Browsing by Author "Rubini, Philip A."
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Item Open Access Application of commercial CFD to improve gasoline port fuel injector design and targeting(Cranfield University, 2002-09) Pierson, S. R.; Rubini, Philip A.The need to meet ever more stringent emission regulations and the desire to improve fuel economy has led to the significant development of the gasoline spark ignition combustion engine in recent years. One area of development has been mixture preparation, and PH (Port Fuel Injection) has been introduced to increase engine responsiveness whilst meeting emission regulations. Successful PH designs however depend upon good targeting of the fuel spray onto the back of the intake valve. Geometric predictions based on injector axis and spray bone angles have been used in the past, but require development to account for the momentum exchange between the spray and the charge air. Alternatively CFD (Computational Fluid Dynamics) can be used. In this study a validated methodology has been successfully developed using the commercial CFD code Fluent5.5, to simulate the spray behaviour from a multi-hole port fuel injector. The approach taken ignored the primary and secondary atomisation phases, and instead droplets were injected at the injector tip position. The droplets velocity and size were then tuned until the predicted spray profile matched the measurement data at 60rnm and 90mm downstream of the injector tip. Having developed a tuned injector model, CFD simulations assessing the injector targeting performance of the Jaguar AJV8 engine were then undertaken. Based upon these assessments some suggestions to improve the engine's injector targeting performance were made. Before this methodology could be developed, a series' of experiments were necessary to characterise a state of the art port fuel injector. A combination of Planar Mie and PDA laser techniques, were used to measure how the spray behaved under different operating and atmospheric conditions. As well as providing spray boundary and validation data, an in depth understanding of the spray structure was gained for both pulsed and continuous injector operations.Item Open Access Assessment of cool thermal storage strategies in Kuwait(Cranfield University, 2006) Sebzali, Mohamed; Rubini, Philip A.The extensive use of air conditioning for indoor cooling in offices and large commercial buildings in Kuwait represents a major part of the power and electricity consumption in that country. The main objective of this research was to investigate ice and chilled water cool thermal storage technologies and operating strategies for air conditioning. This was motivated by the extreme climatic conditions in Kuwait and the necessity to reduce both maximum power demand and energy consumption whilst being economically feasible. This work represents the first such study undertaken. In Kuwait approximately 45% of the total annual exported electrical energy is consumed solely by air conditioning systems as a result of the very high ambient temperatures occurring between March and October. Furthermore, it was estimated air conditioning systems represent about 62% of the peak electrical load. To assess the potential of cool thermal storage, the air conditioning system for an existing clinic building, representing a typical medium size building in Kuwait, was designed with and without a cool thermal storage system. The results demonstrate that internal ice-on-coil and chilled water storage systems are suitable storage technologies that can be implemented in Kuwait. The cooling demand of the clinic building was first estimated using the ESP-r building energy simulation program, following which the different components in the air conditioning systems were sized including chiller, storage tanks, pumps, air handling units for conventional, ice and chilled water storage air conditioning systems operating with load levelling, 50% demand limiting and full storage strategies. The heat gains by different auxiliary components in the air conditioning systems were estimated and the final cooling demand profiles were developed. For each air conditioning design, the power and energy consumption for the design day condition and over the whole year were calculated and analysed. Furthermore, the life cycle costs were determined based on the estimated capital, maintenance, operating costs and a financial analysis was carried out. For the Kuwaiti climate, the results demonstrate ice and chilled water storage systems can reduced the maximum power consumption during the day time when the electricity demand is high and largest reduction in the maximum power achieved full storage strategy. However, the energy consumption of ice storage system operating with 50% demand limiting and full strategies were found were found to be higher than the conventional air conditioning system. Nevertheless, the energy consumption in the ice storage system with a load levelling operating strategy was slightly lower. Chilled water storage system was found to be unlike ice storage system, the energy consumption in all operating strategies improved over the conventional system. Based on the estimated life cycle cost using the actual operating costs for both the government and user, it was established that for the government, ice storage operating with load levelling strategy and all other strategies of the chilled water storage systems would be more economical than conventional systems. However, for the user, load levelling ice storage, load levelling chilled water storage, and 50% demand limiting chilled water storage systems would be more cost effective. Out of all alternatives, chilled water storage system with a load levelling strategy was found to be the most