摘要:

The enhanced mixing observed in excited jets may be used to alter jet flame characteristics. An experimental study was conducted to investigate the effects of strong axisymmetric pulsing on a free, vertical, nonpremixed, turbulent jet flame. The majority of measurements were made with a propane jet flame having an exit Reynolds number of 10,000. The jet flame exit velocity was modulated over a frequency range of 2 to 1340 Hz with pulse amplitudes ranging from 13 to 89% of the centerline exit velocity. Phase-averaged velocity and temperature measurements, and postflame emissions measurements, as well as natural light, schlieren, and laser-plane flow visualizations were obtained to characterize the effects of pulsing on the jet flame. The results show that axisymmetric forcing of the fuel jet significantly alters the structure of a nonpremixed turbulent jet flame. The effects of forcing are frequency dependent, producing a local interaction with the jet flame structure that is described by a preferred-mode coupling characterized by the large-scale jet variables. Forcing with a low frequency near 10 Hz produces a strong coupling with the buoyant structure of the jet flame in the far-field. Pulsing at high frequencies intensifies the local turbulent mixing and fuel consumption near the jet exit. The appearance of enhanced mixing and combustion at these two forcing-frequency regimes reduces the overall length of the jet flame but had no effect on the overall mass emission of oxides of nitrogen.

ISSN:0010-2202    

DOI:10.1080/00102209308935310

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The understanding of the origin of self-excited pressure oscillations in technical combustion systems as well as a successful design of pulse combustors depends on the knowledge of the dynamical behavior of the used flame. In this study the mixture mass flow rate through the burner nozzle was modulated sinusoidally to measure the frequency dependent heat release rate of a premixed turbulent jet flame. The dynamical behaviour of the investigated flames was found to be predominantly a function of a Strouhal number formed with the characteristics of the corresponding steady flames and to be very similar to that of an ideal idle-time model. The discrepancy between this model and the experiments at higher frequencies could be explained by the periodical formation of vortex rings.

ISSN:0010-2202    

DOI:10.1080/00102209308935311

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

An experimental study of burning of natural gas puffs generated by a fully modulated jet, has been conducted. The present study concentrated on the effects of duty-cycle and puff volume, or equivalently the injection time, on the flame length. It is observed that when individual puffs are well-separated, considerably shorter flame lengths, by as much as a factor of four, are obtained compared to a steady jet flame. Furthermore, the flame length of individual puffs scales with the initial volume of puff, in agreement with the previous studies of buoyant puffs in aqueous media. For a given puff volume, as the duty-cycle is increased beyond a given value, the flame length tends to rise rapidly due to the interaction among neighboring puffs. A dimensionless parameter is found which indicates the transition of puff characteristics.

ISSN:0010-2202    

DOI:10.1080/00102209308935312

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

This paper presents the initial results of an investigation of the mechanism that controls the operation of a Rijke type pulse combustor with tangential reactant injection. The study is focused on the role played by reacting vortices, generated in the initial section of the shear layers of the injected reactant flow, upon the driving of the pulsations. Detailed spatial distributions of the reaction rates were obtained by an intensified imaging system. They reveal that reacting, vortex-like, structures are formed near the region where the reactants enter the injection duel when the combustor pressure is near its minimum. Subsequently, these reacting structures grow in size and merge with each other. The merging process is accompanied by a dramatic increase in the reaction intensity that occurs when the combustor pressure reaches its maximum. This satisfies Rayleigh's criterion for driving pulsations by a heat addition process. In contrast, images obtained when the combustor is operated in a steady mode contain no periodic coherent structures.

ISSN:0010-2202    

DOI:10.1080/00102209308935313

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Large amplitude oscillations have been observed in pre-mixed combustion systems, where the flame is stabilized in the recirculation zone behind an obstacle or a sudden expansion (a dump), and have been related to coupling between the combustion dynamics and the acoustics of the system. We have constructed a model, which includes an intake duct connected to a large upstream reservoir at a fixed pressure, a compact combustor with a sudden expansion followed by a sudden contraction (a cavity), and a long exhaust duct connecting the combustor to the atmosphere, to describe a system typical of a pre-mixed dump combustor. The upstream and downstream components are modeled as ID non-reacting flow systems, while the combustor is modeled using a vortex simulation of the 2D Navier-Stokes equations with a low-Mach number, thin flame combustion model. Results indicate that the system dynamics is comprised of the combustor wake-mode, ƒD/U=O(0.l, and several other modes characteristic of the acoustics of the overall system, with the former being one of the lowest subharmonic of the latter. This subharmonic selection mechanism, manifesting the coupling between the convective dynamics in the combustor and the acoustics of the system, is consistent with the fact that although several experiments and numerical simulations exhibit the same low frequency instability, the high frequency contents are different since they are dependent on the system configuration. The origin of the low frequency is the coalescence of a group of eddies, which are shed from the separating shear layer, as they move into the recirculation zone.

