Parametric analysis of excited round jets - numerical study

A computational analysis of excited round jets is presented with emphasis on jet bifurcation phenomenon due to superposition of axial and flapping forcing terms. Various excitation parameters are examined including the amplitudes of the forcing, their frequencies and phase shift. It is shown that alteration of these parameters significantly influences the spatial jet evolution. This dependence may be used to control the jet behaviour in a wide range of qualitatively different flow structures, starting from a modification of the spreading rate of a single connected jet, through large scale deformation of an asymmetric jet, onto jet bifurcation leading to a doubly and even triply split time-averaged jet, displaying different strengths and locations of the branches. We establish that: (i) jet splitting is possible only when the amplitudes of the forcing terms are comparable to or larger than the level of natural turbulence; (ii) the angle between the developing jet branches can be directly controlled by the frequency of the axial forcing and the phase shift between axial and flapping forcing. An optimum forcing frequency is determined, leading to the largest spreading rate

LES of a non-premixed hydrogen flame stabilized by bluff-bodies of various shapes

Dynamics of flames stabilized downstream of different shape bluff-bodies (cylindrical, square, star) with different wall topologies (flat, wavy) is investigated using large-eddy simulations (LES). A two-stage computational procedure involving the ANSYS software and an in-house academic high-order code is combined to model a flow in the vicinity of the bluff-bodies and a flame formed downstream. The fuel is nitrogen-diluted hydrogen and the oxidizer is hot air in which the fuel auto-ignites. After the ignition, the flame propagates towards the bluff-body surfaces and stabilizes in their vicinity. It is shown that the flames reflect the bluff-body shape due to large-scale strong vortices induced in the shear layer formed between the main recirculation zone and the oxidizer stream. The influence of the acute corners of the bluff-bodies on the flame dynamics is quantified by analysing instantaneous and time-averaged results. Compared to the classical conical bluff-body the largest differences in the temperature and velocity distributions are observed in the configuration with the square bluff-body. The main recirculation zone is shortened by approximately 15% and at its end temperature in the axis of the flame is almost 200~K larger. Simultaneously, their fluctuations are slightly larger than in the remaining cases. The influence of the wall topology (flat vs. wavy) in the configuration with the classical conical bluff-body turned out to be very small and it resulted in modifications of the flow and flame structures only in the direct vicinity of the bluff-body surface

Global instability phenomenon as a physical mechanism controlling dynamics of a nitrogen-diluted hydrogen flame

We investigate the dynamics of counter-current reacting flow under varying strengths of the counterflow and report the occurrence of global instability phenomena. This clarifies the role of global instability in the transition between attached and lifted flames. The research is performed with the help of the Large Eddy Simulation method using a high-order numerical method. The combustion process is modelled by applying detailed chemical kinetics for hydrogen combustion with chemical reaction terms computed from the resolved scale. The flow configuration consists of a central jet of nitrogen-diluted hydrogen fuel surrounded by an annular nozzle with a suction slot enforcing counterflow in the vicinity of the fuel stream. Suction strength is controlled by the ratio between the velocity in the suction slot (Usuc) and the velocity of the fuel jet (Uj) (I=Usuc/Uj). The Reynolds number based on the fuel parameters and the central nozzle diameter is Re=1600. Two values of the suction strength are considered (I=0.1,0.2) as well as the case without suction (I=0.0). A hot oxidizer (air) provided in the region outside the co-axial nozzle causes fuel auto-ignition far from the injection system after which the flame propagates downstream. It is shown that increasing the suction strength postpones the ignition and shifts its axial location downstream. Depending on the I parameter the flame stabilizes as attached (I=0.0,0.1) or lifted (I=0.2). It is shown that the flames in the cases with I=0.0,0.1 are very similar to each other appearing as laminar non-premixed flames both in the nozzle vicinity and far downstream. The flame in case I=0.2 is turbulent and much more dynamic. Its lifting is the result of global instability appearing at increased suction in the region of counter-current flow. It is shown that the global instability induces strong toroidal vortices that create a premixed reaction zone at the flame base. This flow structuring prevents upstream flame propagation and intensifies the mixing process ensuring almost complete fuel burning in a short distance from the nozzle.</p

Numerical Study of Hydrogen Auto-Ignition Process in an Isotropic and Anisotropic Turbulent Field

The physical mechanisms underlying the dynamics of the flame kernel in stationary isotropic and anisotropic turbulent field are studied using large eddy simulations (LES) combined with a pdf approach method for the combustion model closure. Special attention is given to the ignition scenario, ignition delay, size and shape of the flame kernel among different turbulent regimes. Different stages of ignition are analysed for various levels of the initial velocity fluctuations and turbulence length scales. Impact of these parameters is found small for the ignition delay time but turns out to be significant during the flame kernel propagation phase and persists up to the stabilisation stage. In general, it is found that in the isotropic conditions, the flame growth and the rise of the maximum temperature in the domain are more dependent on the initial fluctuations level and the length scales. In the anisotropic regimes, these parameters have a substantial influence on the flame only during the initial phase of its development

