Effects Of Hydrodynamic Instabilities Research Articles

Experimental study of laminar and turbulent spherical flame propagation characteristics of hydrogen-rich syngas

Hydrogen-rich syngas, produced from biomass-based feedstocks, is a promising alternative energy source. However, its combustion characteristics are completely different from those of typical hydrocarbon fuels due to the presence of hydrogen. As a result, the kinetic mechanism of hydrogen-rich syngas premixed flame (HRSPF) propagation is very complex. In this work, the propagation of HRSPF was investigated employing different fan speeds (0–4273 rpm), hydrogen fractions (55%–95%), pressures (1.0 bar–3.0 bar), and equivalence ratios (0.6–1.0). It was found that the propagation speed of turbulent HRSPF increased when the mixing process of turbulent flow was enhanced or the stretching effect of flame front was weakened. When the turbulence intensity increased, the effect of flame inherent instability on the propagation speed of HRSPF was weakened. When the relative changes of thermal diffusion and mass diffusion of flame front were considerable, the effect of diffusional-thermal instability on flame acceleration process was more significant. On the contrary, the effect of hydrodynamic instability on flame acceleration process was more significant.

Analysis of explosion and laminar combustion characteristics of premixed ammonia-air/oxygen mixtures

Ammonia is suggested to be a potential carbon-free energy source because of its high volumetric energy density and has attracted attention recently. However, systematic investigations into the explosion characteristics and flame instability of ammonia are still lacking, particularly for low-pressure ammonia-oxygen mixture. The use of low-pressure ammonia-oxygen mixtures could not only benefit for increasing engine's efficiency but also enhance its operational safety. In this study, explosion experiments of premixed ammonia-air/oxygen mixtures are carried out in a 20-L spherical chamber. By adjusting the initial pressure (P0) and equivalence ratio (ϕ), the maximum explosion pressure (Pmax) and maximum rate of pressure rise ((dP/dt)max) are measured. The laminar flame characteristics are analysed simultaneously. The results show that for the ammonia-air mixture, Pmax increases as P0 increases and reaches the peak at ϕ = 1.1; the unstretched laminar burning velocity (SL), Markstein length (Lb), and laminar flame thickness (δ) decrease as the initial pressure increases and reach the peak at ϕ = 1.1. For the ammonia-oxygen mixture, the explosion parameters linearly increase as P0 increases, while the maximum values of the parameters are obtained at ϕ = 1.0; the unstretched laminar burning velocity increases first and then gradually decreases as P0 increases; the maximum SL is 1.14 m/s at P0 = 0.1 MPa and ϕ = 1.0, which is 18 times that of the premixed ammonia-air mixture under the same condition. The schlieren images demonstrate that the buoyancy instability has significant impacts on the premixed ammonia-air flame propagation, while it has negligible impacts on the premixed ammonia-oxygen flame propagation. The effect of hydrodynamic instability on flame propagation could be observed in both the premixed ammonia-air flame at ϕ > 1.0 and the premixed ammonia-oxygen flame at P0 > 0.1 MPa and ϕ < 1.0. In addition, it is proposed that for the premixed ammonia-air flame, the buoyancy instability would remarkably influence the flame propagation when the unstretched laminar burning velocity is < 5 cm/s. The fundamental research on the combustion characteristics of ammonia-air and ammonia-oxygen in this study can provide an experimental reference for the later development of chemical reaction mechanisms, especially for the mechanism suitable for ammonia combustion in oxygen-rich environments. Meanwhile, the combustion characteristics of the ammonia-oxygen mixture at ultra-low pressure also provide a data basis for the possibility of using ammonia fuel as a propellant for hypersonic vehicles in the future.

Self-wrinkling induced by Darrieus-Landau instability in turbulent premixed Bunsen flames from low to moderately high Reynolds numbers

In this experimental work we analyze various turbulent premixed flames under the effects (or lack thereof) of the Darrieus-Landau hydrodynamic instability. In particular, the effects of an increasing level of turbulence are considered by means of different observables. On the one hand we obtain results which can improve the understanding of fundamental mechanisms ruling the interaction between flame fronts, either unstable and stable, and turbulence. On the other hand, conclusions we draw may lead to new definitions or improvement of closure models for numerical simulation of turbulent premixed flames under the effects of hydrodynamic instability.

