Browsing by Author "Newborough, M."
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Item Open Access Electromagnetic heating processes: analysis and simulations(Cranfield University, 1994-08) Calay, Rajnish Kaur; Newborough, M.Electromagnetic heating (EMH) processes are being increasingly used in the industrial and domestic sectors, yet they receive relatively little attention in the thermal engineering domain. Time-temperature characteristics in EMH are qualitatively different from those in conventional heating techniques due to the additional parameters (viz dielectric properties of the material, size and shape of the product and process frequency). From a unified theory perspective, a multi-purpose model has been developed in order to obtain the heating characteristics for an arbitrary processing situation. Theoretical analyses of various EMH processes in materials of various regular geometries and a range of physical properties have been undertaken. Despite the wide spread usage of microwave energy in the food engineering sector. few understand microwaves and their interactions with foods. Much of the published research is largely focussed from the view point of an electrical engineer and aimed at the oven designer. However, trial-and-error methods are usually employed when developing microwavable food products and when using microwave ovens. The presented thesis is focussed from the view-point of the thermal engineer and aimed primarily at food developers and end users. The multi-purpose model was then modified specifically for simulating the heating of food materials in a microwave oven. The validity of the commonly made assumptions was investigated; in particular the variation of dielectriC properties during the heating processes and their likely influence on the model's predictions. Experimental data available in the literature were compiled and analysed to form a set of equations for predicting the dielectric properties of various food materials. Also available correlations for thermal properties were evaluated for a selected set of experimental data of different food materials. Analyses were undertaken to demonstrate and evaluate the effects of various parameters on the heating characteristics of different food materials commonly heated/cooked in microwave ovens. A qualitative comparison of model predictions and experimental measurements is provided to validate the physical basis of the model. Findings from the model lead to a better understanding of the interactions between foods and microwaves. [...cont.]Item Open Access Heat Transfer In Enclosures: Ovens(Cranfield University, 1994-12) Ekundayo, C. O.; Newborough, M.The use of natural convection to effect heat transfers in enclosures is limited principally because of its low heat transfer coefficients. This study aimed to establish a better understanding of the heat transfers occurring in enclosures which are heated by cylindrical heat sources and to identify preferred heating arrangements for improving the rates of natural convection heat transfers. Experimental investigations including interferometric observations were carried out on a square-sectioned enclosure containing a rectangularsectioned, isothermal inner body and cylindrical, sheathed electrical heating elements. The locations of the heaters were varied and the effects on natural convection heat transfers established. The findings conformed to intuitive expectations, concluding that the maximum position for natural convection heat transfer was with the heaters located in the lower half of the sidewalls. The flow patterns and heat transfer characteristics of a single horizontal cylindrical (9.5mm dia) heater in a square-sectioned enclosure were studied by traversing the heater both horizontally and vertically across the enclosure at distinct Rayleigh numbers (7.5x104 and 1.1x105). Optimal positions for maximum and minimum Nusselt numbers were established with respect to heater diameter and vertical height, and horizontal offset from a vertical wall. Flow and Interferometric observations were also carried out on an enclosed single tubular 55mm dia. heater, and two 55mm dia. heaters. Improved arrangements for enhanced heat losses by natural convection from horizontal tubular arrays were deduced, applicable to arrays either totally enclosed or In the proximity of a vertical wall. A square-sectioned (35Ox35Oxl7OOmmtu)n nel oven was designed which would withstand operations under vacuum. Within this enclosure, the heating arrangements were located and varied essentially in the lower half of the enclosure, flow and thermal profiles obtained, a multi-surface radiation model developed and the convective heat transfers validated by evacuating the enclosure. At oven temperatures, over 40% of the heat input was achieved by natural convection.Item Open Access Low-Temperature Thermal-Energy Storage and Transmission Systems Employing Hydrophilic Polymeric Materials(Cranfield University, 1997-04) Augood, P. C.; Newborough, M.; Highgate, D. J.The wide fluctuations that occur in the aggregate electrical demand of a generating utility are punitive with respect to total system efficiency. Demand side management techniques have been applied to reduce such fluctuations including the conversion of electrical energy to thermal energy during periods of low demand for use during peak demand periods. For thermal processes requiring energy above ambient temperature it is feasible to use sensible heat due to the existence of stable storage mediums and efficient methods of heating at the high temperatures required. However where energy is required below ambient