Browsing by Author "O'Callaghan, P."
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Item Open Access Annular flow enhanced two-phase evaporative heat exchanger for space-based systems(Cranfield University, 1991-12) Nikanpour, D.; O'Callaghan, P.The development of multi-kilowatt space-based systems requires the transport of waste heat loads over long distances in micro-gravity conditions. In this context two-phase heat transport systems are attractive enabling a high rate of heat transport with low pump powers compared to single phase systems. In this context the design of a two-phase heat exchanger to transfer heat from a single-phase fluid (water) to a two-phase fluid (Freon 114) is discussed. Until reliable micro-gravity (< 10-3g) test data on the heat transfer and pressure drop in a two-phase flows are available, it is deemed necessary that the design of heat exchangers' passages should promote gravity-independent flow regimes. This would make the design and test data, obtained at ground conditions, applicable in micro-gravity environments. The design concept investigated hinges on utilising a set of helical flow passages (with small cross sectional area) to ensure a predictable flow regime, annular flow, up to high vapour qualities (>0.8), in both micro-gravity and one Ig' environments. The concept was applied to the design of a 5kW helically coiled evaporative heat exchanger for space-based systems, which was subsequently manufactured and tested. Ground tests gave results close to analytical predictions based on computer simulations of the heat transfer and pressure drops in helical flow passages. Finally design guideline for a two-phase evaporative heat exchanger for space-based systems is provided, along with . conclusions and areas to be further researched.Item Open Access Development of a Computer Based Energy Management System(Cranfield University, 1998-09-30) Nagle, Liam; O'Callaghan, P.A prototype computer based expert system has been developed to aid energy managers by speeding the energy auditing process and rapidly identifying potential low cost and fast pay-back energy saving investments for a wide range of businesses. It consists of a generally applicable energy management system based on sound, tried theory and practical experience gained from a number of energy management surveys. These surveys were used to identify key data requirements for the identification of common areas of wastage. The system uses sparse data analysis and the building energy signature model. It produces an entire energy audit and list of economic recommendations for a site based upon minimal input data. This is accomplished by reference to a number of internal databases containing the technical information required, as well as the entire set of algorithms and mathematical routines required for the analyses. The prototype was tested with a synthetic data set derived from the site surveys and with real data from a large tertiary college and it was found to give credible results in line with those produced by extensive and in-depth manual data-gathering and analysis.Item Open Access Development of a solar-energy activated organic Rankine-cycle pilot power plant(Cranfield University, 1984-08) Bala, Eli Jidere; O'Callaghan, P.with the development of a low-grade energy engine in the School of Mechanical Engineering at Cranfield, there began a venture towards utilizing the Sun's power to operate the engine via a thermodynamic cycle, in an Egyptian desert area, i. e latitude 3Q0N. The derived mechanical power is to drive an irrigation pump at the site. The solar thermal system is to operate in a Rankine-cycle using an organic working fluid. The engine is in the form of a multi-vane expander. The solar-energy collector comprises of an array of gravity-assisted heat- pipes each enclosed within a cylindrical high-vacuum glass tube. The irrigation water which is to be sucked from below ground level, is also to cool the shell-and-tube condenser. The first section(i. e. PART A) of this thesis is a presentation of some design optimization concepts in the development of the thermodynamically operated solar-energy system. As in any desert area dust will constitute a problem, reducing the harnessing capability of the collector array. Regular cleaning would therefore be essential. However, cleaning a large array (1000) of such fragile tubes in situ is unlikely to be accomplished without cracks and breakages occurring. This perhaps means that the high-vacuum which is essential for each collectors continued adequate thermal performance could be easily lost. The collectors are also considered expensive. For example, one tube with an effective aperture area of about 0.102ml costs about twenty pounds Sterling. These and the fact that the maximum anticipated working temperature in the organic Rankine-cycle would be about 1201 C, led to the study into a means other than evacuation of reducing thermal losses from the receiver of a flat-plate solar-energy collector in the second section (i. e. PART B). A flat-plate collector employing a simple slatted convection suppression device was studied. It was shown that a flat-plate collector employing an effective convection suppression device and an initial cost of about £1o0/m`would be more cost effective than the evacuated-tube collector, when employed to activate the pilot power plant for operatinq temperaures of (80-X120)° C. The cost per peak