Ionization and nonequilibrium radiation of air behind strong shock waves

It is shown that at high velocities of shock waves (V 9.5 km/sec) an important factor influencing the rate of ionization is the depletion of the number of excited states of the atoms through de-excitation. In the case of low pressures (p 1 torr) and for a bounded and optically transparent region of gas heated by the shock wave (for example, for the motion of gas in a shock tube or in a shock layer near a blunt body), the effective ionization rate k_f depends on the pressure [1], which leads to violation of the law of binary similarity which holds under these conditions without allowance for de-excitation. On leaving the relaxation zone, the gas arrives at a stationary state with constant parameters differing from those in thermodynamic equilibrium. The electron concentration and also the radiation intensity in the continuum and the lines are lower than the values for thermodynamic equilibrium. These considerations explain the results of known experiments and some new experiments on ionization and radiation of air behind a travelling shock wave.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 105–112, January–February, 1980.     

Monte Carlo analysis of dissociation and recombination behind strong shock waves in nitrogen

Computations are presented for the relaxation zone behind strong, one-dimensional shock waves in nitrogen. The analysis is performed with the direct simulation Monte Carlo method (DSMC). The DSMC code is vectorized for efficient use on a supercomputer. The code simulates translational, rotational and vibrational energy exchange and dissociative and recombinative chemical reactions. A new model is proposed for the treatment of three body recombination collisions in the DSMC technique which usually simulates binary collision events. The new model represents improvement over previous models in that it can be employed with a large range of chemical rate data, does not introduce into the flow field troublesome pairs of atoms which may recombine upon further collision (pseudo-particles) and is compatible with the vectorized code. The computational results are compared with existing experimental data. It is shown that the derivation of chemical rate coefficients must account for the degree of vibrational nonequilibrium in the flow. A nonequilibrium chemistry model is employed together with equilibrium rate data to compute successfully the flow in several different nitrogen shock waves.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1990.     

Experimental and numerical studies of rotational relaxation behind a strong shock wave in air

This paper describes the experimental and numerical investigations of unknown characteristics of the rotational nonequilibrium phenomena behind a strong shock wave in air. Experiments were carried out using a piston-driven shock tube with helium as driving gas and air as driven (test) gas, operated as a two-stage shock tube. In the experiments, emission spectra of NO were measured to evaluate the rotational temperature behind a strong shock wave. The numerical calculations use the computational code for the thermal and chemical nonequilibrium flow behind a strong shock wave developed by the present author's group, where 11 chemical species (N, O, NO, N, O, N, O, NO, N, O, e) and 48 chemical reactions of high-temperature air are considered. The thermal nonequilibrium is expressed by introducing an 8 temperature model composed of translational temperature, rotational and vibrational temperatures for N, O, NO, and electron temperature. The coupling of a rotation, vibration and dissociation (CRVD) model was incorporated to take sufficiently into account the rotational nonequilibrium. The calculations were conducted for the same conditions as the experimental ones. From the calculated flow properties, emission spectra were re-constructed using the code for computing spectra of high temperature air “SPRADIAN”. Furthermore, rotational and vibrational temperatures of NO (0,1) were determined from a curve fitting method and compared with the computed results. Received 12 September 2001 / Accepted 18 February 2002     

Effect of vibrational relaxation of molecules on dissociating air characteristics behind a strong shock front

Molecular vibrational relaxation has a considerable effect on the dissociation rate in a gas consisting of molecules of a single type [1]. In gas mixtures such as air, vibrational relaxation also affects the other reaction rates, which may be important in solving several problems of hypersonic aerodynamics. This is due to the fact that in air at temperatures above 5000° the vibrational excitation of nitrogen molecules and the dissociation of oxygen molecules proceed almost simultaneously. We study the effect of vibrational relaxation on the conditions behind a strong shock front.     

Boundary layer effect on thermal NO decomposition behind incident shock waves

The thermal decomposition of nitric oxide (diluted in Argon) has been measured behind incident shock waves by means of IR diode laser absorption spectroscopy. In two independent runs the diode laser was tuned to the=0 =1² _3/2 R(18.5)-rotational vibrational transition and the=1 =2² _3/2 R(20.5)-rotational vibrational transition of nitric oxide, respectively. These two transitions originating from the vibrational ground state (=0) and the first excited vibrational state (=1) were selected in order to probe the homogeneity along the absorption path. The measured NO decomposition could satisfactorily be described by a chemical reaction mechanism after taking into account boundary layer corrections according to the theory of Mirels. The study forms a further proof of Mirels' theory including his prediction of the laminar-turbulent transition. It also shows, that the inhomogeneities from the boundary layer do not affect the IR linear absorption markedly.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.     

Features of nonequilibrium processes of titrogen oxide formation behind strong shock waves in air

The kinetics of NO and NO₂ of behind shock fronts propagating in air are analyzed. It is shown that in certain cases it is necessary to use fairly detailed chemical reaction schemes involving not only N₂, O₂, NO, N, and O, but also NO₂, N₂O, H₂, OH, and H and to take into account the mutual effects of vibrational relaxation and chemical transformations. It is established that neglecting the chemical processes involving NO₂ only can lead to significant errors in the length of the relaxation zone (up to 25 times), the gasdynamic parameters (up to 20%), and the NO concentration (up to 3 times). Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 132–144, January–February, 1999. The work was carried out with financial support from the Russian Foundation for Basic Research (project No. 96-02-18377).     

