B.R. Daniel

Flame Driving of Longitudinal Instabilities in Dump Type Ramjet Combustors

Abstract Coaxial, dump type ramjet combustors are often prone to combustion instability problems that can seriously affect their performance. This study is concerned with the highly detrimental low frequency instabilities which often occur in such engines and investigates the coupling between wedge shaped flames and longitudinal acoustic fields in an experimental apparatus specifically developed for this purpose. The presence of burning vortical structures is observed in the flame region. These structures appear at frequencies close to the first natural acoustic frequency of the apparatus and are believed to be connected with a shear layer type of instability of the flame. Experiments conducted show that the unsteady combustion in these structures is capable of driving the acoustics at the fundamental mode frequency. with increase in fuel-air ratio, a spontaneous instability involving the fundamental mode is observed and explained in terms of increased driv ing associated with the higher, unsteady heat re...

Experimental and theoretical determination of the admittances of a family of nozzles subjected to axial instabilities.

In combustion instability analyses of rocket engines, it is necessary to determine the interaction between the oscillations in the combustor and the wave system in the nozzle. This interaction can be specified once the nozzle admittance is known. The present paper is concerned with the experimental and theoretical determination of the admittances of practical nozzles that are subjected to axial oscillations. The impedance tube technique, modified to account for the presence of a mean flow, was used to experimentally measure the one-dimensional nozzle admittances. The modified impedance tube theory and experimental facility used to evaluate the nozzle admittance are briefly discussed in this paper. Croccos nozzle admittance theory is used to predict the admittances of the tested nozzles for comparison with the experimental data. The theoretical and experimental nozzle admittances are obtained for a family of nozzles having Mach numbers from 0·08 to 0·28, different angles of convergence, and different radii of curvature at the throat and entrance sections. The analytical and experimental results are presented as curves showing the frequency dependence of the real and imaginary parts of the nozzle admittances. Examination of these data shows that the theoretical and experimental admittance values are in good agreement with one another which indicates that existing nozzle admittance theories may be used in practice to predict one-dimensional nozzle admittances.

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...

Measurements of acoustic responses of gaseous propellant injectors

Abstract In a stability analysis of a gaseous propellant rocket combustor the influence of the injector design upon the combustor dynamics must be known quantitatively. The combustor injector interaction is generally specified by the injector response factor which describes the manner in which the propellants burning rate responds to a given pressure oscillation within the combustion chamber. This paper is concerned with the experimental determination of the response factors of a variety of coaxial gaseous propellant rocket injectors. The injector response factors are obtained from injector admittance data measured under cold-flow conditions simulating those observed in rocket motors experiencing axial instability. The experimental data are found to be in reasonable agreement with theoretical predictions obtained by using the Feiler and Heidmann analytical model. Also presented in this paper are data that describe the dependence of the injector response factors upon the open-area ratio of the injector configuration and the length of the injector orifices. These data indicate that although there is no observable change in the magnitude of the response factor at resonance, an increase in the orifice length decreases the injector resonant frequency and the band-width of the injector response curve. Furthermore, it is noted that for a given mass flux through the injector orifices, a change in the injector open-area ratio has a considerable effect on the characteristics of the injector response factor. The relevance of these data to stability considerations of rocket motor combustors and gaseous fuel fired burners is indicated.

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.

Driving of combustor oscillations by gaseous propellant injectors

This paper describes successful quantitative measurements of reactive admittances that describe the capabilities of the combustion processes associated with coaxial gaseous fuel injectors to amplify combustor oscillations. These admittances are needed in (1) stability analyses of rocket motors, gas turbine combustors and industrial burners; (2) for the evaluation of the relative “driving” capabilities of different injectors; and (3) for checking the applicability of an available theoretical model. In this study the modified standing-wave technique has been used to determine the admittances of the combustion processes associated with coaxial injectors utilizing air-acetylene mixtures. Analyses of the measured data indicate that (1) coaxial injectors can sustain combustion instabilities over wide frequency ranges and a range of injector design parameters; (2) the maximum driving capability of a given injector decreases with an increase in the equivalence ratio; (3) the frequency at which maximum driving is observed also decreases with an increase in the equivalence ratio; (4) the characteristic combustion time associated with a given injector design and an operating equivalence ratio is frequency dependent and it decreases in magnitude with an increase in frequency; and (5) the measured data are in good agreement with the theoretical predictions of the Feiler and Heidmann model.

Measurements of Reactive Gaseous Rocket Injector Admittances

Abstract This paper describes the results of an experimental study that was concerned with the quantitative determination of the capabilities of the combustion processes associated with coaxial gaseous propellant rocket injectors to drive combustor pressure oscillations. The data, obtained by employing the modified impedance tube technique with compressed air as the oxidizer and acetylene gas as the fuel, describe the frequency dependence of the admittance of the combined injector-combustion process. The measured data are compared with the predictions of the Feiler and Heidmann analytical model utilizing different values for the characteristic combustion time τb. The values of τb which result in a best fit between the measured and predicted data are indicated for different equivalence ratios. It is shown that for the coaxial injector investigated in this study the τb varies between 0.7 and 1.2 msec for equivalence ratios in the range of 0.57 to 1.31. In addition, the experimental data clearly indicate tha...

Combustion of Heavy Fuel Oils in a Rijke Type Pulse Combustor with a Tangential Injection Stream

This paper describes an investigation of the combustion of heavy fuel oils in a recently developed, Rijke type, pulse combustor that utilizes a tangential fuel and air injection system. This study was stimulated by the need to develop combustors that can burn low quality fuels with high combustion efficiencies and low pollutant emissions. Tests with fuel oil Nos. 5 and 6 revealed that they can be burned in the developed Rijke pulse combustor with combustion efficiencies higher than 99 percent while utilizing less than 10% excess air, which decreases exhaust flow losses and increases the thermal efficiency. Furthermore, the developed combustor could be operated over a wide range of air/fuel ratios, including fuel lean and rich operating conditions. Emissions measurements revealed that the exhaust flow NOx concentrations are low over the whole range of investigated air/fuel ratios and that CO and particulates emissions are low for fuel lean operation. Finally, the paper describes the dependence of the amplitude of the combustor pulsations upon its operating conditions.

Acoustic characteristics of pulse combustor mixing chambers

This paper compares the contrasting behaviors of the apparently similar Helmholtz and the Schmidt type pulse combustors. A “modified impedance tube technique” which takes into account the non-uniform temperature in the impedance tube was used for determining the acoustic characteristics of the components which make up these combustors. These impedance tube studies have shown that there exists an optimum range of air flapper valve settings over which the acoustic driving by the combustion process is the strongest. When a Helmholtz combustor was operated under conditions of strongest driving, the sound pressure level in the combustor was maximized. However, a Schmidt tube produced the smallest pressure amplitudes under similar conditions. This phenomenon can be explained in terms of the acoustic properties of the components which make up these combustors. Cold flow studies indicated that damping by the air flapper valve and, therefore, the mixing chamber assembly decreased with increasing pressure amplitude. However, once the combustion chamber was added its acoustic properties dominated and the damping increased with amplitude. In the limit cycle, the pressure amplitudes in these combustors adjust until the damping in the entire system exactly balances the driving by the combustion process. Hence, strong driving requires strong damping which occurs at high amplitudes in the Helmholtz combustor and at low amplitudes in the Schmidt combustor.