The current study is part of a long term experimental project devoted to investigating single-phase flow pressure drop and heat transfer, flow boiling pressure drop and heat transfer, flow boiling instability and flow visualization of de-ionized water flow in microchannels. The experimental facility was first designed and constructed by S. Gedupudi (2009) and in the present study; the experimental facility was upgraded by changing the piping and pre-heaters so as to accommodate the objectives of the research. These objectives include (i) modifying the test rig, to be used for conducting experiments in microchannels in single and two-phase flow boiling heat transfer, pressure drop and visualization, (ii) redesign metallic single microchannels using copper as the material. The purpose of the redesign is to provide microchannels with strong heaters, high insulation performance and with test sections easy to dismantle and reassemble, (iii) obtaining the effect of hydraulic diameter on single-phase flow, flow pattern, heat transfer and pressure drop, (iv) studying the effects of heat flux, mass flux,and vapour quality on flow pattern, flow boiling heat transfer and pressure drop, (v)comparing experimental results with existing correlations. However, the main focus in this present study is to investigate the effects of hydraulic diameter, heat flux, mass flux and vapour quality on flow pattern, flow boiling heat transfer coefficient and pressure drop. In addressing (iii) many possible reasons exist for the discrepancies between published results and conventional theory and for the scatter of data in published flow boiling heat transfer results: 1. Accuracy in measuring the dimensions of the test section, namely the width, depth and length and in the tested variables of temperature, pressure, heat flux and mass flux. 2. Variations in hydraulic diameter and geometry between different studies. 3. Differences in working fluids. 4. Effects of hydrodynamic and thermal flow development 5. Inner surface characteristics of the channels. Three different hydraulic diameters of copper microchannels were investigated: 0.438mm, 0.561 mm and 0.635 mm. For single-phase flow the experimental conditions included mass fluxes ranging from 278 – 5163 kg/m2 s, heat fluxes from 0 - 537 kW/m², and inlet temperatures of 30, 60 and 90°C. In the flow boiling experiments the conditions comprised of an inlet pressure of 125 kPa (abs), inlet temperature of 98°C (inlet sub-cooling of 7 K), mass fluxes ranging from 200 to 1100 kg/m²s, heat fluxes ranging from 0 to 793 kW/m² and qualities up to 0.41. All measurements were recorded after the system attained steady states. The single-phase fluid flow results showed that no deviation of friction factors was found from the three different hydraulic diameters. The effect of fluid temperature on friction factor was insignificant and the friction factors themselves were in reasonable agreement with developing flow theory. The typical flow patterns observed in all three test sections were bubbly, slug/confined churn and annular, however, based on the observation performed near the outlet, the bubbly flow was not detected. The effects of mass flow and hydraulic diameter on flow pattern for the three test sections investigated in the range of experimental conditions were not clear. The single-phase heat transfer results demonstrated that smaller test sections result in higher heat transfer coefficients. However, for heat transfer trends presented in the form of Nusselt number versus Reynolds number, the effect of hydraulic diameter was insignificant.The flow boiling experiments gave similar heat transfer results; they exhibited that the smaller hydraulic diameter channels resulted in higher heat transfer coefficients. The nucleate boiling mechanism was found for all three test sections, evidenced by the significant effect of heat flux on the local heat transfer coefficient. Moreover, the heat flux had a clear effect on average heat transfer coefficient for the 0.561 mm and 0.635mm test sections, whilst for the 0.438 mm test section, there was no discernible effect. At the same heat flux, increases in mass flux caused heat transfer coefficients to decrease. This could be due to the decrease of pressure inside the test section. When a higher mass flux was tested, the inlet pressure increased, and in reducing the inlet pressure to the original value, a decrease in system pressure resulted. Consequently, the outlet pressure and local pressure became lower. Existing flow pattern maps, flow boiling heat transfer and pressure drop correlations were compared with the experimental results obtained for all three test sections. The comparison showed that the flow pattern map proposed by Sobierska et al. (2006) was the most successful in predicting the experimental data. The local heat transfer coefficient data were compared with existing published correlations. The correlations of Yu et al. (2002), Qu and Mudawar (2003) and Li and Wu (2010) are found to predict the current local heat transfer coefficient better than other correlations tested. Pressure drop results showed that as the heat flux and mass flux were increased, the two-phase pressure drop increased too. These were due to the increase in bubble generations and the inertia momentum effect. As the channel was reduced, the twophase pressure drop increased because the pressure drop related inversely with the channel hydraulic diameter. The pressure and pressure drop fluctuations were indentified in this project, however, the maximum pressure fluctuation was found in the 0.438 mm channel whilst the minimum fluctuation was attained in the 0.561 mm channel. This indicated that the effect of decreasing in hydraulic diameter on pressure and pressure drop fluctuations is not clear and needs to be investigated further. The two-phase pressure drop data were compared with selected correlations. The Mishima and Hibiki (1996)’s correlation was found to predict the current two-phase pressure drop better than the other correlations examined in this study.

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Current concentrated solar technologies are not cost-effective means of generating electricity and would benefit greatly from higher operating temperatures. To reach these temperatures at high efficiencies, a novel receiver design should be used in the plant. As a first step in the design of such a receiver, a sensitivity analysis is useful to determine what parameters most affect the performance of a generic cavity receiver. The results of this sensitivity analysis can be used to develop an optimized cavity receiver geometry intended to operate at a high efficiency (>80%) at extreme temperatures (1,350°C). It was found that limiting re-radiation and convection through the cavity aperture most improve the performance of the receiver; at the same time, the receiver must be designed in such a way as to minimize thermal stresses. Descriptions of many experimental components that have been developed to allow for a successful test of a laboratory scale receiver are also included in this thesis. Additionally, presented here are the results of some initial experiments intended to validate the simulations used to perform the aforementioned sensitivity analysis. Finally, some remarks are proffered detailing additional steps and considerations necessary to scale up the receiver design to an industrial scale.

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