Browsing by Author "Lefebvre, A. H."
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Item Open Access Airblast atomization : the effect of linear scale on the mean drop size(1978-12) El-Shanawany, M. S. M. R.; Lefebvre, A. H.Stationary prefilming-cup airblast atomizers, in which the fuel is first spread into a thin cylindrical sheet and then exposed on both sides to high velocity air, have several important advantages over other common methods of fuel injection in their application to continuous combustion systems. This thesis reports the results of a detailed programme of research on airblast atomization directed primarily to the investigation of the effect of atomizer linear scale, or size, on the mean drop diameter of sprays and, to the provision of a comprehensive picture of the performance of thin-sheet airblast atomizers over a wide range of working conditions. Three geometrically similar atomizers giving cross-sectional areas in the ratio of 1 : 4 : 16 were used; covering the range of prefilming cup diameter, D, from 19.05mm. to 76.20 millimeters, and were capable of handling various liquids at flow rates from 0.003kg/s up to 0.225 kg/s, at fuel pressures below 33 x 10^ N/m^ (about 50 p.s.i.). . It was found that atomizer scale has an appreciable direct effect on atomization quality. The Sauter mean diameter of low liquid viscosity sprays increases with the 0.44 power of atomizer linear dimension (D), while for liquids of high viscosity this effect is slightly higher.Item Open Access An experimental study of an air-blast atomiser(1967-10) Bryan, R. H.; Lefebvre, A. H.; Norster, BrianVisual observations and an optical method of measuring the mean droplet size in sprays have been used to assess the performance of a prototype airblast atomiser, particularly at the low fuel and air flow conditions corresponding to engine cranking. The results obtained from this and a shroud version of the same basic atomiser have enabled design modifications to be made which have resulted in a marked improvement in performance, both in terms of the spraying characteristics of the atomiser at low fuel and air flows, and in terms of the measured droplet size produced. The latter for the final design was very close to that predicted by Wiggs for airblast atomisers operating at the test conditions employed. An apparatus has also been designed to enable tests on the atomisation quality of airblast atomisers to be carried out at pressures up to 250 p.s.i.,, using the same optical technique to make mean droplet size measurements.Item Open Access Factors controlling gas turbine combustion performance at high pressure(Factors controlling gas turbine combustion performance at high pressure, 1965-02) Lefebvre, A. H.In the post most of the research work carried out in gas turbine combustion was mainly concerned with performance problems arising at high altitudes. these were primarily problems of trying to achieve high combustion efficiency, adequate stability and good relighting performance. This research necessitated the use of expensive test facilities to provide large quantities of air at sub atmospheric pressure. For this reason much effort was expended in trying to find methods of simulating low combustion pressures, and in the development of correlating parameters which would allow low pressure performance to be predicted from experimental data obtained at more convenient levels of pressure. In recent years, however, there has been a marked trend towards engines of higher compression ratio. This is illustrated in figure 1 which was reproduce& from a paper by Pearson. This trend has not produced any new problems, with the possible exception of exhaust smoke, but it has aggravated many problems which previously caused no great concern, and it has exposed many deficiencies in our knowledge of certain basic processes, particularly those affecting combustion and heat transfer at high pressure. One object of this paper is to examine the influence of pressure on various aspects of combustion performance. Another object is to derive formulae from which the influence of increasing pressure on various performance criteria can be estimated. One important effect arising at high pressures is an increased dependence of all aspects of combustion performance on fuel spray characteristics. Because of their importance and because so far they appear to have received comparatively little attention, much of this paper is devoted to the effect of pressure on various spray properties such as penetration, droplet size and cone angle.Item Open Access Flame radiation & smoke emissions in gas turbine combustors(1969-09-19) Russell, D. T.; Lefebvre, A. H.The objective of this thesis was to investigate flame radiation and smoke emission phenomena in a combustor typical of those in use in aircraft gas turbine engines. One of the primary