Combustion-driven Oscillations Research Articles

Thermoacoustic Analysis of Combustion-Driven Oscillation.

Thermoacoustic analysis of heat-driven oscillation with steady flow is performed. Comparing the results with Katto's experiment on the Rijke tube and our experiment on combustion-driven oscillation with multispud gas burner which is previously reported, the following results are obtained : (1) In acoustic-type combustion-driven oscillation, one of the generating sources of oscillation is the fluid vibration on temperature boundary layer which promotes the heat transmission through unsteady conduction perpendicular to the temperature gradient. (2) The effects of steady flow velocity on heat driven-oscillation may be summarized by the volume flow rate in the normal state. (3) There are many steady flow regions exciting the oscillation when the path width of an air jet at the heat release zone is large. (4) The effects of heat source position on the oscillation are not so great. (5) The narrower the path width of steady flow and the higher the heating temperature, the smaller the oscillating flow region.

Combustion-Driven Oscillation in a Furnace with Multispud-Type Gas Burners. 4th Report. The Effects of Position of Secondary Air Guide Sleeve and Openness of Secondary Air Guide Vane on Combustion Oscillation Condition.

In the preceeding papers, "the resonance factor", which predicts the degree of combustion oscillation, was proposed, and applied to both the furnace with a single burner and that with multiple burners. It was found that the combustion oscillation occurred in the region of low primary and secondary air flow rates in both furnaces and that the region of combustion oscillation was affected by the openness of secondary air vane and the burner arrangement in a furnace under a fixed flame holder condition. If the operational conditions of the practical burners are not within this region, the furnace will not oscillate at all. This study's objective is to clarify the effects of the position of the secondary air guide sleeve and the openness of the secondary air guide vane on the combustion oscillation region in a multiburner furnace, since the combustion oscillation condition varies with position and strength of the heat release zone is a furnace and thus is governed by the configurations of flame holder which is composed of an impeller, air guide sleeves and air guide vanes. Futhermore, a procedure of burner operation while avoiding combustion oscillation is demonstrated.

Nonlinear stability of combustion-driven acoustic oscillations in resonance tubes

The leading-order fluid motions and frequencies in resonance tubes coupled to a combustion-driven flow source, such as occurs in various types of pulse combustors, are usually strongly related to those predicted by linear acoustics. However, in order to determine the amplitudes of the infinite number of classical acoustic modes predicted by linear theory alone, and hence the complete solution, a nonlinear analysis is required. In the present work, we adopt a formal perturbation approach based on the smallness of the mean-flow Mach number which, as a consequence of solvability conditions at higher orders in the analysis, results in an infinitely coupled system of nonlinear evolution equations for the amplitudes of the linear acoustic modes

Combustion-Driven Oscillation in a Furnace with Multispud-Type Gas Burners. 3rd Report, The Effects of Burner Arrangements on Combustion Oscillation in a Multiburner Furnace.

Combustion-driven oscillation is excited in the burning zone of a furnace, and its magnitude is determined by the position and heat release strength of the heat sources. In previous papers, it was shown that "the resonance factor" is useful in detecting the degree of combustion oscillation in a furnace. In this paper, by applying this method to multiburner furnaces, the effects of burner arrangements, two-stage combustion rate, excess air ratio and gas mixing ratio on the occurence of oscillation are examined. Consequently, the following results are obtained. (1) The oscillation condition is characterized by the fuel flow rate and primary and secondary air flow rates in the burner throat. (2) The low-positioned burner has stronger ability to excite the depth mode oscillation of the gas column in a furnace than the middle and high-positioned ones. (3) The effects of the two-stage combustion rate and excess air ratio on combustion oscillation are found to be represented only by the primary and secondary air flow rates. (4) The exhaust gas mixing combustion method suppresses the combustion oscillation.

Combustion-Driven Oscillation in a Furnace with Multispud-Type Gas Burner. 2nd Report. Oscillation Condition in an Opposed-Firing Furnace.

