Laminar flame in the approximation of weak compressibility of reacting flows

The complete system of hydrodynamic equations describing the development of instability of the reaction front in the hydrodynamic approximation is reduced to a closed system of surface equations with the use of Lagrange variables, integrals of motion, and their analogs. In the adiabatic approximation, it is shown how to take into account the acoustic vibrations of the gas density caused by this motion.     

Nonlinear acoustic effects induced by propagation of intense ultrasound through liquid

Examination was performed into nonlinear acoustic effects (cavitation, acoustic flow, and radiation pressure) induced by propagation of intense (>0.1 W/cm²) kilohertz-range frequency ultrasonic vibrations through liquids. Reported at the Conf. “Liquid-Phase Systems and Nonlinear Processes in Chemistry and Chemical Technology,” Ivanovo, 1999.     

Refining steel in a ladle by inert gas and acoustic blow

Methods are proposed for intensifying mass transfer in blow of steel in a ladle using acoustic vibrations. Industrial tests of blowing metal in a ladle using a lance and imposition of acoustic vibrations generated by an external acoustic radiator are described. An acoustic blow plug (ABP) containing a resonator of jet-acoustic vibrations is proposed for argon blow of steel in the ladle. Model studies and theoretical estimates of factors that contribute to intensification of mixing and decrease in the blow plug erosion are described. The results of industrial testing of blowing steel in a furnace-ladle plant using the ABP at the Severskii Pipe Works are presented. __________ Translated from Novye Ogneupory, No. 8, pp. 12–17, August, 2007.     

Combustion characteristics of unpulverized coal under pulsating and non-pulsating conditions

This paper describes the differences observed when a Rijke type coal combustor was operated under pulsating and non-pulsating conditions. The combustor consisted of an open ended, 108 inch long, 5.5 inch internal diameter, vertical tube with a coal burning bed located in the middle of its lower half. The liberation of heat at this location excites the longitudinal acoustic mode of the tube. Non-pulsating operation was achieved by opening a small hole in the combustor's wall a few inches above the combustion bed. It was verified that the presence of acoustic oscillations results in significant improvements in the combustor performance. For example, switching from non-pulsating to pulsating operation was accompanied by an increase of approximately 20% in the instantaneous combustion efficiency and a decrease of approximately 58% in the relative amounts of exhaust flow particulates.     

On the identification of helical instabilities in a reacting swirling flow

An experimental study of unsteady reacting flow in a lean premixed swirl-stabilized combustor has been performed using a high speed camera, LDV, chemiluminescence and acoustic probes. To discern the role of fluid mechanics instability, isothermal and reacting (propane burning) tests were carried out. A distinct modulation of the combustion process by the hydrodynamical instability was revealed. It was confirmed that the flow unsteadiness is attributed to the precessing left-handed spiral (helical) vortical structure. The abnormal effect of a non-monotonic evolution of precession frequency vs. swirl number, with a region of inverted (decreasing) dependence, was observed regardless of the flow conditions (isothermal/reacting, confined/unconfined). The temporal frequency of the helical mode was treated as a superposition of two terms accounting for its pure rotation and axial translation but weighted with coefficients which are azimuthal and axial wavenumbers, respectively. Using cross-correlation measurements, the impact of swirl was confirmed to increase the helical structure’s axial wavelength. This effect naturally yields the observed decreasing trend in the frequency curve due to the second contributor.     

Forced and self-excited oscillations in a natural gas fired lean premixed combustor

An experimental study of the flame response in a premixed gas turbine combustor has been conducted at room temperature and under atmospheric pressure inlet conditions using natural gas. The fuel is premixed with the air upstream of a choked inlet to avoid equivalence ratio fluctuations. Therefore the observed flame response is only the result of the imposed velocity fluctuations, which are produced using a variable-speed siren. Also, a variable length combustor is designed for investigating characteristics of self-excited instabilities. Measurements are made of the velocity fluctuation in the mixing section using hot wire anemometry and of the heat release fluctuation in the combustor using chemiluminescence emission. The results are analyzed to determine the phase and gain of the flame transfer function.The results show that the gain of flame transfer function is closely associated both with inlet flow forcing conditions such as frequency and amplitude of modulation as well as the operating conditions such as equivalence ratio. In order to predict the operating conditions where the combustor goes stable or unstable at given combustor and nozzle designs, time-lag analysis was tried using convection time delay measured from the phase information of the transfer function. The model prediction was in very good agreement with the self-excited instability measurement. However, spatial heat release distribution became more significant in long flames than in short flames and also had an important influence on the system damping procedure.     

PVC AND INSTABILITY IN SWIRL COMBUSTORS

This paper describes the precessing vortex core (PVC) and other instabilities that are present in swirl generators/burners/combustors at high degrees of swirl to the flow and at high Reynolds number. PVC generates a characteristic sinusoidal signal of velocity and pressure and yields very high levels of regular oscillations of pressure, acoustic oscillations in addition to the high levels of turbulence. A mathematical model is described of the PVC and instability in swirl flows under cold flow conditions by treating the flow to be 2-Dimensional, incompressible, and at low Mach number. Relationships are derived for the various forces acting on the central core of the flow and also the conditions under which PVC is accelerated and hampered. Experimental data are obtained from a swirl combustor in which the degree of swirl and mode of gaseous fuel introduction could be changed. Measurements were made of the noise amplitude-frequency spectra and PVC frequency under cold and hot flow conditions and the radial distribution of temperatures. Comparative data are presented for the effect of combustion upon PVC. Good agreement is found between the calculated and experimentally measured acoustic frequency of the PVC.     