aspects of the investigation was to compare the performance of airblast atomisers with that of conventional swirl atomisers under as near identical conditions as possible. A brief study of combustion chamber processes and carbon formation and thermal radiation mechanisms was made, with consideration of such experimental data as exists in the literature. An airblast atomiser and combustion chamber was developed to a state whereby testing at rich mixtures at moderate combustion pressures could be performed for short periods. This enabled a preliminary comparison to be made monitoring the following parameters:- (i) Flame total radiation. (ii) Exhaust smoke level. (iii) Weak extinction. Data was collected at three different combustion pressures for various air-fuel ratios and chamber velocities. The results showed that the airblast atomiser offers significant advantages over swirl atomisers with regard to total flame radiation and exhaust smoke emissions whilst suffering from a poor weak extinction. A more detailed analysis was performed for swirl atomisers using the Schmidt Method to establish flame temperatures and emissivities. The object of this work was to assess the effects of droplet size and combustion pressure on flame radiation. Owing to the poor atomisation quality and fuel distribution of the airblast atomiser used for the preliminary work, two new injectors were designed. It was hoped that a detailed analysis for these injectors could be performed via the Schmidt Method in order to provide a comparison with the swirl atomisers. Unfortunately, manufacturing was not completed with sufficient time left for this analysis to be completed.Item Open Access The influence of air and liquid properties on airblast atomization(Cranfield University, 1974-09-01) Rizkalla, A. A.; Lefebvre, A. H.This thesis reports the results of a detailed programme of research on airblast atomization carried out using a specially designed atomizer in which the liquid is first spread into a thin sheet and then exposed on both sides to high velocity air. The primary aim of the investigation was to examine the influence of air and liquid properties on atomization quality. The work was divided into four main phases:- (1) The first phase was confined to the effects of liquid properties, namely viscosity, surface tension and density on mean drop size. Special liquids were produced to study the separate effect of each property on atomization quality. They presented a range of values of viscosity from 1.0 to 124 centipoise, while surface tension and density were varied between 26 and 73.5 dynes/cm and 0.8 and 1.8 gm/cm3 respectively. Atomizing air velocities covered the range of practical interest to the designers of continuous combustion systems and varied between 60 and 125 m/sec.(2) To obtain experimental data on the influence of air properties, notably air density, on mean drop size, the air temperature was varied between 23 and 151°C at atmospheric pressure in one series of experiments, while a separate study on the effect of air pressure on atomization quality was undertaken, where tests were conducted at constant levels of air velocity and temperature, using a range of liquid flows from 5 to 30 gm/sec, at various levels of air pressure between 1 and 8.5 atm. (3) In order to provide a comprehensive picture of airb1ast atomizer performance over a wide range of conditions the separate effects of varying air velocity, liquid flow rate, and hence atomizing air/liquid mass ratio on SMD were examined. This study enabled a better understanding of the effects of changes in operation on the atomizer's performance. (4) In all three phases above, velotities of both inner and outer atomizing air streams were kept equal. This last phase was aimed at studying the effect of varying the velocity between the inner and outer air streams. Best atomization quality was achieved when 65% of the total atomizing air was flowing through the outer stream. A detailed description of the light-scattering technique for drop size measurement is included. A discussion on the importance of the results obtained and their direct relevance to the design of airblast atomizers is given. A dimensional analysis and inspection of all the data obtained on the effects of air and liquid properties on atomization quality showed that over the following range of conditions: Liquid viscosity 1.0 to 44 centipoise Liquid surface tension 26 to 73.5 dynes/cm Liquid density 0.78 to 1.5 gm/cm³ Air velocity 70 to 125 m/sec Air temperature 20 to 151 °c Air pressure 1.0 to 8.5 kgf/cm² . Air/liquid ratio 2 to 6 Cont..........Item Open Access Influence of liquid properties on plain-jet airblast atomization(1976-04) Lorenzetto, G. E.; Lefebvre, A. H.This thesis reports the results of a detailed programme of research on airblast atomization carried out using a specially designed