FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1953
1953 - 1656.PDF
810 FLIGHT, 18 December 1953 ASPECTS OF COMBUSTION . . . Rice and Professor B. L. Crawford, Jr. The values of various kinetic parameters deduced by the application of their theories to experimental burning-rate data are compared with values obtained in other ways. A summary of experimental and theoretical information on unstable combustion is also given. The distinctive feature of this phenomenon is an abnormally high burning-rate that accompanies high-frequency pressure oscillations in rocket combustion cham bers. It is concluded mat the theoretical understanding of the combustion of solid propellants is insufficient to be of much practical aid to the propellant chemist seeking formulations with improved properties. A Turbulent Flame Theory Derived from Experiments, by Bela Karlovitz (U.S. Bureau of Mines).—The structure of turbu lent flames burning in explosive mixtures is described and illustrated by examples, and a theory of turbulent burning velocity is developed, based on this flame-structure. The cardinal point of the theory is the calculation of the time interval, during which a small proportion of the instantaneous combustion wave has statistical correlation with the random motion of a certain mass of gas. A comparison of measured values of the turbulent-burning velocity with the predictions of this theory points to the possibility of turbulence generation by the turbulent flame. The mechanism is described by which the turbulent flame can generate additional turbulence and a theory of this process is developed, which permits the calculation of the intensity of flame-generated turbulence. Turbulence intensities calculated from measured turbulent- burning velocity data are in good agreement with turbulence intensities calculated from this theory. For an independent experimental test of both theories the direct measurement of the turbulence intensity in the flame will be necessary, and this appears feasible. A relationship is derived between the burning velocity, scale of turbulence, and the thickness of the turbulent-flame brush. Measurement of the flame-brush thickness by an electronic probe therefore gives information about the scale of turbulence in the flame. Further experimental work is required to complete the experimental test of the theory of turbulent flames, and to establish its range of validity. The Problem of Combustion at High Altitude, by Peter Lloyd (Deputy Director, National Gas Turbine Establishment, M.o.S.) and Brian P. Mullins (Head of Chemical Physics Department, National Gas Turbine Establishment).—Combustion and relight ing problems associated with aircraft gas turbine combustion chambers and jet-pipe reheat systems at altitudes up to 65,000ft are considered. Attention has been focused upon four engine operating conditions: "cruising" and "idling" when discussing combustion, "windmilling" when discussing relighting, and "full speed" when considering reheat. Representative test data under simulated flight conditions are given and these confirm the expec tation that the adverse effect of low pressure and temperature on highly rated combustion systems is sufficient to cause a real deterioration in performance which is particularly serious in the cases of relighting and jet-pipe reheat. Attention is given to the effect of altitude conditions upon the component processes: fuel atomization, mixing of gas streams, spontaneous ignition, spark ignition, flame propagation, droplet evaporation, heat transfer and chemical reaction rate. The effect of reduced pressure upon many of these processes is not known and their relative importance in combustion is not fully under stood; in the authors' opinion this represents an area of worthwhile research. Spectroscopic Studies of Premixed Laminar Flames, by S. S. Penner (AGARD).—A critical review is presented of the results obtained by spectroscopic observations on flames. The objective of the survey is to examine the status, promise and deficiencies of combustion spectroscopy in its relation to (a) elucidation of the mechanism of combustion and (b) the solution of technical com bustion problems. Since important spectroscopic studies have been carried out on low-pressure flames, a discussion of the probable effects of pressure on laminar flame propagation is also included. Combustion Instability in Liquid-Propellant Rocket Motors— a Survey, by Chandler C. Ross (Manager, Liquid Engine Division, Aerojet-General Corporation, Azusa, California), and Paul P. Datner (Senior Engineer).