Gas concentration and temperature in acoustically excited Delft turbulent jet flames

This paper shows the experimental results for changes in the flame structure when acoustic fields are applied in natural gas Delft turbulent diffusion flames. The acoustic field (pulsating combustion) generates zones of intense mixture of reactants in the flame region, promoting a more complete combustion and, consequently, lower pollutant emissions, increase in convective heat transfer rates, and lower fuel consumption. The results show that the presence of the acoustic field changes drastically the flame structure, mainly in the burner natural frequencies. However, for higher acoustic amplitudes, or acoustic pressures, a hydrogen pilot flame is necessary in order to keep the main flame anchored. In the flame regions where the acoustic field is more intense, premixed flame characteristics were observed. Besides, the pulsating regime modifies the axial and radial combustion structure, which could be verified by the radial distribution of concentrations of O₂, CO, CO₂, and NO_x, and by the temperature profile. The experiments also presented the reduction of flame length with the increase of acoustic amplitude.     

Acoustic and pulsating methods and devices in steel production

Methods and devices that produce effective actions on different objects in the course of steel production through the use of acoustic and pulsating vibrations are considered. Data on the results of the use of blowing tuyeres, devices for cleaning blowing tuyeres, a method of generating structuro-acoustic effects in the furnace workspace, and a device for acoustic blasting of the lens of radiation pyrometers are presented.     

Fiberization of Silicate Melts in an Acoustic Field

A new technology for processing silicate melts into superfine fiber using high-intensity acoustic vibrations is considered. The physical mechanism of liquid dispersion in the presence of an ultrasonic acoustic field is studied. An interpretation of the fiberization process in an ultrasonic acoustic field is provided in the context of capillary-wave and cavitation theories. Results of an industrial test of acoustic-blow technology using specially designed ejection heads are described.     

The effect of elastomeric liners on high-pressure liquid propellant combustion oscillations

Liquid propellants are currently being investigated as an alternative to solid propellants in advanced gun designs. The use of liquids may provide advantages in the areas of safety, cost and performance. One area of concern in liquid propellant gun designs is the presence of large pressure oscillations: oscillations of ± 50% of the baseline pressure have been observed. These oscillations may result in a degradation of performance and reliability. The source of pressure oscillations and possible mitigation schemes are being studied in the Sandia Liquid Propellant Injector/Combustor. In this research device, the effect of acoustic liners on pressure oscillations in a high-pressure (128 MPa) liquid propellant spray combustor is studied for a hydroxylammonium nitrate-based propellant. In this study we investigated the effects of orientation and coverage fraction of elastomer liners on pressure oscillation reduction. Results for a range of elastomer/steel liner combinations are presented. along with an acoustic model and a correlation the data. The results indicate that: (1) linears can reduce oscillations by approximately three order of magnitude,(2) the reduction in pressure oscillation is independent of the placement of liners relative to the propellant flow, and (3) the magnitude of the oscillations can be correlated with the acoustic properties of the wall material.     

Helmholtz型脉动燃烧器稳定性研究

脉动燃烧器的稳定性是其应用的前提,其稳定性包括点火稳定性和运行稳定性。本文过对自行研制的Helmholtz型膜片阀式脉动燃烧器的稳定性进行研究。分析了影响脉动燃烧器点火稳定性的因素,提出提高点火稳定性的措施。对所设计脉动燃烧器最小熄火燃气流量即燃烧范围进行了研究。针对脉动燃烧器在空气中长时间燃烧容易熄火的现象做出了分析,认为高温时影响脉动燃烧不稳定的主要因素,并通过对实验结果的分析得出了熄火原因和解决方案。     

Diagnostics of acoustic properties of the combustion zone by means of periodic changes in the nozzle throat area

A mathematical model is proposed, which allows calculating the amplitude of forced pressure oscillations in the combustor of a liquid-propellant rocket engine, caused by periodic changes in the nozzle throat area, with allowance for acoustic properties of the combustion zone. Results calculated for some particular cases are given. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 12–17, November–December, 2008.     

Studies of surface and gas reactions in a catalytically stabilized combustor

A numerical investigation of a catalytically stabilized thermal (CST) combustor was conducted for a multichannel catalyst bed, and both the catalyst bed and thermal combustor were simultaneously modeled. The numerical model handled the coupling of the surface and gas reaction in the catalyst bed as well as the gas reaction in the thermal combustor. The behavior of the catalyst bed was investigated at a variety of operating conditions, and location of the flame in the CST combustor was investigated via an analysis of the distribution of CO concentration. Through parametric analyses of the flame position, it was possible to derive a criterion to determine whether the flame is present in the catalyst bed or the thermal combustor for a given inlet condition. The results showed that the maximum inlet temperature at which the flame is located in the thermal combustor increased with increasing inlet velocity.     

