Showing papers by "Cranfield University published in 1984"

Flamelet Crossing Frequencies and Mean Reaction Rates in Premixed Turbulent Combustion

Abstract: Abstraet–The one-point, one-time description contained in the Bray-Moss-Libby model of premixed turbulent combustion is first generalized to a two-point, two-time formulation which includes information on the time and length scales of the scalar field within turbulent flames. This formulation is then specialized to a one-point, two-time description which is treated in detail so as to yield expressions for the autocorrelation of the progress variable, for the time scale of the scalar field, for the mean crossing frequency of the flamelets and, finally, for the mean rate of chemical reaction. Although the latter expression closely resembles results developed earlier on an intuitive basis, its derivation in the present study permits an assessment of various assumptions and intermediate results by comparison with experimental measurements. Such comparisons as are currently possible are shown to lend support to the analysis and to enhance the prospects of future exploitation of the general formulation.

On the stability of an infinite swept attachment line boundary layer

Abstract: A number of numerical schemes were employed in order to gain insight in the stability problem of the infinite swept attachment line boundary layer. The basic flow was taken to be the classical Hiemenz flow. A number of assumptions for the perturbation flow quantities were considered. In all cases a pseudo- spectral approach was used; the chordwise and spanwise directions were treated spectrally, while an implicit Crank-Nicolson scheme was used temporally. Extensive use of the FFT algorithm has been made.

Spray impaction, retention and adhesion: An introduction to basic characteristics

Abstract: Droplets, falling under gravity through air that is not moving relative to the target, will impact on any object in their path, while charged droplets will be drawn to objects of earth potential along paths normal to the lines of equipotential; thus near the catching surface, they will move directly towards it. If the air is moving relative to the target, it will tend to move the droplets with it. The greater the drag to mass ratio of a droplet, the more rapidly any initial motion it has through the air will cease, and it will move through the air only very slowly under the effects of gravity and any electromagnetic potential. Air flowing past an object is able to change its path rapidly, but droplets moving with the air are less able to do this. Their ability to avoid impact increases with decrease in droplet size and wind speed, and with increase in the size of the catching surface. Thus small smooth stems catch big droplets in a high wind efficiently, but large smooth branches in a light wind will not catch many small droplets. Artificial cylinders and ribbons are poorer at catching droplets than natural surfaces, which are rarely smooth and often hairy. Hairs or spikes on a surface greatly increase the catch efficiency of droplets carried in the wind. A droplet several hundred micrometres in diameter is so dominated by gravity that it will fall in a near vertical path even in a moderate wind, impacting on any horizontal surface that obstructs its path. Its chances of reaching a vertical stem are negligible unless it runs off or splashes from a near horizontal surface. Conversely, a small droplet will be carried almost horizontally in any wind and is most likely to impact on vertical surfaces or flapping leaves. It has a much greater chance of getting inside the canopy without being caught because most leaves are near horizontal, and once there, it must rely on the turbulence induced by the wind for transport and impaction on undersurfaces or hairs. Because turbulence is reduced as the droplet nears the ground, it is very difficult to catch droplets on the lower parts of the crop within the canopy. To bounce, a droplet must have enough surplus kinetic energy to rebound clear of the surface, allowing for the energy losses in deforming the droplet in the bounce process; moreover, the surface must not be significantly wetted by the drop. Thus the droplet must be moderate to large in size, must be moving rapidly relative to the surface, and must have a high surface tension to contain it as a droplet, even at its extreme deformation. Surface condition is of great importance; the presence of hairs and the type of roughness affect the probability of maintaining an air film between the surface and the droplet. In general, droplets below 150 μm diameter are unlikely to bounce, but adding small amounts of surfactant to the droplet formulation can increase this size by several times. Any one plant leaf can vary considerably over its area because of age, abrasion and local surface shape. A film of water on a wet surface ensures an air film is maintained and the droplet will bounce.

Solid Modeling and the Interface Between Design and Manufacture

Abstract: Many challenges must be faced before an automatic process-planning system will be practical for individual application?specifically, the recognition and simulation of a part's ?form features.?

