Natural ventilation has the potential to provide good indoor air quality, thermal comfort for occupants, and can also save energy and reduce CO2 emissions. Computational fluid dynamics (CFD) offers detailed information about indoor flow patterns, air movement, temperature and local draught distribution in buildings, so it has unique advantages as an efficient and cost-effective tool for optimum design in a complex built environment. This thesis shows the use of CFD to simulate the coupled external and internal flow field around a 6m cubic building with two small openings. To study both wind driven and combined wind and buoyancy driven cross ventilation through a full-scale cubic structure, un-structured grid CFD and a steady envelope flow model were applied to calculate mean ventilation rates. To validate the CFD results, full-scale experiments were undertaken under various weather conditions in England. For wind driven ventilation RANS model predictions were proved reliable when wind directions were near normal to the ventilation openings, i.e. 0o~30o. However, when the fluctuating ventilation played a more dominant role than the mean flow (90o) RANS models were incapable of predicting the total ventilation rate. Improved results may be expected by applying more sophisticated turbulence models, such as LES, weighted quasi-steady approximations, or unsteady envelope flow models. In the thesis experience on the modelling of combined wind and thermal effects is outlined and feedback is provided to CFD code developers to enable further improvements for building ventilation studies. The full-scale field testing data from this study is valuable for comparison with wind tunnel results and validation of CFD applications.

The European Union has an ongoing commitment to reducing CO2 emission as highlighted by its agreement at the Kyoto Summit. One approach to achieving these reductions would be to develop alternative energy sources for major energy demanding sectors. In the EU, about 40% of all energy consumed is associated with buildings and of this, about 60% is utilised in the housing sector. A major part of the energy demand of buildings could be met by utilising renewable energy sources, e.g. solar energy. Existing large-scale plants for power generation prevent efficient utilisation of the waste hot water produced. This means that to meet electricity demand, vast quantities of fossil fuels are burnt releasing unwanted pollutants (e.g., CO2 and NOx) into the atmosphere. Over the last decade, small-scale CHP plants have been introduced for many applications with proven environmental and economic benefits. In addition, solar energy has been used to generate electricity and provide hot water in conjunction with the CHP plants. Investigation of a hybrid heat pipe solar collector/CHP system was carried out in this research. The system is powered by solar and gas energy as well as the boiler waste heat to provide electricity and heating for residential buildings. Compared to the relevant system configurations, this system has the following innovative features: The solar collector was integrated with exhaust flue gas channels that allowed both solar energy and waste heat from exhaust gas to be utilised. Heat pipes as high efficiency heat transfer devices were incorporated in the collector panel. Both miniature and normal heat pipes were investigated, and this resulted in two types of collectors, e.g., thin membrane heat pipe solar collector, and hybrid heat pipe solar collector, to be produced for this application. A compact, lightweight turbine was applied in this system. Novel refrigerants, including n-pentane and hydrofluoroethers (HFEs), were employed as the working fluids for the CHP system. Use of the system would save primary energy of approximately 3,150kWh per year compared to the conventional electricity and heating supply systems, and this would result in reduction of CO2 emission of up to 1.5 tonnes. The running cost of the proposed system would also be lower. The research initially investigated the thermal performance of several heat pipes, including micro/miniature heat pipes, normal circular and rectangular heat pipes, with/without wicks. An analytical model was developed to evaluate the heat transport capacity for these heat pipes. A miniature heat pipe with parallel piped channel geometry was proposed. The variation of heat transport capacity for either micro/miniature or normal heat pipes with operation temperature, liquid fill level, inclination and channel geometry were investigated. Investigation of the operating characteristics of the selected heat pipes, e.g., two miniature and one mini heat pipes, and two normal heat pipes, was then carried out using both the numerical technique and experimental testing. It was found that the results from tests were in good agreement with the numerical predictions when the test conditions were close to the simulation assumptions. The research work further involved the design, modelling, construction and tests of two innovative heat pipe solar collectors, namely, the thin membrane heat pipe solar collector and the hybrid heat pipe solar collector. A computer model was developed