watt could be reduced by an average value of about 15% depending upon the'operating temperture. Whereas the break-even time against electricity could be reduced by about 4%. However, the overall efficiency of the power plant could fall by an average value of about 23%, depending upon the operating temperature.Item Open Access Effect on oxides on surface topography and thermal contact resistance(Cranfield University, 1979-11) Mian, M. N.; O'Callaghan, P.The effect of oxides on the topographies of various random engineering surfaces and steady-state thermal contact resistances between oxidefree and oxidized metallic surfaces in contact in high vacua have been investigated. The results of many previous investigations indicated that such problems cannot be dealt with without a detailed study of the surface topographies, resulting in accurate surface characterisation. The distributions of the real micro-contact zones and heat transfer phenomena across interfaces could 'then be predicted. Cylindrical specimens normally mild steel EN3B and commercially pure copper of nominal area 4.908 x 10-4 m2 were oxidized under controlled environmental conditions and subsequently pressed into contact at their flat faces. The oxide film thickness measurements were taken using a high resolution Stereoscan electron microscope. Surface topographies were quantified before and after oxidation to determine the effects of oxide films upon their topological characteristics. Following a detailed analysis, two theories concerning the problem of predicting thermal contact resistances of oxidized joints having Gaussian distribution of surface heights have been developed. The developed theories establish the surface parameters measured to design a predictable thermal joint and apply for thin films of the order of naturally occurring oxides. From a statistical analysis of the experimental measurements for freshly-assembled contacts, an empirical expression (R = 66.0 p-0.945 a-0.128 X0.0346) has been established relating the loading pressure, mean roughness of'the contacting surfaces and oxide film thickness to the thermal resistance of the contacts in high vacua. Finally a method of producing joints with low thermal contact resistance (and vice versa) is suggested.Item Open Access Heat transfer properties of porous materials and insulants(Cranfield University, 1984-03) Batty, William J.; O'Callaghan, P.Buildings are complex thermally-dynamic structures serving aesthetic as well as, utilitarian functions. It is essential that careful planning is undertaken if buildings are to be energy efficient and cheap to run throughout their-expected life-spans. Although regulations have recently been introduced requiring the values of thermal transmittances (i. e. U-values) for walls and roofs of industrial and domestic buildings, to be less than specified limits, there is no guarantee that improved design will result. Also condensation has increasingly become a problem, as natural ventilation has been reduced, because of the introduction of doubleglazing and draught proofing. The use of insulating materials to increase thermal efficiencies " through the reduction of heat losses may also create problems in industrial plant and pipework. Metal structures covered with insulants are thereby hidden from view and so any ensuing corrosion, such as the general attack upon a low-alloy steel or stress-corrosion cracking of a stainless steel, may remain undetected until catastrophic failure occurs. It is, therefore, of utmost importance that the potential for and enhancement of corrosion due to the addition of insulants should be carefully considered. The ingress of water or water vapour into the insulant layer and subsequent leaching is the major cause of corrosion, and it is essential that steps are taken to prevent or reduce the likelihood of this occurring while ensuring that metal surfaces are adequately protected. There is a need to ascertain the heat and mass-transfer behaviours of building and insulating materials. Mathematical models require realistic data to simulate effectively conditions found in real structures. Too often in the past manufacturers' data for thermal properties, measured under laboratory conditions, have been used with little attempt to check on their validity or appropriateness to the conditions which are likely to be experienced. As desk-top computers become cheaper and more powerful these dangers could well increase. The too prevalent trusting attitude that computer predictions are absolutely correct together with a potential lack of understanding of the concepts of heat transfer and moisture mitigation by the users could result in poorer, rather than better, designed buildings. The thermal-probe technique for the measurement of the thermal conductivities of building structural materials has been assessed. This rapid transient and potentially cheap technique could be suited ideally to measurements in such materials. The theoretical basis for the method has been investigated and the accuracies and repeatabilities of thermal-probe instruments have been determined in measurements with paraffin wax. Determinations made with this technique, for masonry and structural components, were found to agree well with the manufacturer's thermal conductivity data. However, further developments need to be made to improve the usefulness of this technique for measuring the effective conductivities of