Shocks generated in a confined gas due to rapid heat addition at the boundary. II: Strong shock waves

Abstract: An inert compressible gas, confined between infinite parallel planar walls, is in an equilibrium state initially. Subsequently energy is added at the boundary during a period that is short compared to the acoustic time of the slot $t'\_a$ (the wall spacing divided by the equilibrium sound speed), but larger than the mean time between molecular collisions. Conductive heating of a thin layer of gas adjacent to the wall induces a gas motion arising from thermal expansion. The small local Mach number at the layer edge has the effect of a piston on the gas beyond. A linear acoustic wave field is then generated in a thicker layer adjacent to the walls. Eventually nonlinear accumulation effects occur on a timescale that is longer than the initial heating time but short compared with $t'\_a$. A weak shock then appears at some well defined distance from the boundary. If the heating rate at the wall is maintained over the longer timescale, then a high temperature zone of conductively heated expanding gas develops. The low Mach number edge speed of this layer acts like a contact surface in a shock tube and supports the evolution of the weak shock propagating further from the boundary. One-dimensional, unsteady solutions to the complete Navier-Stokes equations for an inert gas are obtained by using perturbation methods based on the asymptotic limit $t'\_a$/$t'\_c \rightarrow$ 0, where $t'_c$, the conduction time of the region, is the ratio of the square of the wall spacing to the thermal diffusivity in the initial state. The shock strength is shown to be related directly to the duration of the initial boundary heating.

Novel pseudomonas plasmid involved in aniline degradation.

Abstract: A plasmid of ca. 100 kilobases was detected in a Pseudomonas species which was isolated from soil by growth on aniline as the sole carbon and energy source. The plasmid was shown to be involved in aniline metabolism.

Performance Characteristics of Centrifugal Pumps When Handling Non-Newtonian Homogeneous Slurries:

Abstract: The change in performance characteristics of centrifugal pumps when handling fine granular or homogeneous type non-Newtonian slurries has been investigated using two different slurry pumps handling...

Information systems maintenance: an integrated perspective

Abstract: This article argues that information systems maintenance is a more complex and integrated task that portrayed in the literature It involves not only the maintenance of the applications software, but also all the other elements in an operational system The literature relating to the maintenance of each element is reviewed to reveal substantial underdevelopment in some areas and fragmentation between elements The present practice of focusing upon the maintenance of particular individual elements is criticized and a new focus upon changes in the information inputs to the systems development process is proposed Alternative methods of managing the maintenance operation are examined and the implications of these methods in terms of designing the procedures, staffing the maintenance function, and the need for communication are discussed

From CAP 1 to CAP 2: User‐Initiated Innovation from the User's Point of View

Abstract: User‐initiated innovation is a significant source of new industrial products in certain industries. Recent investigations have been concerned primarily with the identification of user‐innovators and their usefulness to the manufacturers who eventually make and/or market the new product or process generally. The extension of thought to include the activities of customers in the initiation of the industrial innovative process has been based upon the assumption that the firm which initiates the user‐based innovation will play no more than a small role in its commercial exploitation. This article presents a case study of a company, British Aerospace, which has deliberately attempted to exploit commercially some of the innovations developed internally for its own use. The article goes on to suggest an extension to the “customer‐active paradigm” of innovation research to include such pro‐active behaviour.

The relative efficiency of cryogenic fluids used in the rapid quench cooling of biological samples

Abstract: SUMMARY The heat transfer characteristics of various cryogenic fluids used in the rapid quench cooling of biological samples are examined. It is concluded that cryofixation should be achieved during the initial plunge stage of the cooling. Liquid nitrogen maintained near its melting point of 63·1 K at a pressure in excess of the critical value of 33·5 atm will produce the quickest cooling. Nitrogen could also provide the best ultrastructure at atmospheric pressure if the minimum plunge depth and velocity criteria are satisfied. Plunging bare thermocouples into cryogenic fluids will lead to erroneous conclusions about the relative cooling efficiencies of various liquids for cryofixation. A qualitative explanation for the results obtained by Silvester et al. (1982) during the rapid cooling of water drops in various fluids is proposed.

Second Order Theory of Unsteady Burner-Anchored Flames With Arbitrary Lewis Number

Abstract: The stability of burner flames for arbitrary Lewis number is considered on the basis of large activation energy modelling. Previous leading-order-only approaches are now extended to second order in Math (the inverse activation energy). The assumption that the unsteady perturbations are small (order e) means that one must discuss the distinguished limit implicit in the product Math. lt is demonstrated here that different governing equations (and in particular the inner zone equation) are obtained in the two cases, θ1e → 0 and θ1e → ∞. It is shown that there are two complex frequency relations governing the behaviour of flames near burners. It is found that for free flames a dispersion relationship specifically dependent on activation energy is obtained which is similar though not identical to the classical relationship. For near-free flames, the complex frequency obeys a "composite" dispersion relation, the solution to which clearly indicates the destabilizing effect of heat losses to the burner. ...

