Effect of surroundings on the transient energy transfer in a layer of radiating gas

Summary An analysis is presented for the transient cooling or heating of a stagnant layer of hot radiating gas surrounded by a cold gas capable of absorbing and emitting radiation. Scattering of radiation is neglected, and energy transfer by conduction and convection is considered to be negligible compared with radiation. The gas is assumed to be perfect and in local thermodynamic equilibrium. The heating of a cold gas by a diffuse and a collimated radiation flux incident on the boundary of the gas from some external source is considered, and the dependence of physical and radiative properties on temperature is taken into account. The problem is formulated exactly using radiative transfer theory. A scheme is developed for the numerical solution of the nonlinear integrodifferential equations of energy conservation. Starting with arbitrary, but given, initial temperature profiles, temperature distributions and local radiative fluxes are predicted as a function of time for a wide range of physically interesting conditions.     

Corona discharge in a moving gas

Some aspects of the theory of corona discharges in a moving medium are considered. Two situations are analyzed: a corona discharge at a negative electrode under the condition that in the region of electrogasdynamic flow exterior and interior regions of the discharge can be distinguished, the motion of the gas being taken into account only in the exterior region, and corona discharge at a negative electrode under the condition that the effects of the motion of the gas are important in both the exterior and the interior regions of the discharge. For the first situation, a mathmatical generalization is proposed of the traditional model of the interior region, and dimensional and similarity methods are used to obtain functional relationships for the current—voltage characteristics of the discharge in the moving medium. The second situation is investigated for the example of a corona discharge between cylindrical electrodes through which gas is blown or sucked. In this case, the solution to the problem is found without dividing the flow region into exterior and interior regions of the discharge, a system of kinetic equations describing the flow in the complete interelectrode gap being used.     

Optimizing energy input for fracture by analysis of the energy required to initiate dynamic mode I crack growth

A problem for a central crack in a plate subjected to plane strain conditions is investigated. Mode I crack loading is created by a dynamic pressure pulse applied at large distance from the crack. It was found that for a certain combination of amplitude and duration of the pulse applied, energy transmitted to the sample has a strongly marked minimum, meaning that with the pulse amplitude or duration moving away from the optimal values minimum energy required for initiation of crack growth increases rapidly. Results received indicate a possibility to optimize energy consumption of different industrial processes connected with fracture. Much could be gained in for example drilling or rock pounding where energy input accounts for the largest part of the process cost. Presumably further investigation of the effect observed can make it possible to predict optimal energy saving parameters, i.e., frequency and amplitude of impacts, for industrial devices, e.g., bores, grinding machines, etc. and hence significantly reduce the process cost. The prediction can be given based on the parameters of the media fractured (material parameters, prevalent crack length and orientation, etc.).     

Surface waves on the positive column of a gas discharge

Summary The propagation of surface waves along a mercury vapour discharge is investigated theoretically and experimentally. The classical solution of the electromagnetic problem, involving Bessel functions, is compared with a numerical solution. A density profile is used to obtain the theoretical curve that fits measured values best. A surface wave launching device with a minimum of direct radiation is applied, giving a standing wave of pure circular symmetry along the discharge tube. There is no metallic screening tube.     

Gas discharge from a tube into a vacuum with unsteady mass and energy addition

We examine the problem of ideal gas discharge from a cylindrical tube closed at one end into a vacuum, with unsteady mass and energy addition. The gas mass is formed by vaporization of the tube wall material due to the thermal flux from the heated gas. Under the assumption of uniform energy addition throughout the volume varying in accordance with the damped oscillation law, we obtain approximate relations for the mass of the discharging gas and the impulse of the pressure forces on the end wall as a function of time and energy.The resulting impulse values are compared with those determined for unsteady expansion into a vacuum of an ideal gas of given mass when energy is added to it. An increase of the impulse calculated by this technique in comparison with the value calculated using the proposed technique is shown.     

Effect of the design parameters and the atmosphere composition on the electric discharge shape

The effect of the atmosphere composition and the electrode material and design on the discharge shape and variation in the process of its development and in the steady-state regime is investigated. Graphites of different modifications, tungsten, and copper were used as electrode materials and air, argon, and helium as baseline atmospheres. It is established that the discharge shape (the shape of the glow region in the interelectrode gap) considerably depends on the actual composition of the atmosphere which is determined by the composition of the baseline atmosphere and additions introduced in the interelectrode gap either upon electrode evaporation or with a special purpose. The electrode material influences the steady discharge shape via precisely these additions.     