cost effective for the climate of Kuwait and for similar climates of Kuwait. Although, the outcome from this research work can not be generalised however, the method of sizing and energy and economic analysis, which was discussed in this thesis can be generalised and followed to evaluate the impact of cool thermal storage systems on energy performance and economy of the air conditioning systems.Item Open Access A comparison of modified K-ε turbulence models for buoyant plumes(Taylor and Francis, 2001-02-01) Worthy, Jude; Sanderson, V. E.; Rubini, Philip A.The effect of buoyancy on the production and dissipation of turbulent kinetic energy is investigated in variants of the popular k- ε turbulence model. The standard gradient diffusion model is considered for the scalar flux as well as a generalised gradient diffusion model. Also, the addition of the non-isotropic component of an algebraic stress model for the Reynolds stresses is assessed. The relative significance of the various models and terms are demonstrated using different combinations of the models, including the important flux Richardson correction term. The generalised gradient diffusion and algebraic stress models are shown to give a strong increase in turbulence production, although the effect on the flow can be largely controlled by the coefficient of the flux Richardson term. Recommendations are made regarding optimum models and coefficients.Item Open Access A Genetic algorithms based optimisation tool for the preliminary design of gas turbine combustors(Cranfield University, 2002-11) Rogero, J. M.; Rubini, Philip A.The aim of this research is to develop an optimisation tool to support the preliminary design of gas turbine combustors by providing a partial automation of the design process. This tool is to enable better design to be obtained faster, providing a reduction in the development costs and time to market of new engines. The first phase of this work involved the analysis of the combustor design process with the aim of identifying the critical tasks that are suitable for being automated and most importantly identifying the key parameters describing the performance of a combustor. During the second phase of this work an adequate design methodology for this problem was defined. This led to the development of a design optimisation Toolbox based on genetic algorithms, containing the tools required for it's proper integration into the combustor preliminary design environment. For the development of this Toolbox, extensive work was performed on genetic algorithms and derived techniques in order to provide the most efficient and robust optimisation method possible. The optimisation capability of the Toolbox was first validated and metered on analytical problems of known solution, where it demonstrated excellent optimisation performance especially for higher-dimensional problems. In a second step of the testing and validation process the combustor design capability of the Toolbox was demonstrated by applying it to diverse combustor design test cases. There the Toolbox demonstrated its capacity to achieve the required performance targets and to successfully optimise some key combustor parameters such as liner wall cooling flow and NOx emissions. In addition, the Toolbox demonstrated its ability to be applied to different types of engineering problems such as wing profile optimisation.Item Open Access Large eddy simulation of buoyant plumes(Cranfield University, 2003-05) Worthy, Jude; Rubini, Philip A.A 3d parallel CFD code is written to investigate the characteristics of and differences between Large Eddy Simulation (LES) models in the context of simulating a thermal buoyant plume. An efficient multigrid scheme is incorporated to solve the Poisson equation, resulting from the fractional step, projection method used to solve the Low Mach Number (LMN) Navier-Stokes equations. A wide range of LES models are implemented, including a variety of eddy models, structure models, mixed models and dynamic models, for both the momentum stresses and the temperature fluxes. Generalised gradient flux models are adapted from their RANS counterparts, and also tested. A number of characteristics are observed in the LES models relating to the thermal plume simulation in particular and turbulence in general. Effects on transition, dissipation, backscatter, equation balances, intermittency and energy spectra are all considered, as are the impact of the governing equations, the discretisation scheme, and the effect of grid coarsening. Also characteristics to particular models are considered, including the subgrid kinetic energy for the one-equation models, and constant histories for dynamic models. The argument that choice of LES model is unimportant is shown to be incorrect as a general statement, and a recommendation for when the models are best used is given.Item Open Access Numerical simulation of drifting sand(Cranfield University, 2000-12) Alhajraf, Salem; Rubini, Philip A.Two-phase flows are involved in many industrial and natural flow phenomena varying from as specific as the transport of crude oil in pipelines to as general as the dispersion of pollutants in the atmosphere. Numerical modelling based on Computational Fluid Dynamics (CFD), has attracted the attention of scientists and engineers from a wide range of backgrounds over recent decades during which these models have been extensively developed, analysed and applied to many practical applications. Wind blown particles such as sand or snow and their resulting accumulation around buildings, roads, oil field installations and security fences causes severe structural and design problems. These are traditionally addressed based on previous experience, full-scale field investigation or using scale model