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In the current research work, a computational analysis of a high-speed centrifugal compressor stage for turbocharger applications is presented. A detailed investigation about the interactions between backswept impeller and downstream vaneless and vaned diffusers is carried out. ' A unshrouded backswept impeller with splitters was combined with a vaneless diffuser or a number of different designs of vaned diffusers. The CFD solver CFX-TASCow was used. The three-dimensional Reynolds- Averaged Navier-Stokes equations are solved and a pressure correction method is employed to solve the system of equations. A steady simulation and analysis of the interactions between the impeller and the vaneless diffuser is carried out, emphasis is focused on the comparisons of the different interactions at different conditions regarding the flow structures at different radius ratios, effect of rotational speed, mass flow rate and impeller tip clearance. The predicted results were also compared with the available experimental results in terms of radial Velocity, tangential Velocity and flow angle. In general, the predicted results show a reasonable agreement with the experimental data. A steady state simulation and analysis regarding the interaction between the impeller and various vaned diffusers is carried out. For the interface between the rotational impeller outlet and the stationary vaned diffuser inlet, the stage averaging condition is used. A detailed comparison between the predicted and the available experimental data is performed in terms of static pressure rise, total pressure ratio, choking mass flow and efficiency characteristics, and very good agreement is accomplished. In addition, detailed flow distributions are compared, assessed and critically analysed, regarding different number of diffuser vanes, rotational speed, gap between the leading edge of the vaned diffuser and impeller tip, mass flow rate. Emphasis is focused on the steady state study of the effect of the number of diffuser vanes on the stage operating range. Further more, unsteady simulation and analysis regarding the interactions between backswept impeller and downstream vaned diffusers is carried out. In the unsteady simulation, a geometry scaling method is used to modify the diffuser geometry to the nearest integer pitch ratio while keeping the throat area, flow direction and area ratio unchanged in order to deal with the unequal pitch ratio problems which exist in the unsteady simulation. The unsteady investigation was undertaken regarding different number of diffuser vanes, rotational speed, gap between the leading edge of the vaned diffuser and impeller tip, mass flow rate and impeller tip clearance. The detailed interactions at different conditions are compared, assessed and analysed. The studies focus on the analyses of the effect of the different interactions on the stage operating range, peak efficiency, total pressure ratio, level of unsteadiness, flow structures, flow angle or incidence angle, etc. In addition, the' predicted results are compared with available experimental data and a quite good agreement is achieved although the geometry is scaled. On the other hand, a detailed investigation on the differences between the time averaged unsteady simulation results and steady simulation results was performed at different conditions. The comparisons were carried out regarding static pressure, total pressure, speed, flow angle (or incidence angle) and isentropic efficiency. The investigation confirms that unsteady simulation is still quite important, since some of the steady state simulation results are still not similar to the time averaged ones. Designers should take into account the influence of the unsteadiness on the flow fields when they employ the steady state model in the design process.

This thesis reports original research on a novel internal combustion (IC) engine charge air system concept called Turbo-Discharging. Turbo-Discharging depressurises the IC engine exhaust system so that the engine gas exchange pumping work is reduced, thereby reducing fuel consumption and CO2 emissions. There is growing concern regarding the human impact on the climate, part of which is attributable to motor vehicles and transport. Recent legislation has led manufacturers to improve the fuel economy and thus reduce the quantity of CO2 generated by their vehicles. As this legislation becomes more stringent manufacturers are looking to new and developing technologies to help further improve the fuel conversion efficiency of their vehicles. Turbo-Discharging is such a technology which benefits from the fact it uses commonly available engine components in a novel system arrangement. Thermodynamic and one-dimensional gas dynamics models and experimental testing on a 1.4 litre four cylinder four-stroke spark ignition gasoline passenger car engine have shown Turbo-Discharging to be an engine fuel conversion efficiency and performance enhancing technology. This is due to the reduction in pumping work through decreased exhaust system pressure, and the improved gas exchange process resulting in reduced residual gas fraction. Due to these benefits, engine fuel conversion efficiency improvements of up to 4% have been measured and increased fuel conversion efficiency can be realised over the majority of the engine operating speed and load map. This investigation also identified a measured improvement in engine torque over the whole engine speed range with a peak increase of 12%. Modelling studies identified that both fuel conversion efficiency and torque can be improved further by optimisation of the Turbo-Discharging system hardware beyond the limitations of the experimental engine test. The model predicted brake specific fuel consumption improvements of up to 16% at peak engine load compared to the engine in naturally aspirated form, and this increased to up to 24% when constraints imposed on the experimental engine test were removed.