In a previous paper, the resonance factor was proposed to quantitatively estimate the degree of combustion-driven oscillation potential in a furnace with a single gas burner. In this report, the technique is applied to a test furnace, where two multispud-type burners mounted oppositely are fired, and the following results are obtained. (1) The opposed-firing system oscillates stronger than the front-firing one. (2) The mode of oscillation in the opposed-firing system is equivalent to the first mode observed in a closed-ended tube, but slightly different from the mode of a front-firing system. (3) The shorter the furnace depth, the more stable the combustion state.

Combustion-Driven Oscillation in a Furnace with Multispud-Type Gas Burner. (1st Report. Measuring method).

The combustion state in a furnace may appear stable, eventhough an oscillating combustion potential may exist. A new index, called "the resonance factor", is proposed to quantitatively estimate the degree of the combustion oscillation potential, and a measurement technique to display it is developed. The resonance factor is defined as the root meansquare of the ratio of a power spectrum value at a resonant frequency to the mean square value of pressure fluctuation in a furnace. Applying this technique to a combustion test furnace with a single gas burner, it is shown that the resonance factor is useful in clarifying the combustion state where the lower the resonance, the more stable the combustion.

パルス燃焼の技術と応用

Pulse combustion can gain high pressure flue, utilizing the combustion-driven oscillation phenomena. This well-kuown technology has been realized recently by the advance in new technology and also the demand to energy saving. The majour problems in the burner development are how to tune the total system and how to cut noise. The burner was applied as the industrial liquid heater and proved to be highly efficient, not only because of the heat transfer improvement but because of the small heater size. Applications to the painting process and to the galvanizing process were introduced.

Fuel effect on induced vibration in gas turbine engines

In recent years, attempts have been made to evaluate the potential of gas turbine engines for burning different fuels, but without much attention to the induced vibrations when using them. In the present work these vibrations have been studied using a simple turboshaft engine with a free turbine driving dynamometer, burning different fuels, kerosene, gas oil, methanol and methanol-kerosene blend. Vibration patterns over the range of operating conditions were obtained with a real time spectrum analyser. It was found that the combustion-driven oscillations increase with the carbon-hydrogen ratio of the fuel. The frequency and amplitude of the fundamental harmonics decrease with load increase. It was shown that the combustion-induced vibration levels in an engine could be predicted for off-design speeds and fuels; this will be useful for design and diagnostic purposes.

Overview of Combustion Noise

Combustion noise was discussed as early as 1802 in a technical note on singing flames. Occasional papers followed this early work up to about 1950, when jet propulsion spurred interest. Since then there has been a continual broadening of the areas of interest: Now the phenomena related to combustion noise are considered in a host of specific uses of combustion. This review is based on the authors' observations of nonpropulsive combustion in the residential, commercial, and industrial fields, although it is realized that there are many parallels to specific propulsive applications. Three specific areas of combustion noise are discussed in some detail, namely, combustion roar, combustion-driven oscillations, and pulse combustion. Combustion roar, in the absence of acoustic distortion effects, is characterized by a smooth noise spectrum related to the reacting chemistry of the flame and the turbulence level of the combustion region. Combustion-driven oscillations are characterized by a discrete frequency and a feedback cycle to maintain the oscillation. Pulse combustion is the positive application of combustiondriven oscillations. In addition, some other combustion noise phenomena are discussed, such as the interaction with vortex shedding and the combustion amplification of flow phenomena.

Combustion noise in industrial burners and furnaces

Two different types of combustion noise generation and reduction are considered: combustion roar and combustion-driven oscillations. The important characteristic of combustion roar sound generation for industrial-size burners is chemically related amplification where the peak frequency is independent of flow-velocity and jet noise amplification where the peak frequency varies directly with flow velocity. Combustion-driven oscillations have an acoustic feedback mechanism which sustains the oscillations and are characterized by a discrete acoustic frequency with higher harmonics. The efficiency of conversion of chemical energy to acoustic energy is considerably higher for combustion-driven oscillations than for combustion roar and the resulting acoustic pressure amplitudes can cause fatigue failures in furnace structures, damage brickwork, and destroy instrumentation. Understanding of the various sound producing mechanisms and the combustion system operating parameters results in the design of quieter equipment or the retrofit of existing combustion facilities for noise reduction. Elimination of combustion-driven oscillations by design or by retrofit is necessary to prevent structural damage to the system.

