Experimental and Numerical Investigation of Gaseous Detonation in a Narrow Channel with Obstacles

Gaseous detonation propagation in a thin channel with regularly spaced cylindrical obstacles was investigated experimentally and numerically. The wave propagation with substantial velocity deficits is observed and the details of its propagation mechanism are described based on experimental measurements of the luminosity and pressure and on three-dimensional flow fields obtained by numerical simulations. Both experiments and simulations indicate a significant role of shock–shock and shock–obstacle interactions in providing high-temperature conditions necessary to sustain the reaction wave propagation.

ON THE GASDYNAMIC MECHANISMS OF EXOTHERMIC REACTION WAVE FORMATION BEHIND SHOCK WAVES

The paper analyzes the gasdynamic evolution of the test mixture flow in the shock tube at the stage prior to reaction start. The numerical analysis clearly shows that the incepience of reaction kernels is associated with the specific features of flow development in the boundary layer behind an incident shock wave. It is shown that similar to the processes in the gas flow near a solid surface, the gasdynamic instability arises and develops in the flow behind a shock wave. The linear stage of instability development determines the formation of roll-up vortices at a certain distance behind the shock front. Further, at the nonlinear stage, these roll-up vortices transform in more complex structures that diffuse into the bulk flow. Evolution of vortices causes temperature redistribution on the scales of the boundary layer. On the one hand, there is a certain heating due to the kinetic energy dissipation. On the other hand, there are heat losses to the wall. As a result, the temperature field near the wall becomes nonuniform. The reflected shock amplifies temperature perturbations when interacts with the developed boundary layer. This mechanism determines the formation of hot kernels in which the reaction starts. So, the localized sites of exothermal reaction are arising providing conditions for reaction wave formation and propagation in the precompressed test gas.

PENGARUH MODEL DIFRAKSI TERHADAP PERAMBATAN GELOMBANG DETONASI PADA CAMPURAN BAHAN BAKAR HIDROGEN-OKSIGEN DENGAN DILUENT ARGON

Pada sistem pembakaran snpersonik, shock wave dan reaction wave merambat dengan kondisi berhimpit dengan kecepatan di bawah 1 mikro detik. Shock wave yang memiliki tekanan tinggi hingga mencapai 20 kali tekanan awal akan membahayakan bagi keselamatan manusia jika kecelakaan detonasi terjadi. Dengan demikian diharapkan kecelakaan yang diakibatkan oleh gelombang detonasi akan dapat dihindari atau diminimalisasi. Hal ini dilakukan dengan cara mengubah gelombang detonasi menjadi gelombang deflagrasi, yaitu memisahkan shock wave dengan reaction wave akibat proses ekspansi gelombang detonasi. Pada eksperimen ini, model diuji pada pipa uji detonasi (PUD) horizontal berpenampang lingkaran dengan diameter dalam 50 mm dan panjang 6300 mm yang terdiri dari seksi driver sepanjang 1000 mm, seksi driven sepanjang 5300 mm. Pada seksi driven dipasang model facing step 50% dengan bahan alumunium sepanjang 300 mm. Empat unit sensor tekanan yang berfungsi untuk merekam profil tekanan sepanjang proses pembakaran dan empat unit ionisation probe yang berfungsi untuk mendeteksi waktu kedatangan flame front, dipasang masing - masing 2 unit di upstream dan 2 unit di downstream dari model dengan posisi saling berhadapan. Campuran bahan bakar untuk seksi driver yang digunakan pada eksperimen ini adalah campuran hidrogen dan oksigen dengan kondisi stokiometrik dan tekanan awal 100 kPa untuk menjamin terjadinya detonasi pada seksi driver, sedangkan pada seksi driven campuran bahan bakar yang digunakan adalah campuran hidrogen - oksigen dengan diluent argon pada variasi tekanan awal mulai 20 kPa hingga 100 kPa. Dari hasil penelitian diperoleh 3 mekanisme perambatan gelombang detonasi di belakang model.facing step 50%, yaitu a) Reinisiasi detonasi oleh adanya DDT, yaitu kondisi merambatnya kembali gelombang detonasi akibat proses deflagration to detonation transition di daerah downstream dari model setelah sebelumnya quenching detonasi akibat gelombang ekspansi, (b) Reinisiasi detonasi oleh adanya S-W, kondisi merambatnya kembali gelombang detonasi akibat adanya interaksi gelonbang kejut dengan dinding pipa, (c) transmisi detonasi, merupakan proses perambatan gelombang detonasi tanpa melalui proses quenching didaerah downstream dari model.

