Showing papers by "Cranfield University published in 1975"

The Influence of Air and Liquid Properties on Airblast Atomization

Abstract: 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

The influence of flow parameters on minimum ignition energy and quenching distance

Abstract: Experiments have been carried out on the effects of pressure, velocity, mixture strength, turbulence intensity and turbulence scale on minimum ignition energy and quenching distance. Tests were conducted at room temperature in a specially designed closed-circuit tunnel in which a fan was used to drive propane/air mixtures at subatmospheric pressures through a 9 cm square working section at velocities up to 50 m/sec. Performated located at the upstream end of the woring section provided near-isotropic turbulence in the ignition zone ranging from 1 to 22 percent in intensity, with values of turbulence scale up to 0.8 cm. Ignition was effected using capacitance sparks whose energy and duration could be varied independently. The results of these tests showed that rectangular, arc-type sparks of 60 μsec duration gave lower than previously reported values of ignition energy for both stagnant and flowing mixtures. It was found that both quenching distance and minimum ignition energy increased with (a) increase in velocity, (b) reduction in pressure, (c) departures from stoichiometric fuel/air ratio, and (d) increase in turbulence intensity. Increase in turbulence scale either raised or lowered ignition enegy, depending on the level of tubulence intensity. Equations based on an idealized model of the ignition process satisfactorily predicted all the experimental data on minimum ignition energy.

The structure and propagation of turbulent flames

Abstract: The influence of turbulence intensity, scale and vorticity on burning velocity and flame structure is examined by using premixed propane-air mixtures supplied at atmospheric pressure to a combustion chamber 31cm long and lOcmx 10 cm cross-section. The chamber is fitted with transparent side walls to permit flame observations and schlieren photography. Control over the turbulence level is achieved by means of grids located upstream of the combustion zone. By suitable modifications to grid geometry and flow velocity, it is possible to vary turbulence intensity and scale independently within the combustion zone in such a manner that their separate effects on burning velocity and flame structure are readily distinguished. From analysis of the results obtained three distinct regions may be identified, each having different characteristics in regard to the effect of scale on turbulent burning velocity. For each region a mechanism of turbulent flame propagation is proposed which describes the separate influences on burning velocity of turbulence intensity, turbulence scale, laminar flame speed and flame thickness. The arguments presented in support of this 3-region model are substantiated by the experimental data and by the pictorial evidence on flame structure provided by the schlieren photographs. This model also sheds light on some of the characteristics which turbulent flames have in common with laminar flames when the latter are subjected to pressure and velocity fluctuations. Finally the important role of vorticity is examined and it is found that turbulent flame speed is highest when the rate of production of vorticity is equal to about half the rate of viscous dissipation.

A Theoretical Study of the Elastohydrodynamic Lubrication of Reciprocating Rubber Seals

Abstract: This paper describes a method for calculating the pressure distribution, film profile, leakage rate and friction of a rectangular section rubber seal as used in hydraulic rams and jacks. The two basic equations, those describing the hydrodynamic pressure generation and the deflections of the seal surface have been solved simultaneously by numerical techniques. A solution was obtained which predicted film profiles qualitatively similar to those obtained experimentally. Solutions for outstrokes were obtained over ranges of variables as follows: sealed pressure (lbf/in2)…200–1,500500500sliding speed (ft/min)….150.5–25.015rubber hardness (IRHD).858555–85 Presented at the 29th Annual Meeting in Cleveland, Ohio, April 28–May 2, 1974

On interfacial discontinuities in elastic composites

Abstract: The paper contains a succinct analysis of interfacial discontinuities in anisotropic elastic solids. The results are combined with known results on the ellipsoidal inclusion problem to provide some general formulae for the determination of stress (and strain) concentration factors. Some explicit results are given for cavities in an infinite matrix under arbitrary uniform loading at infinity.

Pollution control in continuous combustion engines

Abstract: Brief consideration is given to the formation of smoke, carbon monoxide, unburned hydrocarbons and nitric oxides in continuous combustion systems, and to the manner and extent to which their exhaust concentrations are influenced by changes in engine operating conditions. The main objective, however, is to review the various techniques that have been used or advocated for pollution control. It is shown that the development of low emission combustors is proceeding along two main lines. The simplest and most direct approach is through various minor modifications to established hardware, e.g., by changes in liner geometry and airflow distribution and by the adoption of more sophisticated methods of fuel injection. These modifications may be supplemented, where feasible, by compressor air bleed at low power operation and water injection at high power conditions. The other approach is towards radically new concepts which involve major combustor redesign. Of these the most, promising appear to be variable geometry and staged combustors, and also “prevap/premix” systems in which the fuel is vaporized and thoroughly mixed with all the air required for combustion upstream of the combustion zone. How far the potential of these advanced concepts will be developed must clearly depend on the severity of future legislation on emissions control.

Computer-controlled machine tool

Abstract: A control system for a grinding machine or other machine tool, wherein desired workpiece profiles are stored in a digital computer which controls the position of a tool in relation to a workpiece in accordance with the stored profile and in accordance with the position of a workpiece profile follower to compensate for tool wear.