temperatures, efficiency of cooling limits the use of sensible heat, hence latent heat storage has been adopted. Conventional cold storage systems use ice banks to store cooling energy at 0°C in order to capture the high latent heat of fusion of water. The rate of discharge for such stores is limited by thermal resistance in the store and the thermal capacity of secondary coolants (such as glycol solutions). This investigated the use of hydrophilic materials to overcome the limitations of current cold-storage technology. Such materials have the capacity to absorb and retain up to 95% by mass of water (or other aqueous solutions) regardless of how the materials is subdivided. Furthermore the thermal properties of the polymers in their hydrated state resemble those of the free hydration fluid, including any phase transitions. By supporting the hydrated materials in a non-freeing, non-aqueous fluid the resultant mixture provides a medium for cold storage that can be pumped and used at the point of load, and is not limited by the thermal resistance of an encapsulating material. Three aspects concerning the utilisation of hydrophilic materials for thermal engineering applications have been investigated; (i) the physical properties of the materials in their hydrated state, (ii) methods of fluidising material in a high density store, and (iii) the heat transfer properties of hydrophilic based slurries while undergoing phase transition. Material tests have shown that currently available hydrophilic materials have thermo- physical properties that depend principally upon the hydrating fluid, regardless of particle size, and are stable over long periods (>3years). Suitable hydration fluids can lower the temperature of the phase transition thus extending their potential as storage mediums beyond those of ice-based technologies. Novel materials, of very high water content (95%) have been produced and investigated. These appear to be very suitable for thermal storage because they increase the maximum achievable energy densities of a fluidised storage system and potentially reduce cost. A number of thermal storage devices to utilise hydrophilic based slurries have been designed and evaluated. The resultant devices has been shown to provide a means of taking hydrophilic materials to, and from, a packed bed and feeding them at a controlled rate into a fluid stream. The thermal charge/discharge rates of such a device are limited only by the choice of external heat exchange systems. An experimental apparatus has been designed to investigate the effects of phase change particles on the heat transfer properties of flowing mixtures. The results have shown that (i) at temperatures above the phase transition temperature the presence of the particles causes an increase in the measured heat transfer coefficient for concentrations above 10% by volume, (ii) there is a significant interaction of particles at the heat transfer surface, and (iii) that under high flow rate conditions, with phase change occurring, heat transfer coefficients are considerably enhanced (ie 80%) above those of the support fluid when used alone or with non-active particles. Further work is recommended to extend this study to produce an engineering prototype storage system for trial evaluation.Item Open Access Novel hydrophilic polymer couplant for application in ultrasonic non destructive testing(Cranfield University, 2001) Bourne, Simon James; Newborough, M.; Highgate, D. J.Ultrasonic Non Destructive Testing (NDT) is used to inspect materials and structures for defects. Water is commonly used in NDT as a couplant to improve ultrasonic transfer between an interrogating probe and test piece. Unfortunately, the presence of water can cause corrosion and/or degradation of the test piece material. The aim of this investigation was to evaluate hydrated cross-linked hydrophilic polymers as candidate solid contact ultrasonic couplant for use in the field of ultrasonic NDT. The fact that hydrophilic polymers can absorb and retain large quantities of water suggested that they might demonstrate the desirable ultrasonic properties of water without the risks associated with conventional water coupling. To test this, the ultrasonic properties of a range of hydrophilic polymers were assessed. Excellent results were achieved, attenuation as low as 0.36 and 0.71dB mm-1 at 5 and 1OMHz respectively being measured. Great potential for efficient coupling was established due to acoustic impedance in the region of 1.81VIN S M-3. A polymer dependant coupling pressure of less that 1kg CM-2 was required to achieve optimum coupling to a smooth steel block. Mechanical longevity, evaluated by life testing, showed that polymers of up to 70% equilibrium water content were best suited for dynamic testing applications. Temperature was shown to effect ultrasonic properties; a drop from 5 to -120C caused an increase in attenuation of 3dB mm-1 and velocity of 350m s-1. Pressure demonstrated no influence on attenuation but affected an increase in velocity of 44m s-1 per kg CM-2 . Further investigation into the unique ultrasonic properties of hydrophilic polymers showed that the water sorption process caused an increase in attenuation prior to saturation being reached. This was attributed to the absorption of sound during the polymer transformation from the glassy to rubbery condition observed during hydration. Dehydration from 100 to 37% saturation in a 60% equilibrium water content polymer caused an increase in attenuation of 1.8dB mm-1 at 5MHz. The research concluded with the design and development of a prototype wheel probe employing hydrophilic polymer as