plain-jet atomizer in which the fuel is injected into a high velocity airstream in the form of a discrete jet. Because recent studies on airblast atomization have been mainly confined to sheet and then subjected on both sides to the atomizing action of high velocity air, information was needed to carry out a comparison between^the two mechanisms of atomization. It was in order to obtain such information that the present investigation was undertaken and the study essentially resolved into a detailed experimental exploration of the spray characteristics of 1plain-j et1 airblast atomizers. Specially prepared liquids were employed to distinguish between the separate effects on S.M.D. (Sauter Mean Diameter of spray) of viscosity, surface tension and density. The liquids employed represented a range of values of viscosity from 1 .0 to 76 x 10 Kg/ms,/while surface tension and density were varied -3 3 between 26 and 73 x 10 N/m and 794 and 218oKg/m respectively. Atomizing air velocities covered the range of practical interest to the designers of continuous combustion systems and varied between 70 and 180 m/s. The effect of scale on S.M.D. was studied using several different fuel injectors varying in orifice diameter between 0.39 and 1.58 mm. A detailed description of the light-scattering technique for drop size measurement is included...Item Open Access The influence of swirler design parameters on the aerodynamics of downstream recirculation region(Cranfield University, 1976-05) Kilik, Erol; Lefebvre, A. H.; Ramsden, K. W.The influence of swirler design parameters namely, vane shape (flat or curved), vane outlet angle, aspect ratio and space-to-chord ratio (number of vanes), on the pressure drop-mass flow characteristics and the size and turbulence characteristics of the downstream recirculation region has been determined experimentally. A five-hole spherical pressure probe and hot-wire anemometry have been used for the investigation. A technique for simple and reasonably accurate interpretation of the signals from the hot-wire anemometer has been formulated. Its performance has been tested at first in round jets for which experimental turbulence data are already available for comparison purposes. The method has been applied subsequently to swirling flows issuing from the swirlers; the mean and turbulence characteristics of the flow fields have been determined in this way. In addition to these, the turbulent viscosities Ĭrr, Ĭrz, Ĭrθ and Ĭθθ have been calculated. Also, detailed accounts of the previous experimental work on swirling flows and of some finite difference procedures for the computation of weakly and strongly swirling flows have been given. It has been shown that the hot-wire formulation developed provides a simple, economic and efficient procedure for the measurement of the turbulence quantities in swirling flows. The results have shown that curved vane type swirlers operate more efficiently than flat vane types such that a larger recirculation region and a stronger shear zone are induced at a lower pressure drop. The pressure drop through both the flat and curved vane type swirlers increases with increasing vane angle, decreasing aspect ratio and decreasing space-to-chord ratio. The size of the downstream recirculation region, the reverse mass flow and the decay rates of the axial and swirl velocities increase with increasing vane angle and decreasing aspect ratio in the case of curved vane type swirlers. The effect of the space-to-chord ratio has not been found very significant in the cases of I6 and I2 vanes for the vane angle of 60° and the aspect ratio 0.4. Higher turbulence and stronger shear zones can be obtained in swirling flows with increasing vane angle, decreasing aspect ratio and decreasing space-to-chord ratios. Turbulence is anisotropic in strongly swirling jets issuing from vane type swirlers. The most dominant stresses are in turn ww, uw, and uu; all the other stresses are smaller. In parallel to the anisotropy in the stresses, the turbulent viscosities are also anisotropic; the turbulent viscosity Ĭθθ has shown domination among the four viscosities which have been calculated. Ĭrr values have shown too much scatter. For these reasons, curved vane type swirlers should prove much more efficient than flat vane types in the application to combustion process.Item Open Access The influence of turbulence on the structure and propagation of enclosed flames(College of Aeronautics, 1966-01) Lefebvre, A. H.; Reid, R.Although it has long been established that burning rates can be appreciably increased by turbulence, the actual extent of this increase and the precise mechanism involved are still far from clear. The object of the present research was to examine the effects of turbulence on burning velocity and on the physical structure of the flame surface under flow conditions similar