Influence of the Mesh Topology on the Accuracy of Modelling Turbulent Natural and Excited Round Jets at Different Initial Turbulence Intensities

The paper is aimed at an assessment of the importance of the coordinate system (Cartesian vs. cylindrical) assumed for simulations of free-round jets. The research is performed by applying the large eddy simulation method with spatial discretisation based on high-order compact difference schemes. The results obtained for natural and excited jets at three different turbulence intensity levels, Ti=0.01%,0.1% and 1.0%, are compared. In the case of the natural jet, it is found that both instantaneous and time-averaged results are significantly dependent on the coordinate system only for the lowest Ti. In this case, in the Cartesian coordinate system, the errors introduced by an azimuthal non-uniformity of the mesh seem to have a larger impact on the solutions than the disturbances generated at the nozzle exit. The azimuthal non-uniformity of the mesh also has a substantial influence on the results of the modelling of the excited jets. In this case, the excitation is introduced as time-varying forcing, with the frequency corresponding to half of the preferred mode frequency and the amplitude equal to 5% of the jet velocity. Such an excitation leads to the formation of the so-called side-jets being revealed as inclined streams of fluid ejected outside the main jet stream. Primary attention is paid to the mechanism of the formation of the side-jets, their number and location. The results obtained on Cartesian meshes show that for very low turbulence intensity levels (Ti=0.01%), the number and direction of the side-jets are dependent on the non-uniform distribution of the mesh nodes along the azimuthal direction of the jet. On the other hand, when the cylindrical coordinate system is used, the number of the side-jets and their locations are random and dependent only on inlet parameters. It has been demonstrated that the mechanism of side-jet formation is the same in both coordinate systems; however, its random nature can only be predicted when the cylindrical coordinate system is used

Application of High-Order Compact Difference Schemes for Solving Partial Differential Equations with High-Order Derivatives

In this paper, high-order compact-difference schemes involving a large number of mesh points in the computational stencils are used to numerically solve partial differential equations containing high-order derivatives. The test cases include a linear dispersive wave equation, the non-linear Korteweg–de Vries (KdV)-like equations, and the non-linear Kuramoto–Sivashinsky equation with known analytical solutions. It is shown that very high-order compact schemes, e.g., of 20th or 24th orders, cause a very fast drop in the L2 norm error, which in some cases reaches a machine precision already on relatively coarse computational meshes

Numerical Analysis of a Flow over Spheres Embedded on a Flat Wall

This paper presents the results of numerical simulations of flow in a periodic channel with the walls covered in the central part by spherical elements that have the same overall surface areas but different radii. Two distributions of the sphere are considered, with the subsequent rows placed one after another or shifted. The computations are performed using the high-order code, whereas the solid elements are modelled with the help of the immersed boundary method. For selected cases, the results are validated by comparison with the solutions obtained using the ANSYS Fluent code on a very dense body-fitted mesh. It was found that the increase in the sphere diameter slows down the flow, which is attributed to the larger blockage of the channel cross-section caused by larger spheres and the occurrence of intense mixing (recirculation) between the spheres. The velocity profiles in the vicinity of the sphere are largely dependent on sphere diameter and rise when it increases. It was found that the distribution of the spheres plays an important role only when the spheres are large. In the part of the channel far from the sphere, the velocity profiles are significantly influenced by the sphere diameter but seem to be independent of the sphere distributions

Controlling spatio-temporal evolution of natural and excited square jets via inlet conditions

The paper presents numerical investigations of square jets in a wide range of Reynolds numbers with varying inlet turbulence characteristics. The research focuses on flow characteristics depending on inflow turbulent length/time scales and excitation frequencies in case of excited jets. It is found that the parameters of inlet turbulence affect the solutions qualitatively when the Reynolds number is sufficiently low. In these cases the impact of varying the turbulent time scale is considerably larger than changing the turbulent length scale. It was also observed that at sufficiently high Reynolds numbers the jets become quite independent of the inlet turbulence characteristics. This confirms findings of Xu et al. (Phys. Fluids, 2013) concerning weak/strong dependence of the jet evolution on inflow conditions. In case of excited jets the excitation frequencies play an important role and influence the jet behaviour most strongly at lower values of the Reynolds number. For some forcing frequencies a bifurcation occurs at sufficiently large forcing amplitudes. This phenomenon turned out to be independent of the assumed length and time scales of the turbulent fluctuations, both in terms of robustness as well as amplitude