Critical temperature for volume ignition of deuterium–tritium fuel in inertial confinement fusion: Effects of hydrodynamic instabilities

Volume ignition is an alternative approach to inertial confinement fusion. Due to igniting the whole fuel region rather than the central hot spot compared with the central hot-spot ignition, more laser energy is needed for volume ignition. Therefore, it is much desirable to examine the ignition margin for volume ignition. Hydrodynamic instabilities are major factors responsible for degrading inertial confinement fusion implosion performance. Hydrodynamic instabilities usually bring dramatic deformations of the fuel target, and accordingly, more radiation energy loss leaks from the fuel region. Therefore, the focus of this paper is on how they influence the radiation energy loss and increase critical temperatures for volume ignition. The present results show that critical ignition temperature increases both with the perturbation mode number and the perturbation amplitudes. What is more, we find that perturbations with longitudinal mode have a greater impact than those with latitudinal mode, and targets with lower deuterium–tritium mass are more vulnerable to perturbations. The present results are important and offer support for subsequent ignition-target design.

Self-Acceleration and Self-Similarity of Hydrogen–Methane–Air Flame at Elevated Pressure

ABSTRACT By varying equivalence ratio and methane addition at elevated pressure, this paper is aimed at revealing acceleration characteristics of hydrogen–methane–air flame under thermodiffusive instability and hydrodynamic instability. On the basis of capturing flame characteristics, the flame destabilization mechanism is analyzed using effective Lewis number, density ratio and flame thickness. Then the acceleration characteristics of spherically expanding flame is evaluated using critical flame radius, critical Peclet number and acceleration exponent. Finally, the flame front evolutions of Leeff < 1.0 and Leeff > 1.0 are compared experimentally and theoretically. The results demonstrate that as the methane addition increases, the hydrogen–methane–air flame of Φ = 0.8 and Φ = 1.0 tends to be stable due to decreasing destabilization effect of both thermodiffusive instability and hydrodynamic instability. For Φ = 1.4, the stabilization effect of thermodiffusive instability and destabilization effect of hydrodynamic instability reduce monotonously with increasing methane addition, which gives rise to the transition from cellular flame to smooth flame. Both critical flame radius and critical Peclet number continue to increase with increasing methane addition. For Φ = 0.8 and Φ = 1.0, the self-similarity are emerged at x CH4 ≤ 0.5. For Φ = 1.4, the self-similarity could be only observed at x CH4 = 0.1. Especially as the methane addition increases, maximum acceleration exponent in self-similarity stage has a decreasing tendency. The flame front evolution of Leeff < 1.0 is correlated to short wavelength and the flame front evolution of Leeff > 1.0 is strongly determined on flame size.

Testing a one-dimensional prescription of dynamical shear mixing with a two-dimensional hydrodynamic simulation

The treatment of mixing processes is still one of the major uncertainties in 1D stellar evolution models. This is mostly due to the need to parametrize and approximate aspects of hydrodynamics in hydrostatic codes. In particular, the effect of hydrodynamic instabilities in rotating stars, for example, dynamical shear instability, evades consistent description. We intend to study the accuracy of the diffusion approximation to dynamical shear in hydrostatic stellar evolution models by comparing 1D models to a first-principle hydrodynamics simulation starting from the same initial conditions. We chose an initial model calculated with the stellar evolution code GENEC that is just at the onset of a dynamical shear instability but does not show any other instabilities (e.g., convection). This was mapped to the hydrodynamics code SLH to perform a 2D simulation in the equatorial plane. We compare the resulting profiles in the two codes and compute an effective diffusion coefficient for the hydro simulation. Shear instabilities develop in the 2D simulation in the regions predicted by linear theory to become unstable in the 1D model. Angular velocity and chemical composition is redistributed in the unstable region, thereby creating new unstable regions. Eventually the 2D simulation settles in a symmetric, steady state, which is Richardson stable everywhere, whereas the instability remains for longer in the 1D model due to current limitations in the 1D code. A spatially resolved diffusion coefficient is extracted by comparing the initial and final profiles of mean atomic mass. The presented simulation gives a first insight on hydrodynamics of shear instabilities in a real stellar environment and even allows us to directly extract an effective diffusion coefficient. We see evidence for a critical Richardson number of 0.25 as regions above this value remain stable for the course of the simulation.