Effect of energy supply to a gas on laminar boundary layer separation

A decelerated flow in a supersonic boundary layer containing a heat source modeling an electric discharge is studied numerically. Calculations are performed for a wide range of the source power. The possibility of controlling the boundary layer separation is demonstrated. The boundary layer separation on cooled walls is found to occur substantially later than on thermally insulated walls.     

Effect of the mode of gas-surface interaction on intensive monatomic gas evaporation

The direct Monte Carlo simulation method is used for investigating the effect of the thermal accommodation coefficient α _E on the relations on the Knudsen layer boundary in the presence of intensive evaporation. The model of mirror reflection of molecules from the surface is considered. It is shown that diffuse reflection with α _E = 0 leads to almost the same relations on the Knudsen layer boundary as mirror reflection. The accuracy of the moment method is estimated in application to the problems of intensive evaporation with diffuse and mirror reflection from the surface.     

Dependence of the natural frequencies and mode shapes of vibrations of an ideal gas on the acoustic resonance parameters

The problem of plane wave propagation through a circular hole is studied in the framework of long-wave approximation. The constructive notion of “apparent mass of holes” (Rayleigh; Fok) is used to construct a mathematical model of gas vibrations in an acoustic resonator and determine and analyze the natural frequencies and mode shapes for the velocity potential depending on the relative geometric parameters of the system. The high-precision calculations of the boundary value problem for the natural frequencies and mode shapes in the parametric approximation to the cross-section are based on a numerical-analytical accelerated convergence method. Two models are analyzed and compared, and the basic qualitative properties of gas vibrations are revealed depending on the basic parameters such as the mode number, relative size of the hole, and the dividing wall location.     

Theory of nonequilibrium inductive high-frequency discharge in a gas flow

The global nonequilibrium flow in the discharge chamber of an induction plasma generator is modeled. The problem for an equilibrium discharge was considered in [2, 3]. Here, on the basis of a numerical solution of the combined system of Navier-Stokes, Maxwell, energy, ionization kinetics and electron-gas energy balance equations, the structure of the nonequilibrium discharge is analyzed and the results obtained within the framework of the local one-dimensional approach [1] and on the basis of global numerical modeling of the flow are compared. As distinct from [2, 3], in finding the electromagnetic field distribution in the discharge chamber the boundary-value problem for the two-dimensional Maxwell equations is solved.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 153–160, March–April, 1991.The author is grateful to V. V. Lunev and G. N. Zalogin for their constant interest and useful discussions.     

Analytical calculation of the parameters of a molecular gas on a surface in the Smoluchowski problem

An analytical solution of the classical Smoluchowski problem on the temperature jump in molecular (monatomic, diatomic, and polyatomic) gases is presented. The gas occupies a half–space above a flat wall, with a constant temperature gradient and evaporation rate from the gas—condensed phase interface set far from this wall. The distribution function is explicitly constructed both in the half–space and at its boundary. Formulas for the concentration and temperature at the interface are derived; in the case of diatomic and polyatomic gases, formulas for temperatures determined by translational and rotational degrees of freedom of molecules are obtained. Numerical calculations are performed.     

Growth of a nanosecond pulsed discharge in a gas with one-electron initiation

Pulse breakdown on gaps of millimeter order at substantial overvoltages is explained in terms of a discharge mechanism involving photoelectric emission from the cathode followed by collisional multiplication in the gas to give avalanches. The mechanism is used to deduce a theoretical equation for the time of discharge buildup in one-electron mutation, which is compared with experiment.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 1, pp. 40–45, January–February, 1971.     

Mechanism of energy separation in a gas ejector

A new phenomenon is revealed — the rotation of an ejecting jet, discharging from a nozzle and adhering to the wall of the mixing chamber, in an axisymmetric gas ejector in modes with zero and negative ejection coefficients — and a possible mechanism for its origin is discussed. It is suggested that the rotation of an adhering jet, which induces axisymmetric vortex motion of the gas in the injector, is responsible for the inverse separation of the initially energetically homogeneous stream into heated and cooled sections.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 145–151, November–December, 1977.