Differential surface tension is a common phenomenon in many chemical and biomedical processes. Localised surface tension gradients due to differential surface loading in thin films give rise to a moving shock front in the direction of higher surface tension. Existence of a background flow enhances the shock wave giving rise to wave breaking and wave separation mechanisms. The effect of a background flow field on Marangoni stress induced shock fronts were investigated in this thesis. Furthermore, a numerical procedure to find approximate solutions to the fully nonlinear flow problem that arises due to Marangoni spreading is proposed. A set of surface evolution equations that incorporates the effects of the background flow field is studied in two major respects: (i) breaking the horizontal symmetry and (ii) nonlinear accretion leading to shock front breaking or separation. The evolution of the surface is evaluated by numerical simulations for a wide range of parameter values. The investigation showed that there are two breaking mechanisms switched by the value of Peclet number. Furthermore it showed that the life time of the shock front is determined by the volumetric flow rate of the film. It is shown here that a weak Marangoni force generates a pure capillary gravity wave that propagates faster than the surfactant front. It is customary to use the lubrication approximations to simplify thin film problems. As a result, the inertial terms in flow equations and nonlinear terms in surface stress balances become excluded. To analyse the fully nonlinear flow, a finite element (FEM) analysis is proposed. The simulations shows that the lubrication theory holds globally in predicting the spreading rates but fails to do so locally until a quasi-steady state is reached. The FEM model shows the formation of two counter-rotating vortices at the beginning which diminish as time evolves. The FEM results are compared with the lubrication theory simulations. FEM model shows rapid film thinning forming extremely thin films within a short period of time. Though detailed transport mechanisms differ, both methods are in close agreement in predicting the spreading rates.

The phase behaviour and thermodynamic properties of simple model mixtures are examined using the statistical associating fluid theory as extended to chain molecules interacting with potentials of variable range (SAFT-VR), and by computer simulation. The SAFT-VR approach is based on an accurate and compact representation of the free energy of chain molecules. We present the SA FT -VR methodology as applied to mixtures of non-conformal molecules. A series of mixing rules are presented, beginning with the van der Waals one-fluid prescription and including more complex treatments. The vapour-liquid equilibria of a mixture consisting of hard spheres and square-well monomers is examined with the SAFT-VR equation of state, together with the liquid-liquid equilibria of a symmetrical square-well mixture with no unlike interactions. Additionally, we examine the vapour-liquid equilibria of a square-well monomer-dimer mixture, composed of equal-sized segments, both with the SAFT-VR approach and by Gibbs ensemble Monte Carlo simulation. The simulation data are used to determine the vapourliquid critical line of the mixture. An extension of the SAFT-VR approach to describe the phase behaviour of chain molecules interacting with a soft repulsive potential and an attractive well of variable range is presented. We focus on the vapour-liquid properties of Lennard-Jones chains using a simple recipe for the evaluation of the chain free energy. We also perform a case study for a specific class of phase equilibria exhibited by binary mixtures, where systems are seen to posses a region of closed-loop immiscibility in their phase diagrams. We examine the nature of this type of pl1ase behaviour using the SA FT· VR equation of state and Gibbs ensemble simulation for a simple model system with an anisotropic bonding site, which is seen to be the governing factor in the appearance of the region of low-temperature miscibility for this system. The model is chosen in order to mimic the physical features of real systems which exhibit this type of re-entrant phase behaviour. The critical regions of this model are examined using a finite-size scaling analysis performed in the semigrand canonical ensemble.

This document describes the development of a computational model to study the movement and breakup _of droplets in turbulent two-component flows. The aim is to produce a suitable model which will be economical of computing resources and practical for engineering applications. The application of particular interest here is that of water droplets in fully developed turbulent pipe flows of oil. The computational method uses Vortex filaments to produce, in a novel way, instantaneous fluctuating velocities within the flow domain. The trajectory of a particle within this field is predicted by integrating the theoretical law of motion for the particle. In addition, the breakup of a fluid particle in the turbulent field may be predicted using an empirical criterion formulated using data obtained from a series of experiments. The tests were designed to study the deformation and breakup of a single water droplet in oil subjected to shear. Wherever possible the results of each development stage of the model were compared with work published in the literature.

