Volume 54, Nº 3 (2018)

Results of an experimental study of hydrodynamics and diffusion combustion of hydrocarbon jets are presented. Various regimes of instability development both in the jet flame proper and inside the source of the fuel jet are considered. The experiments are performed for the case of subsonic gas jet expansion into the air from a long tube 3.2 mm in diameter in the range of Reynolds numbers from 200 to 13 500. The fuel is the propane–butane mixture in experiments with a cold jet (without combustion) and pure propane or propane mixed with an inert dilutant (CO₂ or He) for the jet flame. The mean velocity and velocity fluctuations in the near field of the jet without combustion are measured. Among four possible regimes of cold jet expansion (dissipative, laminar, transitional, and turbulent), three last regimes are investigated. The Hilbert visualization of the reacting flow is performed. The temperature profiles in the near field of the jet are measured by a Pt/Pt–Rh thermocouple. An attached laminar flame is observed in the transitional regime of propane jet expansion from the tube. In the case of combustion of C₃H₈ mixtures with CO₂ or with He in the range of Reynolds numbers from 1900 to 3500, the transitional regime is detected in the lifted flame. Turbulent spots formed in the tube in the transitional regime exert a significant effect on the flame front position: they can either initiate a transition to a turbulent flame or lead to its laminarization.

The dynamic problems of the theory of combustion and explosion are classified from the point of view of competition of the characteristic times of bulk chemical reaction, frontal combustion, heat transfer, gas flow from the vessel, compression (motion of the piston), and heating (cooling) rate. We consider dynamic similarity criteria such as the ratio of the characteristic times in problems of classical thermal explosion, dynamic thermal explosion, flammability limits, combustion in communicating vessels, competition of frontal and volumetric combustion, autoignition by adiabatic compression, competition of frontal combustion and piston motion, as well as compound similarity criteria (such as functions of simple dynamic similarity criteria). The problem of autoignition of a mixture by compression is considered, and a method for obtaining an analytical solution of the problem and an algorithm for the approximate solution based on a special differential criterion are proposed.

Results of an experimental study of the spatial structure of a reacting flow during combustion of a propane–air mixture in a turbulent swirling jet escaping into atmospheric air are presented. The fuel-to-air equivalence ratio is φ = 0.7, and the Reynolds number of the jet is Re = 5 · 10³. The time-averaged spatial distributions of velocity, local density, and concentrations of the main species of the gas mixture are measured in low-swirl and high-swirl flows. In both cases, the flame front is stabilized in the internal mixing layer formed by the axial region of jet retardation, where hot combustion products are concentrated. In a high-swirl flow, the temperature distributions in the cross section y/d = 0.5 show that the region with the maximum temperature of the gas is located at the periphery of the central recirculation zone. Moreover, in the case of a high-swirl flow, there exists a recirculation zone at the axis, and the CO₂ concentration is twice higher than in a low-swirl jet. The opposite situation is observed for O₂.

A series of experiments on a black non-charring polymer in the low-pressure chamber is conducted under different external heat fluxes. The surface and bottom temperatures and the mass loss of the sample are measured. A parameter T_p is introduced to describe the impact of pressure on the surface temperature. There is a loose layer of the char residue left with significant pyrolysis bubbles under the low heat flux, and the bubble size decreases with pressure. The parameter T_p is found to exhibit a significant decline trend with increasing pressure, and the mass loss rate of the sample decreases apparently as the pressure increases. However, under a high heat flux, the char residue is denser, and the pyrolysis bubbles are not observed. The value of T_p and the mass loss rate of the sample have no obvious relationship with pressure. The average pyrolysis rate is linearly proportional to p^a.

This paper presents the results of an experimental and theoretical study of heat and mass transfer during ignition of wet wood particles in a high-temperature gas medium. Experiments were carried out in a setup which provides conditions similar to the combustion spaces of boiler units. The main heat transfer parameters (ambient temperature) and integrated ignition characteristics (ignition delay) were measured. The measurement error of these parameters did not exceed 18%. The convective transfer of water vapor formed during evaporation of pore moisture and pyrolysis products were found to have an insignificant effect on the ignition characteristics and conditions. From the results of the experiments, a mathematical model of the ignition process was developed which describes the simultaneous occurrence of the main processes of thermal preparation under conditions of intense phase (evaporation of water) and thermochemical transformations (thermal decomposition of the organic part of the fuel, thermochemical interaction between water vapor and carbon coke, ignition of volatiles) taking into account the convective diffusion of water vapor and pyrolysis products in the near-wall gas area during the induction period. The theoretical ignition delay is in satisfactory (within the confidence interval) agreement with the experimental value. The numerical model of the diffusion flame adequately (good agreement between experimental and theoretical ignition delays) describes the ignition of a wet wood particle.

A review of some investigations performed in the field of mechanics of reacting heterogeneous media with micro- and nanostructures at the Khristianovich Institute of Theoretical and Applied Mechanics of the Siberian Branch of the Russian Academy of Sciences is presented. Some new results are also reported.

Combustion tests of a ramjet model 1.05 m long and 0.31 m in diameter with an expanding annular combustor operating in the regime of detonation combustion of hydrogen are described. The tests are performed in a short-duration wind tunnel at free-stream Mach numbers of the incoming air flow from 5 to 8 and stagnation temperature of 290 K. Continuous detonation and longitudinally pulsating regimes of hydrogen combustion with characteristic frequencies of 1250 and 900 Hz, respectively, are observed. The maximum measured values of the fuel-based specific impulse and the thrust generated by the engine are 3600 s and 2200 N, respectively.

The paper presents basic technical solutions for the firing system and design of the burners of boilers PK-39-IIM and BKZ-420-140-5 when firing Ekibastuz coal. The developed low-NO_x firing systems ensure low nitrogen oxide emissions both at low cross-section heat release rates and high furnace cross-section heat release rates. Technical solutions developed for the firing systems are based on the simulation of coal combustion in furnaces of the mentioned boilers using ANSYS Fluent software.