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The paper presents the results of experimental studies of scenarios of diffusion combustion of a microjet of hydrogen flowing out of a cylindrical micronozzle with a diameter of 200 μm. The hydrogen microjet is ignited near the nozzle exit and at a distance. The experimental data are compared with the data previously obtained, and the results are expressed in dimensionless parameters (in terms of Reynolds numbers). It is established that the stabilization of flame during the outflow of a circular micro jet of hydrogen is associated with the presence of heating of the nozzle by a spherical flame region covering the nozzle section. The hysteresis of the diffusion combustion process of circular hydrogen microjets is shown depending on the ignition location of the microjet (near or far from the nozzle exit) and the change in the flow rate (growth or decrease).

The effect addition of inert (He, N2, Ar, CO2) and reacting (СН4, O2, CF3Br, (CH3O)PO) gases in a hydrogen or in coflowing air stream on lift-off diffusion flame conditions of hydrogen micro jet effluxed from the round micro nozzle was experimentally studied. Using the schlieren technique a critical hydrogen flow rate was established at which the flame of the hydrogen microjet detaches from the nozzle when introducing additives of the studied gases into both air and hydrogen. It has been established that the addition of all the studied gases to the hydrogen leads to a decrease in the velocity range of its microjet at which flame stabilization is possible, regardless of whether the gases introduced into hydrogen are inert or reactive. It is shown that in the case of the addition of various gases to the hydrogen, the main factor determining the critical flow rate at flame lift-off from the micro nozzle is the average molecular weight of the H2 gas mixture with additives. At addition of the studied gases into the coflowing air, the critical flow rate of H2 is determined by their affect on the chemical reactions of hydrogen oxidation (inhibition effectiveness), as well as by a decrease in oxygen concentration due to dilution of air by additives. The data obtained are of interest to hydrogen energy in terms of determining the limits of sustainable combustion of the hydrogen microjet, as well as determining the minimum phlegmatizing concentrations of additives of inhibitors and fire suppressant in the air, preventing the ignition and explosion of hydrogen in case of emergency at its leak.

Results of experimental studies of the round hydrogen microjet interaction with a coaxial air for supersonic speed jets efflux are presented in this work. It is shown that interaction of round and coaxial air jets at their supersonic efflux leads to appearance of a train small/large-scale and, on the contrary, large/small-scale supersonic cells on a resultant jet with growth and reduction of speed of its efflux, respectively. It is found that the result of the round and coaxial air jets interaction at their supersonic efflux does not depend on geometrical parameters of a round and coaxial exit nozzle.

Results of experimental studies of features of the round hydrogen microjet combustion in a coaxial air jet are presented in this work. It is shown that the combustion scenario is connected with existence of the «bottleneck flame region». This fact correlates with the similar scenarios of the diffusion hydrogen microjet combustion at subsonic efflux velocity investigated by us earlier. It is revealed that the spherical shape of the “bottleneck flame region” is transformed to a cylindrical shape. It is found that the round hydrogen microjet combustion in a coaxial air jet at supersonic efflux velocity is accompanied by existence of supersonic cells both in a hydrogen microjet and in a wake of coaxial air jet. Round hydrogen microjet supersonic combustion in a coaxial air jet is connected with a flame separation from a nozzle exit.

Results of experimental studies of the round air microjet interaction with a coaxial hydrogen jet at its combustion for supersonic speed jets efflux are presented in this work. It is revealed that combustion of the coaxial hydrogen jet with growth of its speed efflux is accompanied by all scenarios, observed at study of the round and plane hydrogen microjets diffusion combustion. However, “bottleneck flame region” undergoes considerable geometrical deformations because of specifics of a flame of a coaxial jet. It is shown that “bottleneck flame region” is transformed from Y-shaped to spherical shape in the activity of growth of a coaxial jet speed efflux. It is found that a round air microjet interaction with a coaxial hydrogen jet at its combustion is accompanied by several new phenomena: existence of cone-shaped area a coaxial jet combustion near a nozzle exit; existence of small-scale supersonic cells on a resultant flame; absence of the hydrogen combustion efflux from combustion region of a coaxial jet near nozzle exit; flame-out from combustion region of a coaxial jet near nozzle exit that leads to hydrogen ignition downstream, its intensive combustion and sharp acoustic noise occurrence; existence of a turbulent flame, to its separation from a nozzle exit and transition to supersonic combustion of a resultant jet.

Results of the experimental studies of a round hydrogen microjet combustion in a coaxinal jet of air and SiO2 nanopouder mixture efflux from a slot-hole (multinozzle) coaxial nozzle at subsonic and supersonic efflux velocity are presented. It is shown that scenarios of a hydrogen microjet combustion in a coaxinal jet of air and nanopouder mixture are similar to scenarios of diffusion combustion of a hydrogen microjet at subsonic and supersonic efflux velocity. Existence of “bottleneck flame region” is revealed at subsonic and a flame separation from a nozzle exit at supersonic efflux velocity. It is found that with efflux velocity growth of jets it is possible to observe intensification of the luminescence of the flame at the boundary between the jet of hydrogen and air / nanopowder mixture and existence of «bottleneck flame region» as in a laminar coaxial jet and in a flame of a hydrogen microjet combustion. On the other hand, it is possible to observe actually disappearance of the «bottleneck flame region» at approach to transonic efflux velocity.