Showing papers on "Shock tube published in 2022"

A Detailed Chemical Insights into the Kinetics of Diethyl Ether Enhancing Ammonia Combustion and the Importance of NOx Recycling Mechanism

Abstract: In this work, we investigated the combustion characteristics of ammonia (NH3) by blending it with various proportions of diethyl ether (DEE). We measured laminar flame speed of various NH3/DEE blends (DEE, 10–40% by mole) using a constant volume spherical vessel at Ti = 298 K and Pi = 3 and 5 bar and Φ = 0.8–1.3. We developed a detailed kinetic model to describe the trends of the current and previously published experimental data. For the robustness of the model, we first developed a comprehensive diethyl ether kinetic mechanism to accurately characterize neat DEE oxidation behavior. We validated the kinetic model using a large pool of experimental data comprising shock tube, rapid compression machine, jet-stirred and flow reactors, freely propagating, and burner-stabilized premixed flames. The developed kinetic model performs remarkably in capturing the combustion behavior of pure DEE and NH3. Importantly, our model captures the experimental data of laminar flame speed and ignition delay times of various NH3/DEE blends over a wide range of conditions. We found that DEE is a promising candidate to promote the combustion characteristics of NH3. A small portion of DEE (10%) enhances the laminar flame speed of NH3 by a factor of 2 at Pi = 1 bar, Ti = 298 K, and Φ = 1.0. A further doubling of the DEE mole fraction to 20% did not enhance the laminar flame speed of NH3 with the same propensity. At low temperatures, adding 5% DEE in NH3 blend has significantly expedited the system reactivity by lowering the autoignition temperature. A further 5% increment of DEE (i.e., 10% DEE in NH3) lowers the autoignition temperature by ∼120 K to achieve the same ignition delay time. The “NONO2” looping mechanism predominantly drives such reactivity accelerating effect. Here, the reactions, NO + HO2 = NO2 + OH and NO2 + H = NO + OH, appear to enhance the reactive radical pool by generating OH radicals. We observed that the HNO path is favored more with increasing DEE content which eventually liberates NO. Other key reactions in “NONO2” looping mechanism are: CH3 + NO2 = CH3O + NO, CH3O2 + NO = CH3O + NO2, C2H5 + NO2 = C2H5O + NO, C2H5O2 + NO = C2H5O + NO2. In addition, CH3 + NH2(+M) = CH3NH2(+M) reaction is also one of the important cross-reactions which leads to the formation of HCN. Therefore, cross-reactions between the nitrogen and carbon family are crucial in accurately predicting autoignition timing. This work provides a detailed chemical insight into the NH3 and DEE interaction, which could be applied to other fuel blends of NH3. The kinetic model is also validated for several C1C3 fuels including their interaction with NOx.

State-of-the-art review on ultra high performance concrete - Ballistic and blast perspective

Abstract: The world is experiencing multiple blasts and ballistic attacks in the last few decades leading to huge loss of civilians and warriors in fringe territories. In this context, this paper reviews the state-of-the-art information on ultra-high-performance concrete (UHPC) against blast and ballistic impact. The first part briefly discusses the ingredients with special emphasis on fibres behaviour in UHPC. Secondly, the review of dynamic characterization of UHPC carried with the aid of Split Hopkinson Pressure Bar (SHPB), covering pulse shaper technique, dynamic increase factor (DIF) and high strain behaviour . Thirdly, from ballistic perspective, various prediction models, penetration depth analysis and various research to tackle high-velocity impact are addressed. Further, shock tube studies and in-field blast testing of UHPC is summarized under blast perspective. Performance of reinforcement bars in UHPC for mitigation of ballistic and blast attacks is reviewed. Subsequently, numerical simulation and technology to enhance ballistic and blast resistant efficiency, such as inducing prestressing, use of metallic foam and blast resistant design are discussed. This review endeavours to help researchers identify areas of improvement needed in UHPC and highlight the benefits of UHPC for designers and consultants to use in structures to mitigate the effect of impact and blast. • Dynamic characterization of UHPC using the SHPB technique is comprehensively discussed. • Available prediction models for determination of penetration depth due to ballistic load are critically evaluated. • Performance of UHPC against the lab. based (shock tube) and field-based blast tests are reported. • Improvement of UHPC performance against ballistic and blast performance are also discussed. • Numerical simulations of ballistic and blast loading on UHPC are reviewed.

