Junwei Li

Feedback Control of Combustion Instabilities Using a Helmholtz Resonator with an Oscillating Volume

A feedback control strategy is developed for mitigating combustion instabilities using a Helmholtz resonator with an oscillating volume. This is based on the fact that the frequency at which the resonator provides maximum damping can be controlled by oscillating its cavity volume. For this, two algorithms are developed. One is a real-time plane-wave decomposition algorithm; the other is a finite impulse response filter, its coefficients being optimized by the least-mean-square method but with a variable step size. The filter uses the decomposed incident wave to determine the optimum actuation signal. The performance of the control strategy, carried out with off-line system identification, is evaluated via a numerical model of an unstable combustion system with a dominant longitudinal mode. It is successfully demonstrated that the control strategy is more robust and capable of stabilizing the combustion system at a faster rate than that of conventional filters with fixed step size.

Theoretical Modeling and Numerical Study for Thrust- Oscillation Characteristics in Solid Rocket Motors

Todiscover thrust-oscillation characteristics in solid rocketmotors, analyticalmodeling andnumerical simulation are carried out by an experimentalmotor in the vonKarman Institute for FluidDynamics. The numericalmethod by means of amesh sensitivity analysis is proposed for validation. Velocity profiles, oscillation frequencies, and pressure amplitudes were obtained by numerical simulations and then compared with the experimental data. Various cases with different inlet temperatures are proposed to investigate the influences of parameters on the oscillation characteristics. The results indicate that it is not a necessary condition for vortex-shedding frequency to approach a certain acoustic frequency when periodic oscillations are generated. Oscillations are more severe if the vortexsheddingphenomenon coupleswithhigh-order acousticmodes.Velocitymagnitude in the combustion chamber is the main factor that influences the vortex-shedding frequency; meanwhile, the pressure amplitude is mostly determined by themeanMach number. Theoretical modeling in conjunction with numerical calculations proves that the ratio of dimensionless thrust amplitude to pressure amplitude is predominantly determined by the throat-to-port-area ratio J, and it varies inversely as J. An integrated formula is presented to describe the relationship between thrust amplitude and pressure amplitude.

Diffusion Combustion of Liquid Heptane in a Small Tube with and without Heat Recirculating

In order to understand diffusion flame characteristics in a small tube, combustion of liquid n-heptane and air was experimentally and numerically studied. A tube of ID 4 mm and OD 6 mm made of quartz was used as the burner. Liquid n-heptane was delivered into a capillary from a syringe pump. Stable flames were established inside the burner with and without heat recirculating. Additionally, numerical simulations were conducted, and effects of equivalence ratio and external heat loss coefficient on diffusion flame were studied. Results show that, for a diffusion flame of liquid n-heptane in a small tube, as fuel flow rate increases, the flammable limits increase. The diffusion flame position moves downstream with increasing air flow, eventually stabilizing at the bottom of the outer tube until extinction. When the flame passes in the tube, the peak temperature would occur on the wall. If there is heat recirculating, the wall temperature of the inner tube is higher than the boiling point of liquid n-heptane. It is conducive to the pre-evaporation of liquid n-heptane. In contrast, if there is no heat recirculating, liquid fuel will be accumulated in the tube. The heat loss coefficient has a great influence on flammable limits of the tube burner without heat recirculating.

Numerical Analysis on Oscillation Characteristics in a Tailpipe Nozzle Solid Rocket Motor

DOI: 10.2514/1.48867 Based on vortex–acoustic coupling theory, large-eddy simulation with wall-adapting local eddy-viscosity model and finite element method are carried out to study the internal flowfield and acoustic field, respectively, in a tailpipe nozzle solid rocket motor with transition-section grain configuration. The numerical method by means of a mesh sensitivityanalysisisproposedforvalidation.Theinstantaneous flowfieldcharacteristicsinthecombustionchamber and tailpipe are analyzed. The excited low frequencies are close to that observed in experiment. The phenomenon in which acoustic signals, superimposed on the vortex-shedding motions, couple with an internal flowfield is proven to be one of the main reasons contributing to oscillation in the motor. According to fast Fourier transform, low frequenciespredominateinthecombustionchamber;however,highfrequenciespredominateinthetailpipe.Dozens of cases with different geometrical configurations are presented to investigate parameters that have impact on the low-frequency oscillation characteristics. The results indicate that the oscillation characteristics are mainly influenced by upstream mean velocity, transition-section angle, distance between vortex source and impingement points, tailpipe radius, and convergence angle of the nozzle.

