Flow Of Combustion Products Research Articles

Effect of the condensed particles on a flow of combustion products in a Laval nozzle

The specialized multifunctional software package is developed for the numerical simulation of the flow of products of combustion in a Laval nozzle. The complex has a friendly graphical user interface of input-output and wide possibilities for visualization of simulation results directly in the process of calculating. Particles moving in the gas stream are exposed to the action of hydro-mechanical forces due to its interaction with the carrier gas environment, as well as mass forces caused by the gravitational field. Performed parametric calculations have shown that the interaction of particles with the combustion products, even with a relatively small volume fraction may lead to a qualitative change in the internal flow in the Laval nozzle, and thereby influence the characteristics of the nozzle.

Ignition and combustion of pyrotechnic compositions based on microsized and ultra-nanosized aluminum particles in a moist medium in a two-zone gas generator

This paper presents the results of an experimental study of the ignition and combustion of pyrotechnic compositions based on microsized and ultra-nanosized aluminum particles in a model two-zone gas generator using water as oxidizer. The flow of combustion products from the gas generator was video recorded, and the condensed products sampled behind the nozzle exit were studied by chemical and particle-size analyses. It was found that the replacement of microsized aluminum particles by ultra-nanosized particles in the samples led to a ≈17% decrease in the active aluminum content in the condensed phase, a 10–15% increase in the efficiency of the process in the gas generator (completeness of conversion of the pyrotechnic composition to combustion products), and a factor of about three decrease in the ignition delay.

Combustion of solid propellant with micron-sized Aluminium under the acceleration force

The paper presents a physical-mathematical model for metallized solid propellant combustion under acceleration directed along the normal to the burning surface. The model takes into account the thermal effect of decomposition of the condensed phase, convection, diffusion, the exothermic chemical reaction in the gas phase, heating and combustion of Al particles in the gas flow, the flow of combustion products, the particle velocity lag relative to the gas and the field effect of acceleration on the motion of Al particles. The effect of the Al particle size and mass fraction, emitted from the burning surface, on the burning rate is also taken into consideration. The impact of the Al particle size, emitted from the burning surface, on the linear burning rate has been investigated under acceleration. The study results showed that with increasing acceleration the burning rate increased. It was also revealed that the larger was the size of aluminum particles emitted from the burning surface, the higher was the response of the burning rate to acceleration. The results showed that increasing the mass fraction of aluminum in the propellant composition led to an increase in the response of the relative burning rate. It was also found that the relative burning rate sensitivity increased with pressure above the burning surface. The results obtained are in a qualitative agreement with those described in the scientific literature.

Large-Eddy Simulation of a Reacting Jet in Cross Flow With NOx Prediction

The reduction of full and part load emissions and the increase of the turndown ratio are important goals for gas turbine combustor development. Combustion techniques, which generate lower NOx emissions than unstaged premixed combustion in the full load range, and which have the potential of reducing minimum load while complying with emission legislation, are of high technical interest. Therefore, axial-staged combustion systems have been designed, either with or without expansion in a turbine stage between both stages. In its simpler form without intermediate expansion stage, a flow of hot combustion products is generated in the first stage of the premixed combustor, which interacts with the jets of premixed gas injected into the second stage. The level of NOx formation during combustion of the premixed jets in the hot cross flow determines the advantage of axially staged combustion regarding full load NOx emission reduction. Employing large-eddy simulation in openfoam, a tool has been developed, which allows to investigate staged combustion systems including not only temperature distribution but also NOx emissions under engine conditions. To be able to compute NOx formation correctly, the combustion process has to be captured with sufficient level of accuracy. This is achieved by the partially stirred reactor model. It is combined with a newly developed NOx model, which is a combination of a tabulation technique for the NOx source term based on mixture fraction and progress variable and a partial equilibrium approach. The NOx model is successfully validated with generic burner stabilized flame data and with measurements from a large-scale reacting jet in cross flow experiment. The new NOx model is finally used to compute a reacting jet in cross flow under engine conditions to investigate the NOx formation of staged combustion in detail. The comparison between the atmospheric and the pressurized configuration gives valuable insight in the NOx formation process. It can be shown that the NOx formation within a reacting jet in cross flow configuration is reduced and not only diluted.