Nonequilibrium Vibrational, Rotational, and Translational Thermometry via Megahertz Laser Absorption of CO

Abstract: A mid-infrared laser absorption strategy for simultaneously measuring translational, rotational, and vibrational temperatures of carbon monoxide (CO) at high speeds was developed for application to high-temperature nonequilibrium environments relevant to Mars atmosphere entry. Rapid-tuning scanned wavelength techniques were used to spectrally resolve the R(0,66), P(0,31), P(2,20), and P(3,14) lines of the CO fundamental vibrational bands at a rate of 1 MHz to infer multiple temperatures of CO behind incident and reflected shock waves in a shock tube. A distributed feedback quantum cascade laser was used to probe the P-branch transitions near and an external cavity quantum cascade laser was used to probe the R-branch transition near , both using bias-tee circuitry. The sensing method is shown to resolve each targeted transition with temporal and spectral resolution sufficient for quantitative multi-temperature measurements over a wide range of temperatures and pressures (2100–5500 K, 0.03–1.02 atm), including behind incident shock waves traveling up to 3.3 km/s. Measured temperature results were compared to equilibrium and nonequilibrium simulations.

Shock-tube laser absorption measurements of N2O time histories during ammonia oxidation

Abstract: The oxidation of ammonia was studied experimentally by monitoring the time history of the intermediate N 2 O species using laser absorption spectroscopy. Experiments were conducted in a shock tube for mixtures of NH 3 /O 2 diluted in ∼96.7% Ar for equivalence ratios of 0.54, 1.03, and 1.84. The equivalence ratios were determined accurately using spectroscopic measurements of NH 3 with another laser before each experiment. Experiments were performed at an average pressure of 1.2 atm and covered a temperature range of 1829 to 2198 K. For the same temperature, experiments revealed that increasing the equivalence ratio leads to less N 2 O formation. The time-history profiles showed that N 2 O is formed at the beginning of the experiments, mainly from the formed NO, until reaching a peak. The N 2 O is then fully consumed, mainly via its reaction with H-atom. Characteristic parameters, such as the N 2 O peak time and mole fraction, were extracted from the N 2 O profiles and compared with 15 recent NH 3 kinetics models. The comparison revealed that none of the existing kinetics models were able to correctly predict both the peak N 2 O time and mole fraction together. Two of the models were selected to perform a chemical analysis, and an improvement of the predictive capability of one model is proposed. The N 2 O profiles reported herein are excellent validation targets that offer stringent constraints for the improvement of future NH 3 kinetics models.

Three-dimensional numerical simulation on near-field pressure evolution of dual-tube underwater detonation

Abstract: The detonation-powered underwater engine, with the advantages of high specific impulse, high speed, and simple structure, has very broad application prospects in the field of underwater propulsion, and dual-tube combination is an effective means to improve its propulsion performance. In this work, near-field pressure evolution of shock waves and high-pressure zones between two detonation tubes is numerically studied. The two-fluid model and three-dimensional conservation element and solution element method are adopted to reveal the formation, intersection, and interaction of shock waves. Detonation waves generated by two detonation tubes decouple into shock waves after penetrating into water and form a high-pressure zone near each tube exit. The two leading shock waves intersect with each other in the propagation, creating the second high-pressure zone between two tubes. Then, a propagating forward merged new shock wave covers the two original wave-fronts and maintains higher pressure. Pressure evolution under different tube intervals, ignition delays, and filling conditions is also presented to discuss their influence on the performance of dual-tube detonation. The intensity and directivity of shock waves are found to be sensitive to these factors, complexly affecting the thrust components, which provides a depth understanding of dual-tube combination in the application.