to analyse the heat transfer in the collectors. Two collector efficiencies, η and η1, were defined to evaluate their thermal performance, which were all indicated as the function of a general parameter (tmean-ta)/In. Effects of the top cover, manifold as well as flue gas temperature and flow rate (for hybrid collector only) on collector efficiencies were investigated using the computer model developed. Laboratory tests were carried out to validate the modelling predictions and experimentally examine the thermal performance of the collectors. Comparison was made between the modelling and testing results, and the reasons for error formation were analysed. The research then considered the issues of the micro impulse-reaction turbine, which was another part of the integrated system. The structure configuration, coupling pattern with the generator as well as internal geometry contour of the turbine were described. The velocity, pressure and turbulent kinetic energy of the flow in the turbine were determined using numerical CFD prediction. In addition, experimental tests were carried out using a prototype system. The results of CFD simulation and testing show good agreement. This indicates that CFD can be used as a tool of optimizing turbine geometry and determining operating conditions. The research finally focused on the integrated system which brought the heat pipe solar collector, boiler and micro turbine together. The individual components, configurations and layout of the system were illustrated. Theoretical analysis was carried out to investigate thermodynamic cycle and heat transfer contained in the combined system, which is based on the assumption that the system operated on a typical Rankine cycle powered by both solar and gas energy. Tests for the prototype system was carried out to realistically evaluate its performance. Two types of turbine units were examined; one is an impulse-reaction turbine, and the other is a turbo-alternator. The turbo-alternator was found to be too small in capacity for this system thereby affecting its output significantly. The micro impulse reaction turbine was considered a better option. A typical testing showed that the majority of heat required for the turbine operation came from the boiler (7.65kW), and very little (0.23kW) from the solar collector. The gas consumption was 8.5kW. This operation resulted in an electricity output and domestic hot water generation, which were 1.34kW and 3.66kW respectively. The electrical efficiency was 16% and the thermal efficiency was 43%, resulting in an overall efficiency of 59%. Increasing the number of the collectors used would result in reduced heat output from the boiler. This would help in improving system performance and increasing efficiencies. In this application, number of collectors used would be 4 as the flue gas flow rate would only be sufficient to provide 4 to 5 such collectors for heat recovery. The research resulted in the proposal of another system configuration. The innovative concept is illustrated in Chapter 8, and its key technical issues are discussed.

This thesis investigates a novel combination of a jet-pump refrigeration cycle and a thermal (ice) storage (TIS) system that could substantially reduce the electrical energy requirements attributable to comfort cooling.Two methods of TIS were identified; spray ice TIS would use evaporative freezing to store ice on a vertical surface,and encapsulated ice TIS would freeze a bed of encapsulated elements by sublimation freezing.Thestudy also investigates jet-pump refrigeration at partload and a convergent-divergent design manufactured from a thermoset plastic to make recommendations for performance enhancement for a system that has a low COP. An experimental rig was built to investigate the novel concepts in the laboratory. Encapsulated ice TIS was superior to spray ice TIS because, for the same nominal secondary flow, sublimation freezing causes an increase in coolth storage rate of about 10 % compared to evaporative freezing. Encapsulated ice stores experience difficulties in fully discharging their coolth (approximately 6% in this case), but spray ice TIS can be used to produce an ice/brine slurry enabling all of the ice to be used, and so may be more suitable if the unmelted ice represents a large proportion of the cooling capacity. Approximately 85 % to 90 % of the ice formed on the vertical surface during spray ice TIS testing was formed by evaporative freezing from a falling film. At high saturation conditions, heat is transferred mainly by conduction across the falling film. Both the growth of an ice layer on a vertical surface and freezing of encapsulated elements were found to be successful, but a large data spread was observed during spray ice TIS testing. It was thought that a variation in the steady-state saturation conditions in the evaporator/ice store was caused by variability of droplet size distribution from the spray nozzle flow, which may make a full-scale system unreliable. The COP of the spray ice TIS system was approximately 0.15 compared to a COP of approximately 0.25 found during encapsulated ice TIS testing. The difference was because of the use of an