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Heat management in high thermal-density systems such as CPU chips, nuclear reactors and compact heat exchangers is confronting rising challenges due to ever more miniaturized and intensified processes. While searching for heat transfer enhancement, micro-channel flow boiling and the usage of high thermal potential fluids such as nanofluids are found to be efficient heat removal approaches. However, the limited understanding of micro-scale multiphase flows impedes wider applications of these techniques. In this thesis work, liquid-vapour phase change and multiphase flow heat transfer in micro-channels were experimentally investigated. Included are studies on the single phase friction, vapour dynamics, liquid meniscus evaporation, two-phase flow instabilities and heat transfer. An experimental system was built. Rectangular microchannels with different hydraulic diameters (571 μm, 762 μm and 1454 μm) and crosssectional aspect ratios were selected. Transparent heating was utilised by coating the micro-channels with a layer of tantalum on the outer surfaces. FC-72, n-pentane, ethanol, and ethanol-based Al2O3 nanofluids were used as working fluids. Pressures and temperatures at micro-channel inlet and outlet were acquired. Simultaneous visualisation and thermographic profiles were monitored. Single phase friction of pure liquids and nanofluids mostly showed good agreement with the conventional theory. The discrepancies were associated with hydrodynamic developing flow and the early transition to turbulent flow, but nanoparticle concentration showed minor impact. After boiling incipient, the single vapour bubble growth and flow regimes were investigated, exploring the influences of flow and thermal conditions as well as the micro-channel geometry on vapour dynamics. In addition, liquid meniscus evaporation as the main heat transfer approach at thin liquid films in micro-channels was studied particularly. Nanoparticles largely enhanced meniscus stability. Besides, flow instabilities were analyzed based on the pressure drop and channel surface temperature fluctuations as well as the synchronous visualization results. Moreover, study on flow boiling heat transfer was undertaken, the corresponding heat transfer characteristics were presented and the heat transfer mechanisms were elucidated. Furthermore, ten existing heat transfer correlations were assessed. A modified heat transfer correlation for high aspect ratio micro-channel flow boiling was proposed. The crucial role of liquid property and microchannel aspect-ratio on flow boiling heat transfer was highlighted.

Direct injection spark ignition (DISI) engines are the next generation of gasoline fuelled engines. Their greater fuel economy and reduced CO2 emissions compared with port fuel injection (PFI) engines has led to their popularity. However, DISI engines produce a greater number of particulate matter (PM) emissions than PFI engines. Concern over the health effects of PM emissions, and forthcoming European legislation to regulate them from gasoline powered vehicles has led to an increased interest in the study of PM formation, measurement, and characterisation. A model was developed by Aikawa et al, the PM index, correlating PM emissions with fuel composition. PM emissions are thought to be linked both to the vapour pressure (VP) and the double bond equivalent (DBE) of the components of the fuel. However, there was no independent control of these parameters and the study was undertaken on a PFI engine. In this thesis, experiments have been conducted to validate this model and extend it, as the PN index, to DISI engines. Fuels have been designed using Raoult’s law and UNIFAC (with careful consideration of octane number) such that the DBE and VP of the fuel mix could be varied independently. The design of the fuels was such that the component parts would co-evaporate upon injection into the cylinder, ensuring a homogeneous mixture of the components at the point of ignition. The PN index has been tested on a single cylinder engine, at a matrix of test points, using these model fuels, and their PM emissions have been analysed using a Cambustion DMS500. The results show that the PN index is followed closely using model fuels, provided that these model fuels contain a ‘light-end’ (in this case 5 % v/v n-pentane). Imaging of in-cylinder evaporation and in-cylinder measurement of hydrocarbons shows how the composition of model fuels affects their PM emissions. The PN index has also been tested using commercial fuels on a single cylinder engine and a Jaguar V8 engine; the results again show that the PN index is also an excellent predictor of PN emissions for market fuels from both of these engines. PN emissions have been evaluated from two fuels representing the EU5 reference fuel specification, developed using the PN index to give a difference in PM emissions. Testing these fuels on both a single cylinder engine and a Jaguar V8 engine has shown up to a factor of three variation in observed PN emissions. This has important implications for forthcoming European emissions legislation. The results of these tests were fed into the recommendations for the EU6 reference fuel specification. The PN index has also been investigated in a Jaguar V6 engine with five different fuels with a spread of calculated PN indices over a simulated NEDC. Here the PN emissions have been measured using two PN, and one PM instrument and the results compared. The results show that the trends of the PN index are followed, but not as closely as predicted. Detailed analysis shows that this discrepancy is due to other effects, for example cold start, dominating the PN emissions in certain phases. PN emissions have been measured from a highly boosted engine at a variety of operating points using 14 different fuels. It has been shown that for a large variety of engine operating parameters PN emissions from highly boosted engines behave as expected. When changing the fuels, the results show that a variation of over three orders of magnitude can be observed. The predictions of the PN index are inconclusive however, with further work suggested to fully evaluate the PN index on highly boosted engines.

