Home – Home – Jornals – Combustion, Explosion and Shock Waves 2018 number 3

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 exhaustion into the air from a long tube 3.2 mm in diameter in the range of Reynolds numbers from 200 to 13500. The fuel is the propane-butane mixture in experiments with a cold jet (without combustion) and pure propane or propane mixed with an inert diluter (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 exhaustion (dissipative, laminar, transitional, and turbulent), three last regimes are investigated. Schlieren 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 exhaustion 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 detached 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.

This paper presents a method of testing structural materials for spark safety and the results of testing this method for several pairs of materials and samples of combustible gases and vapors. Hydrogen, acetylene, AI-92 gasoline vapor, methane and liquefied petroleum gas were used as combustible gases and vapor, and samples of various types of steel, aluminum, and brass as structural materials.

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

The structure and dynamics of a natural convection turbulent diffusion flame near a vertical surface with combustible gas exhaustion are numerically studied by using the FDS model and computer code. The flame is considered near the surface through which gaseous propylene is injected with a prescribed flow rate. Requirements are determined for the grid cell size in the near-wall region, which ensures a sufficient spatial resolution of the boundary layer structure. It is shown that the computed value of the total heat flux on the surface agrees with the measured results. Investigations of ignition and combustion of a vertical plate of completely gasifiable thermal plastic (polymethylmetacrylate) with allowance for the material pyrolysis reaction shows that the heater-igniter parameters determine the duration of the transitional period, but weakly affect the growth rate of the heat release intensity and the height of the pyrolysis region at the stage of developed combustion. Significant effects of the heater shape, size, and temperature, as well as lateral entrainment of air on the velocity of flame propagation upward over the plate surface and on the shape of the pyrolysis front. The existence of critical parameters of the heater separating flame decay from developed combustion is demonstrated. Three regimes of flame propagation with different pyrolysis front shapes are identified.

Results of an experimental study of the spatial structure of a reacting flow during combustion of a propane-air mixture in a turbulent swirled jet escaping into atmospheric air are presented. The fuel-to-air equivalence ratio is  = 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 under the conditions of moderate and intense swirling of the flow. In both cases, the flame front is stabilized in the internal mixing layer formed by the axial region of jet deceleration, where hot combustion products are concentrated. In the case of intense swirling of the 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. In the case of intense swirling of the flow, there is a recirculation region at the axis, and the CO2 concentration is twice higher than in a moderately swirled jet. The opposite situation is observed for O2.

The principles of calculation of the characteristics of both coarse and fine fractions of condensed combustion products formed at the propellant surface and evolving as part of the multiphase flow in the combustion chamber. The characteristics of the condensed products at the propellant surface are determined by modeling the processes involved in their formation using a decision-making system based on the ideology of expert systems. An evolution model under inhomogeneous flow conditions with the mutual influence of the characteristics of condensed and gaseous combustion products is developed to determine the characteristics of condensed products in the multiphase flow of combustion products. The developed tools allow the characteristics of condensed products to be determined depending on the propellant composition, characteristics of the propellant charge, and combustion chamber parameters. Parametric analysis of the developed models for conditions in a combustion chamber was performed.

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 Tp 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 Tp 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 Tp 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 experimental results for the thermal decomposition of hydroxylammonium nitrate (HAN) in the presence of activated carbon with a high specific surface (up to 3000 m2/g) obtained by activation of rice husk with potassium hydroxide at a temperature of 700oC in the rotating spherical furnace. With addition of activated carbon, the temperature of the beginning of the decomposition of HAN reduced from 185 to 86±0.5oC. The burning rate of HAN doped with activated carbon increases to 400 mm/s at an overpressure of 6 MPa. It has been shown that the addition of activated carbon reduce the amount of NO x gases produced by decomposition to 30%.