Analysis of combustion efficiency in CFB coal combustors

Fluidized bed technology is well known for its high combustion efficiency and is widely used in coal combustion. In this study, the combustor efficiency has been defined and investigated for CFB coal combustor based on the losses using a dynamic 2D model. The model is shown to agree well with the published data. The effect of operating parameters such as excess air ratio, bed operational velocity, coal particle diameter and combustor load and the effect of design variables such as bed height and bed diameter on the mean bed temperature, the overall CO emission and the combustion efficiency are analyzed for the small-scale of CFBC in the presently developed model. As a result of this analysis, it is observed that the combustion efficiency decreases with increasing excess air value. The combustion efficiency increases with the bed operational velocity. Increasing coal particle size results in higher combustion efficiency values. The coal feed rate has negative effect on the combustion efficiency. The combustor efficiency considerably increases with increasing combustor height and diameter if other parameters are kept unchanged.     

Study of vibratory behavior of interconnected porous PZT by impulse method

Performances in ultrasonic active transducers of interconnected porous lead zirconate titanate (PZT) piezoelectric disks with a porosity ranging from 30 to 70%, and polarized along their axial axis, are investigated. The characterization method used is based on the measurement of the voltage, which appears between the two faces of the piezoelectric element when it is excited by a current impulse. The device used, allows the acquisition of axial and radial vibrations of the transducer, and from these data, electromechanical and acoustic parameters are deduced. One observes that interconnected porosity causes the disappearance of the radial vibrations, and for large porosities the disk vibrates exclusively according to the axial mode. k_t is increased, the acoustic impedance is reduced, and the axial propagation velocity reaches 2500 m s⁻¹ for 30% of porosity. These results show that interconnected porous PZT are suitable for making ultrasonic active transducer, such as biomedical imaging devices.     

煤粉低尘燃烧器内燃烧特性的数值模拟

介绍了一种用于中小型工业窑炉的新型煤粉低尘燃烧技术,利用计算机数值模拟考察了煤粉低尘旋流燃烧器的特性. 在合理选择气相流动、固相流动、煤燃烧及NO的生成等模型的同时,针对旋流燃烧场中固体颗粒在壁面附近的碰撞及熔融特性,探讨了煤粉在壁面处的运动模型,并以此为基础考察了燃烧场的两相流动特性,模拟了燃烧器内煤粉的燃烧过程及各物理量的分布. 在与实验比较的基础上,对燃烧器的结构进行了改进. 结果表明,在低化学计量比下,改进后的燃烧器性能更好,颗粒在燃烧器内充分燃尽,在保证液排渣效果的同时,NO的排放远低于常规液排渣旋风器的NO排放量.     

Continuous spin detonation of a coal-air mixture in a flow-type plane-radial combustor

Regimes of continuous spin detonation of coal particles in an air flow in a flow-type plane-radial combustor 500 mm in diameter are studied. The tested substance is fine-grained cannel coal from Kuzbass having a particle size of 1–7 µm and containing 24.7% of volatiles, 14.2% of ashes, and 5.1% of moisture. A certain amount of hydrogen is added for coal transportation into the combustor and promotion of the chemical reaction on the surface of solid particles. To reduce air pressure losses in channels connecting the manifold and the combustor, their cross section is increased to limiting values (25 cm²), whereas the combustor exit diameter is reduced. The angle of the air flow direction and the combustor geometry are also varied. The minimum pressure difference in the air injection channels (16%) is reached with stability of continuous spin detonation in the combustor being retained. The domain of continuous spin detonation regimes in the coordinates of the fuel flow rate and specific flow rate of the mixture is constructed. The results of studying detonation burning of solid fuels can find applications in power engineering, chemical industry, and environmental science, in particular, contamination by combustion products.     

Experimental and numerical studies of mixing and flame stability in a micro-cyclone combustor

A 30-W-class micro-cyclone combustor was developed as a heat source for a 1-W thermoelectric power generator (TPG). Methane gas was used as a fuel instead of liquefied fuel in this feasibility study for convenience. The combustion stability of the combustor was measured, and the flame shapes were visualized experimentally. Numerical simulations were performed to examine the details of the flame structure and flame stabilization mechanism inside the micro-cyclone combustor. The micro-cyclone combustor burned the supplied fuel stably inside the combustion chamber in the range where the combustor generated 30 W of heat energy. The mixing and flow characteristics of non-reacting and reacting flows in the combustor were examined using the simulation results. The mixing of the fuel with air in a non-reacting flow field was enhanced by increasing the equivalence ratio for a fixed fuel flow rate. For non-reacting flow, a recirculation region and a small negative axial velocity region near the injection ports were formed. The recirculation region became wider with decreasing equivalence ratios. For reacting flows, however, the recirculation region disappeared and the only small negative axial velocity region was formed near the fuel injection ports. The flame was stabilized inside the combustor because the flame base was anchored near the negative axial velocity region near the fuel injection ports.