Induction Zone Research Articles

Oblique detonation waves induced by two symmetrical wedges in hydrogen-air mixtures

An understanding of oblique detonation waves (ODWs) has inherent research value for high-speed compressible reacting flow and hypersonic propulsion. In this paper, the interactions between two ODWs induced by two symmetrical finite wedges in hydrogen-air mixtures are investigated numerically by solving the reactive Euler equations and considering a detailed chemical model. Interferences between abrupt transition patterns are analyzed. The flowfield involves complex phenomena, such as interactions among shock waves, combustion waves, ODWs, and slip lines. The effects of the expansion waves and the distance between the two wedges on the flowfield characteristics are investigated, including the wave structures, initiation characteristics, as well as the temperature and density distributions. The numerical results demonstrate that the induction zone is shorter in the double-wedge configuration than the single-wedge configuration, and oblique detonations are more likely to occur during interactions. Shock polar analysis and numerical simulations show that the ODW interactions result in a Mach interaction, which is a regular interaction between the oblique shock waves (OSWs).

Characteristics of the oblique detonation flow field induced by a complex wave structure

In this paper, the initiation characteristics of the oblique detonation flow field induced by single- and double-wedge surfaces of finite length in a confined space are investigated. Numerical simulations with a detailed H2/air reaction and theoretical shock polar analyses are combined to study the influence mechanism of a complex wave system structure on the characteristics of the oblique detonation. The effects of expansion waves on the oblique detonation waves (ODWs) and their flow field characteristics for different equivalent ratios and geometric sizes are analyzed in single-wedge and double-wedge structures with the same inflow parameters. The results show that the length of the induced ODW is shorter in the double-wedge structure than in the single-wedge structure. For the single-wedge structure, the strength of the expansion wave increases, the wall temperature drops, and the characteristic length of the induction zone decreases with increasing deflection angle of the second wedge. If the strength of the expansion wave is sufficiently large, the ODW is initiated. For the double-wedge structure, the ODWs interact and form a complex wave system structure, consisting of a Mach stem, two reflected detonation waves and slip lines. The length and the temperature before and after the Mach stem decrease with an increase in the strength of the expansion waves. The effects of the expansion waves on the flow field of the ODW are relatively small at a large equivalent ratio and significantly larger at a small equivalent ratio.

Morphology of oblique detonation waves in a stoichiometric hydrogen–air mixture

Abstract

Microstructure and growth characteristics of Al2O3/Er2O3/ZrO2 solidified ceramics with different compositions

Directionally solidified Al2O3/Er2O3/ZrO2 ceramics were prepared with high frequency induction zone melting. Their phase components, microstructure and growth characters were investigated. The results indicated that the raw material powder with uniform oxides’ distribution could be obtained via the green body’s crushing and second ball-milling, which was benefited to get the fined and unified eutectic microstructure. The phase components of each sample were consistent with the prediction originated from the position of their composition in the phase diagram. The microstructures of eutectic and hyper/hypo-eutectic were basically a network with interpenetrated crystal phases, and a small amount of coarsened phase existed in the form of colony structure in the latter, the microstructure was refined with the increase of the ZrO2 concentration. The quasi-eutectic with colony structure could be prepared at the molar ratio of raw materials determined by the volume fraction. In addition, a large number of coarsened phases appeared in the microstructures of the samples with the eutectic molar ratios, so the corresponding samples were non-eutectic. These results demonstrated that the composition might be more important than the growth rate in the preparation of non-eutectic samples with eutectic microstructure, and the selection of the composition of solidified ceramics should not depend too much on the phase diagram. The eutectic growth could be divided into four processes according to the microstructures of precursor and solidified ceramics.

Modeling of destructive interaction of hot jet and solid plate

Based upon the results of a literature review, a set of physical parameters suitable for quantitative comparison in experimental and numerical studies is determined. The review showed the feasibility of modeling the process of the outflow of an immersed jet of inert gas into the surrounding space and the evolution of a free shear layer formed by hot and cold gases to analyze the process of convective and diffusion transport of various types of particles. In the model formulation we consider the problem of solid destruction by a melt of the same substance, as well as by a nitrogen stream at high temperature. The test simulation of the problems of two-phase interaction of a immersed jet of molten liquid and hot gas, on the whole, qualitatively showed the reproduction of the main characteristics of the flow: the generation of large-scale vortices around the jet in the induction zone, the oscillations of the jet when it collides with an obstacle, and the shape of the cavity formed.

