1,009 research outputs found
Unsteady dynamics of wedge-induced oblique detonations under periodic inflows
No full textTwo-dimensional, wedge-induced oblique detonation waves (ODWs) subject to periodic inflow are simulated using the reactive Euler equations with a two-step induction-reaction kinetic model. The focus of this work is how the periodic unsteadiness of a sinusoidal density disturbance with varying frequency and amplitude influences an initially established ODW structure. Three fundamental ODW structures with different transition types and inflow Mach numbers are disturbed, resulting in two types of triple-point formations: the main triple point (MTP) and the train of triple points (TTP). The TTP features multi-triple points arising almost simultaneously and traveling together, which has never been observed before. A parametric study and frequency analysis reveal that the MTP derives from forced destabilization, while the TTP derives from the combined effect of surface instability and inflow disturbance. Furthermore, a new phenomenon of MTP degeneration is observed for a proper inflow Mach number and disturbance amplitude. Finally, the oscillation amplitudes of unsteady ODWs are analyzed with respect to the Mach number and inflow disturbance, demonstrating the effects of transition type on surface unsteadiness
Effects of injection parameters on propagation patterns of hydrogen-fueled rotating detonation waves
No full textTwo-dimensional rotating detonation waves (RDWs) with separate injections of hydrogen and air are simulated using the Navier-Stokes equations together with a detailed chemical mechanism. The effects of injection stagnation temperature and slot width on the detonation propagation patterns are investigated. Results find that extremely high temperatures can lead to a chaotic mode in which detonation waves are generated and extinguished randomly. Increasing the slot width can reduce the number of detonation waves and finally trigger detonation quenching at a low injection stagnation temperature. But increasing the slot width can change the RDW propagation pattern from a chaotic to a stable mode under high injection temperature. Furthermore, the kinetic parameter t (representing the chemical reactivity of the mixture) and the kinematic parameter a (representing the mixing efficiency of hydrogen and oxygen) are introduced to distinguish the RDW propagation patterns.(c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved
Cellular Aluminum Particle-Air Detonation Based on Realistic Heat Capacity Model
No full textModeling aluminum (Al) particle-air detonation is extremely difficult because the combustion is shock-induced, and there are multi-phase heat release and transfer in supersonic flows. Existing models typically use simplified combustion to reproduce the detonation velocity, which introduces many unresolved problems. The hybrid combustion model, coupling both the diffused- and kinetics-controlled combustion, is proposed recently, and then improved to include the effects of realistic heat capacities dependent on the particle temperature. In the present study, 2D cellular Al particle-air detonations are simulated with the realistic heat capacity model and its effects on the detonation featured parameters, such as the detonation velocity and cell width, are analyzed. Numerical results show that cell width increases as particle diameter increases, similarly to the trend observed with the original model, but the cell width is underestimated without using the realistic heat capacities. Further analysis is performed by averaging the 2D cellular detonations to quasi-1D, demonstrating that the length scale of quasi-1D detonation is larger than that of truly 1D model, similar to gaseous detonations
Effects of thermal stratification on detonation development in hypersonic reactive flows
No full textGaseous detonation waves in a uniform mixture have been studied widely, but uniformity is seldom realized in practical applications such as detonation-based engines. Nonideal scenarios involving incomplete mixing and curved intake compression lead to the thermal stratification of reactants. Local high-temperature regions first trigger the reactant autoignition and even result in the untimely formation of a detonation wave. Using the two-dimensional Euler equations and a detailed H2-air reaction mechanism, we examine the effects of reactant thermal stratification on the autoignition wave morphology in hypersonic reactive flows. Three flow regimes, namely, the autoignition-driven reaction front, detonation wave, and decoupling shock/reaction front, are observed. These flow regimes are determined by the temperature gradient, and only a moderate temperature gradient can trigger an oblique detonation wave. The oblique detonation can stabilize in hypersonic inflows, primarily because the upstream autoignition region acts as an anchorage point. Comparisons of one- and two-dimensional autoignitions confirm that supersonic flow enhances the formation of pressure waves. An analysis of the reaction front propagation speeds reveals that the detonation development is determined by two aspects. One is the convergence of compression waves originating from thermal expansion and supersonic flows, and another is the reaction front propagation speed at the early stage, which must exceed the local sound speed to promote positive feedback between pressure waves and heat release