Experimental estimation of the influence of the droplet evaporation process on the conditions of movement in an oncoming high-temperature gas flow

By means of high-speed video registration, the cross-correlation system, and panoramic optical methods of trace visualization, experimental estimation of the influence of liquid (water) droplet evaporation on the conditions of droplet movement (acceleration and deceleration) through the high-temperature (about 1100 K) gases was made. The experiments were conducted with droplets about 1–6 mm in diameter at start velocities of 1–5 m/s. We compare the integral characteristics of the droplet movement in the air at a temperature of about 300 K (in the ongoing flow and through the steady gas medium) and in the combustion product flow at a temperature of about 1100 K. The gas and the air flow velocities were about 1.5 m/s. The typical difference in the droplet velocities under essentially different ambient temperatures was discovered. The contribution of water evaporation and the ongoing gas movement into droplet deceleration was discovered.

Effect of gas temperature and oxygen concentration on single particle ignition behavior of biomass fuels

This work focuses on single particle ignition behavior of biomass fuels. Five biomass fuels were studied, namely, wheat straw, kiwi branches, vine branches, sycamore branches and pine bark. Coal was also tested for comparison purposes. Tests were carried out in an optical flat flame McKenna burner able to produce a confined laminar flow of combustion products, in which single particles were injected upward through a central hole. The equivalence ratio and the thermal input of the burner were adjusted to yield the following operating conditions in the ignition zone: five mean temperatures (1500, 1575, 1650, 1700 and 1800K) and three mean dry oxygen concentrations (3.5, 5.1 and 6.5vol. %). The ignition mode and the ignition delay time were evaluated through the analysis of images obtained by means of a CMOS high-speed camera. The effects of gas temperature and oxygen concentration on the ignition mode and ignition delay time were assessed. For the studied conditions, the ignition of the biomass particles generally occurred in the gas-phase, although surface ignition was also observed in some cases. The results suggest that the critical diameter for the ignition mode transition varies with the fuel type. All particles show a consistent trend of decreasing ignition delay time with increasing gas temperature, with the biomass particles presenting slightly higher values than the coal tested. The estimated drying times were subtracted from the total ignition delay time and the trends shifted, with the pine bark and wheat straw presenting a lower remaining time than the coal. Finally, for the studied oxygen concentration range, its variation did not have a significant impact on the ignition mode and delay time at high temperatures.

Formation of Vortex Structures in the Prenozzle Space of an Engine with a Vectorable Thrust Nozzle

A numerical simulation of the hydrodynamic effects arising in the process of work of the vectorable thrust nozzle of a solid-propellant rocket engine has been performed. The fields of the flows of combustion products in the channel of a charge, the prenozzle space, and the nozzle unit were calculated for different angles of vectoring of the nozzle. The distributions of the gasdynamic parameters of the flow of combustion products in the prenozzle space, corresponding to their efflux from the cylindrical and star-shaped channels of charges, were compared. The formation of a vortex flow in the neighborhood of the back cover of the nozzle was considered.

The integral characteristics of the deceleration and entrainment of water droplets by the counter flow of high-temperature combustion products

This paper presents the results of experimental investigations on the deceleration and entrainment of water droplets during their motion in the counter flow of high-temperature combustion products (up to 1900K). High-speed video cameras (105 frames per second), specialized software applications (with continuous tracking functions), as well as panoramic optical methods (Particle Image Velocimetry, Stereoscopic Particle Image Velocimetry, Particle Tracking Velocimetry, Shadow Photography) registered the processes under study. We used several typical oils, gasoline, kerosene, acetone, and industrial alcohol to generate combustion products with a controlled high temperature. The initial sizes (radius) of droplets and their velocities were varied from 0.05mm to 0.35mm and from 0.5m/s to 5m/s. The velocities of counter motion of combustion products were varied from 0.1m/s to 2.5m/s. In this paper we have also determined the characteristic trajectory length of the droplets of different sizes until their complete stop (and subsequent entrainment) in the counter flow of high-temperature gases. As a result of studies, we summarized the research results through the establishment of Weber and Reynolds numbers ranges for droplets and gases, when the full stop and entrainment of droplets may occur. This paper also covers a comparison of the characteristics of water droplet deceleration under the conditions of non-stationary (when the temperature of combustion products varies from 1900K to 400K in channel) and nearly stationary (when the temperature is 1100±30K) heat transfer. Thus, it has been found out that the values of these parameters correlate well (deviation did not exceed 7%) under such conditions (stationary and nearly stationary) during short heating (less than 0.5s).