The results of an experimental and theoretical study of heat and mass transfer occurring during ignition of wet wood particles in a high-temperature gas environment. Experiments were carried out in a facility which provides conditions corresponding to the combustion spaces of boiler units. The main heat transfer parameters (ambient temperature) and integrated ignition characteristics (delay time) were measured. The measurement error of these parameters did not exceed 18%. It is found that the convective transfer of water vapor formed during evaporation of intraporous moisture and pyrolysis products does not have a significant effect on the ignition characteristics and conditions. The results of the experiments were used to develop a mathematical model of the ignition process, which describes the joint flow 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 of 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 one. The numerical model of the diffusion flame adequately (good agreement between experimental and theoretical ignition delays) describes the ignition of a wet wood particle

The high efficiency of a combined cycle and the availability of systems for deep purification of synthesis gas before combustion allow for the study of gasification-combined cycle plants as a promising solution to increase the efficiency and environmental friendliness of coal power engineering. The key element of the device is a gasifier. A Mitsubishi Heavy Industries gasifier is selected as an initial design for modernization, which consisted in heating the blast air up to 900oC and steam supply with a temperature of 900oC. The impact of modernization on the gasifier characteristics is determined by using zero-, one-, and three-dimensional models. Modernization of the gasifier allows increasing the thermal power by the synthesis gas and chemical efficiency from 77.2 to 84.9% and increasing the ratio H2/CO from 0.34 to 0.6.

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 Russian Academy of Sciences is presented. Some new results are also reported.

This paper describes the ignition of high-energy materials (HEMs) on the basis of perchlorate and ammonium nitrate and an active fuel-binder, containing Al powders (base composition), B, AlB2, AlB12, and TiB2, upon initiation of the process by a CO2 laser in the heat flux density range 90-200 W/cm2. The delay time of ignition and surface temperature of the reaction layer during the heating-up and ignition of HEMs in air. It is obtained that the complete replacement of the micron-sized aluminum powder by amorphous boron as part of HEMs significantly reduces the delay time of the sample (by 2.2-2.8 times) with the same density of the heat flux, and this occurs due to the high chemical activity and difference between mechanisms of oxidation of boron particles. The use of aluminum diboride in HEMs reduces the ignition delay time by 1.7-2.2 times in comparison with the basic composition. The ignition delay time of a HEM sample with titanium diboride decreases slightly (by 10-25%) relative to the ignition delay of the basic composition.

Firing 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 test 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 streamwise-oscillating 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 Navier-Stokes equations were used to carry out the numerical modeling of chemically reactive gas flow in an annular flow chamber. The model was based on laws of conservation of mass, momentum, and energy for nonstationary two-dimensional compressible gas flow in the case of axial symmetry with a tangential component of the gas velocity. Viscosity, thermal conductivity, and turbulence were taken into account. Fuel and oxidizer were fed into the chamber separately, and heat release in the chemical reaction zone was largely determined by the rate of turbulent mixing of the gas components. The possibility of burning out of the mixture in the chamber was demonstrated numerically. Detonation failure can occur if reagents temporarily cease to be fed into the chamber, which can be caused by high pressure in the reaction zone. With short combustion chamber lengths, there are the underburning of fuel and the release of unreacted hydrogen into the atmosphere.

This paper describes engineering solutions for a combustion system and structure of the furnace burner devices of PK-39-IIM and BKZ-420-140-5 boilers. The developed low-emission schemes of combustion make it possible to reduce emissions of nitrogen oxides both at low and high values of the heat stress of the cross section of the furnace space. The basis of the developed engineering solutions for combustion systems is the mathematical modeling of aerodynamics and combustion of coal in the furnaces of these boilers, carried out in an ANSYS Fluent software complex.

Results of an experimental study of conditions and characteristics of ignition of fuel suspensions prepared on the basis of typical waste products of coal enrichment and oil refining are reported. The main attention is paid to analyzing the differences in the characteristics and conditions of ignition of organic coal-water fuels prepared on the basis of coal enrichment waste products (lean, caking, low-caking, fat, gas, and long-flame coals are considered). The ignition delay time and the minimum (threshold) temperature of ignition of the examined fuels are estimated. Conclusions on the influence of the properties and concentrations of the fuel components on the combustion initiation characteristics are formulated.