Evolution of weakly unstable oblique detonation in disturbed inflow
No full textThe surface instability of oblique detonation waves (ODWs) without perturbations has been extensively investigated, yet the impact of external perturbations remains under-explored. Utilizing reactive Euler equations coupled with a two-step induction-exothermic reaction model, this study conducts a numerical examination of the evolution of unstable ODW surfaces subjected to a continuous sinusoidal density/temperature perturbation inflow. The results show that, without inflow perturbations, the ODW can evolve into triple points in the downstream due to detonation instability, similar to previous work. However, a small continuous perturbation can induce a significant forward movement of the ODW unstable position. Surprisingly, as the perturbation magnitude increases, the changes in the unstable position become progressively less pronounced. By increasing the perturbation frequency, the oscillation amplitude first increases, but a decreasing period/stage occurs with a modest frequency. To investigate the response of ODW to the increase in perturbation, the frequency characteristics and numerical smoked cells of detonation surfaces are examined and analyzed using Fast Fourier Transformation. The power spectral density indicates the presence of two distinct oscillation modes within oblique detonation. Low-frequency, small-amplitude perturbations serve to amplify the instability of the detonation, and more irregular oscillations could be observed. Conversely, high-frequency, large-amplitude perturbations suppress the development of small-scale waves on the detonation wavefront and lead to a relative regular oscillation, indicating that the wavefront pressure oscillations are entirely determined by inflow perturbations and become predictable. These findings have significant implications for the control of intrinsically unstable ODWs, providing valuable insights into the regulation of ODW dynamics
Numerical investigation on the initiation of oblique detonation waves in stoichiometric acetylene-oxygen mixtures with high argon dilution
No full textOblique detonation waves (ODWs) in stoichiometric acetylene-oxygen mixtures, highly diluted by 81-90% argon, are studied using the reactive Euler equations with a detailed chemistry model. Numerical results show that the incident Mach number M-0 changes the ODW initiation structure, giving both the smooth transition in the case of M-0 = 10 and the abrupt transition in the case of M-0 = 7. By comparing results of numerical simulation and theoretical analysis, the initiation processes are found to be chemical kinetics-controlled regardless of M-0, different from those in hydrogen-air mixtures which are wave-controlled in the low M-0 regime. The argon dilution effect on the initiation morphology is investigated, showing that the structures are determined by the dilution ratio and M-0 collectively. However, the initiation length is found to be independent of the dilution ratio and only determined by M-0, which is attributed to the competing effect of the high density and high temperature. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved
Numerical study of wedge-induced oblique detonations in unsteady flow
No full textOblique detonation waves (ODWs) have been studied widely to facilitate their employment in hypersonic propulsion, but the effects of continuous unsteady inflow have never been addressed so far. Thus, the present study investigates wedge-induced oblique detonations in unsteady flow via numerical simulations based on the reactive Euler equations with a two-step induction-reaction kinetic model. As a first step, the chemical and flow parameters are chosen for the simplest structure such that the ODW initiation occurs under a smooth transition with a curved shock. After a steady ODW with smooth initiation transition is established, the inflow is then subject to a continuous sinusoidal density/temperature disturbance. Cases with single-pulse inflow variation are also simulated to clarify whether the observed phenomena are derived solely from the continuous disturbance. Two aspects are analysed to investigate the features of ODWs in unsteady flow, namely, the formation of triple points on the surface, and the movement of the reactive front position. On the formation of triple points, the continuous disturbance generates at most one pair of triple points, less than or equal to the number of triple points in single-pulse cases. This indicates that the effects of continuous disturbance weaken the ability to generate the triple points, although there appear more triple points convected downstream on the surface at any given instant. On the