Studies on Effect of Head Cavity on Resonance Damping Characteristics in Solid Rocket Motors

In order to discover the effect of head cavity on resonance damping characteristics in solid rocket motors, large-eddy simulations with Wall-Adapting Local Eddy-viscosity subgrid scale turbulent model are implemented to study the oscillation flow field induced by vortex shedding on the foundation of VKI (von Karman Institute) experimental motor. The numerical method by means of a mesh sensitivity analysis is proposed for validation. Pressure oscillation frequencies and amplitudes are obtained, and then compared with the experimental data. It is investigated that oscillation amplitudes reduce remarkably after adding a cavity at the head-end. The results indicate that cavity volume, location and configuration have cooperative effect on the oscillation amplitude. Rayleigh criterion is proved to be of guiding significance of resonance damping. The substance of altering grain configuration is a comprehensive process of adding and abstracting mass. The suppression effect is not caused by the complicated flow field at the head-end. Additionally, it is neglected whether altering grain configuration at the acoustic pressure node. It is concluded that large mass flux added at pressure antinode could attribute to significant amplitude; meanwhile, the damping effect of cavity is stronger if the distance between cavity and pressure antinode becomes shorter. At last, this method is adopted by an engineering solid rocket motor. Ground test reflects that the oscillations are suppressed by the head cavity.

Experimental and Numerical Studies on Methane/Air Combustion in a Micro Swiss-Roll Combustor

To understand effects of an air groove on working characteristics of micro Swiss-roll combustors, combustion of premixed CH4/air is conducted in 2 micro Swiss-roll combustors, one with an air groove and the other without. Experimental results show that stable combustion of premixed CH4/air in 2 combustors can be achieved and the flame is kept in combustors center. An air groove can extend flammable limit of the combustor and make it work at a larger excess air coefficient and a lower methane flow rate. Furthermore, it increases the radial surface temperature gradient on outer wall of the combustor. Additionally, the combustor with an air groove is numerically simulated. Numerical results indicate that hot combustion products play a strong role in heating incoming mixtures. On one hand, it makes premixed flame surface inclined across inlet channel; on the other hand, it makes flame position vary with flow velocity, excess air coefficient and heat loss to the environment.

Experimental and Numerical Study on Oxygen Enhanced Methane Combustion in a Rapidly Mixed Tubular Flame Burner

To promote energy and environment security through combustion efficiency improvement as well as CO2 capture and sequestration (CCS), in this study oxygen enhanced combustion of methane has been investigated by using an inherently safe technique of rapidly mixed tubular flame combustion. As a new type of flame, the tubular flame has excellent flame characteristics such as negligible heat loss, aerodynamic stability and thermodynamic stability. Various applications have been proposed and demonstrated for determining the flammability limits, stabilizing a flame in a high speed flow, and obtaining a uniform and large-area laminar flame to heat iron slab or to reduce steel sheet surface. Especially, by individually injecting the fuel and the oxidizer into a cylindrical burner through four tangential slits hence, hence without flame flashback, the rapidly mixed tubular flame burner has been applied to analyze the characteristics of oxygen enhanced methane flame.To make a fundamental investigation, methane oxygen combustion has been attempted under various oxygen mole fractions with nitrogen and carbon dioxide as the diluents respectively. At first, nitrogen was added to the oxygen stream, and the oxygen mole fraction in the oxidizer was increased from 0.21 to 1.0. A stable, laminar tubular flame can be obtained from lean to rich when the oxygen mole fraction is no more than 0.4. And the maximum adiabatic flame temperature reaches around 2700 K. To enhance the mixing of fuel and oxidizer, nitrogen was also added to the fuel inlet to increase the injection velocity of fuel stream. The results show that by assigning the nitrogen to both the fuel and oxygen inlets to approach the same injection velocity, the flames become more uniform and stable. However, the range of stable tubular flame in equivalence ratio remains almost the same.Secondly, instead of nitrogen, carbon dioxide was used to dilute the methane/oxygen flames. Thus, the NOX emissions introduced by nitrogen will be greatly reduced, in addition, the main exhaust will be carbon dioxide and steam, which is beneficial for CCS. When carbon dioxide was only added into the oxygen stream, a stable tubular flame was obtained from 0.9 to 1.2 in equivalence ratio at the oxygen mole fraction of 0.21. With an increase of oxygen mole fraction, the stable tubular flame range enlarges in equivalence ratio, and up to the oxygen mole fraction of 0.50, stable tubular combustion could be achieved from lean to rich. By adding carbon dioxide to both the fuel and oxygen inlets to approach the same injection velocity, the upper limit of stable tubular flame increases much. Up to the oxygen mole fraction of 0.86, the stable combustion can be achieved at the stoichiometry, which gives a flame temperature around 3000 K.To fully understand the flame characteristics above, the chemical effects of carbon dioxide are numerical analyzed in comparing with the nitrogen diluted flames using the CHEMKIN PREMIX code with the GRI kinetic mechanism.Copyright

