mixing layer as well as the conditions for diffusion flame extinction and edge-flame propagation along the mixing layer. Although in the reacting case variable density effects due to heat release play an important role inside the mixing layer, in this paper the analysis of the inner structure is carried out using the constant density model, which provides good qualitative understanding of the mixing layer response