Capturing energy from ventilation air methane a preliminary design for a new approach

Methane is a potent greenhouse gas (GHG), discharged to the atmosphere by coalmining, the natural gas industry and natural biological processes, second only to carbon dioxide; thus, any reduction in atmospheric methane would be globally beneficial. The capture or use of ventilation air methane (VAM) is challenging because it is a high volume low concentration methane source. This results in the routine discharge of methane into the atmosphere.A review of VAM mitigation technologies is provided and the main disadvantages of the existing technologies are discussed. In the proposed VamTurBurner© system, the heat from the combustion chamber is transferred to the preheating zone either by a heat exchanger or by redirecting the combustion products to mix with the ventilation air stream from a coalmine. Gas turbines (GT) are used to produce electricity with the exhaust gases directed to mix with the incoming ventilation airflow. The turbulence introduced by the GT exhaust assists with mixing of the incoming ventilation airflow and the return flow of combustion products from the combustion chamber. The combustion products are a source of heat, which increases the temperature of the incoming ventilation air to a value high enough for the methane to undergo flameless combustion upon encountering the igniters.The high temperature combustion products enter a multi-generation system. The multi-generation system is based on mature engineering technology such as heat exchangers and steam turbines. The residual heat provides additional heat based products such as industrial scale drying, chilling by an absorption chiller or simply hot water.The VamTurBurner© uses the energy from the GT, igniters and VAM to provide clean efficient energy while mitigating the atmospheric emissions of methane. The opportunity to collect carbon credits may improve the economics. Since the VAM is a free energy source, the output of the system is greater than the purchased energy.

Dynamics of gas combustion in a channel with combustion product flow through a porous wall

Gas flow from a reservoir with porous permeable walls in which combustion occurs is considered. Gas combustion in the reservoir causes an increase in the pressure in it and filtration gas flow through the permeable porous wall into the surrounding space. A model is proposed which simultaneously describes the pressure increase in the reservoir due to combustion and the associated filtration gas flow through the porous walls. Data on the maximum gas pressure in the reservoir, the discharge time, and flow characteristics as functions of the permeability of the medium were obtained by numerical solution of a one-dimensional problem.

Modeling of spontaneous condensation in high-speed expansion of gaseous mixture

Many efforts have been undertaken to model the condensation phenomena in high-speed steam flow in a steam turbine. Spontaneous vapor condensation takes place not only in high-speed flows involving pure steam such as in a steam turbine but also in other gaseous flows such as the flow of hydro-carbon combustion products through a supersonic nozzle. In the present paper, the condensation models for pure steam flow have been modified and extended to include spontaneous vapor condensation in flows of gaseous mixture containing that vapor. The modified condensation models are implemented into the Eulerian/Eulerian approach within CFD++, the commercial CFD software suite from Metacomp Technologies. It is shown that condensation would occur in a supersonic nozzle when hydro-carbon combustion products expand through the nozzle such that the water–vapor pressure at the nozzle exit is well above the saturation pressure at the local temperature. Another numerical investigation on the condensation taking place in transonic moist air flowing over a NACA 0012 airfoil with an angle of attack illustrates that there is a stationary double shock system in the flow over the upper surface of the airfoil.

Dust reactor for limestone calcination

Experimental modeling of calcination of dusty carbonate particles is performed in an isothermal flow of combustion products in a reactor holding the necessary temperature due to the intermediate introduction of inflammable gas into afterburning. The decomposition kinetics of carbonate particles in combustion products is analyzed applying the available publications in order to predict the efficiency of such reactors for practical use and to determine optimal process modes. Special attention is paid in this study to comparison of theoretical and experimental data in boundary calcination regimes, i.e., at parameters, below which calcination in combustion products is already impossible. Taking into account that one important purpose of rapid dust calcination is the preparation of active lime, which effectively binds “acidic” gases in combustion products of power installations, control over the removal of SOx from gases by dust in a reactor was performed.