Effects of external heating on flame stability in a micro porous combustor fuelled with heptane

ABSTRACT Effects of external heating on flame stability were investigated experimentally in a micro porous combustor. The flame stability limits, extinction process, and low-temperature oxidation were studied at various external thermal inputs, from 0.32 W to 0.94 W, and corresponding recirculation efficiency is 3.58–10.53%. Results show that external heating can extend flame stability limits obviously in micro porous combustor. Flame stability limits are 0.343 and 4.285 when external thermal input is 0.94 W, which is wider than that at standard condition. Porous medium temperature decreases gradually after stopping external heating, which causes flame to oscillate and extinguish. Time between stopping external heating and flame extinction increases from 15.7 s to 24.2 s with heating power from 0.32 W to 0.94 W. When temperature of gas mixtures is lower than 233.2°C, n-heptane hardly reacts with oxygen in porous medium. When temperature is higher than 233.2°C, low-temperature oxidation reaction of n-heptane occurs in porous medium and gas temperature increases. Temperature rise is 5.8°C when gas mixtures temperature is 324.9°C.

Experimental Study on Flame Stability and Thermal Performance of an n-Heptane-Fueled Microscale Combustor

ABSTRACT In this work, we study the stabilization behavior of micro-diffusion flame of n-heptane formed in a combustor with the inside diameter of 4 mm, in order to elucidate the unique stability mechanism due to miniaturization of diffusion flame downstream porous medium. Effects of incoming Reynolds number and fuel flow rate on overall flame shape, exhaust temperature, and wall temperature are examined experimentally. Furthermore, an energy balance model of the micro combustor is established and optimal working conditions are proposed. Liquid n-heptane is used as fuel and two types of outer tubes are employed in order to examine the role of the heat recirculation. It turns out that the outer tube increases the wall temperature and broadens the flame stability limits. The incoming Reynolds number changes flame position and energy balance in the micro combustors. At low Reynolds number, the outer tube allows the flame to stay close to the porous medium and, accordingly, the porous medium is substantially heated up. Then, the fuel flowing through the porous medium “receives” the heat from the burner (heated by flame) effectively to enhance the reactivity, resulting in improving the stability. At high Reynolds number, the outer tube allows the flame to stay close to the bottom wall of the outer tube and, hence, more radiative heat is transferred through the outer tube, ideal for micro-photovoltaic systems. Additionally, in the case of fixed equivalence ratio, with increasing of the fuel flow rate, combustion releases more heat and the flame is blown toward the bottom of the outer tube. More energy is transferred to the surroundings via the outer tube wall and the maximum value is up to 72.7% of the total combustion heat release.

The Combustion Characteristics of a Non-Premixed Combustor with Opposed Methane Jets

In order to understand the combustion condition of the opposed jetting combustor in different flow conditions, a non-premixed combustor with opposed methane jets was studied in this paper by using the commercial fluid dynamic software based in N-S equation. It was found that, in the non-premixed combustor, when the velocity of methane remain unchanged, the flame moved to downstream with the air velocity increasing; the temperature difference between the steel wall and quartz wall is positively correlated to the air velocity. Under the medium or huge methane velocity, the methane efficiency gets higher with the increase of air velocity.