Surface-mounted heat flux sensors have been used in the intake port of a fuel injected, spark ignition engine to investigate heat transfer between the surface, the gas flows through the port, and fuel deposited in surface films. This investigation has been carried out with a single-cylinder engine on which the cylinder head is from a production four valve per cylinder engine with a bifurcated intake port. The objective has been to establish how engine operating conditions affect trends in surface heat transfer rates both with and without fuel deposition on the surfaces, and to relate these to the mechanisms involved in the transport of fuel into the engine. The effects on these mechanisms of injector type and fuel characteristics have also been studied. Fuel transport has been characterised using the τ and X parameters, and experimental studies have been carried out to examine these for fully-warm and warm-up engine operating conditions, with a range of injector types representative of those currently used in service. This data has been compared to the results of a photographic study of the fuel distribution pattern produced by each injector type, and these combined results used to decide upon suitable positions within the inlet port for the heat flux sensors. The dynamic response characteristics of the surface-mounted heat flux sensors have been determined, and measured heat flux data corrected accordingly to account for these characteristics. Details of the model and data processing technique used, are described. Corrected intra-cycle variations of heat transfer to fuel deposited have been derived for engine operating conditions at 1000 RPM covering a range of manifold pressures, fuel supply rates, port surface temperatures, and fuel injection timings. Both pump-grade gasoline and isooctane fuel have been used. The influence on heat transfer rates of the deposited fuel and its subsequent behaviour has been examined by comparing fuel-wetted and dry-surface heat transfer measurements. With both fuel types, the heat transfer rate to the fuel reaches peak values up to around 50 kW/m2 during the engine cycle, and is typically 5 kW/m2 on average in regions of heavy fuel deposition. The effects of operating conditions on the magnitude and features of the heat flux variations are described Integration of this heat flux data has provided values of heat transfer per cycle, allowing direct comparisons of operating condition and injector type effects to be made. For dry-port conditions heat transfer per cycle varies between 0 and 300 J/m2 depending on location, towards the surface at low temperatures and away from the surface at fully-warm conditions. During warm-ups with fuel deposition, as coolant temperature increases from 0 to 90°C, values of heat transfer to the fuel typically increase from 300 J/m2 to 1000 J/m2. For a given coolant temperature, heat transfer values generally increase as manifold absolute pressure (MAP) is lowered or fuel flow rate increases. The effect of fuel deposition on heat transfer has been characterised by a function of MAP, fuel flow rate and coolant temperature. When running on isooctane fuel the heat transfer measurements were made using a heat flux gauge bonded to the intake port surface in the region where highest rates of fuel deposition occur. Heat transfer changes are consistent with trends predicted by convective mass transfer over much of the range of surface temperatures from 20°C to 100°C. Towards the upper temperature limit, heat transfer reaches a maximum limited by the rate and distribution of fuel deposition. The inferences drawn from the isooctane results are discussed and related to characteristics observed when gasoline is used.

