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The combustion of clouds of fuel droplets is of great importance in many industrial applications, such as gasoline and diesel engines, gas turbines and furnaces. Here, efficient combustion has to be combined with minimum noxious emissions. Aerosols also might produce a particularly hazardous explosion risk. To optimise their performance a fundamental understanding of the complex processes in aerosol combustion systems is necessary. A fundamental study of aerosol combustion has been conducted to quantify the parameters of importance. For this, a novel aerosol combustion apparatus was developed, that offers a well controlled environment with respect to aerosol properties, temperature, pressure and turbulence. Aerosols were generated using the Wilson cloud chamber principle of expansion cooling, which produces a homogeneously distributed, near monodisperse droplets cloud. Drop sizes of 10 to 30μm, pressures between 100 and 360kPa and temperatures of 263 to 292K were used. Laminar mixtures between the overall equivalence ratios of 0.8 and 1.2 were studied. A considerable burning velocity enhancement of up to 420% was observed. This enhancement was shown to be a function of drop size and liquid fraction. From the study, it was concluded that burning velocity enhancement probably is caused by the increase in surface area due to wrinkling, caused by the development of instabilities. At low temperature (<275K) the formation and destruction of wrinkles and cells was random. At higher temperatures (>290K) cell formation and division was progressive and traceable, like that observed in gaseous flames. Cellular acceleration at these temperatures was similar to that of gaseous flames. Stretch appeared to have a damping effect on the instabilities, caused by the aerosol. Oscillating flames were observed for some experimental conditions and these also showed enhanced flame speeds. These oscillations were possibly caused by aerodynamic interaction between droplets and gas motion ahead of the flame. Also Stretch and radiation probably influenced these oscillations. Inert glass particles in a gaseous fuel-air mixture had no effect on flame speed or structure. However, water aerosols caused significant burning velocity enhancement (50%). These findings contradict the hypotheses that fuel rich pockets, flame propagation through "easy-toburn" regions or a "grid-effect" trigger instabilities in aerosols. Comparison with a linear stability analysis of heat loss from the flame (Greenberg et al.,1998), yielded good qualitative agreement with the data of the present work.

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A novel concept which couples ID and 2D CFD models in a simulation of unsteady lC engine flows was investigated, and such a coupled model was developed. Two unified solution procedures which are capable of predicting mixed compressible and incompressible flow fields found in an engine were developed and comparatively studied. One is the pressure correction algorithm, the other is the block implicit algorithm. They provided platforms for the implementation of coupled models. Second order spatial and Euler backward time differencing schemes were adopted. The comprehensive comparative studies were performed on a variety of benchmark flows ranging from steady to unsteady, incompressible to compressible. The data documented have shown that the prediction qualities of the two algorithms were comparable in all calculations. The block implicit procedure required more storage memory generally but it converged faster in all cases except the incompressible flow calculations. General strategies to couple the ID CFD model with the 2D CFD model in one calculation were proposed. They were successfully incorporated in both of the unified solution procedures. The predictions from these coupled models for a series of unsteady benchmark flows were competitive in quality with those from single 2D CFD models, however, the computing costs involved were comparatively much lower. In these calculations, the coupled models integrated in the block implicit procedure produced faster convergence than those in the pressure correction procedure, but required more computing resource. In addition, the implicit coupling stragety was more efficient compared to the explicit counterpart. A ID and 2D coupled model integrated in the pressure correction procedure was applied to simulate a realistic cylinder-valve-pipe flow. The overal prediction quality is satisfactory compared with experimental measurements. Some discrepancies which occurred were largely attributed to numerical representations of valve mechanism and the lack of turbulence models. For this engine application, the coupled model has shown advantages in computing cost or straightforwardness over a conventional uniform 2D model or boundary condition model.

A Hilton combustor was substantially modified to a suitable symmetrical configuration for research purposes. Provisions for swirl, preheat and injection of LCV gases were incorporated with appropriate burner management systems for safe operation. Instrumentation for temperature, velocity and concentration measurements was developed and fully automated by interfacing to a microprocessor for rapid data acquisition. Flame stability limits were determined over a wide range of operating conditions by varying swirl, secondary air temperature and excess air levels while maintaining the burner momentum constant. Addition of swirl up to a limit of O~ 69 generally improved stability. Preheating secondary air alone was beneficial only if the temperature was raised to at least 250 oC. A combination of intermediate swirl and moderate preheat of the secondary air resulted in satisfactory flame stability over a wide range of calorific values of the fuel. Thus, existing concentric pipe burner systems may be easily modified at low cost to burn LCV gases of variable compositions. With LCV gas flames the excess air factor (EAF) had a major influence on values of temperature, species concentration and velocity. Unburnt hydrocarbon (UHC) and CO not surprisingly increased in concentration close to the blow-off limits under the majority of operating conditions. This indicated incomplete combustion probably resulting from the lowered flame temperatures and partial flame lift-off. On the other hand, burnout efficiencies at the exhaust were reasonably high for most operating conditions involving LCV gases. The combustion data were analysed to extract the characteristic mixing and chemical reaction times the ratio of which gave the parameter epsilon, originally proposed for unconfined flames. Close to the blow-off limit epsilon took the value 4.9 compared with 6.2 for fully stable flames. This finding showed that the criterion was also valid for confined flames, supporting the extinction mechanism proposed by Peters and Williams, and providing an important basis for predicting stability limits and burner design parameters.

Two nominally 15, and 30 kW Helmholtz-type pulsed combustors were designed and constructed. These were bench mounted with the heat exchangers (i.e. combustion chamber and tailpipe) immersed in the water bath. Their design was based upon the design of a nominally 7.5 kW pulsed unit previously developed at Middlesex University. The design enabled the lengths of the combustion chambers to be varied so that various combustion chamber volumes could be achieved. This provided a new dimension to the study of pulsed combustors which is lacking in many reported works. It was found that the required input rates could be achieved by scaling up or down each combustion chamber dimensions linearly by a factor of 1.5, while maintaining the geometry identical. Tests showed that the present design of pulsed combustors can operate successfully at various input rates of mains natural gas (93 % methane) with a maximum turn-down ratio of 1.8:1. Results indicated that the three developed combustors would generally operate in the fuel-lean condition. Interestingly, these tests revealed that the amount of excess air reduced as the combustion chamber volume (CCV) was increased. Systematic investigation on the three developed combustors showed that the temperature within the combustor was principally controlled by the air-to-fuel ratio (A/F). Analysis of the average measured NOx concentrations at various operating conditions indicated that NOx emission in this type of pulsed combustor is principally controlled by combustion temperature with no significant influence of combustion chamber volume, tailpipe length or scale of the combustors except in so far as these influenced the A/F and hence the temperature within the combustor. The dominant role of temperature on NOx production from these combustors become more evident when nitrogen or argon was injected into the system resulting in reduced NOx emissions at a given A/F. Systematic analysis of data indicated that as the amount of diluent increased, the temperature within the combustor decreased. Almost all the NOx values recorded were in the form of NO which is believed to be as a result of the high flame temperature (typically above 1850K). The minimum recorded NOx value was 5 ppm at the upper limiting value of excess air ratio, λ ; importantly it was round that at these high A/F values there was no significant reduction in overall efficiency of the pulsed units, showing calculated values above 90%. Analysis of data indicated that combustion temperature is also a primary factor controlling CO emissions from the present design of pulsed combustors. CO concentrations exhibited U-shaped characteristics when plotted vs λ, showing maximum values at the lowest and highest λ values. By changing water bath temperature (WBT) and hence modifying heat losses to the combustion chamber wall, it was shown that the quenching of the combustion reactions and incomplete mixing of air and gas prior to combustion are contributing factors to CO formation in this type of pulsed combustor. The developed pulsed combustors were operated successfully with standard test gases. The composition and flame stability of these test gases were similar to the standard test gases G21 (incomplete combustion gas), G222 (light back gas) and G23 (flame lift gas). Analysis of the exhaust gas composition showed similar trends to those obtained when burning mains natural gas; as the heat input was increased, O2 levels decreased while CO2 and NOx emission levels increased. Similarly, CO concentrations showed U-shaped characteristics when plotted against firing rate. Measurements of peak pulsing pressure and frequency were used as a guide to operation and stability performance of the pulsed units. It was found that the operating frequency was a function of configuration of the combustors and temperature of the internal gases. Frequency of operation showed a reciprocal correlation with volume of combustion chamber and tailpipe length and increased as the heat input was increased. Pulsing pressure amplitude also was influenced by change of configuration of the combustors, increasing as the CCV and tailpipe length were decreased. Analysis of experimental data obtained at fixed configuration of the combustors showed that the peak pulsing pressure was a strong function of the heat release per cycle in the present design of pulsed combustors. A major drawback of the use of pulsating combustors is the high noise level which is associated with their operation. It was found that it is possible to reduce overall noise levels of the pulsed burners to acceptable values by configuring the system appropriately. This included the use of expansion chambers at the inlet and the exhaust outlet which reduced the overall noise levels to a minimum value of 65 dBA.

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