V. F. Nikitin

Investigation of Self-Sustaining Waves in Metastable Systems: Deflagration-to-Detonation Transition

Self-sustaining waves can propagate in metastable media; energy needed to support such waves is released by the wave itself. The examples are waves of combustion and waves of boiling in overheated liquids. As a rule, two regimes of propagation exist: subsonic and supersonic. The difference is based on the different mechanisms of medium activation. Processes of transition between those regimes were less studied up to now, in comparison with pure subsonic or supersonic modes. Knowing mechanisms of controlling detonation initiation is important to work out effective preventive measures, such as suppressing deflagration-to-detonation transition in the case of combustible mixture ignition, and mitigation of a detonation wave in case it is already developed. On the other hand, the advantages of burning fuel in a detonation regime in comparison with slow burning at constant pressure attract increasing attention to pulse detonation burning chambers and to their possible application to new generation engines. The deflagration-to-detonation transition can be a principal stage of the work cycle in a pulse detonation engine, and the knowledge of details ofthis process and means of control can significantly decrease the predetonation distance and optimize the device. This work contains a review of the results obtained in theoretical and experimental investigations of deflagration-to-detonation transition processes in gases. Influence of internal geometry and flow turbulization on the detonation onset is considered; the influence of temperature and fuel concentration in the unburned mixture is discussed. Transitional processes of overheated liquid boiling up are also analyzed.

Instability and mixing flux in frontal displacement of viscous fluids from porous media

The goal of the present study is to investigate the instability of viscous fluid displacement by a less viscous one in a two-dimensional channel, and to the determine the characteristics of displacement quality and entrapment zones. Experiments on miscible displacement of fluids in Hele-Shaw cells were conducted under microgravity conditions. Extensive direct numerical simulations allowed to investigate the sensitivity of the displacement process to the variation of values of the main governing parameters. Validation of the code was performed by comparing the results of model problems simulations with experiments and with the existing solutions published in literature. Numerical simulations allowed to explain new experimental results on the pear shape of fingers and periodical separation of their tip elements from the main body of displacing fluid. These separated blobs of less viscous fluid move much faster than the mean flow of the displaced viscous fluid. The results of numerical simulations processed ...

Ignition and combustion of turbulized dust – air mixtures

A mathematical model for turbulent combustion and ignition of polydispersed dust-air mixtures is developed. The worked out mathematical model takes into account the two-way coupling effects in gas-particles interactions and combines both deterministic and stochastic approaches. The equations of motion for particles take into consideration the influence of random turbulent pulsations in the gas flow. The models for phase transitions and chemical reactions take into account thermal destruction of dust particles, vent of volatiles, chemical reactions in the gas phase, and heterogeneous oxidation of carbon by O2 and CO2 influenced by both diffusive and kinetic characteristics. The model is validated in comparison with experiments on organic dust combustion in confined volumes, under different levels of initial turbulization of the mixture. The developed mathematical model makes it possible to investigate the peculiarities of polydispersed organic dusts ignition and combustion and the influence of flow nonuniformities on the ignition limits

Filtration in artificial porous media and natural sands under microgravity conditions

The goal of the present paper is to investigate experimentally and theoretically the capillary driven filtration in porous media with homogeneous and inhomogeneous porosity and permeability under microgravity conditions. The motivation having determined the choice of the environment was to study nonequilibrium phenomena in two-phase filtration in porous media. The paper contains the results illustrating the sensitivity of capillary forces to variations of porous media characteristics. The experimental results obtained for fluid imbibition into unsaturated artificial and natural porous media are compared. Theoretical and experimental results on determination of mixing fluxes in two-phase filtration are discussed.

Microgravity investigations of instability and mixing flux in frontal displacement of fluids

The goal of the present study is to investigate analytically, numerically and experimentally the instability of the displacement of viscous fluid by a less viscous one in a two-dimensional channel, and to determine characteristic size of entrapment zones. Experiments on miscible displacement of fluids in Hele-Shaw cells were conducted under microgravity conditions. Extensive direct numerical simulations allowed to investigate the sensitivity of the displacement process to variation of values of the main governing parameters. Validation of the code was performed by comparing the results of model problems simulations with experiments and with the existing solutions published in literature. Taking into account non-linear effects in fluids displacement allowed to explain new experimental results on the pear-shape of fingers and periodical separation of their tip elements from the main body of displacing fluid. Those separated blobs of less viscous fluid move much faster than the mean flow of the displaced viscous fluid. The results of numerical simulations processed based on the dimensions analysis allow to introduce criteria characterizing the quality of displacement. The functional dependence of the dimensionless criteria on the values of governing parameters needs further investigations.

