A bluff-body has been employed as the flame stabilization scheme for many combustion devices such as gas turbines and aviation engines. Although the bluff-body flame holder has a key advantage of generating a hot gas recirculation zone behind it and assist in stable combustion, it also induces flow field and combustion instabilities such as unstable vortex shedding, which can adversely affect the flame stability and lead to blow-off. The understanding of the physical mechanism of flame stabilization and blow-off processes has been one of the critical subjects in premixed combustion systems under highly turbulent conditions. As considering this, the present dissertation presents insight of flame stabilization and blow-off mechanisms using several series of computational studies and detailed analysis using diagnostic approaches. Two-dimensional direct numerical simulations are conducted to examine flame/flow and blow-off dynamics in lean premixed hydrogen-air and syngas-air flames stabilized on a meso-scale bluff-body in a square channel. Several distinct effects on flame stabilization and blow-off dynamics are investigated, such as reduced confinement, hydrodynamic instability, flame time scale, and differential diffusion effects. For the analysis, a proper time scale analysis is attempted to characterize the flame blow-off mechanism, which turns out to be consistent with the classic blow-off theory of Zukoski and Marble. The combined approach of computational singular perturbation and tangential stretch rate is applied to examine chemical characteristics in blow-off dynamics. As an extension from Eulerian to Lagrangian viewpoint, Lagrangian particle tracking analysis of post-processing the pre-computed results is performed to examine the local characteristics during the critical transient event of local extinction and recovery.

The compromise between ride comfort and handling for a passive suspension system is a well-known and often researched problem. Semi-active suspension systems offer significant improvements to this compromise. One example of a semi-active system, that can change both spring and damper characteristics between two discrete values is the 4-state semi-active hydro-pneumatic suspension system. This system can switch between a ”ride comfort mode” (soft spring and low damping) and a ”handling mode” (stiff spring and high damping) within 100ms, improving both ride comfort and handling. The discrete 4S4 could be improved upon further by adding continuous variable damping. Work on this topic showed great promise but also posed its challenges in achieving this in a safe and controllable manner. In order to make continuous variable damping a reality a new configuration for the 4S4 is proposed. This new configuration incorporates a blow-off damper in parallel with a proportional flow control valve. The system ensures that, in the highly non-linear closing region of the proportional flow control valve, adequate damping for handling is maintained and uncontrollable peak pressure differences are avoided. Experimental work conducted showed that the system was capable of achieving the required spring and variable damping characteristics in a safe and controllable manner. The experimental data was used for parametrizing and validating a physics based mathematical model of the suspension system. The mathematical model incorporates the: pressure drop vs: flow characteristics for both the blow-off and proportional valves, response time for the on-off valves as well as the gas pressure vs: flow characteristic incorporating the compressibility of the oil and thermal properties of the gas. This model can be used to make informed decisions on further prototype development or in full vehicle simulations. The system makes continuous variable damping possible ranging from the optimal damping characteristic for handling to the low damping characteristic required for ride comfort. The system also shows a significant reduction in friction. / Dissertation (MEng)--University of Pretoria, 2020. / VDG / University of Pretoria / Mechanical and Aeronautical Engineering / MEng / Unrestricted