The turbocharger is a performance enhancing device which uses the hot exhaust gas mixture from the engine exit to spin the turbine blades for driving the compressor blades to compress the ambient air from the atmosphere. During the operation, the turbine blade surfaces will have severe contaminated deposit accumulations due to the usage of the low-grade diesel fuel for the engine combustion. Hence, this will adversely impact on the turbocharger operating efficiency with some significant performance losses. In this aspect, the development of efficient and cost-effective deposit removal techniques has become one of the urgent tasks for the turbocharger design and manufacture industries. Apparently, an engineering solution to the problem is to apply a built-in “online” water washing system for diesel turbocharger deposits removal. While this method has been used for a while now, the system performance is not entirely satisfactory and further improvements are very demanding. The intention of this study is to review the current know-how industry practice of deposit removal techniques, in particular the “online” water washing technique, in order to understand the underlying mechanisms. The objective is to provide detailed information and flow characteristics for turbocharger designers to improve the system performance. To achieve these targets, a research programme of combining numerical simulation and experimental test has been proposed and carried out on a medium sized diesel turbocharger model to investigate both single-phase gas mixture flow and two-phase gas/liquid flow characteristics. In the latter case, the study is focused on the droplet trajectory, the size distribution and the water droplet coverage area on the blade ring. The experimental tests were based on an in-house test rig at Napier Turbochargers. The measurements were conducted over a wide range of working conditions with and without water washing injections. The test results were used for data analysis and validation of numerical predictions. It was found that in general the water coverage area decreases with the increase of turbocharger loading speed. By using three water injectors evenly distributed in a circumferential direction, maximum ten blades have been washed (i.e. “wetted” blades), while the remaining fourteen blades stayed “dry”. This indicated that more water injectors might be needed to increase the water coverage area on the blade ring. It was also noted that the test rig employed has some limitations of test loading speeds and temperature range, preventing its application to wider operation conditions. The numerical simulations of single-phase gas mixture flow and two-phase gas/liquid flow have been carried out using a commercial computational fluid dynamics (CFD) software ANSYS-CFX with ‘ad hoc’ subroutines of the CFX Command Language (CCL). For a single-phase gas mixture flow, the numerical predicted blade leading-edge temperatures were in good agreement with the experimental measurements with max 1% discrepancies. The results also revealed some influences of three upstream guide vanes on the downstream flow field. For a two-phase flow, the computation has been carried out using the coupled Eulerian/Lagrangian method. Similar to the single-phase flow, simulation results of two-phase flow were generally in good agreement with the experimental observations qualitatively and quantitatively, e.g. the water coverage increases with the decrease of the turbocharger loading speed. For all simulation cases, the water droplet movements in the computational domain have been visualised by the particle trajectory tracking and the 3D iso-surface plotting, etc. A standard conical shaped of water spray cones were clearly seen. Other aspects of the studies were the validation and the assessment of various models adopted in the simulations. The numerical optimisation of the water nozzle configuration was also performed by parametric studies, such as the injector geometry, the injection location and the inclination angle. For a water injector at two inclination angles of 30[sup]o and 90[sup]o against the mainstream flow with different loading speeds, simulation results were in good agreement with the experimental measurements at corresponding test conditions. It was found that for the 90[sup]o inclination angle with the standard nozzle, the number of “wetted” blades was similar to that from the 30[sup]o inclination angle case, but the water coverage area was shifted to the upper half of the blade ring. For the nozzle injection angle at 90[sup]o with a spacer inserted, the numerical predictions have been performed with success and the water coverage has shown some regular patterns as those seen in the standard nozzle simulations. However, surprisingly, this did not entirely agree with the experimental observations, where the water droplet particle distributions on the blade surfaces were randomly distributed without any clear patterns. To understand the underlying cause, some further studies are needed, e.g. the nozzle injection point and the droplet break up model influences. Similar to other numerical simulations, limitations of CFD work are mainly due to the influence of turbulence models and other sub-models used in this study. Further improvement could be done through model refinement study and validation against wider range of test data. In conclusion, the present study has shown that numerical predictions using CFD techniques are generally in good agreement with the experimental measurements in terms of the blade leading-edge temperature distributions and the blade ring water coverage. The flow details such as the water droplet particle trajectory and the size distribution are obtained and they are valuable for the design engineers to improve the current “online” water washing system. Furthermore, the simulation models developed in this study can be used for modelling other conditions that are difficult to perform on the current test rig (mainly due to the safety constraints).