Direct-drive inertial confinement fusion: A review

The direct-drive, laser-based approach to inertial confinement fusion (ICF) is reviewed from its inception following the demonstration of the first laser to its implementation on the present generation of high-power lasers. The review focuses on the evolution of scientific understanding gained from target-physics experiments in many areas, identifying problems that were demonstrated and the solutions implemented. The review starts with the basic understanding of laser–plasma interactions that was obtained before the declassification of laser-induced compression in the early 1970s and continues with the compression experiments using infrared lasers in the late 1970s that produced thermonuclear neutrons. The problem of suprathermal electrons and the target preheat that they caused, associated with the infrared laser wavelength, led to lasers being built after 1980 to operate at shorter wavelengths, especially 0.35 μm—the third harmonic of the Nd:glass laser—and 0.248 μm (the KrF gas laser). The main physics areas relevant to direct drive are reviewed. The primary absorption mechanism at short wavelengths is classical inverse bremsstrahlung. Nonuniformities imprinted on the target by laser irradiation have been addressed by the development of a number of beam-smoothing techniques and imprint-mitigation strategies. The effects of hydrodynamic instabilities are mitigated by a combination of imprint reduction and target designs that minimize the instability growth rates. Several coronal plasma physics processes are reviewed. The two-plasmon–decay instability, stimulated Brillouin scattering (together with cross-beam energy transfer), and (possibly) stimulated Raman scattering are identified as potential concerns, placing constraints on the laser intensities used in target designs, while other processes (self-focusing and filamentation, the parametric decay instability, and magnetic fields), once considered important, are now of lesser concern for mainline direct-drive target concepts. Filamentation is largely suppressed by beam smoothing. Thermal transport modeling, important to the interpretation of experiments and to target design, has been found to be nonlocal in nature. Advances in shock timing and equation-of-state measurements relevant to direct-drive ICF are reported. Room-temperature implosions have provided an increased understanding of the importance of stability and uniformity. The evolution of cryogenic implosion capabilities, leading to an extensive series carried out on the 60-beam OMEGA laser [Boehly et al., Opt. Commun. 133, 495 (1997)], is reviewed together with major advances in cryogenic target formation. A polar-drive concept has been developed that will enable direct-drive–ignition experiments to be performed on the National Ignition Facility [Haynam et al., Appl. Opt. 46(16), 3276 (2007)]. The advantages offered by the alternative approaches of fast ignition and shock ignition and the issues associated with these concepts are described. The lessons learned from target-physics and implosion experiments are taken into account in ignition and high-gain target designs for laser wavelengths of 1/3 μm and 1/4 μm. Substantial advances in direct-drive inertial fusion reactor concepts are reviewed. Overall, the progress in scientific understanding over the past five decades has been enormous, to the point that inertial fusion energy using direct drive shows significant promise as a future environmentally attractive energy source.

Characteristic regimes of premixed gas combustion in high-porosity micro-fibrous porous media

Dynamical behaviour of the premixed flame propagating in the inert high-porosity micro-fibrous porous media has been studied numerically. Effects of mixture filtration velocity, equivalence ratio and burner transverse size on the flame structure have been investigated and the regions of existence of different combustion regimes have been determined. It was found that the influence of the hydrodynamic instability on the flame dynamics is significant in the case of the moderate and high filtration velocities and this effect is negligible at the low velocities. At the moderate filtration velocities the effect of hydrodynamic instability manifests in the flame front deformation and in particular in the flame inclination. It was found that the flame can be stabilized within the whole interval of the filtration gas velocity, whereas in the ordinary porous media the standing wave is settled only at fixed value of gas filtration velocity. This finding is in line with recent experimental results on combustion in micro-fibrous porous media (Yang et al., Combust. Sci. Tech. 181 (2009), 1–16). Possible physical interpretation of the flame anchoring effect may be given on the base of present numerical analysis. At the high filtration velocities the hydrodynamic instability manifests itself in periodical appearance of the moving wrinkles on the flame front surface which forms non stationary high temperature trailing spots behind the leading part of the flame front. Such dynamics may be associated with splitting wave structures which were revealed in previous experiments (Yang et al., Combust. Sci. Tech. 181 (2009), 1–16).

Reprint of: Mean Flow Boundary Layer Effects of Hydrodynamic Instability of Impedance Wall

The Ingard-Myers condition, modelling the effect of an impedance wall under a mean flow by assuming a vanishing boundary layer, is known to lead to an ill-posed problem in time-domain. By analysing the stability of a mean flow, uniform except for a linear boundary layer of thickness h, in the incompressible limit, we show that the flow is absolutely unstable for h smaller than a critical hc and convectively unstable or stable otherwise. This critical hc is by nature independent of wave length or frequency and is a property of liner and mean flow only. An analytical approximation of hc is given for a mass-spring-damper liner. For an aeronautically relevant example, hc is shown to be extremely small, which explains why this instability has never been observed in industrial practice. A systematically regularised boundary condition, to replace the Ingard-Myers condition, is proposed that retains the effects of a finite h, such that the stability of the approximate problem correctly follows the stability of the real problem.

The formation of high-density core plasma in non-spherical implosion using high-resolution two-dimensional integrated implosion code

Fast ignition is an epoch-making scheme for inertial fusion energy. In this scheme, the formation of high-density core plasma is one of the key issues which must be solved, because the implosion dynamics of the fast ignition target, which is non-spherical with a conical target, is quite different from that of the conventional central ignition target. Some previous works showed the possibility of formation of a high-density core. However, the effects of hydrodynamic instability were not discussed in those works. In this paper, we simulate the whole implosion dynamics of a perturbed non-spherical target, and the effect of radiation transport instability is estimated using the two-dimensional integrated implosion (radiation hydrodynamics) code PINOCO. In the result, we have found that the hydrodynamic instabily has less of an effect on the formation of high-density core plasma in the cone-guided implosion, in comparision with the spherical implosion.

Computational study of implosion physics and target design for the fast ignition experiment FIREX-I

The fast ignition is an epoch-making scheme for inertial fusion energy. In this scheme, the formation of high-density DT fuel is one of the key issues which must be solved, because the implosion dynamics of the fast ignition target which is non-spherical with conical target is quite different from that of the conventional central ignition target. Some previous works showed the possibility of formation of high density core. But the effects of hydrodynamic instability and DT cryogenic target were not discussed in those works. In this paper, we simulate the whole implosion dynamics of a perturbed non-spherical DT cryogenic target, and evaluate the possibility of formation of high-density DT fuel using 2-D integrated implosion code PINOCO.

Effect of hydrodynamic instabilities on solute transport in a membrane system

The effect of the gravitational instability of the concentration boundary layers (CBLs) on the substance transport through a membrane is reported. The solute transfer is caused by the diffusion through the membrane and enhanced by the convective mass transport due the hydrodynamic vertical flow in near-membrane regions. The study has been carried out with binary mixtures—aqueous ethanol solution using laser interferometric technique. Measuring the concentration profiles, we have found that the transition from diffusive to diffusive–convective transport of solute at CBLs is related with stable or unstable behaviour of the CBLs. Their Rayleigh number determines the stability of these layers. The method to determine the critical concentration Rayleigh number is based on equation, which includes the diffusive permeability of the membrane, ω m, solution parameters (density, ρ, kinematics viscosity, ν, diffusion coefficient, D) and critical value of the concentration boundary layers thickness, δ crit.

The Characteristics of Spiral Vortex Flow at High Taylor Numbers

This experimental investigation is devoted to the study of combined axial and rotational flow in a concentric annular gap, of radius ratio 0.8, formed by a stationary outer and a rotatable inner cylinder. Taylor numbers varying from the critical to an order of 106 will be considered. The investigation is divided into three parts, illustrating different aspects of spiral vortex flow. Firstly, the evolution of the flow with increasing Taylor number at a constant axial Reynolds number is studied by the analysis of the spectrum of the signal from a hot-wire anemometer. Secondly, the wave length and drift velocity of the spiral vortices are determined for the axial direction. Thirdly, the effects of hydrodynamic instability on the mean flow are investigated. It should be noted that the first and second parts are under adiabatic conditions, while the third is both adiabatic and diabatic, heat being transferred isothermally through the outer wall of the annular gap. Also, all of the measurements were made in the fully-developed region of the flow.