wind tunnel experiments, all of which incur high cost. In this study, wind blown particles are considered as a two-phase flow system. A finite volume based CFD code is developed using two-phase flow theory and is employed to numerically simulate the drifting of sand and snow around obstacles of different geometry. The model solves the governing transport equations in three dimensional space. Three different approaches are investigated to represent and solve the secondary flow phase, particles, within the flow field; a particle tracking model, based on a Lagrangian reference frame and the homogenous and the mixture models, based on an Eulerian reference frame. The capabilities and limitations of each of these models are investigated for flow fields involving drifting particles around obstacles of different geometry. Particles transported by wind both in suspension and saltation are modelled based on the physical characteristic and the threshold condition of the particle. Their effect on the flow field is incorporated through separate source terms contributing to the particle transport equation. The Eulerian based models are coupled with the Fractional Area/Volume Obstacle Representation (FAVOR) as a mean of representing the solid boundary formed by deposited particles separating the flow field from the accumulation zones. The FAVOR treatment allows the flow field to respond to the changes in the geometry of the deposition regions and further calculations take into account the erosion and deposition processes that have previously occurred. The model can be calibrated to match specific flow conditions through several controlling parameters. These controlling parameters are identified and analysed for four distinct case studies. Model results are compared with field and wind tunnel observations available in the literature and with field measurements conducted as a part of this study in the desert of the State of Kuwait. Qualitatively good agreement between the model and the observations is obtained in two as well as three dimensions. Although the mixture and particle tracking models show the potential capability to simulate such flow systems, the homogenous model is found to be the most appropriate model due to its relative simplicity compared to the mixture model and its lower computational cost compared to the Lagrangian particle-tracking model. In conclusion, a practical CFD tool has been developed and validated, incorporating novel physical and numerical models. The tool can be utilised by scientists and engineers to further understand the real world problem of drifting sand and snow in urban and industrial environments.Item Open Access Optimisation of Combustor Wall Heat Transfer and Pollutant Emissions for Preliminary Design Using Evolutionary Techniques(Sage, 2003) Rogero, J. M.; Rubini, Philip A.This paper presents the concept and application of a design optimisation toolbox, based upon evolutionary techniques, for the preliminary design of a gas turbine combustor. The toolbox has been designed to interface with existing analysis packages and to perform optimisation in parallel over a heterogeneous network of workstations. The optimisation capabilities of the toolbox are demonstrated for gas turbine combustor design by automatically attaining twenty-two performance targets in a combustor design whilst performing minimisation of wall cooling flow and NOx emissions.Item Open Access Preliminary gas turbine combustor design using a network approach(Cranfield University, 1997-02) Stuttaford, Peter J.; Rubini, Philip A.Gas turbine combustor design represents an ambitious task in numerical and experimental analysis. A significant number of competing criteria must be optimised within specified constraints in order to satisfy legislative and performance requirements. Currently, preliminary combustor flow and heat transfer design procedures, which by necessity involve semi-empirical models, are often restricted in their range of application. The objective of this work is the development of a versatile design tool able to model all conceivable gas turbine combustor types. A network approach provides the foundation for a complete flow and heat transfer analysis to meet this goal. The network method divides the combustor into a number of independent interconnected sub-flows. A pressure-correction methodology solves the continuity equation and a pressure-drop/flow-rate relationship. A constrained equilibrium calculation, incorporating mixing and recirculation models, simulates the combustion process. The new procedures are validated against numerical and experimental data within three annular combustors and one reverse flow combustor. A full conjugate heat transfer model is developed to allow the calculation of liner wall temperature characteristics. The effects of conduction, convection and radiation are included in the model. Film cooling and liner heat pick-up effects are included in the convection calculation. Radiation represents the most difficult mode of heat transfer to simulate in the combustion environment. A discrete transfer radiation model is developed and validated for use within the network solver. The effects of soot concentration on radiation is evaluated with the introduction of radial properties profiles. The accuracy of the heat transfer models are evaluated with comparisons to experimental thermal paint temperature data on a reverse flow and annular combustors. The resulting network analysis code represents a powerful design tool for the combustion engineer incoporating a novel and unique strategy.Item Open Access A Study of LES Stress and Flux Models Applied to a Buoyant Jet.