—Three different types of combustion instability in liquid-propellant rocket engines—the low-frequency "chugging," the high-frequency "screaming" and the low- frequency "divergent" type—are briefly described. The pertinent information available in the literature is presented, and the problems encountered in securing reliable experimental data on the phenomena, using conventional instrumentation, are reviewed. A discussion of low-frequency instability, resulting from the inter action of propellant feed system oscillations and combustion gas vibrations, is given, based on available test results. Some data on high-frequency combustion oscillations are presented, indicating that the vibrations of the combustion gases correspond to the propagation of finite pressure disturbances in the combustion chamber, which appear to originate in the com bustion zone. The modes of combustion gas oscillations of both high and low frequencies are described as similar to those of acoustic oscillations taking place in cavity resonators, and as main tained by the interaction between the rates at which propellants are consumed and fluctuations Li combustion chamber pressure, resulting in strong reactive shock-waves. It is noted that com bustion oscillations may be excited by disturbances of the random-noise type, inherent in the combustion, or by a shock such as a "hard start." It is pointed out that all published theories on the phenomena agree on two principles: (1) The oscillations of combustion gases are maintained by periodic variations in the propellant consump tion rate, resulting in a periodicity in the rate of heat release. All theories are based on Rayleigh's principle that vibration in a confined mass of gas can be maintained by periodic heat release if variations in heat release and in pressure oscillations are in phase, but that the oscillations will be impeded if the fluctuations in heat release and pressure are out of phase. (2) The significance of the combustion time-lag with respect to combustion instability is generally recognized, and the rocket engines under discussion are considered to be dynamic systems with delayed reaction because of this inherent time lag. It is shown that the Nyquist criterion of stability could be applied successfully for predicting the dynamic behaviour of rocket systems if sufficient data were available concerning the physical and chemical kinetics of the conversion of propellants into burned gases in the combustion chamber. Combustion of a Single Droplet and of a Fuel Spray, by D. B. Spalding (Department of Engineering, University of Cambridge). —The physical and chemical aspects of liquid-fuel burning are shown to be capable of separate treatment. Considerable progress has been made in understanding the former aspect, and the burn ing rate of an isolated droplet can be predicated with acceptable precision if the natures of the fuel and atmosphere and of their relative motion are specified. The relative motion within a spray is, however, too complex to permit analysis as yet, and it is not even certain whether droplet burning or jet-mixing processes control the flame length in gas-turbine combustion chambers. Knowledge of the conditions for the stable burning of liquid fuels is still less advanced, and is restricted to the laminar diffusion flame adjacent to a fuel surface. It appears that the extinction condition is related to the laminar flow speed but not to the spontaneous ignition characteristics of the fuel. The contribution to design of the fundamental knowledge obtained so far has been largely negative in character. More direct application of this knowledge is only likely if the design of combustion systems is so modified as to be amenable to analysis. In the present state of the art this means restricting the use of fuel sprays. Diffusion Processes as Rate-controlling Steps in Laminar Flame Propagation, by Dorothy M. Simon (National Advisory Committee for Aeronautics).—Correlations between equilibrium concentra tions of atoms and free radicals and laminar burning velocities of pre-mixed gases are reviewed. Such correlations exist for both non-hydrocarbon and hydrocarbon systems under conditions of changing composition, initial temperature, pressure and type of diluent. Critical examination of the physical significance of the correlations shows that diffusion effects do not usually give unique explanations of the experimental data and that, in general, an alternate correlation with a thermal mechanism of flame propaga tion may be devised. Also, it is clear that there are important limitations of the interpretations, such as the lack of chemical kinetic data, the use of simplified equations based on drastic assumptions, and the similarity between equations based on diffusion mechanisms. However, within these limitations and those imposed by the accuracy of measurement of burning velocity, two types of burning-velocity data—the effect of inert gases and the effect of the addition of water to combustible systems—slightly favour a diffusion mechanism over a thermal mechanism. Other flame properties which may be influenced by diffusion are reviewed; namely, flame structure instabilities, limits of inflam mability, quenching distance, minimum ignition energy and minimum ignition pressure. Neither does the use of flame pro perties other than burning velocity to resolve the mechanism of propagation provide any decisive evidence. It is probable that both heat conduction and the diffusion of atoms and free radicals contribute to flame propagation. In order to evaluate which process is more important in controlling the burning velo city under a particular set of experimental conditions, indepen dently measured kinetic factors and well-established thermal properties are needed. If, however, reasonably good predictions of the effects of variations in experimental conditions on the laminar burning velocity of pre-mixed gases are desired, then simplified equations such as the Tanford-Pease equation or the Semenov equation containing empirical factors are useful. (Further summaries will appear in a forthcoming issue)
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