A field study of the predators of the grain aphid, Sitobion avenae (F.) (Hemiptera: Aphididae), in winter wheat in Britain.

Abstract: In a field study of Sitobion avenae (F.) in winter wheat at Cranfield in southern England, the main predators were found to be the larvae of the staphylinid Tachyporus sp. and syrphids. Most predation occurred overnight. These predators were active during the period when the wheat crop was most suitable for aphid population increase, and in 1982 were apparently responsible for preventing an outbreak at Cranfield.

Cavitation in an Oscillatory Oil Squeeze Film

Abstract: This paper records observed features of cavitation arising in an oscillatory oil squeeze film In the experimental apparatus, two nondeformable surfaces contained the oil film The square upper surface oscillated, normally to the oil film, at any frequency between 5 and 50 Hz A transparent lower surface together with a viewing and synchronising system enabled cavitation bubble patterns in the oil film to be observed and photographed at any point in the oscillatory cycle Three different behavioral regimes (designated 1, 2, and 3) have been observed, each characterized by the method of forming cavitation bubbles together with particular features in the oil film pressure, thickness and bubble extent-time cycle Descriptions are given of the salient features of each regime, and the transition from one to another The paper contains photographs of cavitation bubble patterns at important points in the typical oscillatory cycles together with their location in the oil film pressure and thickness time histories

Monitoring physical properties of a fluid

Abstract: Un moteur (12) a l'interieur d'une chambre (1 et 2) fait tourner a une vitesse angulaire constante une turbine (4) placee sur un arbre d'agitation (3). A motor (12) within a chamber (1 and 2) is rotated at a constant angular velocity a turbine (4) provided on a stirring shaft (3). Les vibrations induites dans l'arbre (3) en raison de l'action d'agitation de la turbine (4) sont detectees par un transducteur (5) dont la sortie est conditionnee par l'appareillage (6) pour emettre des signaux representant la vibration de l'arbre (3). Induced vibrations in the shaft (3) due to the action of stirring of the turbine (4) are detected by a transducer (5) whose output is conditioned by the apparatus (6) for emitting signals representing the vibration of the shaft (3). Un appareillage d'analyse (7) deduit les conditions a l'interieur de la chambre (1 et 2) a partir de ces signaux. An analysis apparatus (7) derives the conditions inside the chamber (1 and 2) from these signals. La presente invention peut etre appliquee a des procedes se deroulant dans un recipient de reaction et a un liquide se trouvant dans une chambre de pompage. The present invention can be applied to processes taking place in a reaction vessel and a liquid in a pump chamber. Comme l'excitation est une vitesse angulaire constante et que l'effet induit est une vibration, il n'y a pas d'interference entre l'excitation et l'effet induit, malgre le fait que le transducteur (5) est monte sur l'arbre d'agitation. Since the excitation is a constant angular velocity and the induced effect is a vibration, there is no interference between the excitation and the induced effect, despite the fact that the transducer (5) is mounted on the stirrer shaft.

Flux and the Continuous Casting of Steel

Abstract: Equation representant la distribution de temperature dans une piece coulee en acier, qui satisfait aux conditions de l'interface avec l'acier liquide et prevoit que cet interface est parabolique

Effect of curvature on three-dimensional boundary layer stability

Abstract: The problem of the stability of a three-dimensional laminar boundary layer formed on a curved surface is considered. A calculation scheme, which takes account of the curvature of the flow streamlines and of the surface is proposed for the prediction of the development of small amplitude temporal disturbances. Computations have been performed for the flow over the windward face of an infinitely long yawed cylinder and comparisons have been made with experimental data. These indicate that the theory correctly predicts many of the features of the unstable laminar flow. The theory also suggests that transition, in this particular situation, is dominated by traveling disturbance waves and that, at the experimentally observed transition location, the wave which has undergone greatest total amplification has an amplitude ratio of approximately e to the 11th. When the effects of curvature are omitted the maximum amplitude ratio at transition is about e to the 17th.

Lectures on plane waves in reacting gases

Abstract: On decrit certains effets que les reactions chimiques ou le transfert d'energie relativement lent vers la structure interne d'une molecule peuvent avoir sur la dynamique d'un gaz. Description des lois de conservation et des lois constitutives. Etude des ondes infinitesimales dans une atmosphere en equilibre. Analyse des proprietes des courbes d'Hugoniot. Ondes de choc, detonation, deflagration. Comportement acoustique des gaz poussiereux, analogie avec les gaz reactifs ou en relaxation