the tyre. Operation at 5MHz in pulse echo mode demonstrated results competitive to conventional immersion testing. An MMA-VP cross-linked hydrophilic polymer of approximately 60% equilibrium water content was found most suitable to this application. This thesis suggests that there is a clear role for hydrophilic polymers in ultrasonic NDT. The success of the wheel probe design developed as a result of this research has resulted in patent application in both the UK and USA.Item Open Access Refrigeration appliances : Performance enchancements via novel thermal-energy storage(Cranfield University, 1999-10) Lawson, Chukwuemeka Jonathan Adeolu; Newborough, M.; Highgate, D. J.In recent * years there has been increasing concern for the environmental impacts of economic and technological development. This (principally socio-political) con- cern has led to the well-known series of environmentall ,y motivated (global and local) conventions, agreements and legislation put forward in a bid to manage these environmental impacts, which in turn has led to research and commercial activ- ity on environmental issues. One of the major activities identified as adversely impacting the environment is the manner in which energy is currently harnessed, inter-converted and utilised. Cont/d.Item Open Access Rescheduling electricity demands in domestic buildings(1995-01) Deering, Stuart; Newborough, M.Utilisation of electricity within the domestic sector is examined. The characteristically time-dependent behaviours o f domestic consumers and their associated usages of household appliances, result in "peaky" daily electricity-demand curves. This is not conducive to achieving (i) a high-efftciency electricity supply, (ii) low rates o f financial investment in new generating plant or (iii) curtailing rates of pollutant emissions. A relatively energy-efficient, environmentally-clean, electricity-supply system can only be realised when the total demand (i.e. the total for the domestic, commercial and industrial sectors) versus time curve does not exhibit rapid changes in gradient. In order to achieve this goal, existing and more appropriate electrical-load management techniques need to be implemented, especially for the domestic sector. Thus opportunities fo r demand-side load-management are assessed together with the prospective benefits obtainable by domestic consumers and electricity-supply companies. The operation o f appliances, which are significant contributors to the typical daily household electricity-demand profile, are examined in detail. Demand profiles for individual appliances are presented and, where appropriate, examples of thermal, motive and control sub-profiles are provided. Patterns of appliance use within households are discussed with various recommendations for achieving reduced load profiles, both for specific appliances and households.Item Open Access Thermo-mechanical behaviour of heavy-duty disc brake systems(Cranfield University, 2001-10) Chen, J. P.; Newborough, M.; Kyriacou, S.In heavy-duty disc brake systems, braking is a transient, non-linear and asymmetrical thermo-mechanical process. Surface cracking, rather than wear, is the major factor limiting the brake disc's life. The disc material (cast-iron), heat transfer boundary conditions and pad-disc frictional reactions are characteristically non-linear and asymmetrical during the friction process. Non-uniform deformation and surface cracks in brake discs result from the accumulation of excessive residual stress/strain. During braking processes, many factors affect the distributions of the residual stress and strain in discs, and hence the propagation of the surface cracks. The disc material, structure and boundary conditions are three of the crucial aspects. From the structure, a brake disc could be either solid or ventilated. In practice, solid structures always have higher anti-cracking performance than the same class of ventilated designs. However solid discs cost more material and have lower cooling efficiency. This thesis presents an improved finite element analysis for heavy-duty disc brakes and identifies design improvements. As the friction pads slide against the disc's surfaces continuously, the thermal and mechanical loads are functions of time and spatial coordinates. A 3-D asymmetrical finite element model was developed to achieve more accurate simulations of the thermo-mechanical behaviour of brake discs during braking processes. A non-linear inelastic material model for cast-iron was employed in the FE model. Permanent plastic stress and strain fields were predicted and analysed for multi-stop drag operations. The residual stress/strain fields in the discs are investigated to understand the differences between solid and ventilated discs in terms of the cracking resistance ability. Several engineering solutions are recommended for optimising the performance of the disc brake system. _ The thesis is organized in five chapters. Chapter One introduces the background concepts about the commercial disc brake system. In this part, the brake structure, material and previous researches are reviewed. The goals for this investigation are also summarised at the end of this chapter. Chapter Two introduces the general finite element modelling knowledge, procedures and the modelling boundary conditions and material models. Chapter Three presents an analysis of the disc brakes thermo-mechanical behaviour and the affecting factors. Chapter Four is focused on the residual stress field prediction and cracking behaviour analysis. The project conclusions and further research recommendations are presented in Chapter Five.