A numerical investigation of oblique detonation waves in hydrogen-air mixtures at low mach numbers

Oblique detonation waves (ODWs) have potential applications in hypersonic propulsion, but the boundary of steady ODWs has not yet been examined comprehensively. In this study, Euler equations coupled with detailed chemical reaction models are used to simulate ODWs in hydrogen-air mixtures with relatively low flight Mach numbers (from 7 to 8) and different flight altitudes (30 km and 25 km). Decreases in the flight Mach number and altitude are shown to result in unsteady ODWs in a stoichiometric hydrogen-air mixture. These ODWs can be re-stabilized by decreasing the fuel‒air equivalence ratio. Regardless of different parameters of Mach number and altitude, unsteady ODWs appear only when the velocity in the induction zone exceeds that of the corresponding Chapman-Jouguet detonation. A low equivalence ratio also induces a long initiation length, limiting the availability of decreases in the equivalence ratio to maintain a steady ODW in practical applications. A flow‒combustion criterion is proposed for the application of ODWs, based on the steadiness of ODWs and fast initiation.

Initiation characteristics of oblique detonation waves from a finite wedge under argon dilution

In this paper, the flow field characteristics of Oblique Detonation Waves (ODWs) induced by a finite wedge under argon dilution are studied by solving the Euler equations with a detailed chemical model of hydrogen and air. First, the effects of the expansion waves, argon concentration, geometric parameters, and Mach number on the ODW are discussed. The results show that the changes of these parameters may make the oblique detonation not be initiated. Then, the ODW initiation criterion of the finite wedge is summarized, as the characteristic length of the induction zone LC and the characteristic length of the oblique wedge LW meet the condition LC/LW < 1, the initiation of the ODW occurs; otherwise, it does not occur. What’s more, the Constant Volume Combustion (CVC) theory is applied to study the characteristic length of induction zone. It is found that CVC theory is more suitable for the “smooth transition” type of ODW flow field, the theoretical and numerical characteristic length in induction regions are in good agreement. This work is of great significance for the design of oblique detonation engines for hypersonic vehicles.

Study on initiation mode of oblique detonation induced by a finite wedge

Oblique detonation engines attract more and more attention due to their potential use in hypersonic propulsion, but the initiation of detonation still requires comprehensive study concerning the complicated interaction of wedge, supersonic flow, and heat release. In this study, the effects of the wedge angle on the initiation mode of oblique detonation have been investigated theoretically and with numerical simulation. Detailed chemical reaction models are used, and constant-volume combustion theory that considers the vibration excitation of the gas molecules is applied to study the characteristic length of the induction zone. It is found that the theoretical and numerical characteristic lengths in induction regions are in good agreement. Two initiation modes—kinetics-controlled and wave-controlled—are analyzed in depth at different Mach numbers and wedge angles. The results show that reducing the wedge angle can cause the initiation mode to change from kinetics-controlled to wave-controlled at different Mach numbers. The detailed induction zone length and the transit wedge angles between two initiation modes of wedge-induced oblique detonation have also been found.

Pulsating detonative combustion in n-heptane/air mixtures under off-stoichiometric conditions

Numerical simulations of one-dimensional pulsating detonation in off-stoichiometric n -heptane/air mixtures are conducted by solving the reactive Navier–Stokes equations with a skeletal chemical mechanism. The effects of mixture equivalence ratio, initial pressure and temperature on pulsating detonations are studied. The results show that the pulsating instabilities in n -heptane/air mixtures are strongly affected by equivalence ratio. It is seen that pulsating instability only occurs in the fuel-lean or fuel-rich cases, whereas stable detonation is obtained for near-stoichiometric mixtures. Low-frequency pulsating detonations with single mode are observed, and decoupling / coupling of the reaction front and leading shock front occur periodically during the pulsating detonation propagation. The heat release and flame structure at the reaction front of the fuel-lean case differ from those in the fuel-rich case, and thus affects the DDT process of the reaction front. The pulsating detonation frequency is considerably influenced by equivalence ratio, initial pressure and temperature. The results of chemical explosive mode analysis and budget analysis of energy equation reveal that the mixture between the reaction front and shock front is highly explosive and thermal diffusion would promote the periodic dynamics of the reaction front and shock front. It is also found that the chemical explosion mode in the induction zone consists of two parts, i.e. the autoignition dominated reaction immediately behind the leading shock front and a following propagating reaction front.