over-expanded primary nozzle, which restricted secondary flow and increased momentum losses. A primary nozzle that expands close to the design evaporator saturation conditions should be used to maximise entrainment ratio. The COP of a jet-pump TIS is low, but a system designed to operate at off-peak periods could increase the COP to about 0.8 by taking advantage of the lower ambient conditions. The measurement of entrainment ratio was used successfully to determine ice storage rate and COP. This was valid because of the assumption that the saturation conditions in the evaporator/ice store approached steady-state. However, over longer periods that would be found in large-scale systems, the ice storage rate and entrainment ratio may fall substantially. The steady-state assumption could still be used to observe the change in evaporator conditions by sampling over short time intervals (30 minutes). At part-load, increases in evaporator saturation temperature could increase entrainment ratio substantially (50 % increase) for only a small reduction in critical pressure lift ratio Ns *(15 % reduction). A variation in chilled water temperature could be used to boost entrainment ratio at the peak demand. The variation in Ns* is too small to use this strategy to control the jet-pump with respect to condenser operating conditions. The entrainment ratio is approximately proportional to the diff-user to primary nozzle area ratio. A doubling of entrainment ratio was attained for only a 15% reduction in Ns*. The change in geometry from a constant area throat to a convergent-divergent design caused the flow through the jet-pump to vary with outlet conditions indicating that secondary flow was not choked. Higher entrainment ratios and pressure lift ratios were observed, but the entrainment ratio varied with outlet conditions in the form of peaks and troughs, making its operation unpredictable. This was thought to be caused by the restriction in secondary flow area due to the interaction of the primary jet and the curved wall. The convergent-divergent design manufactured from a thermoset plastic was successfully tested, showing that a plastic material can be used as a material of construction. In principle, a large number of jet-pump units could be manufactured from a single mould, reducing the first cost. The investigation proved the concept of jet-pump TIS. Waste-heat could be utilised over 24 hours and year round, increasing the efficiency of the process. The use of a convergent-divergent throat design, multiple geometry jet-pumps and operation at off-peak periods can maximise the performance over a cooling season, and be competitive with other TIS and chiller systems. The mass production of jet-pumps using injection moulding techniques could reduce substantially the capital cost of a system. All of these factors should encourage the development of such systems, so that the harmful emissions caused by the use of air conditioning systems can be minimised.

This thesis describes a theoretical and experimental investigation of the jet-pump refrigerator, and the application of Computational Fluid Dynamics (CFD) to improve the performance of the jet-pump, which lies at its heart. Within this thesis a number of new studies aimed at improving the COP of jet-pump refrigerators are carried out. These include an investigation of a novel jet-pump design methodology, (the Constant Rate of Momentum Change method), the application of CFD in the design of jet-pumps, the experimental testing of two new refrigerant fluids and finally, a comparative experimental investigation of performance benefits resulting from two cycle improvements that had not been tested before - these are the introduction of (i) a pre-heater (or recuperator) between the jet-pump and condenser, to preheat the liquid flow to the vapour generator, and (ii) a pre-cooler (or economiser) in the suction line between the evaporator and jet-pump, in order to sub-cool the liquid refrigerant in the line between the condenser and evaporator. Literature studies of jet-pump refrigerator technology and jet-pump design methodology are reviewed and discussed. A CFD model has been developed, assessed and validated against given experimental data. Simulations of a jet-pump that is part of a jet-pump refrigerator cycle was carried out to investigate the refrigerant flow structure and to assess the dominant influence of operating conditions and geometry. The validated CFD code was then used to optimize the design of the jet-pump for two new refrigerants (R236fa and R245fa). The resulting optimized jet-pumps were manufactured and tested experimentally over a wide range of operating conditions, using an adaptable test rig that was purpose-developed as part of this research work. Detailed experimental studies were carried out. All the experimentally determined results were compared to the CFD predicted values, and these showed good agreement for all the jet-pumps tested. These results showed that CFD has the potential to be an effective and powerful tool for simulating and optimising jet-pumps. The results also show that the jet-pump refrigerator should be considered if sources of low-grade heat are available.