This thesis describes an experimental investigation of the response of lean turbulent swirling flames with different degrees of premixedness (i.e. different mixture patterns) to acoustic forcing using the same burner configuration and varying only the fuel injection strategy. Special emphasis was placed on the amplitude dependence of their response. Also, the behaviour of self-excited fully premixed flames was examined. kHz OH* chemiluminescence was used to study qualitatively the heat release response of the flames, while kHz OH Planar Laser Induced Fluorescence (PLIF) was employed to understand the response of the flame structure and the behaviour of the various parts of the flame. The Proper Orthogonal Decomposition (POD) method was used to extract the dominant structures of the flame and their periodicity. In the first part of the thesis, self-excited oscillations were induced by extending the length of the duct downstream of the bluff body. It was found that the longer the duct length and the higher the equivalence ratio, the stronger the self-excited oscillations were, with the effect of duct length being much stronger. The dominant frequencies of the system were found to increase with equivalence ratio and bulk velocity and decrease with duct length. For some conditions, three simultaneous periodic motions were observed, where the third motion oscillated at a frequency equal to the difference of the other two frequencies. A novel application of the POD method was proposed to estimate the convection velocity from the most dominant reaction zone structures detected by OH* chemiluminescence imaging. For a range of conditions, the convection velocity was found to be in the range of 1.4-1.7 bulk flow velocities at the inlet of the combustor. In the second part, the response of fully premixed, non-premixed with radial fuel injection (NPR) and axial fuel injection (NPA) flames was investigated and compared. All systems exhibited a nonlinear response to acoustic forcing. The highest response was observed by the NPR flame, followed by the fully premixed and the non-premixed with axial fuel injection flame. The proximity of forced flames to blow-off was found to be critical in their heat release response, as close to blow-off the flame response was significantly lower than that farther from blow-off. In the NPR and NPA systems, it was shown that the acoustic forcing reduced the stability of the flame and the stability decreased with the increase in forcing amplitude. In the fully premixed system, the flame area modulations constituted an important mechanism of the system, while in the NPR system both flame area and equivalence ratio modulations were important mechanisms of the heat release modulations. The quantification of the local response of the various parts of the flame at the forcing frequency showed that the ratio RL (OH fluctuation at 160 Hz to the total variance of OH) was greater in the inner shear layer region than in the other parts in the case of NPR and NPA flames. In fully premixed flames, greater RL values were observed in large regions on the downstream side of the flame than those in the ISL region close to the bluff body. The ratio of the convection velocity to the bulk velocity was estimated to be 0.54 for the NPR flame, while it was found to be unity for the respective fully premixed flame. In the last part of the thesis, the response of ethanol spray flames to acoustic oscillations was investigated. The nonlinear response was very low, which was reduced closer to blow-off. The ratio RL was the highest in the spray outer cone region, downstream of the annular air passage, while RL values were very low in the inner cone region, downstream of the bluff body. Unlike NPR and fully premixed flames, in case of spray and NPA systems, it was found that forcing did not affect greatly the flame structure. The understanding of the nonlinear response of flames with different degrees of premixedness in a configuration relevant to industrial systems contributes to the development of reliable flame response models and lean-burn devices, because the degree of premixedness affects greatly the flame response. Also, the understanding of the behaviour of forced spray flames is of great interest for industrial applications, contributing to the development of thermoacoustic models for liquid fuelled combustors. Finally, the estimation of the convection velocity is of importance in the modelling of self-excited flames and flame response models, since the convection velocity affects the flame response significantly.

Possible areas of improving the efficiencies of the Lincoln University flame weeder are identified and investigated. The Hoffmann burner initially used in the Lincoln University flame weeder was found not to entrain sufficient air to allow complete combustion of the LPG used. A new burner, the Modified Lincoln University burner, was designed to improve the entrainment of air. Results show that the new design entrained sufficient air to theoretically allow complete combustion of the LPG, and this resulted in a 22.7% increase in heat output per Kg of LPG used over the Hoffmann burner. Temperature x time exposure constants required to kill weeds 0 - 15, 15 - 30, and 30 - 45 mm in size, were found to be respectively 750, 882, and 989 degrees Celsius.Seconds. These constants can be used to calculate the maximum speed of travel an operator can use a flame weeder at, once the temperature profile underneath its shields are established at various travel speeds, and therefore ensure that the flame weeder is used at its maximum efficiency. The constants can also be used to establish the cost efficiency of any flame weeder (in $/Ha), depending on the size of the weeds to be treated. The materials and methods used in establishing the temperature x time exposure constants can be used to establish the temperature x time exposure constant of any weed species at any size.