KAUST DepartmentApplied Mathematics and Computational Science Program
Permanent link to this recordhttp://hdl.handle.net/10754/566154
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AbstractWe consider a simplified model for the dynamics of one-dimensional detonations with generic losses. It consists of a single partial differential equation that reproduces, at a qualitative level, the essential properties of unsteady detonation waves, including pulsating and chaotic solutions. In particular, we investigate the effects of shock curvature and friction losses on detonation dynamics. To calculate steady-state solutions, a novel approach to solving the detonation eigenvalue problem is introduced that avoids the well-known numerical difficulties associated with the presence of a sonic point. By using unsteady numerical simulations of the simplified model, we also explore the nonlinear stability of steady-state or quasi-steady solutions. © 2014 The Combustion Institute.
SponsorsThe research reported here was supported by King Abdullah University of Science and Technology (KAUST).
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Enhancement of the transition to detonation of a turbulent hydrogen–air flame by nanosecond repetitively pulsed plasma dischargesGray, Joshua A.T.; Lacoste, Deanna (Combustion and Flame, Elsevier BV, 2018-11-02) [Article]This work provides proof of concept for the use of nanosecond repetitively pulsed (NRP) plasma discharges to accelerate a propagating turbulent flame, resulting in enhanced deflagration-to-detonation transition and significant reduction in run-up length. The investigations are conducted on a stoichiometric hydrogen-air mixture at near ambient conditions. The effect of plasma actuation on the flame velocity is investigated using time-of-flight measurements of the propagating flame and detonation wave. The flame velocity shortly after the application of the NRP plasma discharges is more than double that obtained in cases in which no plasma is applied. High-speed imaging of OH* chemiluminescence in the electrode area confirms this result and provides insight about the mechanisms of plasma action. While the volumetric energy deposited during plasma actuation is sufficiently low as to not ignite the combustible mixture prior the arrival of the flame, the chemical and thermal enhancement of the gas is efficient enough to significantly accelerate the transition to detonation. The decrease in the run-up length to transition to detonation is obtained for a plasma power of less than 0.14% of the thermal power of the flame. This result indicates that low-energy active devices using NRP discharges might be suitable for replacing passive devices such as orifice plates or Shchelkin spirals.
Detonation onset in a thermally stratified constant volume reactorSow, Aliou; Lee, Bok Jik; Hernández Pérez, Francisco E.; Im, Hong G. (Proceedings of the Combustion Institute, Elsevier BV, 2018-10-17) [Article]Understanding detonation development from a flame kernel initiated by a pre-ignition event is important for modern internal combustion (IC) engines operating at boosted conditions. To provide fundamental insights into the effects of bulk gas temperature stratification on the characteristics of detonation development, one-dimensional high fidelity simulations were conducted for a constant volume reactor filled with a thermally stratified reactive stoichiometric hydrogen/air mixture. A linear temperature variation in the upstream end-gas was introduced to represent the thermal stratification of the bulk mixture, and the evolution from the initial deflagration flame front to detonation development was examined. The results showed that the bulk-gas temperature gradient has a significant effect on the run-up time and intensity of the developing detonation. Detailed analyses further revealed that the mechanism of detonation development is qualitatively different for the positive and negative temperature gradient cases. In the former, the detonation development is initiated by the end-gas autoignition at the wall, while the latter exhibits detonation development following the process of the self-acceleration of the flame similar to the deflagration-to-detonation transition. This behavior is attributed to the longer residence time in the end-gas allowing the reinforcement by the interaction of incident and reflected pressure waves during the flame propagation, and results in the peak pressure even higher than the case with the same level of positive temperature gradient. Furthermore, yet another detonation development pattern was observed for the negative temperature gradient condition in the presence of a uniform temperature region just ahead of the flame. In this case, autoignition was found to start in the middle of the bulk end-gas, and subsequently leads to the transition to detonation. The results demonstrate the importance of the bulk gas conditions in predicting the detonation development, which corroborate the existing theoretical framework.
On the viscous boundary layer of weakly unstable detonations in narrow channelsSow, Aliou; Chinnayya, Ashwin; Hadjadj, Abdellah (Computers & Fluids, Elsevier BV, 2018-11-19) [Article]The present study investigates, via high performance computing simulations, detonations propagating in small channel filled with a working fluid representative of the thermodynamic and transport properties of a stoichiometric mixture of propane and oxygen. With the help of a high-order compressible Navier–Stokes solver based on Weighted Essentially Non-Oscillatory (WENO5) scheme coupled with the Strang splitting method, we investigate the 2D mean structure of weakly unstable non-ideal detonations. The mean procedure is conducted on the instantaneous position of the shock. To overcome the expensive CPU time needed due to the long lengths required to get a self-similar solution that is independent from the initial solution, we implemented a recycling block technique (RBT). The RBT combined with the addition of a negative inflow in the detonation propagation direction allows to reduce the domain length by a factor of ten. Moreover, the investigation of the viscous boundary layer characteristics using different channel heights and different activation energies show that the displacement thickness scales in Rex−α,α ≈ 0.56–0.65. For the skin-friction coefficient we find a scaling in Rex−1.