This thesis describes the work undertaken over a 3 year period under a Department of the Environment, Transport and the Regions (DETR) Partners in Technology programme in collaboration with four industrial partners. The aim of the work was to investigate the two-phase flow heat transfer and fluid dynamic performance of the non-ozone depleting refrigerants and lubricating oil mixtures in a horizontal tube. A single tube test facility was designed and built to determine local heat transfer coefficients and overall pressure drop for evaporation and condensation. A circulating pump system pressurised through an accumulator charged with nitrogen was selected to enable accurate setting of the evaporation and condensation conditions. The system was validated using R22. Tests were undertaken for R404A, R407C, Isceon 59 with a Polyol-ester lubricating oil. The experimental data consist of two-phase local heat transfer coefficients and overall pressure drops inside a smooth copper tube. Numerous data were obtained for evaporation and condensation, and constituted a large database of two-phase heat transfer coefficients. The experimental results were compared with several existing correlations traditionally used in heat exchanger design, to assess their suitability with the new refrigerants. As a result of these comparisons offering a poor agreement, a model characterising the variation of the local heat transfer coefficient was developed. The evaporation model consists in a modification of the Kattan et al. model (1998a, b & c). The new model predicts the experimental results well, with a standard deviation of 6.1% for refrigerant R407C and Isceon 59. The effects of lubricating oils on two-phase flow heat transfer are also discussed qualitatively. This work provides a design tool when dealing with the non-ozone depleting refrigerants, and recommendations are made on the use of several correlations.

published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

Increased consciousness of the environmental problems has aroused people’s interest of renewable energy systems, especially the application of green features in buildings. The demand for air conditioning / cooling in domestic and non-domestic buildings is rising throughout the world; this increases the reliance on conventional fuels and the global warming effect from greenhouse gas emissions. Passive cooling and energy efficient design can substantially reduce reliance on fuel based heating and cooling. Passive and Hybrid Downdraught Cooling, in different forms, is now technically viable in many parts of the world. This has been established through a combination of research projects. In some hot arid regions, a major part of the energy consumed consists of air-conditioning requirements. Alternative methods, using passive cooling techniques, can assist in reducing the conventional energy consumption in buildings. Evaporative cooling, which can be tracked back several hundreds of years in ancient Egypt and Persia [1–3], is one of the most effective strategies, because of the enormous latent heat needed for evaporation of water. Green features are architectural features used to mitigate migration of various air-borne pollutants and transmission of air from outside to indoor environment in an advantageous way [9]. The reduction of fossil fuel consumption and the associated decrease in greenhouse gas emissions are vital to combat global warming and this can be accomplished, in part, by the use of natural ventilation. To assess the performance of several innovative cooling systems devices and to develop improved models for more established technology, quantitative measurement of output was necessary. This was achieved in this study by the development of simply constructed low energy cooling systems which were calibrated by the innovative use of wind and water as a source. These devices were found to be consistent and accurate in measuring the temperature and cooling load from a number of devices. There were some problems in the original evaporative units. Therefore, a number of modifications have to be made to enhance the systems performance. The novel Windcatcher – PEC cooling system was assessed and different cooling loads were achieved.

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