Formation of vortex structures in channels with mass injection and their interaction with surfaces in solid-fuel rocket engines

The topological features of the structure of combustion products flow in the flow paths with different shapes of channel cross sections at power installations are considered. The results of mathematical modeling of internal gas dynamics of the flow paths of power installations are compared with experimental data.

On flames established with air jet in cross flow of fuel-rich combustion products

Advances in combustor technologies are driving aircraft gas turbine engines to operate at higher pressures, temperatures and equivalence ratios. A viable approach for protecting the combustor from the high-temperature environment is to inject air through the holes drilled on the surfaces. However, it is possible that the air intended for cooling purposes may react with fuel-rich combustion products and may increase heat flux. Air Force Research Laboratory (AFRL) has developed an experimental rig for studying the flames formed between the injected cold air and the cross flow of combustion products. Laser-based OH measurements revealed an upstream shift for the flames when the air injection velocity was increased and downstream shift when the fuel content in the cross flow was increased. As conventional understanding of the flame stability does not explain such shifts in flame anchoring location, a time-dependent, detailed-chemistry computational-fluid-dynamics model is used for identifying the mechanisms that are responsible. Combustion of propane fuel with air is modeled using a chemical-kinetics mechanism involving 52 species and 544 reactions. Calculations reveled that the flames in the film-cooling experiment are formed through autoignition process. Simulations have reproduced the various flame characteristics observed in the experiments. Numerical results are used for explaining the non-intuitive shifts in flame anchoring location to the changes in blowing ratio and equivalence ratio. The higher diffusive mass transfer rate of hydrogen in comparison to the local heat transport enhances H2–O2 mixing compared to thermal dissipation rate, which, in turn, affects the autoignition process. While increasing the blowing ratio abates the differences resulting from non-equal mass and heat transport rates, higher concentrations of hydrogen in the fuel-rich cross flows accelerate those differences.

ДИНАМИКА ГОРЕНИЯ ГАЗА В КАНАЛЕ С ИСТЕЧЕНИЕМ ПРОДУКТОВ СГОРАНИЯ ЧЕРЕЗ ПОРИСТУЮ СТЕНКУ

ДИНАМИКА ГОРЕНИЯ ГАЗА В КАНАЛЕ С ИСТЕЧЕНИЕМ ПРОДУКТОВ СГОРАНИЯ ЧЕРЕЗ ПОРИСТУЮ СТЕНКУ

The use of infrared thermography to study the optical characteristics of flames from burning vegetation

Some optical phenomena observed in the flames from the burning of flammable vegetation materials and ethyl alcohol are analyzed. Recommendations on the optimal spectral bands for temperature measurements of hot objects obscured by flames are given. Frequency spectra of temperature pulsations in flames are obtained, thus revealing a spatial structure in the flow of combustion products, as well as the degree of turbulence in a high temperature fluid medium.

A Comparative Computational Fluid Dynamics Study on Flamelet-Generated Manifold and Steady Laminar Flamelet Modeling for Turbulent Flames