ISSN:0010-2202    

DOI:10.1080/00102209308935314

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

This paper presents descriptions and analysis of novel acoustic phenomena that have been observed during the combustion of particle clouds in flame tubes in a reduced gravity environment. It has been found that flame propagation is steady for fuel-rich particle mixtures. However, for near-stoichiometric mixtures of fuel and air, the flame propagates in an unsteady “chattering” or vibratory fashion accompanied by high amplitude periodic pressure oscillations. A longitudinal mode combustion instability analysis of the system is formulated to explain this behavior. Predictions from the model are in good agreement with experimentally obtained data. In particular, the oscillation frequency is well predicted and it is shown that stoichiometrically rich particle mixtures may adversely affect the excitation of the flame-acoustic instability and contribute to the observed disappearance of chattering flames for fuel-rich mixtures.

ISSN:0010-2202    

DOI:10.1080/00102209308935315

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A theoretical study of the limit cycle characteristics of a gas fired, mechanically valved, Helmholtz pulse combustor is presented. The analysis is carried out in the frequency domain rather than the time domain in order to develop a performance prediction program that can be run on a personal computer. The pulse combustor is treated as a feedback system. The forward branch of the system consists of the acoustic resonator while the feedback loop consists of the combustion process and heat losses through the pulse combustor walls. The model is based upon an energy balance of the combustion chamber and an analysis of the acoustics of the tail pipe. A previously developed nonlinear model is used to describe the periodic inflow of reactants through the flapper valves and experimental data is used to develop a relationship between the reactants inflow and the magnitude of the oscillatory heat addition by the combustion process. The model predicts that the energy needed to drive the combustor oscillations near resonance is much smaller than the energy supplied by the combustion process. An order of magnitude analysis shows that known turbulent convective heat transfer processes cannot account for the difference between the predicted combustor energy utilization and the energy supplied by the combustion process. Consequently, the combustor cannot work near resonance unless the heat transfer through its walls is an order of magnitude larger than that predicted by known mechanisms and/or the phase difference between the pressure and the velocity oscillations in the tail pipe is significantly different than 90 degrees.

ISSN:0010-2202    

DOI:10.1080/00102209308935316

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

This paper concerns the simulation of unsteady combustion processes. It gathers up-to-date results of simulation activities, part of a long-term research program between the Italian National Institute for Electric Power (ENEL) and the University of Padova. The software used is ‘Fluent’1; the hardware a Vax 8530 and a Cray X-MP/14.

ISSN:0010-2202    

DOI:10.1080/00102209308935317

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The flow field in a pulse combustor can over the major part of the combustor be approximated with an oscillatory “plug” flow, indicating that the flow can be simplified to a one-dimensional flow. Hence, a natural way to start developing theoretical models for pulsating combustion would be to formulate and examine one-dimensional models; such simplified models can serve as a theoretical instrument for designers of pulse combustors.

ISSN:0010-2202    

DOI:10.1080/00102209308935318

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Pulsating turbulent gas-particle flow in a circular tube is investigated by both experimental and numerical methods. In experiments, the ensemble averaged centerline gas and particle velocities ate measured by using laser Doppler anemometry. The amplitude and the frequency of pulsations are controlled via the diameter and the RPM of a rotating butterfly valve. It was found that significant variations could be obtained along the axial position in the amplitude and the phase of the fluid velocity deviation from its mean. Both the amplitude and the phase shift behavior was a function of the imposed pulsating frequency and amplitude. Particle velocity measurements showed that the slip velocities between the fluid and particles are dependent on frequency and position along the axial direction. The experiments are simulated using a one-dimensional transient model which consists of one-dimensional compressible flow equations in an Eulerian, and particle momentum equation in a Lagrangian frame of reference. The explicit MacCormack method is used for the gas phase and Gear's method is used for the particle phase equations. An assessment of the one-dimensional, transient model is presented. It is shown that the experimentally observed dependence of both the fluid and the particle velocities on the frequency of forced oscillations as well as the significant variations with the axial distance can be simulated well by this simple model. The model can predict the instantaneous slip velocity between the fluid and solid particles in good agreement with measurements. This information is essential for combustion analysis of pulsed combustors utilizing pulverized coal.

ISSN:0010-2202    

DOI:10.1080/00102209308935319

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor