This thesis discusses the production and use of laser-machined boiling grids that provide controlled nucleate boiling and enhanced heat transfer characteristics for application primarily to IC engine cooling systems. The surface features of heated plates are known to have a significant effect on nucleate boiling heat transfer and bubble growth dynamics. Nucleate boiling starts from discrete bubbles that form on surface imperfections, such as cavities or scratches. The gas or vapours trapped in these imperfections serve as nuclei for the bubbles. After inception, the bubbles grow to a certain size and depart from the surface. The bubble departure process significantly increases heat transfer rates compared to pure convection. In this work, special heated surfaces were manufactured by laser machining cavities into polished aluminium plates. This was accomplished with an Nd:YAG laser system, which allowed drilling of cavities of a known diameter. The size range of cavities was 25 to 300 micrometers. The resulting nucleate pool boiling was analysed using a high-speed imaging system comprising an infrared laser and high resolution CCD camera. This system was operated up to a 2 kHz frame rate and digital image processing allowed bubbles to be analysed statistically in terms of departure diameter, departure frequency, growth rate, shape and velocity. Data were obtained for heat fluxes up to 150 kW.m'2. Bubble measurements were obtained working with water at atmospheric pressure. The surface cavity diameters were selected to control the temperature at which vapour bubbles started to grow on the surface. The selected size and spacing of the cavities was also explored to provide optimal heat transfer. Insights into the interaction and interseeding mechanism were obtained. The research has demonstrated that nucleate boiling can be controlled by optimally sized and spaced laser-machined cavities in heated metal surfaces.

An investigation has been carried out into the formation of nitric oxide in high pressure combustion environments. Experiments have been conducted using a constant volume combustion bomb, which enabled the effect of pressure to be decoupled from the effect of temperature. Experiments have also been performed using a single cylinder spark-ignition engine to provide data for comparison. A thermodynamic and multi-reaction chemical kinetics model has been developed to calculate burned gas temperatures and NO concentrations. The model has been used to assess the effects of thermochemical data, kinetic rate data, radical concentrations and various chemical reactions on predicted NO concentrations, and the results have been compared to measured NO data. A search of the literature revealed that existing models describing NO formation in engines often fail to predict measured concentrations of NO, especially under high load conditions. Although the effect of pressure on NO formation has previously been subject to theoretical study, the mechanisms of NO formation at high pressure remain subject to uncertainty and in some cases empirical modifications to theoretical data have been used to obtain agreement with experimental results. The combustion bomb experimental results showed clear evidence that an increase in pressure causes a decrease in NO concentrations under lean conditions. However, this effect was not observed in the engine owing to the high temperatures that were generated under high load conditions. It was found that the pressure effect was less significant under stoichiometric and rich conditions in the combustion bomb. The model showed that the commonly-used extended Zeldovich mechanism was unable to accurately predict NO concentrations in non-stoichiometric conditions in either the bomb or the engine. It was found that the time evolution of temperature had a significant effect on calculated NO emissions, with high temperatures at the end of combustion generating much higher NO concentrations than high temperatures earlier in the combustion process. An existing comprehensive model of NO kinetics, the super-extended Zeldovich mechanism, has been subject to a sensitivity study which found that only a small number of reactions play a significant role in NO formation and destruction under the conditions tested here. calculations showed that reactions involving N02 made a significant contribution to NO formation and destruction under lean conditions, and accurately predicted NO concentrations under lean conditions in the bomb. However, the same reactions were found to accelerate NO formation in the engine.

This thesis is addressed to the problem of predicting the emission of exhaust smoke from the diesel engine. A simulation program based on a zonal phenomenological combustion model has been developed, permitting analysis of soot modelling techniques. For the first time, a comparative study of the common soot model expressions has been undertaken. Model sensitivities and behaviour have been critically assessed in order to determine the key model parameters and to establish a more solid predictive capability. Validation of both the combustion and soot predictions was made by means of comparison with the extensive experimental data-set of Kamimoto. The combustion model results showed a very good match between -predicted and experimental heat release curves. The only notable weakness derived from the method chosen to represent the effect of air swirl on the jet. Otherwise, the combustion predictions were deemed to be sufficiently accurate to serve as an effective platform for soot model development and analysis. The predictions of exhaust smoke for different operating conditions revealed the importance of accurately describing the overall air-to-fuel ratio in the spray. The effect of load variation was poorly represented due to neglect of the transfer of combustion products between the model zones. Soot rate predictions were generally quantitatively poor, thus requiring expression calibration. The comparative study of soot expressions identified a ranking of sensitivities of the formation expressions. Though oxidation is conceptually simpler, more distinct qualitative differences were observed in the behaviour of the expressions. The predictions of exhaust soot were found to be highly sensitive to the 'matching' of the formation and oxidation expressions over the period of the combustion process, and with poorly matched expressions, a very high sensitivity to the soot model constants was shown. The best results were obtained by use of simple quasi-chemical rate expressions.