The increasing demand for the domestic refrigeration system urges the development of greener form of refrigeration. The eighty-year-old single pressure absorption refrigeration system invented by Albert Einstein and Leo Szilard is attractive as it has no mechanical moving parts and can be driven by heat alone. However, the literature on either the refrigeration system or its components is scarce. The bubble pump is the crucial component of the refrigeration system, but it is poorly understood as its mass flow rate cannot be readily predicted. Two new time correlations in the mass flow rate prediction are presented to increase the accuracy when heat losses occur in the bubble pump. These time correlations are verified with the experimental results. When either the heat input or submergence ratio increases, the accuracy of the prediction increases. The percentage of error for the high heat input or submergence ratio is within ±10%. Working conditions and system dimension have a direct influence to the bubble pump performance. For instance, the bubble pump experimental results show that the mass flow rate of the bubble pump increases when either the submergence ratio or the concentration of ammonia increases. However, the performance of the bubble pump drops when the tube diameter or the system pressure increases. The Einstein refrigeration system has only been rebuilt once since it invention. In order to redesign and rebuilt a practical Einstein refrigeration system, some challenges are revealed. With the combination of the good features of the designs of Einstein and Delano, a new prototype has been rebuilt and tested. The practical results obtained from the five experimental setups are the first set of experimental result that has ever been presented. The highest cooling capacity and coefficient of performance (COP) obtained are 5 W and 0.04 respectively.

Large-scale bed features are often encountered in coastal waters, and include sandbanks and spoil heaps. The morphodynamic development of such features involves complicated nonlinear interactions between the flow hydrodynamics, sediment transport, and bed profile. Numerical modelling of the morphodynamic evolution and migration of large-scale bed features is necessary in order to understand their long-term behaviour in response to changing environmental conditions. This thesis describes detailed measurements of the morphodynamics of sand mounds in unidirectional and oscillatory (tidal) flows, undertaken at the U.K. Coastal Research Facility (UKCRF). High quality data were collected, including water velocities, water levels and overhead images. The parameters tested are: three types of mound shape (circular and elliptical in plan shape, and Gaussian, cosine and triangular in cross-section); underlying fixed or mobile bed conditions; and initial crest height (submerged, surface-touching and surface-piercing). Peak flow velocities are about 0.5 m/s, the sand median grain size is 0.454 mm, and transport occurring mostly as bedload. When analysing the data, the bed contours are determined by digitising the shoreline at different water levels. From these plots, the volume, height, and centroid position of the mound are calculated. A large-scale fit method, based on a Gaussian function has been used to separate small-scale ripples from the large-scale bed structure during the evolution of an isolated sand mound or spoil heap. The bed profile after the ripples are removed is comparable to typical predictions by shallow-flow numerical solvers. The UKCRF experiments investigated the morphodynamic response of a bed mound to hydrodynamic forcing: shape changes, migration rates, volume decay and sediment transport rates. The measured migration rate and decay of a submerged sand mound in the UKCRF are found to be in satisfactory agreement with results from various theoretical models, such as the analytical solution derived by De Vriend. Numerical predictions of mound evolution by a commercial code, PISCES, are also presented for a fully submerged sand mound; the bed evolution is reasonably similar to that observed in the UKCRF. The data provided as a result of the research reported in this thesis provide insight into the behaviour of sand mounds in steady and unsteady flows at laboratory scale, and should also be useful for benchmark (validation) purposes to numerical modellers of large-scale morphodynamics.