Piloted ignition and extinction for solid fuels

Piloted ignition of solid fuels is investigated by simulating the transport and chemical reaction in a counter-flow arrangement where a known fuel (methane) is supplied through a porous burner and the power and the location of the igniter are varied. The porous burner arrangement simulates a pyrolyzing solid fuel at constant temperature by separating the gas phase from the solid conduction and pyrolysis phenomena. An Arrhenius one-step global reaction and a simplified transport model with Lewis number equal to one were used in the simulation. Only quasi-steady conditions are considered for the gas phase in this work because the response time for the solid phenomena is, in general, much larger than the response diffusion time for the gaseous phenomena. The relation of piloted ignition to extinction is also investigated. The effect of Damköhler number on ignition and extinction and the effect of the igniter on ignition are presented through a characteristic S curve obtained by plotting the evolving maximum temperature as a function of fuel mass flux. Based on the S-shaped curve (representing the maximum temperature in the system versus the mass flux of fuel), the relationship between the piloted ignition and extinction turning points and mass fluxes has been demonstrated in this paper. The piloted ignition turning point gradually approaches the extinction turning point with increasing Damköhler number and also with increasing power of the igniter. The ignition mass flux is found to depend basically on three parameters, Damköhler number, the location of the igniter and the power of the igniter all expressed in dimensionless forms.     

Counterflow ignition and extinction of FACE gasoline fuels

The demand for petroleum-derived gasoline in the transportation sector is on the rise. For better knowledge of gasoline combustion in practical combustion systems, this study presents experimental measurements and numerical prediction of autoignition temperatures and extinction limits of six FACE (fuels for advanced combustion engines) gasoline fuels in counterflow flames. Extinction limits were measured at atmospheric pressures while the experiments for autoignition temperatures were carried out at atmospheric and high pressures. For atmospheric pressure experiment, the fuel stream consists of the pre-vaporized fuel diluted with nitrogen, while a condensed fuel configuration is used for ignition experiment at higher chamber pressures. The oxidizer stream is pure air. Autoignition temperatures of the tested fuels are nearly the same at atmospheric pressure, while a huge difference is observed as the pressure is increased. Unlike the ignition temperatures at atmospheric pressures, minor difference exists in the extinction limits of the tested fuels. Simulations were carried out using a recently developed gasoline surrogate model. Both multi-component and n-heptane/isooctane mixtures were used as surrogates for the simulations. Overall, the n-heptane/isooctane surrogate mixtures are consistently more reactive as compared the multi-component surrogate mixtures. Transport weighted enthalpy and radical index analysis was used to explain the differences in extinction strain rates for the various fuels.     

Diffusional-thermal instability in strained diffusion flames with unequal Lewis numbers

Diffusional–thermal instability is analysed for near-extinction counterflow diffusion flames to examine the instability characteristics of strained diffusion flamelets in turbulent flames, with the additional intention of providing a guideline to future experimental investigations. Attention is focused on the linear stability of the instability patterns appearing in the unstrained direction of two-dimensional counterflow diffusion flames, which is treated by the near-equilibrium regime of activation-energy asymptotics with Lewis numbers close to unity. The effects of unequal Lewis numbers for fuel and oxidizer are also taken into account by introducing an effective Lewis number. The resulting formulation describing linear stability of the harmonically decomposed disturbances turns out to be identical to the formulation derived previously for equal fuel and oxidizer Lewis numbers. For effective Lewis numbers less than unity, cellular instability is predicted for the entire range of the equivalence ratio, and the threshold Lewis number maintains a value slightly less than unity. On the other hand, for effective Lewis numbers sufficiently greater than unity, two types of oscillatory instabilities are found. As the effective Lewis number increases from unity, a travelling instability is first encountered for a range of finite wavelengths, and a pulsating instability emerges immediately above the travelling instability. These two types of oscillatory instabilities are predicted only for equivalence ratios sufficiently greater than unity because the threshold Lewis numbers for these instabilities are found to be infinity at unity equivalence ratio. For large values of the equivalence ratio, which is typical of most hydrocarbon flames, oscillatory instabilities are predicted only for flames burning extremely heavy hydrocarbon fuels or for flames heavily diluted by light inert gases.     