Kinetics and oxidation mechanism of cyclopentadiene behind the shock waves

Oxidation of cyclo-C₅H₅ was investigated by monitoring O atoms and also CO molecules behind the reflected shock of cyclo-C₅H₆ / O₂ /Ar and C₆H₅OCH₃ /O₂ /Ar mixtures. In both mixtures we could observe the formation of O atoms produced by the reaction of cyclo-C₅H₅ + O cyclo-C₅H₅O + O and from the kinetic analysis of these time profiles the rate constant for this reaction was determined to be kJ/RT) cm³ mol^-1 s^-1. The possibility of CO formation by the reaction of cyclo-C₅H₅O C₄H₅ + CO was also investigated but because of the subsequent reactions forming CO molecules no direct information for the reaction of cyclo-C₅H₅O C₄H₅ + CO was derived.Received: 3 February 2003, Accepted: 17 June 2003, Published online: 5 August 2003     

IR-diode laser measurements on the decomposition of NO behind shock waves

The decomposition of NO diluted in Ar was measured in the temperature range 2,500 K T 3,500 K at pressures between 0.5 and 1.9 bar using the shock tube method. The gas mixtures containing 10 to 25% NO in Ar were shock heated and the time-dependent NO concentrations were monitored by using a tunable IR-diode laser. The measured NO concentration profiles in the post-shock reaction zone were interpreted by means of computer simulations. With a reaction mechanism developed by Thielen and Roth (1984) nearly complete agreement between calculated and measured NO concentration profiles was obtained.     

Atomization of liquid droplets on surfaces exposed to moving shock waves

Many engineering applications involve the stripping of liquid droplets from surfaces, one example being the entrainment of surface fuel from the inlet valves, ports, cylinder walls and piston crowns of internal combustion engines during the induction process. This configuration is likely to exhibit differences from the more commonly studied case of suspended droplets. In order to study the atomization of liquids from surfaces, shock waves at low Mach numbers (M = 1.05 and 1.12) have been used in the present work to initiate the flow over water droplets with visualization obtained from shadowgraph photographs, high-intensity flash photography and a CCD camera. Visualization paths both normal and angled at ±45° to the flow were used in order to obtain improved examination of the atomization details. Surface wave formation and a specific pattern of droplet distortion followed by stripping, was observed. There are similarities in the processes to those of suspended droplets that are modified by the boundary layer effects. At the Weber numbers considered, a cave-like formation occurs near the wall due to surface flow around the droplet with a major liquid flow directed tangentially across the air flow towards the cave peak where bag or chaotic type break-up and stripping takes place.     

Emission and radiative cooling of xenon plasma behind a strong shock front

A numerical and experimental investigation of the emissivity and radiative cooling of xenon plasma in strong shock waves with Mach numbers M=16–45, including experimentally up to M=28, has been made. It is shown that under these conditions the equilibrium temperature behind the shock wave can be reduced by cooling by 1.5–2 times.Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 155–162, January–February, 1992.     

Normal interaction of shock waves in the presence of vibrational relaxation

The effect of nonequilibrium physicochemical processes on the flow resulting from the normal collision and reflection of shock waves is studied by the example of nonequilibrium excitation of molecular oscillations in nitrogen. It is shown that the thermal effect of vibrational relaxation is small and the problem can be linearized around a known solution [1]. A similar approach to the solution of the problem of flow around a wedge and certain one-dimensional non-steady-state problems was used earlier in [2–4]. The solution of these problems was constructed in an angular domain, bounded by the shock wave and a solid wall (or the contact surface) and was reduced to a well-known functional equation [6]. The solution of this problem, because of the presence of two angular domains divided by a tangential discontinuity, reduces to a functional equation of more general form than in [6]. The results are obtained in finite form. In the special case of shocks of equal intensity, the normal reflection parameters are obtained.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 90–96, July–August, 1976.     

The use of driver inserts to reduce non-ideal pressure variations behind reflected shock waves

Non-ideal shock tube facility effects, such as incident shock wave attenuation, can cause variations in the pressure histories seen in reflected shock wave experiments. These variations can be reduced, and in some cases eliminated, by the use of driver inserts. Driver inserts, when designed properly, act as sources of expansion waves which can counteract or compensate for gradual increases in reflected shock pressure profiles. An algorithm for the design of these inserts is provided, and example pressure measurements are presented that demonstrate the success of this approach. When these driver inserts are employed, near- ideal, constant-volume performance in reflected shock wave experiments can be achieved, even at long test times. This near-ideal behavior simplifies the interpretation of shock tube chemical kinetics experiments, particularly in experiments which are highly sensitive to temperature and pressure changes, such as measurements of ignition delay time of exothermic reactions.     

Optical measurement of velocity and drag coefficient of droplets accelerated by shock waves

The drag coefficient of micron-sized droplets accelerated by a shock wave has been investigated. The motion of the droplets was studied by an optical measurement system, and an inertial relaxation in the mist flow is discussed in detail. An expansion-shock tube was employed in the present experiment, in which water droplets were produced by a homogeneous condensation when humid nitrogen gas expanded adiabatically in the test section. The local mean diameter and local number density of the droplet cloud were 1.0 m and on the order of 10¹² particles/m³, respectively, as estimated using a light scattering measurement in a preliminary experiment. The droplet cloud accelerated behind a shock wave was observed using a direct shadowgraph method with a spatial filter. Since the intensity of transmitted light through the mist flow is a function of the radius and number density of droplets, we can obtain the locally averaged number density distribution under an adequate approximation. The transmitted light intensity was related to the velocity distribution of droplets under the adequate assumption. So, the acceleration of droplets was estimated from the velocity ratio between the droplets and gas flow. Then, the drag coefficient was calculated for the particle Reynolds number. The experimental result was also compared to a numerical prediction.