Jeff Jagoda

Active Control of Lean Blowout for Turbine Engine Combustors

A complete, active control system has been developed to permit turbine engine combustors to operate safely closer to the lean-blowout (LBO) limit, even in the presence of disturbances. The system uses OH chemiluminescence and a threshold-based identification strategy to detect LBO precursor events. These nonperiodic events occur more frequently as the LBO limit is approached. When LBO precursors are detected, fuel entering the combustor is redistributed between a main flow and a small pilot, so as to increase the equivalence ratio near the stabilization region of the combustor. This moves the effective LBO limit to leaner mixtures, thus increasing the safety margin. The event-based control system was demonstrated in an atmospheric pressure, methane-air, swirl-stabilized, dump combustor. The NOx emissions from the piloted combustor were found to be lower than those from the unpiloted combustor operating at the same safety margin and same nominal velocity field. The controller minimizes the NOx at constant total power by keeping the pilot fuel fraction at the lowest value needed to limit the number of precursor events to an acceptable level.

Periodic mixing and combustion processes in gas fired pulsating combustors

Abstract Although gas fired pulse combustors for home and water heating have been in use for a number of years, little research has been carried out, to date, to develop an understanding of the fundamental physical and chemical processes which control the operation of the pulse combustors. Consequently, the past developments of such combustors have been based upon costly trial-and-error approaches. This paper describes some results of an ongoing investigation which studies controlling processes in valved pulse combustors. Specifically, this paper provides experimental data which describe the mixing of the reactants, the time dependence of the combustion process heat release rate, its phase relationship with the combustor pressure, and the spatial characteristics of the combustion process. High speed shadow and Schlieren photography was carried out on a partially transparent pulse combustor. The combustion was observed to take place largely in the mixing chamber. Fuel and air jets enter the mixing chamber prior to the combustor reaching its minimum pressure. The two jets impinge, and mix and the new reactants are ignited by entrained remnants of reacting pockets from the previous cycle. This ignition takes place at the location and time at which the two reactant streams first mix. The resultant reactive stream breaks into two opposing vortices which fill the mixing chamber. Finally, data from CH and CC radiation measurements showed that the reaction is periodic and that it remains nonzero throughout the entire cycle. Furthermore, the reaction rate is in phase with the combustion pressure oscillations, satisfying Rayleighs criterion for wave driving by heat addition.

A Theoretical Investigation of the Behavior of Droplets in Axial Acoustic Fields

This paper describes a theoretical investigation of the behavior of small droplets in an acoustic field. It was motivated by the increasing interest in the use of pulsations to improve the performance of energy intensive, industrial processes which are controlled by rates of mass momentum and heat transfer. The acoustic field is expected to enhance heat and mass transfer to and from the droplets, probably because of the relative motion between the droplets and the gas phase. Relative motion is traditionally quantified by an entrainment factor which is defined as the ratio between the amplitude of the droplet and the gas phase oscillations, and a phase delay. In an alternate approach, these two quantities are combined into a single quantity called the “degree of opposition” (DOP), which is defined as the ratio of the amplitude of the relative velocity between the droplet and the gas phase to the amplitude of the acoustic velocity. The equation for the droplet motion is solved using two methods; by numerical integration and by using a spectral method. Despite the nonlinear nature of the problem, the results were found not to be sensitive to initial conditions. The DOP was predicted to increase with increasing droplet diameter and frequency. In other words, larger diameters and higher acoustic frequencies reduce the ability of the droplets to follow the gas phase oscillations. The DOP also decreases with increasing acoustic velocity. It was shown that the amplitude of the higher harmonics are very small and that the droplet mean terminal velocity decreases with increasing acoustic velocity. Theoretical predictions were compared with experimental data and good agreement was observed.

An Experimental Investigation of the Behavior of Droplets in Axial Acoustic Fields

This paper describes an experimental investigation of the behavior of water droplets in axial acoustic fields. It was motivated by the increasing interest in the use of pulsations to improve the performance of energy intensive, industrial processes. The presence of an acoustic field is believed to enhance heat and mass transfer to and from the droplets, probably because of the relative motion between the droplets and the gas phase. This relative motion is characterized by the ratio of the amplitude of the oscillatory droplet velocity to that of the acoustic velocity (entrainment factor), and by the phase between the droplet and gas phase oscillations. An experimental set-up was developed to investigate the effect of acoustic oscillations on the motion of individual droplets. In these experiments a droplet produced by a piezo-ceramic droplet generator is allowed to fall through a transparent test section in which an acoustic field has been set up using a pair of acoustic drivers. Images of the droplets in the test section acquired at consecutive instants using a high speed, intensified imaging system were used to determine the time dependent droplet trajectory and velocity. The acoustic velocity was calculated from measured acoustic pressure distributions. The entrainment factor and the phase difference were then determined from these data. The results show how the entrainment factor decreases and the phase difference increases with increasing droplet diameter and frequency, indicating that larger diameters and higher frequencies reduce the “ability” of the droplets to follow the gas phase oscillations. The measured data are in excellent agreement with the prediction of the Hjelmfelt and Mockros model. Both theoretical predictions and measured data were correlated with the Stokes number, which accounts for the effects of droplet diameter and frequency. It was also shown that acoustic oscillations decrease the mean terminal velocity of the droplets.

Vortex Shedding and Periodic Combustion Processes in a Rijke Type Pulse Combustor

Abstract 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 Rayleighs criterion for driving pulsations by a heat addition process. In contrast, images obtained when the combustor is operated in a steady...