The influence of water on extinction and ignition of hydrogen and methane flames

The influence of water vapor on critical conditions of extinction and autoignition of premixed and nonpremixed flames is investigated. The fuels tested are hydrogen (H₂) and methane (CH₄). Studies on premixed systems are carried out by injecting a premixed reactant stream made up of fuel, oxygen (O₂), and nitrogen (N₂) from one duct, and an inert-gas stream of N₂ from the other duct. Critical conditions of extinction are measured for various amounts of water vapor added to the premixed reactant stream. The ratio of fuel to oxygen is maintained at a constant value, and the amounts of water vapor and nitrogen are so chosen that the adiabatic temperature remains the same. This ensures that the physical influence of water is the same for all cases. Therefore, changes in values for the critical conditions of extinction are attributed to the chemical influence of water vapor. Studies on nonpremixed systems are carried out by injecting a fuel stream made up of fuel and N₂ from one duct ,and an oxidizer stream made up of O₂ and N₂ from the other duct. Critical conditions of extinction are measured with water vapor added to the oxidizer stream. The concentrations of reactants are so chosen that the adiabatic temperature and the flame position stay the same for all cases. Critical conditions of autoignition are measured by preheating the oxidizer stream of the nonpremixed system. Water vapor is added to the oxidizer stream. Numerical calculations are performed using a detailed chemical-kinetic mechanism and compared with measurements. Experimental and numerical studies show that addition of water makes the premixed and nonpremixed flames easier to extinguish and harder to ignite. The chemical influence of water is attributed to its enhanced chaperon efficiency in three body reactions.     

Spatiotemporal growing wave fronts in spatially stable boundary layers

In fluid dynamical systems, it is not known a priori whether disturbances grow either in space or in time or as spatiotemporal structures. For a zero pressure gradient boundary layer (also known as the Blasius boundary layer), it is customary to treat it as a spatial problem, and some limited comparison between prediction and laboratory experiments exist. In the present work, the two-dimensional receptivity problem of a Blasius boundary layer excited by a localized harmonic source is investigated under the general spatiotemporal framework, by using the Bromwich contour integral method. While this approach is seen to be equivalent to the spatial study for unstable systems, here we show for the first time how spatially stable systems show spatiotemporally growing wave fronts.     

Electron-hole liquid in strained SiGe layers of silicon heterostructures

The electron-hole liquid has been found in strained SiGe thin films of Si/Si_1?x Ge_x/Si heterostructures. The density and binding energy of the electron-hole liquid have been determined. Owing to the presence of internal strains in the SiGe layer, the density and binding energy are significantly smaller than the respective quantities for the electron-hole liquid in a bulk single crystal of the solid solution of the same composition. The critical temperature of the transition from the exciton gas to the electron-hole liquid is estimated using the experimental data. The Mott transition (from the exciton gas to electron-hole plasma) occurs above the critical temperatures for high excitation intensities.     

Critical thickness and stress relaxation in YBaCuO (123) strained epitaxial layers and YBaCuO based strained superlattices

An energy model has been used to calculate the critical thickness h _c of YBaCuO thin films and YBaCuO based superlattices within an isotropic or anisotropic approximation. The critical thickness of single layers calculated from the anisotropic model (16 nm) is in good agreement with the previously published experimental values which are spread out from 4 to 20 nm. In the case of superlattices, relaxation appears to be governed by the critical thickness of the elementary sub-layers and is then better evaluated through the calculation performed for YBaCuO single layers. XRD measurements on YBa₂Cu₃O₇/PrBa₂Cu_3?xGa_xO₇ superlattices grown on {100{ SrTiO₃ have evidenced a tetragonal stress in the YBaCuO ab plane which remains expanded when the YBaCuO elementary layer thickness is lower than 4.8 nm (4 YBaCuO cells). However the critical temperature of the shortest period superlattices is only slightly affected by this expanded stress in contrast to the effect of an elastic stress externally applied along the ab plane of YBaCuO thin films.     

Catalytic ignition and extinction of hydrogen: comparison of simulations and experiments

Surface ignition and extinction of hydrogen/air mixtures on platinum surfaces are modeled using a detailed surface kinetic mechanism and transport phenomena. It is shown that the platinum surface can be poisoned by different adsorbates, and the dynamic process of catalytic ignition and extinction is associated with a phase transition from one poisoning species to another. For certain temperatures, multiple poisoned states of the surface coexist. Comparison of simulations with experiments is conducted, and it is shown that the self-inhibition of hydrogen catalytic ignition is caused by poisoning of platinum by atomic hydrogen.     

Adjoint-based sensitivity and ignition threshold mapping in a turbulent mixing layer

Successful ignition in non-premixed turbulent flows remains a fundamental challenge in combustion systems. Current design strategies typically rely on iterative testing to map the spatial distribution of ignition probability. We propose to accelerate this by formulating the adjoint of the perturbed and linearised governing equations in such a way that sensitivity of an ignition indicator can be obtained with a cost comparable to the flow solution. A space–time discrete adjoint method for multi-component chemically reacting flows is developed, and the gradient formed via the corresponding adjoint solution is used to identify regions favourable to ignition in a direct numerical simulation of non-premixed turbulent free shear flow. This approach requires a specific definition of an ignition metric, although this can be problematic because ignition either succeeds or fails after some period and thus gradients for some metrics become ill-defined near the ignition threshold. To this end, a quantity of interest is designed to provide short-time sensitivity in conjunction with an indicator function over a long-time period that informs whether successful ignition occurred. The gradients are used in a line-search algorithm to map the ignition boundary under specific constraints. Finally, parametric sensitivity is evaluated at different flow realisations to analyse factors governing local sensitivity in unsteady chemically reacting flows.     

间接驱动快点火锥壳靶锥体材料燃料混合问题研究

与中心点火相比,快点火将压缩和点火过程分开,大大放宽了对压缩对称性和驱动能量的要求。通过在神光Ⅱ激光装置上开展了快点火锥壳靶预压缩实验研究,利用X射线背光分幅照相方法观察到了清晰完整的快点火锥壳靶内爆压缩过程,并利用阿贝反演结合剩余烧蚀质量的方法得到了不同时刻燃料密度、面密度分布数据,当前实验条件下获得的最大压缩密度和面密度分别为30g/cm3和50mg/cm2;为解决金柱腔M带对导引锥的预热以及由此导致的燃料-锥体材料混合问题,提出了一种在锥体表面镀低Z材料的方法,实验和辐射流体数值模拟结果验证了该方法的有效性,该方法的成功解决了间接驱动快点火激光聚变的重要关键技术问题。     

间接驱动快点火锥壳靶锥体材料燃料混合问题研究

与中心点火相比,快点火将压缩和点火过程分开,大大放宽了对压缩对称性和驱动能量的要求。通过在神光Ⅱ激光装置上开展了快点火锥壳靶预压缩实验研究,利用X射线背光分幅照相方法观察到了清晰完整的快点火锥壳靶内爆压缩过程,并利用阿贝反演结合剩余烧蚀质量的方法得到了不同时刻燃料密度、面密度分布数据,当前实验条件下获得的最大压缩密度和面密度分别为30 g/cm3和50 mg/cm2;为解决金柱腔M带对导引锥的预热以及由此导致的燃料-锥体材料混合问题,提出了一种在锥体表面镀低Z材料的方法,实验和辐射流体数值模拟结果验证了该方法的有效性,该方法的成功解决了间接驱动快点火激光聚变的重要关键技术问题。     

Computational and experimental study of the two-phase mixing in gas-dynamic ignition system

The work is devoted to the determination of main peculiarities of the two-phase mixture formation in the flow duct of the gas-dynamic ignition system. The paper presents a mathematical model and the results of a numerical and experimental investigation of the peculiarities of the unsteady gas flow as well as the processes of the fragmentation and evaporation of droplets in the resonance cavity of the gas-dynamic ignition system. Different configurations of injectors for liquid supply are considered, and the influence of the most significant factors on heat release and concentration of the evaporated liquid in the resonance cavity is investigated. The obtained data may be used for choosing the injectors and the regimes of the liquid fuel supply, which enable one to ensure the stable conditions for igniting two-phase fuel mixtures in the gas-dynamic ignition system.