A Simple Hydrodynamic-particle Method for Supersonic Rarefied Flows

Abstract: In the practical aerospace industry, the supersonic rarefied effect presents multiscale characteristics from the near-continuous regime to the free molecular regime. In this paper, a simple hydrodynamic-particle method (SHPM) is proposed to efficiently capture the multiscale properties for the supersonic rarefied flow. To combine the conventional computational fluid dynamics (CFD) solver with the particle-based method, the weights are theoretically derived from the integral solution of the Boltzmann-BGK equation. The present numerical method is validated by test cases of supersonic shock wave structure, Sod shock-tube and supersonic flow around the circular cylinder. Numerical results demonstrate that the SHPM could capture the multiscale properties from the continuum regime to the rarefied regime.

Experimental investigation on shock wave propagation and self-ignition of pressurized hydrogen in different three-way tubes

Abstract: This paper experimental investigated the shock wave propagation characteristics and self-ignition produced by the high-pressure hydrogen release in the three-way tubes. Two Y-shaped tubes (60°, 120°) and one T-shaped tube (180°) were used in the experiments and the initial release pressure was 3–8 MPa. The pressure and photoelectric signals in tubes were recorded by the sensor. The results showed that the intensity of shock wave was enhanced or attenuated during the entire releasing process, but the dominant effect was distinct under different conditions and the two effects synergistically affected the occurrence possibility of self-ignition. The critical release pressure for self-ignition in the three-way tubes decreased with the increasing of the bifurcation angle, and the most difficult to occur the self-ignition was the 60° Y-tube in this study. In addition, quenching occurred in the 60° Y-tube when the initial release pressure was 6 MPa, because the temperature of the mixture dropped by the expansion effect. Furthermore, the intensity of the reflected shock wave was not strong enough to promote hydrogen rekindled. This experimental results have reference value for the safety of high-pressure hydrogen production, storage and transportation, and are helpful to understand the influence of bifurcation structure on self-ignition in energy application.

The influence of shock speed variation on radiation and thermochemistry experiments in shock tubes

Abstract: Abstract Shock tubes are a crucial source of experimental data for the aerothermodynamic modelling of atmospheric entry vehicles. Notably, many chemical-kinetic and radiative models are validated directly against optical measurements from these facilities. Typically, the incident shock speed at the location of the experimental measurement is taken to be representative of the test slug; however, the shock velocity can vary substantially upstream of this location. These variations have been long posited as a source of disagreement with computational predictions, although a definitive link has proved elusive. This work describes a series of experiments which aim to isolate and confirm the importance of the shock deceleration effect. This is achieved by generating different shock trajectories and comparing the post-shock trends in atomic oxygen emission and electron density. These trends are shown to be directly linked to the upstream shock speed variations using a recently developed numerical tool (LASTA). The close agreement of the comparisons confirms the importance of shock speed variation for shock tube experiments; these findings have direct and potentially critical relevance for all such studies, both past and present.

Effect of NO2 on ignition characteristics of methanol and chemical kinetics analysis

Abstract: Based on the shock tube experiments, the influence of NO2 on the ignition characteristics of methanol fuel was systematically studied. The ignition delay time of the CH3OH/NO2/O2/Ar mixture with equivalence ratios of 0.5, 1, and 1.5 was measured at the reflected shock temperatures of 1100 ~ 1650 K and the reflected shock pressure of 0.20 MPa. The methanol–NO2 mechanism was newly constructed based on previously developed mechanisms for methanol and NO2 reaction pathways. The new combined reduced mechanism could well predict the ignition delay time of the CH3OH/NO2/O2/Ar mixture. Based on the newly constructed methanol–NO2 mechanism, the reaction path, sensitivity, and important elementary reactions of the stoichiometric mixture were analyzed, and the reasons why NO2 promoted the ignition of methanol fuel were found out: the increase of OH radicals generation and rapid accumulation in the early ignition stage enhanced the activity of the reaction system, interfered the ignition process of CH3OH/NO2/O2/Ar mixture in advance, and promoted the ignition of methanol fuel.