Ultra Low Emissions Combustor with Non-Premixed Reactants Injection

This paper describes a novel Stagnation Point Reverse Flow (SPRF) combustor concept that can burn gaseous or liquid fuels in premixed or non-premixed modes of combustion with ultra low NOx emissions. The combustor consists of a tube with open and closed ends. Contrary to most combustors, the reactants and products enter and leave this combustor at the same (open) end. In the investigated configuration, the reactants are injected along the combustor center line, moving towards the closed end, where the flow velocity must be zero. This creates a low velocity region towards the closed end of the combustor that helps stabilize the combustion process. Furthermore, the presence of a closed end forces the generated combustion products and burning gas pockets to reverse their flow direction and move towards the open (exhaust) end of the combustor. Thus, a portion of the hot products, laden with radicals, are entrained back into the incoming reactants to form a more chemically reactive mixture. The presence of radicals in the mixture lowers its ignition temperature and, thus, the lean blowout limit of the combustor. Thus, the SPRF combustor’s geometry produces a combination of stagnation and reverse flow entrainment that allows this combustor to operate stably at very low temperatures with ultra low NOx emission in the 1 ppm range and below. It is also shown that these low NOx emissions can be attained with premixed or non-premixed modes of combustion. Finally, it is shown that the developed combustor can operate with high combustion intensities, in the 30-40 MW/m range at atmospheric pressure, without experiencing combustion instabilities.

Performance of a Gas Burning Rijke Pulse Combustor With Tangential Reactants Injection

Abstract This paper describes an investigation of the dependence of the performance of a propane burning Rijke pulse combustor upon the design of a recently developed tangential injection system. Two injection systems were investigated. In one the fuel and air were rapidly premixed before injection into a duct where most of the combustion occurred. In the second air and fuel were injected separately into the duct. In both cases the pulse combustor could be operated for air/fuel ratios a between 0.5 and 1.8. However, when the reactants were premixed, the combustor pulsations were dominated by the fundamental acoustic mode when α was low, and by its first harmonic when 1.1 < α < 1.6. In contrast, the fundamental acoustic mode dominated for all α when the reactants were injected separately. This suggests that this pulse combustor does not behave like a true Rijke tube, at least while operated in the premixed mode. In addition, exhaust flow analyses revealed that the combustors combustion efficiency practica...

Controlling the rich limit of operation of pulse combustors

This paper describes an investigation of the characteristics of the driving processes and the mechanisms which control the existence of a rich limit of operation of a gas fueled, valved, Helmholtz type, pulse combustor. Oscillating pressures and heat release rates were measured and the flow field was visualized using high speed shadowgraphy. The driving efficiency of the combustion process was quantified over a wide range of operating conditions. The results indicate that the processes which control the lean and rich limits of operation of the pulse combustor are fundamentally different. Near the lean limit of pulsations, the driving of the pulsations by the combustion process is low because of the large phase angle between the pressure and heat release oscillations. Near stoichiometric conditions, the driving process is most efficient. Nevertheless, a rich limit is reached. This occurs because as the equivalence ratio is increased the ignition of the new fuel is delayed. A rich limit is then reached when insufficient time remains to complete the mixing and combustion of the reactants before the reaction is quenched by the backflow of combustion products. The rich limit may be extended if the time available for combustion is prolonged either through lengthening the periol of oscillations or by accelerating the mixing.

STABILIZATION OF A COMBUSTION PROCESS NEAR LEAN BLOW OFF (LBO) BY AN ELECTRIC DISCHARGE

Electrical discharges for the stabilization of lean, propane-air, premixed combustion have been investigated experimentally on a laboratory burner. Low power arcs significantly lowered the lean limits of flammability but were accompanied by unacceptable electrode consumption. High frequency spark discharges caused similar enhancement of combustion at lean blow off without significant corrosion of the electrodes and with an order of magnitude less electrical input requirements. The plasma does, however, generate its own NOx. On the other hand, leaner combustion partially offsets the spark NOx during the time that flame augmentation is needed. In addition the HF spark can be easily and quickly switched on and off making suitable to prevent loss of flame even under extreme lean blow off conditions.

Oscillatory velocity response of premixed flat flames stabilized in axial acoustic fields

Abstract This article describes a combined theoretical-experimental investigation of the processes that drive axial instabilities in solid propellant rocket motors. In this study, the solid propellant flame has been simulated by a premixed flat flame that has been stabilized on the porous side-wall of a duct. The driving processes have been investigated by studying the interaction of the premixed flame with an axial acoustic field. Using experimentally determined acoustic pressures, burner surface admittances, and steady-state flame temperature distributions as input data, the developed model was used to determine the characteristics of the velocity field in the flame region under a variety of test conditions. The predicted velocity field was then compared with LDV velocity measurements to check the validity of the model and determine the flame driving. These studies reveal that the investigated flame responds to the presence of an axial acoustic field by producing an oscillatory velocity component, ν′, normal to the duct wall, that can drive or damp the acoustic field. Comparison of the measured data with the model predictions reveal satisfactory agreement. These studies also showed that driving and damping of the acoustic field occur simultaneously in different regions of the flame. The net effect of the flame upon the acoustic field depends upon the relative magnitudes of these opposing tendencies. It is also shown that the flame driving depends upon the acoustic admittance of the side-wall surface and the frequency of the acoustic field.