Effect of hydroxyl radical precursor addition on LTC-affected detonation in DME–$$\hbox {O}_{{2}}$$–$$\hbox {CO}_{{2}}$$ mixtures

The effects of hydrogen peroxide ( $$\hbox {H}_{2}\hbox {O}_{2}$$ ) and tert-butyl hydroperoxide (TBHP), both hydroxyl radical precursors, on the characteristic length-scales of low-temperature-chemistry-affected (LTC-affected) detonation propagating in dimethyl-ether–oxygen–carbon-dioxide (DME– $$\hbox {O} _{2}$$ – $$\hbox {CO}_{2}$$ ) mixtures were investigated using the Zeldovich–von Neumann–Doring model. A three-step energy release is observed when the $$\hbox {CO}_{2}$$ content is above a critical value, with and without $$\hbox {H}_{2}\hbox {O}_{2}$$ or TBHP addition. The effect of these two additives on the energy release dynamics and chemical kinetics has been analyzed in detail. The $$\hbox {H}_{2}\hbox {O} _{2}$$ addition induces a similar reduction of the different induction zone lengths, as well as an increase of the energy release rate. The addition of TBHP induces both a thermal and a chemical effect. Thermally, since the effective equivalence ratios of the mixtures are higher, higher Mach numbers and higher von Neumann temperatures are achieved. Chemically, an “LTC chain propagation loop” leads to a significant decrease of the induction zone length along with a substantial increase of the energy release rate, especially for the first stage of energy release. Hence, TBHP seems to be a promising additive for experimentally observing LTC-affected detonation with multi-stage energy release.

Enhanced DDT mechanism from shock-flame interactions in thin channels

We show experimentally and numerically that when a weak shock interacts with a finger flame in a narrow channel, an extremely efficient mechanism for deflagration to detonation transition occurs. This is demonstrated in a 19-mm-thick channel in hydrogen-air mixtures at pressures below 0.2 atm and weak shocks of Mach numbers 1.5 to 2. The mechanism relies primarily on the straining of the flame shape into an elongated alligator flame maintained by the anchoring mechanism of Gamezo in a bifurcated lambda shock due to boundary layers. The mechanism can increase the flame surface area by more than two orders of magnitude without any turbulence on the flame time scale. The resulting alligator-shaped flame is shown to saturate near the Chapman–Jouguet condition and further slowly accelerate until its burning velocity reaches the sound speed in the shocked unburned gas. At this state, the lead shock and further adiabatic compression of the gas in the induction zone gives rise to auto-ignition and very rapid transition to detonation through merging of numerous spontaneous flames from ignition spots. The entire acceleration can occur on a time scale comparable to the laminar flame time.

Effect of boundary layer losses on 2D detonation cellular structures

We evaluate the effect of boundary layer losses on two-dimensional H2/O2/Ar cellular detonations obtained in narrow channels. The experiments provide the details of the cellular structure and the detonation speed deficits from the ideal CJ speed. We model the effect of the boundary layer losses by incorporating the flow divergence in the third dimension due to the negative boundary layer displacement thickness, modeled using Mirels’ theory. The cellular structures obtained numerically with the resulting quasi-2D formulation of the reactive Euler equations with two-step chain-branching chemistry are found in excellent agreement with experiment, both in terms of cell dynamics and velocity deficits, provided the boundary layer constant of Mirels is modified by a factor of 2. A significant increase in the cell size is found with increasing velocity deficit. This is found to be very well captured by the induction zone increase in slower detonations due to the lower temperatures in the induction zone.