No description available.

A numerical approach for modelling variable stator stagger in multistage stage axial flow compressors is presented. The development of such an approach has been motivated by the requirements of an optimisation methodology for stator vane setting and active control of instability using controlled stator vane setting. The optimisation methodology has been further developed but active control approaches are discussed as future considerations. Varying upstream stator vane stagger . changes the incident flow angle on . the downstream rotor thus affecting the entire flow distribution within the compression systems. The approach therefore begins by investigating the effect of a change in stator stagger setting on stage performance. A meanline method was used for nu- merical prediction of stage characteristics as it can simulate the effect of a change in stagger settings and ( or) in rotational speeds. Overall compressor performance was obtained by stacking the (experimental or predicted) stage characteristics and the surge conditions predicted using a stage-by-stage dynamic compression model where the compressibility was considered explicitly. This approach for variable stagger set- ting was incorporated into a FORTRAN code and validated using the data from the 12-stage HP SPEY jTAY variable geometry compressor. To optimise the setting, a direct search method incorporating a Sequential Weight Increasing Factor Technique (SWIFT) algorithm was incorporated into the variable stagger model. The objective function in this optimisation is penalised externally 11 with an updated factor which helped to accelerate convergence. The methodology has been incorporated into a FORTRAN program and its validations were conducted using the data from the 7-stage LP OLYMPUS and the 12-stage HP SPEY /TAY compressors. Results have demonstrated that variable stagger setting is a powerful method to rematch stages and which can be used to improve the desired overall performance, and that the potential benefits of introducing additional rows of variable setting vanes can be achieved. Future work arising from the present study has been discussed and highlighted, which involves the enhancement of the model capacity and development of active control approaches. In addition the thesis involves several reviews focusing on different topics. Most reviews contain considerable information and it is expected that the information can be of help for the interested readers to trace more relevant references. These reviews consist of a general review in chapter 1; a brief review on stage characteristics modelling in chapter 2; a comparative review on incompressible and compressible surge models in chapter 3; a review of various optimisation methods for practical problems, especially for constrained non-smooth problems, in chapter 4; and a review of the state-of-the-art active approaches in chapter 7. The suitability of various approaches has been highlighted. Steinke's meanline method is suitable for investigating the in- influence of stagger resetting on stage performance. To predict the surge conditions for a (high-speed) multistage environment, the stage-by-stage compressible models are III more promising. For constrained non-smoothed optimisation, the SWIFT algorithm can be an alternative. The controlled stator vane regulated through nonlinear control law will permit the robust control of compressor instabilities.

For the efficient design of transonic turbomachinery systems, understanding of the complex flow phenomena inherent in the flow passages is essential. In the present study, a computational technique is adopted to meet this formidable goal. A code using a time-marching technique has been developed first for quasi three-dimensional cascades (that is two-dimensional computation with the varying streamtube height in the third dimension taken into consideration) and then extended to fully three-dimensional flows within the rotating flow passages. Each code has a built-in switch for in viscid and viscous flows. The basis of the codes is the conservative form of the Reynolds- averaged Navier-Stokes equations in a rotating framework. This is supplemented by either the Baldwin-Lomax (algebraic) or the k-e (two-equation) turbulence model. For solving the hyperbolic type governing equations, spatial derivatives are first discretized on the easily-constructed H-type grid system using a central-difference finite-volume approximation with the flow variables stored at the cell centre. An explicit multistage Runge-Kutta scheme is then employed for the time integration o f the resulting ordinary differential equations. The accuracy of the quasi three-dimensional code is initially evaluated by predicting the flows through cascades with simple geometry. Its robustness is then confirmed by two realistic configurations with a wide range o f operating conditions. Finally the fully three-dimensional code is applied to two highly loaded transonic rotors with complicated geometry at peak efficiency and near stall operating conditions. An extensive comparison in terms of detailed flowfield and overall performance between the predictions and experiments with laser anemometry and conventional probes shows the accuracy o f the codes and also indicates that the present study has great potential to be a viable aerodynamic design and analysis tool in the development of transonic turbomachinery systems.