The determination of fracture strains of growing surface oxides on mild steel at high temperatures

Abstract: A new technique for measuring the tensile properties of surface coatings at high temperatures is described. It is shown to be capable of measuring the properties of growing surface scales on metals and has been applied initially to the measurement of fracture strains of growing surface oxides on mild steel at high temperatures. The fracture strains determined ranged from 1.1×10−4 to 2.3×10−4 over the temperature range 600–900°C.

Thermal Rectification Due to Distortions Induced by Heat Fluxes across Contacts between Smooth Surfaces

Abstract: Predictions have been derived for the distribution of true contact area and the associated constriction resistance for a pressed contact between two surfaces which are flat when isothermal but become bowed in the presence of heat fluxes across the contact. Induced thermal strains of the interfacial surfaces can lead to the preferential passage of heat in one direction across the contact. Experimental measurements of the thermal resistance of a contact between a specimen of En58B stainless steel and one of commercially-pure aluminium corroborate this theory. The study has identified some of the mistakes which can be made unconsciously, and remain unrecognized subsequently, in using standard measuring techniques for the thermal resistance of a pressed contact.

Communication within the organization: organizational development, team making and informal meetings

Abstract: In this paper I hope to outline the reasons for the growth of the strategy known as organization development, describe some of the methods it uses and briefly review its current status. It is not intended to be an exploration in great depth—nor is it intended to be a sales pitch for consultancy work. Its intention is that readers shall be able to understand a little of the strategy and optimistically be able to see how they can take some of the methods outlined further in their own organizations. In order that we can get to grips with the subject, I shall only rarely acknowledge references and reduce the amount of jargon.

Packaging as a Source of Profit

Abstract: This monograph eschews the view that packaging is an exclusively marketing tool, or for that matter the concern of the distribution function alone The author insists that a total systems view should be taken of the multiple purposes which the pack must meet This poses very real problems for at least three levels—the conceptual acceptance of the proposition, its interpretation into operational terms, and implementation/control of the packaging activity This discussion is intended for all directors of companies with a substantial recurrent expenditure in packaging as well as for members of the distribution function itself Accordingly, it takes a managerial rather than a technical view of packaging It stresses the critical importance of the state of goods on arrival with the customer; the aesthetics and sales power of packaging; the informative role of the pack; issues of unitisation; handling problems at all stages in the channel; and the ecological/environmental aspects in contemporary society At each juncture, sensible plans of action are proposed

An exploratory study of determinant and indicant beliefs in attitude measurement

Abstract: In the central equation of Fishbein's attitude theory the overall affect attached to an object is equated with the sum of evaluation × belief strength for the salient beliefs held by the individual concerned. Two previous studies have shown equally good prediction of attitude whether the beliefs used are those spontaneously elicited by each individual (ISB) or those most frequently elicited by the population (MSB). This result is replicated here in the context of an attitude change study. Fishbein's explanation for the efficiency, and occasional superiority, of modal sets of beliefs relative to individuals' own sets of beliefs, i.e., that MSB contain less non-salient items than ISB which result from inaccurate, forced elicitation, cannot account for the results in the present study since it is shown that a large proportion of MSB were non-salient for many of the subjects. Further, certain sets of beliefs known to be inappropriate, i.e., which were neither modally salient nor individually salient, are shown to be good predictors of attitude using the Fishbein equation. The implications of these findings for the theory and the practical use of the Fishbein technique are discussed.

Alternative fuels for aviation

Abstract: Aviation is an advanced technology, constantly pursuing higher performance, and backed by painstaking research into new materials and techniques. The target of “performance” has changed with the period, but the extent to which energy can be provided for propulsion has always been rated of major importance. Since propulsive energy is stored on board in a fuel medium, the more obvious criteria for the fuel are the energy density (based on mass and/or volume) and the proportion of this energy usable within the propulsion system. At the same time, the fuel must respond safely to reasonable handling procedures. The more subtle requirements to emerge over recent years include those of a minimum potential for pollutant formation and, in the case of very high speed flight, a maximum capacity for heat absorption.

The Thermal Behaviour of Gauzes as Interfacial Inserts between Solids

Abstract: The distribution of contact points formed between a flat solid surface and a metallic gauze has been examined, and the thermal resistances of contacts with gauze inserts have been measured. The resistance of a steel-to-steel contact in an ambient air environment was increased by the insertion at the interface of a gauze made from soft, high-conductivity metallic wires, but similar inserts led to a decrease of the overall resistance of the assembly in a high vacuum.

Velocity and Temperature Distributions for Cold Air Jets Issuing from Linear slot Vents into Relatively Warm Air

Abstract: Two-dimensional jets having various exit conditions were studied with a 15 cm field of view, laser-stimulated Mach Zehnder interferometer which provided isothermal contour maps, and with a pitot and thermoprobe traverse. At low discharge velocities (around 3 ms***), the onset of turbulence in the free air stream occurred further downstream of the nozzle than for higher efflux rates, and for vent widths of 2 mm and 4 mm the hypothetical source appeared downstream of the nozzle. However, at these low velocities, jet diffusion was more rapid after transition than for higher velocity cases. In general, the thermal boundary layer breadths were greater for narrower nozzle widths. To avoid excessive entrainment, the nozzle exit Reynolds number should exceed 1000.