Wind speed measurement for absolute power curve determination from induction zone lidar measurements

There is growing interest in turbine performance assessment using nacelle mounted lidars. In the past, this has involved measuring the free stream wind speed, on the periphery of the operating turbine’s induction zone. As turbines increase in size, measuring at correspondingly longer ranges can start to introduce uncertainties in the wind speed, especially in complex terrain or when a turbine is part of an array. One way of reducing these uncertainties is to measure wind speeds inside the turbine’s induction zone, closer to the turbine’s rotor, then use various techniques to adjust these measurements to generate free stream equivalent windspeeds. This paper describes the application of both induction zone model and transfer function approaches. The results from various campaigns are presented and the two methods are compared using correlations. Both yield good results in terms of accuracy (usually gradients of 1.00 ± 0.004) and coefficients of determination (typically 0.99).

Wind farm blockage effects: comparison of different engineering models

The work presents four engineering methods to estimate the induction zone in front of a wind turbine and account for the wind farm blockage effect. The methods comprise the vortex cylinder model, vortex dipole model, self-similar model, and wake projection model. The majority of the models presented account for yaw misalignments and ground effect. Actuator disk simulations are used to verify the individual models. The performance of each model is evaluated both in terms of precision and computational time. The induction models are coupled to wake models within the FLOw Redirection and Induction in Steady State framework to provide the full velocity field within a wind farm. Sample wind farm computations are presented, and the impact of including induction effects into wind farm performance predictions is reported. The different codes are publicly available online.

Longitudinal coherence and short-term wind speed prediction based on a nacelle-mounted Doppler lidar

The spatial structure of turbulence in atmospheric boundary layer flows is highly relevant to wind energy. In particular, wind turbine control strategies based on inflow preview measurements require knowledge of the longitudinal evolution of turbulent flow as it approaches the rotor. These upstream measurements are usually obtained with nacelle-mounted wind lidars. In contrast to traditional in situ anemometry, lidars collect measurements within a probe volume which varies in size depending on the technology of the commercial system being used. Here, we address two issues related to the use of wind lidar to measure the incoming flow to a wind turbine: (i) whether existing longitudinal coherence models can be used to predict flow at the rotor, based on measurements performed at a distance away from the rotor; and (ii) what effect probe-volume averaging has on the inflow predictions. These two questions are critical to the design and implementation of robust wind turbine control strategies. To address these questions, we perform field measurements and large-eddy simulations to determine which incoming flow structures can be readily predicted with existing coherence models, and which require additional corrections to account for lidar volumetric averaging effects. Results reveal that the wind turbine induction zone has a negligible impact on the longitudinal coherence and first-order turbulence quantities, such as the standard deviation of velocity fluctuations. However, the phase of the signal, from which advection time periods of the turbulent structures are derived, is affected by the rotor blockage effect.

Detonation propagation across a stratified layer with a diffuse interface

A study was conducted to examine detonation propagation in a stratified layer of hydrogen-oxygen-nitrogen above an inert gas in a horizontal narrow channel. The stratified layer was produced by a gravity current, generated by retracting a door initially separating a hydrogen-oxygen-nitrogen mixture in the predetonator and a heavier inert gas in the test-section. A steady detonation wave generated in the predetonator was transmitted into the stratified layer. The reactivity of the predetonator mixture was varied via the hydrogen-oxygen equivalence ratio and the amount of nitrogen dilution. Schlieren photography was used to visualize the detonation front in the test-section, and soot foils were used to obtain the cellular structure. Schlieren imaging showed a curved detonation front that decoupled at about mid channel height, into a shock wave and trailing contact surface. Both the hydrogen-oxygen-nitrogen reactivity and the type of inert gas initially in the test-section affected the distance travelled by the detonation wave in the stratified layer. The mixture composition distribution within the test-section before ignition was obtained via a three-dimensional CFD simulation. The lateral extent of the cellular structure captured on the soot foil, coincided with the calculated inert gas mole fraction contour that corresponds to a sharp increase in the ZND induction zone length, e.g., 70% argon dilution for a stoichiometric hydrogen-oxygen predetonator mixture.