to those experienced in turbojet afterburner systems.Item Open Access Some experimental techniques in the study of flame stabilization(College of Aeronautics, 1964-02) Norster, E. R.; Lefebvre, A. H.The purpose of this note is to describe one or two techniques which are currently being used at Cranfield in various investigations into the effects of flameholder geometry and flow parameters on the process of flame stabilization. Much of the report is concerned with a description of the nitrogen dilution technique as a means of simulating low combustion pressures, and a comparison is made between this and the now established water injection technique. It is shown that many aspects of flame stability can be effectively studied by local injection of nitrogen directly into the combustion zone, with appreciable economies in the amounts of nitrogen required. A further application of this local injection method is also described which has proved useful in an investigation of the factors governing the amount of air entrained in the recirculation zone.Item Open Access Spark ignition studies in flowing liquid fuel-air mixtures(Cranfield University, 1974-03) Rao, K. V. L.; Lefebvre, A. H.The thesis describes a programme of research to determine ignition energy requirements in flowing kerosene spray air mixtures under various flow conditions such as velocity, mixture strength, spray mean drop size and drop distribution. In order to determine the spray characteristics of mixture ratio, mean drop size and drop distribution in the flowing fuel-air spray accurately, new designs have been evolved and employed. A single rectangular pulse of constant power was employed for initiating ignition in the mixture. The flow parameters were found to have considerable influence on spark characteristics and hence on the energy released in the spark. The breakdown voltage requirements of the spark in the two phase flow differed from they of premixed gaseous mixtures. In the range of weak mixtures investigated, optimum spark duration was found to vary depending on spray drop size between 30 and 60 microseconds. Minimum ignition energy increased with increase in velocity, but decreased with reduction in drop diameter and with increase in equivalence ratio. Low flow number atomisers giving small drop sizes extended the weak ignition limits for a given spark energy. Spray drop size and distribution were found to have a singularly large influence on all aspects of ignition. The thesis includes a detailed account of the designs and procedures employed in carrying out the investigation, and also a discussion on the significance of the results on the practical aspects of liquid fuel ignition in aero engine combustion system.Item Open Access Theoretical aspects of aero-engine altitude relighting performance(College of Aeronautics, 1968-03) Lefebvre, A. H.The purpose of this note is to review some current theories of ignition in stagnant and flowing mixtures, and to present an alternative theory to describe the process that occurs in a turbojet combustion chamber when the engine is being relighted after a flame-out at high altitudes.Item Open Access Theoretical aspects of gas turbine combustion performance(College of Aeronautics, 1966-08) Lefebvre, A. H.A correlating parameter for gas turbine combustion performance, based on a 'burning velocity' theory for primary zone combustion is derived using a more direct approach than that originally employed by Greenhough and Levebvre.1 The various applications of this parameter are discussed and it is shown that the shape of correlated performance curves is directly related to the combustion processes taking place in the various zones of the chamber. An alternative, more basic, theory is presented in which it is assumed that the low-pressure performance of a spray-type combustor is controlled by a balance between the separate effects of chemical reaction, fuel evaporation and mixing. It is argued that combustion efficiency is a function of p2 /M where x = 2.0, 1.7 or 1.0 depending upon whether the rate of heat release is governed by chemical reaction, fuel evaporation or mixing respectively. It is postulated that the amount by which values of x determined experimentally fall below 1.7 provides a useful practical indication of the extent to which mixing is intervening in the overall combustion process. At high pressures the mixing process predominates, x = 1, and it is shown that, for any given fuel-air ratio, the rate of heat release depends only on flame-tube geometry and mode of fuel injection, and is independent of chamber size, pressure loss factor and the operating conditions of pressure, temperature and velocity. The basic principles involved in the design of primary combustion zones for maximum volumetric heat release rate and maximum stability in terms of wide burning range are discussed.