fibrous insulants. Also the thermal-probe technique has been assessed for use in moist materials. Initial investigations with wet-day specimens showed that the probe diameter had no significant effect on the indicated values of the apparent thermal conductivity. Tests to measure the apparent thermal conductivities of aerated concrete blocks, at various moisture contents, gave results that compared well with other published data. Attempts to reduce national energy demands have led to increases in insulation thicknesses in roof spaces in Northern Europe and North America. It has generally been assumed that the apparent thermal conductivity of each material used has been a constant and equal to the value obtained in the testing laboratory. Examination of the temperature profiles through various horizontal thicknesses of loose-fill mineral wool insulants suggests that radiation effects and convection in the upper surface layers exposed to free air result in much larger apparent thermal conductivity values than those generally quoted in the literature, and the magnitudes of these effects also increase with the thickness of the insulant layer. Heat transfer mechanisms have been examined to explain these phenomena. Mathematical models of heat transfers through multi-phase materials have been examined. A model is proposed to describe the thermal conductivity of high-porosity cellular insulant which includes heat transfers by conduction through the solid and gaseous phases and by radiation. Predictions were found to agree well with experimental data for airfilled polystyrene foams and to be of the correct order of magnitude for air/fluorocarbon filled polyurethane foams.Item Open Access Industrial Energy Analysis and Conservation(Cranfield University, 1989-03) Dorling, John; O'Callaghan, P.This work is concerned with the analysis of industrial energy use for the purposes of reducing that use to a minimum within the constraints imposed by the existing plant. The work is divided into two parts. Firstly, a general analysis of the energy used in an industrial plant is made for the year 1986. Based on monthly records of energy consumption and environmental parameters, it is primarily intended to heighten management awareness of energy flows and the relative contributions from each energy type towards the total annual consumption and cost. It is performed using a suite of interactive linked computer programs and the data is reduced to a graphical form for presentation in a report. The analysis is a first approximation, assuming steady state conditions with many parameters time and space averaged, but is justified by the findings that monetary savings approaching 5% of the annual energy bill could be made with changes to operational procedures alone. As a result of this work, it was revealed that the boiler-house supplying the majority of the plant with high pressure hot water for process and space heating requirements used more than half of the total annual energy requirements and that potential energy savings were likely. Secondly, a detailed analysis of the energy used by the boiler-house is- carried out. A real time monitoring and targeting system which records and displays boiler, environmental and plant parameters is developed and installed. Data gathered over a period of six months enables (i) specific cases of energy wastage resulting from operational procedures to be identified and (ii) trends of performance indicating poor energy efficiency to be traced. The system developed may be used to provide the information necessary to enable improved control. A general result which may be inferred from this work would be that the combined approaches of a topdown general analysis and a bottom-up detailed analysis of energy use can provide significant opportunities for energy conservation resulting solely from the improved operation of large energy consuming plant. The work was performed at a major manufacturing plant in the U. K. with an annual energy consumption of the order of 600 gigawatt-hours and an annual energy bill of the order of E10 million.Item Open Access Low grade energy engines(Cranfield University, 1981-09) Mohey El-Din, K. H.; O'Callaghan, P.It is clear that there is no long-term solution to energy supply problems other than planned and continuous conservation. The use of low-grade energy as an unlimited resource will play an important part in maintaining the balance between the future of the world for both developed and developing countries alike. energy supply and demand A literature survey reviewing the most , the construction of low-grade energy engines has been conducted. The V promising approaches to' validity of each feasible system has been examinéd and the of operation described principles cycle utilizing common suitable refrigerants (e.g. halo-carbons) as A feasibility study concerning the adoption_of the organic Rankine ` working fluids has been conducted. In particular, the use of a multiple vane expander as the prime mover has been A suite of computer programs has been developed to fully investigated; describe working fluid properties optimise the geometrical and dynamical parameters of the Rankine cycle to achieve the most efficient operation in both steady and`transient states ' ' optimise the mechanical design of the expander depending upon the mode of operation and-the source and sink characteristics ' fully flexible experimental test facility