Shock-Driven Decomposition of Polymers and Polymeric Foams

Polymers and foams are pervasive in everyday life, as well as in specialized contexts such as space exploration, industry, and defense. They are frequently subject to shock loading in the latter cases, and will chemically decompose to small molecule gases and carbon (soot) under loads of sufficient strength. We review a body of work—most of it performed at Los Alamos National Laboratory—on polymers and foams under extreme conditions. To provide some context, we begin with a brief review of basic concepts in shockwave physics, including features particular to transitions (chemical reaction or phase transition) entailing an abrupt reduction in volume. We then discuss chemical formulations and synthesis, as well as experimental platforms used to interrogate polymers under shock loading. A high-level summary of equations of state for polymers and their decomposition products is provided, and their application illustrated. We then present results including temperatures and product compositions, thresholds for reaction, wave profiles, and some peculiarities of traditional modeling approaches. We close with some thoughts regarding future work.

DNS Study of the Bending Effect Due to Smoothing Mechanism

Propagation of either an infinitely thin interface or a reaction wave of a nonzero thickness in forced, constant-density, statistically stationary, homogeneous, isotropic turbulence is simulated by solving unsteady 3D Navier–Stokes equations and either a level set (G) or a reaction-diffusion equation, respectively, with all other things being equal. In the case of the interface, the fully developed bulk consumption velocity normalized using the laminar-wave speed SL depends linearly on the normalized rms velocity u′/SL. In the case of the reaction wave of a nonzero thickness, dependencies of the normalized bulk consumption velocity on u′/SL show bending, with the effect being increased by a ratio of the laminar-wave thickness to the turbulence length scale. The obtained bending effect is controlled by a decrease in the rate of an increase δAF in the reaction-zone-surface area with increasing u′/SL. In its turn, the bending of the δAF(u′/SL)-curves stems from inefficiency of small-scale turbulent eddies in wrinkling the reaction-zone surface, because such small-scale wrinkles characterized by a high local curvature are smoothed out by molecular transport within the reaction wave.

The Zeldovich spontaneous reaction wave propagation concept in the fast/modest heating limits

Quantitative mathematical models describe planar, spontaneous, reaction wave propagation (Zeldovich, Combust. Flame, vol. 39, 1980, pp. 211–214) in a finite hot spot volume of reactive gas. The results describe the complete thermomechanical response of the gas to a one-step, high-activation-energy exothermic reaction initiated by a tiny initial temperature non-uniformity in a gas at rest with uniform pressure. Initially, the complete conservation equations, including all transport terms, are non-dimensionalized to identify parameters that quantify the impact of viscosity, conduction and diffusion. The results demonstrate unequivocally that transport terms are tiny relative to all other terms in the equations, given the relevant time and length scales. The asymptotic analyses, based on the reactive Euler equations, describe both induction and post-induction period models for a fast heat release rate (induction time scale short compared to the acoustic time of the spot), as well as a modest heat release rate (induction time scale equivalent to the acoustic time). Analytical results are obtained for the fast heating rate problem and emphasize the physics of near constant-volume heating during the induction period. Weak hot spot expansion is the source of fluid expelled from the original finite volume and is a ‘piston-effect’ source of acoustic mechanical disturbances beyond the spot. The post-induction period is characterized by the explosive appearance of an ephemeral, spatially uniform high-temperature, high-pressure spot embedded in a cold, low-pressure environment. In analogy with a shock tube the subsequent expansion process occurs on the acoustic time scale of the spot and will be the source of shocks propagating beyond the spot. The modest heating rate induction period is characterized by weakly compressible phenomena that can be described by a novel system of linear wave equations for the temperature, pressure and induced velocity perturbations driven by nonlinear chemical heating, which provides physical insights difficult to obtain from the more familiar ‘Clarke equation’. When the heating rate is modest, reaction terms in the post-induction period Euler equations exhibit a form of singular behaviour in the high-activation-energy limit, implying the need to use a nonlinear exponential scaling for time and space, developed originally to describe spatially uniform thermal explosions (Kassoy, Q. J. Mech. Appl. Maths, vol. 30, 1977, pp. 71–89). Here again the result will be the explosive appearance of an ephemeral spatially uniform high-temperature, high-pressure hot spot. These results demonstrate that an initially weak temperature non-uniformity in a finite hot spot can be the source of acoustic and shock wave mechanical disturbances in the gas beyond the spot that may be related to rocket engine instability and engine knock.