The laminar flamelet model (LFM) (Peters, 1986, “Laminar Diffusion Flamelet Models in Non-Premixed Combustion,” Prog. Energy Combust. Sci., 10, pp. 319–339; Peters, “Laminar Flamelet Concepts in Turbulent Combustion,” Proc. Combust. Inst., 21, pp. 1231–1250) represents the turbulent flame brush using statistical averaging of laminar flamelets whose structure is not affected by turbulence. The chemical nonequilibrium effects considered in this model are due to local turbulent straining only. In contrast, the flamelet-generated manifold (FGM) (van Oijen and de Goey, 2000, “Modeling of Premixed Laminar Flames Using Flamelet-Generated Manifolds,” Combust. Sci. Technol., 161, pp. 113–137) model considers that the scalar evolution; the realized trajectories on the thermochemical manifold in a turbulent flame are approximated by the scalar evolution similar to that in a laminar flame. This model does not involve any assumption on flame structure. Therefore, it can be successfully used to model ignition, slow chemistry, and quenching effects far away from the equilibrium. In FGM, 1D premixed flamelets are solved in reaction-progress space rather than physical space. This helps better solution convergence for the flamelets over the entire mixture fraction range, especially with large kinetic mechanisms at the flammability limits (ANSYS FLUENT 14.5 Theory Guide Help Document, http://www.ansys.com). In the present work, a systematic comparative study of the FGM model with the LFM for four different turbulent diffusion/premixed flames is presented. The first flame considered in this work is methane-air flame with dilution air at the downstream. The second and third flames considered are jet flames in a coaxial flow of hot combustion products from a lean premixed flame called Cabra lifted H2 and CH4 flames (Cabra, et al., 2002, “Simultaneous Laser Raman-Rayleigh-LIF Measurements and Numerical Modeling Results of a Lifted Turbulent H2/N2 Jet Flame in a Vitiated Coflow,” Proc. Combust. Inst., 29(2), pp. 1881–1888; Lifted CH4/Air Jet Flame in a Vitiated Coflow, http://www.me.berkeley.edu/cal/vcb/data/VCMAData.html) where the reacting flow associated with the central jet exhibits similar chemical kinetics, heat transfer, and molecular transport as recirculation burners without the complex recirculating fluid mechanics. The fourth flame considered is a Sandia flame D (Barlow et al., 2005, “Piloted Methane/Air Jet Flames: Scalar Structure and Transport Effects,” Combust. Flame, 143, pp. 433–449), a piloted methane-air jet flame. It is observed that the simulation results predicted by the FGM model are more physical and accurate compared to the LFM in all the flames presented in this work. The autoignition-controlled flame lift-off is also captured well in the cases of lifted flames using the FGM model.

Subgrid-scale model for radiative transfer in turbulent participating media

The simulation of turbulent flows of radiating gases, taking into account all turbulence length scales with an accurate radiation transport solver, is computationally prohibitive for high Reynolds or Rayleigh numbers. This is particularly the case when the small structures are not optically thin. We develop in this paper a radiative transfer subgrid model suitable for the coupling with direct numerical simulations of turbulent radiating fluid flows. Owing to the linearity of the Radiative Transfer Equation (RTE), the emission source term is spatially filtered to define large-scale and subgrid-scale radiation intensities. The large-scale or filtered intensity is computed with a standard ray tracing method on a coarse grid, and the subgrid intensity is obtained analytically (in Fourier space) from the Fourier transform of the subgrid emission source term. A huge saving of computational time is obtained in comparison with direct ray tracing applied on the fine mesh. Model accuracy is checked for three 3D fluctuating temperature fields. The first field is stochastically generated and allows us to discuss the effects of the filtering level and of the optical thicknesses of the whole medium, of the integral length scale, and of the cutoff wave length. The second and third cases correspond respectively to turbulent natural convection of humid air in a cubical box, and to the flow of hot combustion products inside a channel. In all cases, the achieved accuracy on radiative powers and wall fluxes is about a few percents.

Investigation of Choking and Combustion Products' Swirling Frequency Effects on Gas Turbine Compressor Blade Fractures

Premature fracture failure of blades occurred in four of a refinery's gas turbine compressors. In order to evaluate the probability of combustion instability's effects on failure of the blades; i.e., choking and chamber resonance problems, 3D models of the combustion chamber structure and combustion flow were studied with finite element and computation fluid dynamics codes, respectively. Comparison of results of combustion chamber natural frequencies with combustion swirl frequency showed that the chamber structure is not under resonance. In order to verify probability of choking, the combustion product flow's Mach number was studied. Results of the Mach number distribution showed that the flow is subsonic in the transition piece area but, due to existence of supersonic flow conditions near the swirl vanes it may become supersonic in some critical conditions. Thus, it is suggested to operators that, for avoiding choking probabilities, it is better that engine operation be maintained close to optimum design conditions. Results of simulations showed that the fracture of the blades is not due to combustion problems.

Numerical study of intrachamber processes dynamics at nozzleless solid propellant rocket engine actuation

The results of the comprehensive numerical analysis for dynamics of intrachamber processes that appear at nozzleless solid propellant rocket engine (SPRE) actuation are presented. A complete cycle of rocket engine operation is analyzed. We solve a conjugate problem involving the igniter actuation; heating, ignition and following combustion of a solid propellant charge; a combustion product flow in the combustion chamber; depressurization of the combustion chamber, and the subsequent motion of the rocket engine blank; variation of the combustion surface geometry at the expense of the gradual and nonuniform burnout of solid propellant web.