(Taylor and Francis Ltd, 2005-09-03) Worthy, Jude; Rubini, Philip A.Large eddy simulation (LES) stress and scalar flux subgrid scale models are evaluated in the context of buoyant jets. Eddy viscosity, eddy diffusivity (including formulations of the generalized gradient diffusion hypothesis), “structure” (Bardina and Leonard), mixed, and dynamic models are scrutinized. The performance of the models is examined in terms of the main flow variables and also with respect to the “internal” behavior of the models in terms of the relative contributions to the turbulent kinetic energy budget.Item Open Access Synthetic Turbulence Generation for LES on Unstructured Cartesian Grids(Cranfield University, 2007) Bin Mohamad Badry, Ahmad Badarudin; Rubini, Philip A.A parallel CFD code to solve incompressible fluid flow on unstructured Cartesian meshes has been developed almost from ground up. Turbulence statistics have been computed using the Large Eddy Simulation technique. The new code was subjected to some validation where results are compared to available reference data. An analysis on the iteration and discretisation errors was carried out. This code was then applied to predict the lid driven cubical cavity flow in at a bulk Reynolds number of 10,000. Three different mesh sizes were used to investigate independence of results on grid size. Amongst others, turbulence statistics were checked against Kolmogorov -5/3 law. A detailed study of synthetic turbulence methods was carried out and applied to the prediction of flow in a duct with square cross section using an inlet and outflow boundaries. Three different turbulence generation methods were investigated namely the artificial turbulence generation method, random perturbation method and a novel hybrid particle-wave method also termed as the enhanced vortex particle method in this study. The mean and instantaneous field variables together with the turbulence statistics from each method were compared and analysed. Finally, the code was used to solve turbulent flow over arrays of wall-mounted obstacles with mesh densities comparable to previous studies. The velocity profiles and vector fields at various locations in the domain were compared to data obtained from recent LES simulations. The artificial turbulence generation case was applied for the first time to produce turbulence at the inlet. The turbulence kinetic energy spectrum distribution agrees well with reference data. Important findings from this study are clarified and some suggestions for future work are given in the conclusions section.Item Open Access A thermal analysis of the horizontal zone refining of indium antimonide(Elsevier Science, 2004-11-15) Roussopoulos, G. S.; Rubini, Philip A.Single crystals of semiconductors and optical materials are very important in the field of computer technology and communications. The properties of such crystalline materials significantly depend upon the quantity and the type of impurities contained in their structure. The principal objective of this work was to further the understanding of the heat transfer mechanisms in the horizontal zone refining process and, by successfully modelling the process, to obtain a correlation between the length of the molten zone and typical operational parameters, for example, the physical dimensions of the process equipment, the heating element and the required energy input. Results are presented from the associated parametric study accomplished through a three-dimensional pseudo-steady-state simulation of the zone refining process for indium antimonide. The conclusions will be of value when optimizing the operating efficiency of horizontal zone refining where the length of the molten zone is a significant factor.Item Open Access Turbulence modelling of turbulent buoyant jets and compartment fires(Cranfield University, 2001-02) Sanderson, V. E.; Rubini, Philip A.Turbulent buoyant jets are a major feature in fire hazards. The solution of the Reynolds Averaged Navier-Stokes (RANS) equations through computational fluid dynamic (CFD) techniques allow such flows to be simulated. The use of Reynolds averaging requires an empirical model to close the set of equations, this is known as the turbulence model. This thesis undertakes to investigate linear and nonlinear approaches to turbulence modelling and to apply the knowledge gained to the simulation of compartment fires. The principle contribution of this work is the reanalysis of the standard k- ε turbulence model and the implementation and application of more sophisticated models as applied to thermal plumes. Validation in this work, of the standard k- ε model against the most recent experimental data, counters the established view that the model is inadequate for the simulation of buoyant flows. Examination of previous experimental data suggests that the measurements were not taken in the self-similar region resulting in misleading comparisons with published numerical solutions. This is a significant conclusion that impacts of the general approach taken to modelling turbulence in this field. A number of methods for modelling the Reynolds stresses and the turbulent scalar fluxes have been considered and, in some cases for the first time, are applied to nonisothermal flows. The relative influence of each model has been assessed enabling its performance to be gauged. The results from this have made a valuable contribution to the knowledge in the field and have enabled the acquired experience to be applied to the simulation of compartment fires. The overall conclusion drawn from this thesis is that for the simulation of compartment fires, the most appropriate approach with current computational resources, is still the buoyancy corrected standard k- ε model. However, the turbulence scalar flux should be modelled by the generalised gradient diffusion hypothesis (GGDH) rather than the eddy-diffusivity assumption.