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No description available.

A design-point thermodynamic model of the Brayton-cycle gas-turbine under assumptions of perfect chemical equilibrium is described. This approach is novel to the best knowledge of the author. The model uniquely derives an optimum work balance between power turbine and nozzle as a function of flight conditions and propulsor efficiency. The model may easily be expanded to allow analysis and comparison of arbitrary cycles using any combination of fuel and oxidizer. The model allows the consideration of engines under a variety of conditions, from sea level/static to >20 km altitude and flight Mach numbers greater than 4. Isentropic or polytropic turbomachinery component efficiency standards may be used independently for compressor, gas generator turbine and power turbine. With a methodology based on the paper by M.V. Casey, “Accounting for losses” (2007), and using Bridgman’s partial differentials , the model uniquely describes the properties of a gas turbine solely by reference to the properties of the gas mixture passing through the engine. Turbine cooling is modelled using a method put forward by Kurzke. Turboshaft, turboprop, separate exhaust turbofan and turbojet engines may be modelled. Where applicable, optimisation of the power turbine and exhaust nozzle work split for flight conditions and component performances is automatically undertaken. The model is implemented via a VB.net code, which calculates thermodynamic states and controls the NASA CEA code for the calculation of thermodynamic properties at those states. Microsoft Excel® is used as a graphical user interface. It is explained that comprehensive design-point cycle analysis may allow novel approaches to off-design analysis, including engine health management, and that further development may allow the automation of cycle design, possibly leading to the discovery of opportunities for novel cycles.

The successful design of cooling systems for gas turbine engines is a key factor to feasibility of new projects, as the trend for increasing turbine entry temperatures implies requirements for more sophisticated cooling methods. This work focuses on the prediction of cooling performance of turbines, starting from local heat transfer effects at the surface of blades and vanes and expanding to performance simulation of cooled high pressure turbines and engines. In this context, this thesis establishes a new method that investigates the following topics: • The connection between the gas flow field around a cooled blade or vane and the prediction of cooling requirements of the setup. • The connection between a detailed gas flow field around a cooled blade or vane and a preliminary estimation of its metal temperature. • The effect that blade cooling requirements prediction has towards the performance simulation of a cooled turbine and the difference in results between turbine models of different axial resolution. • A simulation platform that includes the aforementioned topics under a web-based gas turbine performance simulation program. The first two objectives are tackled by developing a preliminary cooling design framework, which performs the needed convective and conductive heat transfer calculations between the gas and the blade, the blade and the coolant, and within the blade material. The method divides the geometry into a finite number of volumes, where heat transfer calculations are performed for steady-state conditions. One- and two-dimensional results show a good agreement with previous experimental work. The results suggest that chord resolution for blade heat transfer prediction is essential for a more accurate coolant temperature and mass flow rate prediction. In addition, conduction modelling has a dominant effect in heat transfer prediction of blades with steep temperature gradients. The third objective is achieved by associating the coolant state before mixing with the main stream and the results in turbine performance. The coolant temperature and mass flow rate prediction have a significant impact on turbine work and thermodynamic efficiency, figures highlighted as well for different turbine axial resolution methods. The results suggest that as the coolant heats up through a blade or vane and eventually mixes with the main flow, it contributes significantly towards the predicted turbine work, affecting as well the overall engine performance results, such as specific fuel consumption and specific thrust. A multistage turbine model is most suitable for capturing these effects, but it requires a number of additional inputs. Finally, the thesis suggests that a simulation framework such as the aforementioned, it can be of high usability and applicability if implemented on the cloud, rather than locally installed.

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No description available.