Homogeneous ignition of H2/CO/O2/N2 mixtures over palladium at pressures up to 8 bar

The catalytic and gas-phase combustion of fuel-lean H2/CO/O2/N2 mixtures over palladium was investigated experimentally and numerically at a global equivalence ratio φ = 0.285, H2:CO volumetric ratios 1-4, pressures 1-8 bar and catalyst surface temperatures 950-1200 K. In situ planar laser induced fluorescence (PLIF) of the OH radical monitored homogeneous combustion inside a channel-flow catalytic reactor, while 1-D Raman measurements of main gas-phase species concentrations across the channel boundary layer assessed the heterogeneous processes. Simulations were carried out with a 2-D numerical code using detailed heterogeneous and homogeneous chemical reaction mechanisms and realistic transport. The simulated and measured transverse species profiles attested to a transport-limited catalytic conversion of H2 and CO at all operating conditions. The OH-PLIF measurements and the simulations confirmed the establishment of appreciable homogeneous combustion only for p < 4 bar, with progressively diminishing gas-phase contribution as the pressure increased from 4 to 8 bar. This strong pressure dependence reflected the complex pressure/temperature dependence of the homogeneous ignition chemistry as well as the competition between the catalytic and gaseous reaction pathways for H2 and CO consumption. Over the gaseous induction zones (x < xig), the wall temperatures were below the pressure-dependent upper temperature limit for the decomposition of PdO to metallic Pd. Even though palladium catalysts exhibited a “self-regulating” temperature effect due to the decomposition of PdO, the attained temperatures were still sufficient to ignite homogeneous combustion of the H2/CO/O2/N2 mixtures, in contrast to hydrocarbon fuels for which gas-phase combustion was largely suppressed over PdO in the pressure range 1-8 bar. The results indicated that for the elevated pressures and preheats of syngas-fueled hetero-/homogeneous combustion power systems, gas-phase chemistry cannot be ignored during reactor design.

Effect of oxygen atom precursors addition on LTC-affected detonation in $${\hbox {DME}}{-}{\hbox {O}}_{2}{-}{\hbox {CO}}_{2}$$ mixtures

The effects of ozone $$({\hbox {O}}_{3})$$ or nitrogen dioxide $$({\hbox {NO}}_{2})$$ as oxygen atom precursors on the characteristic length-scales of low-temperature chemistry (LTC)-affected detonation propagating in dimethyl $${\hbox {ether}}{-}{\hbox {O}}_{2}{-}{\hbox {CO}}_{2}$$ mixtures were investigated using the Zeldovich–von Neumann–Doring model. The effect of these two additives on the energy release dynamics and chemical kinetics was analyzed. Under some conditions, up to three steps of energy release were observed. Ozone strengthens the LTC which results in a decrease in the induction zone length along with an increase of the energy release rate. The addition of $${\hbox {NO}}_{2}$$ provides chemical pathways which lead to a bypass of the intermediate-temperature chemistry and a decrease in the separation distance between the first and the second steps of energy release. An increase of the energy release rate is also observed for the two first peaks. Among the two additives tested, ozone appears as the most promising to experimentally observe LTC-affected detonation with multi-stage energy release.

Inductive sounding of layered earth

Research aim. The research results in the development of a technology of continuous multi-frequency induction profiling measuring magnetic field components on long survey routes and detailed elaboration of sections by means of remote induction soundings. Research methodology. Theoretical and experimental studies of magnetic field components change patterns of harmonic vertical magnetic dipole over non-uniform layered earth. Mathematical simulation of harmonic vertical magnetic dipole magnetic field in non-uniform layered earth. Analysis of results. Information value of harmonic vertical magnetic dipole magnetic field components characteristics in low-frequency induction zone has been shown when studying non-uniform geoelectric sections together with the obtained results correspondence to the type of the non-uniform section when carrying out remote inductive sounding. 30 "Izvestiya vysshikh uchebnykh zavedenii. Gornyi zhurnal". No. 4. 2020 ISSN 0536-1028 Scope of results. for extended sections of route research (road and railway, oil and gas pipelines, power transmission lines, etc.) process sequence of vertical magnetic dipole magnetic field characteristics measurement has been determined to determine geoelectric properties of a layered section by means of continuous multi-frequency profiling and determination of homogeneous sections of medium structure with their detailing by discrete analysis by the method of remote inductive sounding.

AORN Position Statement on Managing Distractions and Noise During Perioperative Patient Care

<scp>AORN</scp> Position Statement on Managing Distractions and Noise During Perioperative Patient Care