movement of the reactive front, oscillatory behaviours are induced in either single-pulse or continuous disturbance cases. However, more complicated dynamic displacements and noticeable effects of unsteadiness are observed in the cases of continuous disturbance, and are found to be sensitive to the disturbance wavenumber, . Increasing results in three regimes with distinct behaviours, which are quasi-steady, overshooting oscillation and unstable ODW. For the quasi-steady case with low , the reactive front oscillates coherently with the inflow disturbance with slightly higher amplitude around the initiation region. The overshooting oscillation generates the most significant variation of downstream surface in the case of modest , reflecting a resonance-like behaviour of unsteady ODW. In the case of high , the disturbed ODW surface readjusts itself with local unstable features. It becomes more robust and the reactive front of the final unstable ODW structure is less susceptible to flow disturbance
Steadiness of wave complex induced by oblique detonation wave reflection before an expansion corner
No full textOblique detonation wave (ODW) reflection before an expansion corner leads to a sophisticated wave complex, whose steadiness is critical to achieve a practical oblique detonation engine. Both steady and unsteady wave complexes have been observed before, but the features of unsteady wave dynamics with related unsteadiness rules are still unclear so far. In this study, the ODW reflections before an expansion corner have been simulated using the reactive Euler equations with a two-step induction-reaction kinetic model, and the wave complex structures and dynamics have been analyzed correspondingly. Three subsonic zones have been distinguished, and their interactions were found to determine the wave complex steadiness. The main subsonic zone derives from the ODW reflection, which locates behind the Mach stem, while two other subsonic zones form due to the shock reflection downstream. The two downstream subsonic zones might travel upstream and combine with the main subsonic zone, resulting in two different unsteadiness modes. These wave complex dynamics were analyzed with respect to the deflection location, deflection angle and inflow Mach number, leading to the boundaries of combustion modes and ascertaining the rule of mode regime. Some transient phenomena related with the flow instability have been also discussed, clarifying fine flow structures further. (C) 2021 Elsevier Masson SAS. All rights reserved
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Novel fuzzy logic controllers with self-tuning capability
No full textTwo controllers which extend the PD+I fuzzy logic controller to deal with the plant having time varying nonlinear dynamics are proposed. The adaptation ability of the first self tuning PD+I fuzzy logic controller (STPD+I_31) is achieved by adjusting the output scaling factor automatically thereby contributing to significant improvement in performance. Second controller (STPD+I_9) is the simplified version of STPD+I_31 which is designed under the imposed constraint that allows only minimum number of rules in the rule bases. The proposed controllers are compared with two classical nonlinear controllers: the pole placement self tuning PID controller and sliding mode controller. All the controllers are applied to the two-links revolute robot for the tracking control. The tracking performance of STPD+I_31 and STPD+I_9 are much better than the pole placement self tuning PID controller during high speed motions while the performance are comparable at low and medium speed. In addition, STPD+I_31 and STPD+I_9 outperform sliding mode controller using same method of comparison study
Numerical study on reflection of an oblique detonation wave on an outward turning wall
No full textOblique detonation waves (ODWs) have been studied widely as the basis of detonation-based hypersonic engines, but there are few studies on ODWs in a confined space. This study simulates ODW reflection on a solid wall before an outward turning corner for a simplified combustor-nozzle flow based on a two-step kinetic model. Numerical results reveal three types of ODW structures: stable, critical, and unstable. When the reflection occurs at the turning point, the stable ODW structure remains almost the same as before reflection. When the wave reflects at the wall before the turning point, either the critical structure or the unstable structure arises, which has never been investigated before. Both structures have the same initial two-section detonation surface: but the critical one becomes stationary at a certain position, while the unstable one keeps traveling upstream. By adjusting the location of the expansion wave and degree of the turning angle, the difference of the two structures is attributed to the thermal choking appearing only in the unstable structure. The thermal choking is achieved by the merging of subsonic zones, whose dependence on the various parameters is discussed
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