Combustion-driven oscillations in a small tube

Velocity and noise characteristics have been measured in the flow in a model combustor, which allows the simulation of combustion oscillations similar to those observed in afterburners. Local values of the mean axial velocity and the rms of the corresponding fluctuations have been obtained by laser-Doppler anemometry for a range of operating conditions resulting in flames with and without discrete-frequency oscillations. The mean velocities and normal stress distributions, but not the turbulence intensity distributions, are shown to increase considerably with combustion. The intensity and frequency of the sound pressure were also measured as a function of equivalence ratio, velocity, and added gas. The results showed, for example, that velocity and sound pressure fields are related and that the intensity of the oscillations could be suppressed by injection of helium gas downstream of the stabilising ring. This suppression led to a small decrease in the corresponding normal stress; to a first approximation, the resulting thrust was not altered.

Measurements of Wall Heat Transfer in the Presence of Large-Amplitude Combustion-Driven Oscillations

Two independent methods were used to measure the rate of heat transfer from a hot gas to the walls of a cylindrical combustion chamber in which large-amplitude combustion-driven oscillations accompanied a mean flow. The results obtained from the two methods were in good agreement and indicated a definite increase in the heat transfer in the presence of oscillations. The increase was found to vary approximately as the square root of the oscillation amplitude and as the fourth root of the frequency. A correlation in terms of dimensionless variables was obtained using the thickness of the acoustic boundary layer as the appropriate length scale.

Application of the Galerkin Method in the Solution of Non-linear Axial Combustion Instability Problems in Liquid Rockets

Abstract A nonlinear analysis of the stability of longitudinal combustion-driven oscillations in liquid propellant rocket motors is presented. A modified version of the Galerkin method is used to obtain solutions valid for moderate amplitude instabilities in rocket combustors having a low Mach number mean flow. Linear stability of a variety of liquid-rocket motors is investigated. Computed nonlinear results show that the resultant instability will exhibit a shock-type behavior with the number of shocks present in the system determined by the characteristics of the unsteady combustion process. These predictions are in qualitative agreement with available experimental data. Contrary to other solution techniques, the approach presented in this paper can predict both the transient and final periodic behavior of the instability. The final limit cycles are independent of the nature of the initial conditions. The relationship between the final limit cycles and the characteristics of the unsteady combustion process are discussed. Finally, the application of the Galerkin Method requires relatively little computation time and it offers considerable physical insight into the behavior of the instability.

Combustion-driven acoustic oscillations in a gas-fired burner

Longitudinal pressure oscillations in the acoustic modes of a cylindrical chamber can be excited and amplified by the combustion of premixed methane, nitrogen, and oxygen. In this study, the gas mixture was introduced through a stainless-steel frit at one end of the chamber; a blind flange closed the other end of the chamber, and a sonic vent was located in the cylindrical section midway between the ends. The exponential growth constant of the acoustic pressure amplitude, d(ln p)/dt, is taken as the measure of the tendency to oscillate—or, as is commonly said, a measure of the acoustic instability of the combustion system. Process variables that can be independently varied are pressure, oscillation frequency, mass-flow rate, and energy release per unit mass of gas. The effects on growth constant of frequency, flow rate, and mixture composition are such as to suggest that a sophisticated treatment of gas-phase kinetics will be required for an adequate explanation. This being the case with the gas-fired burner, one is led to suppose that similar considerations may be necessary for understanding the acoustic instability of solid rocket propellants.

Convective Heat Transfer in a Gas-Fired Pulsating Combustor

The effect of longitudinal combustion-driven oscillations on convective heat transfer rates was studied using a tubular, propane-fired combustor which resonated as a quarter-wavelength organ pipe at a frequency of 100 cps. The combustor was provided with damping tubes of variable length which enabled the amplitude of the oscillations to be varied, to the extent of damping them out completely. Heat transfer coefficients were measured with and without the presence of combustion-driven oscillations. It was found that heat transfer coefficients were highest at a position of maximum velocity amplitude, where improvements of over 100 percent were obtained. Satisfactory prediction of the effects of the oscillations was obtained by using the quasisteady-state theory.