No description available.

Oil-air separation is a key function in aero engines with closed-loop oil systems. Aero-engine separators are employed to separate oil from air before being released overboard. Typically, these devices make use of a porous medium such as an open-cell metal foam, in order to enhance oil separation. Although quite scarce, there has been some research aimed at developing a suitable modelling framework for aero-engine separators. However, numerical modelling of the air/oil flow through the open-cell metal foams employed in aero-engine separators has never been properly addressed. This thesis presents the development of a pore-scale numerical modelling approach to determine the transport properties of fluid flow through open-cell metal foams. Micro-computer tomography scans were used to generate 3D digital representations of several commercial open-cell metal foams. A code was developed in Matlab to render the CT images into 3D volumes and perform morphological measurements on the samples. Subsequently, conventional finite volume simulations are carried out in order to obtain the airflow and compute the pressure gradient across the investigated samples. Simulations were performed for a wide range of Reynolds numbers and the feasibility of using Reynolds-averaged Navier-Stokes (RANS) turbulence models is investigated. Validation was done by comparing the pore-scale pressure gradient results against experimental measurements. Further simulations were carried out to isolate and analyse particular effects in more detail, such as wall and entrance effects, fluid compressibility, time-dependent flow features, anisotropy of the foam structure and the impact of porosity and surface area on the pressure gradient. The oil phase within aero-engine separators has the form of disperse droplets. Thus, the oil phase in the pore-scale simulations was modelled using a Lagrangian particle tracking approach. Lagrangian simulations were run in steady state and one-way coupled, due to the low mass fraction of oil normally present within aero-engine separators Converged airflow pore-scale solutions were employed as the base flow for the Lagrangian tracking approach. A simplified oil capture criterion assumed the droplet trajectory to be terminated upon collision against the foam solid ligaments. The focus of the present work was on separation of small droplets with a diameter smaller than 10 microns. Hence, a series of calculations were performed using a representative droplet diameter range, and multiple flow velocities. The outcome of such approach was a qualitative evaluation of the oil separation effectiveness for several commercial open-cell metal foams under a representative range of flow regimes. Furthermore, rotational effects which are experienced by the metal foams within aero-engine separators were modelled using a moving frame of reference (MRF) approach. Finally, a methodology for upscaling the results obtained by the detailed pore-scale simulations into a simple macroscopic porous medium model is described, showing promising results. One of the aims of this work was to develop a numerical modelling framework able to provide an accurate representation of the airflow and a qualitative assessment of the oil capture within aero-engine separators. The feasibility of using the current state-of-the-art modelling framework is assessed. The separator design and geometry are based on the oil separation test rig located at the Karlsruhe Institute of Technology (KIT). Experimental measurements of the overall pressure drop and oil capture performed at KIT are used to validate the simulations. The methodology presented here overcomes some limitations and simplifications present in previous similar studies. The upscaled macroscopic porous medium model was applied to full aero-engine separator CFD simulations. Experiments and simulations were conducted for three different separator configurations, one without a metal foam, and two with metal foams of different pore sizes. For each configuration, a variation of air flow, shaft rotational speed and droplet size was conducted. The focus was on the separation of droplets with a diameter smaller than 10 \textmu m. Single-phase air flow simulation results showed that overall pressure drop increases with both increased shaft speed and air flow, largely in agreement with the experiments. Oil capture results proved to be more difficult to be captured by the numerical model and indicate that droplet re-atomization might play a significant role in the oil separation phenomena. Re-atomization, droplet-droplet collisions and droplet breakup were not considered at the present stage, but could be subject of future work. The modelling framework described here should not be seen as a definite answer but as an improvement upon the current state-of-the-art methodology, providing important lessons and recommendations for future work on aero-engine separators.

No description available.

No description available.