was constructed so ., as to be capable of testing a wide variety of prime movers. This has _ been operated for a real time test period in excess of 300 hours. ` The test results are encouraging. Efficiencies were measured V in_accordance with the mathematical predictions and a portfolio of - and_reliable expander capable of utilising low has been produced. , Fruitful links with relevant have been forged. ,"Demonstrations of the system are planned the UK and overseas. L A ' ~ within British industries grade thermal energy suggested modifications towards a high efficiency, low-cost, robustItem Open Access The management and implementation of energy-thrift in hospitals(Cranfield University, 1989-07) Adderley, A. E.; O'Callaghan, P.The 1984 Auditor Genera1's report on energy~thrift in the Health Service claimed that energy~thrift programmes in hospitals were not achieving their performance targets. In order to determine the reasons for this failure, twenty hospital energy~audits were analysed. It was discovered that the two principle factors impairing the performance werez- (a) The thermal conflict between thrift measures implemented on the same thermal system, and, (b) inefficient implementation strategies. In order to investigate the influences of thermal conflict and implementation strategy on the out~comes of energy~thrift programmes,four objectives were defined:~ (i) To develop a computer model which was capable of simultaneously thermally modelling the implementation of energy-thrift measures on several hospital sites. (ii) To use the model to predict the thermal and financial out~comes of various implementation strategies. (iii) To test the sensitivity of the outrcomes to changes of unit fuel prices and capital costs, and, (iv) to compare the predicted results with those actually obtained. Data from four hospital sites and ninety thrift measures were entered into the model for analysis. It was deduced that a law of diminishing returns existed between capital investment and annual savings and that thermal conflict was responsible for an average annual financial loss of 15.5%. The financial returns on the capital invested were considerably enhanced (in one case by 3ÖØ%) by spreading the financial resources over all four sites rather than by concentrating the same capital outlay on a single site. Furthermore it was discovered that there was a level of capital investment that yielded an optimal net present value over the selected project life. The programmes were more sensitive to reductions of unit fuel prices than increases of capital costs. Most of the thrift programmes remained viable (N.B. N.P.V. > Ø using the public sector discount rate _of 5% and a project life of 5 years) after the unit fuel prices had been reduced by 5Ø% of their 1985 levels. When the results of the implemented programmes were compared with the models' predictions, the accuracy of the estimated savings ranged from a 4.5% under-estimate t 2.5% over-estimate. The software was designed to find the best-fit Ventilation rate and`base temperature for each hospital site and for each thermal zone within the hospital. When the post-implementation assessments of overall air-change rate and site base temperature were compared with those predicted, the model detected changes closely matching the predictions. â The results indicated that the technique developed in this thesis has potential as a monitoring and targeting system as well as a thermal model for predicting the out- comes of energy-thrift programmes.Item Open Access Thermal energy accumulation in stratified hot water stores(Cranfield University, 1986-03) Cohen, R. R.; O'Callaghan, P.Hot water thermal energy stores have the potential to improve and extend the performance of many kinds of energy system. Waperature stratification in the store is likely to affect the system's efficiency. A basic but accurate computer model of the hot water store under various inlet flow conditions is a requisite means of assesiing promising applications of hot water storage by system computer simulation techniques. A microprocessor-controlled test facility has been constructed to evaluate the performance of a 3m 3 hot water store under a wide range of inlet flow conditions, using a temperature step input approach. Three types of inlet/outlet ports have been examined: horizontal, vertical and distributors. The results show that two distinct regions evolve within the store: a fully-mixed region adjacent to the inlet port and a region of smooth 'plug-flow' in the remaining volume of the store. The performance of the store is shown to be defined by the initial depth of the fully-mixed region which in turn is seen to be closely related to the buoyancy and momentum fluxes of the inlet flow. The behAviour of the store and the evident correlations have enabled a one-dimensional computer model of the store to be developed, taking into account the turbulent mixing, vertical heat conduction and heat losses to the surrounding areas. The model has been successfully validated against the results from the step input experiments. The model has been integrated into a computer simulated central heating system which incorporates a hot water store. Predictions have been made, using the simulation, of the energy savings which may be achieved with the use of storage in comparison to a conventional system, and an assessment has been made of the economic viability of the application.