Microgravity investigations of capillary-driven imbibition and drainage in inhomogeneous porous media

Abstract The main focus of the microgravity engineering sciences is the fundamental results obtained in space experiments, which enable one to solve burning terrestrial problems. The goal of the present paper is to investigate experimentally and theoretically the role of capillary forces in filtration of fluids in porous media with inhomogeneous permeability and to learn how the porosity and permeability non-uniformity affects displacement and entrapment of wetting fluids. Microgravity environment having been chosen for the experimental investigations provides the possibility for upscaling pores, thus revealing the processes taking place at a pore level. The presence of capillary forces is the most important factor allowing water and solutions migration to the upper soil layers, thus delivering life to all the plants on the planet. The obtained knowledge of capillary-driven filtration mechanisms allows one to develop optimal irrigation strategies, which is most important in hot desert regions, and to prevent liquid contaminants migrating to the roots of plants. The paper contains the results illustrating the sensitivity of capillary forces to variations of porous media characteristics. The experimental results obtained for fluid imbibition into unsaturated artificial and natural porous media are compared. The mathematical model for multiphase filtration in porous media under non-equilibrium conditions is being developed along with experimental procedures to determine the influence of capillary forces and mixing fluxes. In contrast to the existing theories, the present model does not rely on relative permeability functions for phases. Experimental and theoretical investigations show that zones of lower permeability could serve as capillary traps for wetting fluids.

Motion and Sedimentation of Particles in Turbulent Atmospheric Flows above Sources of Heating

In this paper, theoretical investigation of the problem of particle evolution and sedimentation in turbulent gas flows above the zones of large-scale instabilities caused by heating from below is undertaken. The mathematical model takes into account the two-way coupling effects in gas-particle interactions and combines both deterministic and stochastic approaches. To simulate the gas-phase flow the k -epsilon model is used with accounts of the mass, momentum, and energy fluxes from the particulate phase. The equations of motion for particles take into consideration random turbulent pulsations in the gas flow. The mean characteristics of those pulsations are determined with the help of solutions obtained within the frames of the k -epsilon model. Contrary to the existing theories, the present approach enables us to take into account the polydispersed character of the mixtures. The models for phase transitions and chemical reactions take into account thermal destruction of dust particles, vent of volatiles, chemical reactions in the gas-phase, and heterogeneous oxidation of particles influenced by both diffusive and kinetic characteristics. The obtained results make it possible to analyze the influence of inert and chemically reacting particles on the flow field induced by heating from below and by sedimentation and to determine the influence of sources of heat release on dispersion of particles and dynamics of the reaction zone.

The Role of Geometrical Factors in Deflagration-to-Detonation Transition

This chapter discusses the influence of geometrical characteristics of the ignition chambers and flow turbulization on the onset of detonation and the influence of temperature and fuel concentration on the unburned mixture. The operation mode of the pulsed detonation wave generator is closely related to the periodical onset and degeneration of a detonation wave. The unsteady-state regimes should be self-sustained to guarantee a reliable operation of such devices. Minimizing the predetonation length and ensuring stability of the onset of detonation enables an increased effectiveness of pulsed detonation devices. The presence of one or two turbulizing chambers in the ignition section, the chambers of wider cross-section mounted in the ignition section of the detonation tube and filled with lean hydrocarbon-air mixtures, and the preheating of the mixture promotes the deflagration-to-detonation transition (DDT) and shortens the predetonation length. The presence of turbulizing chambers makes the DDT process less sensitive to the variation of ignition conditions. The absence of turbulizing chambers increases the predetonation length for similar initial conditions and makes the transition process very unstable with the onset of detonation having a sporadic character, the predetonation length varying stochastically, and making the DDT process very sensitive to ignition conditions.

Evolution of detonation propagation through an annular gap into a combustion chamber

Some results of theoretical studies of detonation processes in combustible gaseous mixtures are discussed for a model geometry of large combustion chambers of detonation engines in the case of mixtures of hydrogen and oxygen-enriched air. The effect of geometric characteristics on the operation of pulse detonation engines is analyzed. In particular, the propagation of detonation waves in tubes of small diameter to larger volumes and the evolution of detonation under the action of converging shock waves are considered.

Two-dimensional simulation of a hydraulic fracture crack cleaning process with consideration of lateral inflow of a fluid from the surrounding porous saturated rock

The cleaning of a hydraulic fracture crack filled with a fluid injected through a well is studied as one of the stages of oil extraction. A crack is considered as a porous medium whose permeability is much higher than that of the surrounding rock and whose length is several times larger than its width and is many times larger than its thickness. A two-dimensional model of this process is used; in this model it is assumed that a less viscous fluid displaces a more viscous fluid in a porous medium with consideration of inflow through the lateral surface of the crack.