This thesis conforms conventional and advanced experimental techniques for the measurement of and mathematical prediction of velocity pressure-loss factors (k-factors) for fittings used in heating, ventilation and air-conditioning (HVAC) systems. After an extensive study of different tracer-gas experimental techniques, the constant injection method is applied to various duct fittings on a small scale HVAC system situated in a laboratory. The results are compared with those of experiments performed using a more conventional technique using a Pitot-static tube. The basis of the experimental procedure is to achieve an accurate method of measuring the mean air velocity within a duct. This allows an accurate estimate of the velocity pressure-lossf actor to be obtained. A wide variety of duct fittings are investigated experimentally and numerically including bends, transitions, branches, inlets, outlets and obstructions such as orifice plates, wire mesh and lateral pipe obstructions. Computational fluid dynamics (CFD) is applied to each duct fitting tested in the lab. A commercially available package FLUENT is used with a high powered computer to simulate the airflow through various duct fittings. The pressure loss and velocity vectors are predicted for each particular duct fitting and therefore a prediction for k-factors is obtained. k-factor predictions are compared with experimental results and published data given in ASHRAE and CIBSE guides in order to assess the accuracy of CFD prediction. It is shown that as an accurate method for prediction of k-factors in duct fittings, CFD is a useful tool for the design and development of HVAC systems. The application of CFD allows the designer to vary any duct component with ease to observe the effect on a particular duct fitting without incurring the expense of laboratory experimentation. It is also shown that values of current published kfactors are greatly over estimated leading to oversizing of HVAC system fans. Experimentally produced k-factors obtained using the tracer-gas method and CFD predictions are approximately 20% lower than current data available to HVAC system designers. CFD may be applied to various applications in the field of heat-pumps and refrigeration systems. A detailed investigation is carried out here to compare CFD prediction and experimental results of several low pressure and high pressure ejectors commonly found in refrigerator absorption cycles. The compressible flow of refrigerants was modelled through an ejector to obtain a prediction of the entrainment ratio ( i. e. the ejector's ability to entrain a refrigerant from an evaporator using a hot main flow through a nozzle). These predictions were then compared with experimental results and this indicated that CFD could serve as a useful tool in the design of refrigeration systems. Application of CFD has also been studied in relation to the investigation of pressure loss through different types of evaporator/condensecr oils found in heat pump systems; here the design of such coils is important to the operating efficiency. The pressure loss across heat-pipes found in ducted flows is also predicted using CFD; in this case the geometry and the thermal conditions play an important role in the overall pressure loss.

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This study investigated the effectiveness of wind-driven natural ventilation in courtyard and atrium-type buildings, particularly in the context of ventilative cooling. Courtyard and atrium buildings are currently enjoying great popularity. Perhaps a primary reason for their revival comes from the energy and environmental awareness of the current period, in which courtyard and atrium concepts are emerging as very promising. Wind-driven ventilation is one of the most basic and probably among the most efficient ways to prevent overheating, and provide cooling in the summer season, especially in humid climates. A review of previous works showed that little attention has been given to the wind-driven natural ventilation capability of these structures, and to the means of maximizing this ventilation. This study was thus aimed to fill part of the gap in this subject. In order to evaluate the wind-driven ventilation effectiveness of these structures, and to examine some of the influential parameters, experimental wind tunnel tests were made. Actual indoor air flows were measured in small replica models of four-storey courtyard and atrium buildings by means of small calibrated orifice plates. A parametric study of the geometry of the courtyard was made in isolation conditions, where the depth and breadth of the courtyard were systematically varied. Several atrium ventilation modes were tested both in isolation and in urban terrains. The tests involved different roof geometries and various roof porosities. The measurements were followed by a discussion on the validity of simple computational methods to predict airflow in atria. The investigation portrayed the importance of some factors, such as the wind orientation rather than the courtyard geometry, for enhancing the flow in these structures. The superiority of some atrium designs over the courtyard types, particularly in sheltered sites, was underlined. The study concluded with a discussion of design guide-lines and referred the reader to an application as an example, describing a simple step-by-step method to estimate the cooling benefits of these structures in a particular site, and making use of the measurement data obtained from the study.

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