Mechanical Engineering - Theses
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ItemEffect of solidity on momentum and heat transfer of rough-wall turbulent flows( 2020)A major area of interest in engineering is the skin-friction drag and convective heat transfer of surfaces in turbulent flow, such as turbine blades, marine vehicles, and airplanes. While these surfaces may appear smooth, they almost always have some form of roughness, for example, pitting on the surface of a turbine blade, barnacles on the hull of a marine vehicle, or rivets on the wings of an airplane. For a given roughness and flow speed, the Moody diagram can be used to find the frictional drag or pressure drop. A similar diagram can be constructed to find heat transfer. Although widely used, the biggest limitation of the Moody diagram is that the Nikuradse equivalent sand grain roughness has to be known for the rough surface in question. Another limitation of the Moody diagram is in predicting skin friction for transitionally rough surfaces, owing to the unrepresentative Colebrook fit. Also, while the effects of varying key roughness topographical parameters on momentum transfer have been studied extensively, relatively little is known on heat transfer. Over the years, researchers have used computational and experimental methods to investigate the flows over a number of roughness types. This thesis expands on the computational works on sinusoidal roughness by systematically investigating the effect of varying roughness solidity on both momentum and heat transfer in turbulent air flow, and the underlying flow physics that give rise to the observed behaviour. Rough-wall flows transfer more momentum and heat when compared to smooth-wall flows, and it is found that an increase in solidity for a matched equivalent sand-grain roughness height causes a greater increase in heat transfer than the increase in momentum transfer due to increased wetted area and increased recirculation region that facilitates mixing.
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ItemFlame Wall Interactions for Flames Diluted by Hot Combustion Products( 2020)Flames diluted by combustion products can reduce emissions such as Carbon Monoxide (CO) and Oxides of Nitrogen (NOx) in industrial applications. In applications such as gas turbines, these flames are confined in a combustor and can interact with relatively cold walls. This interaction can quench the flame, producing incomplete combustion products. In this study, Flame-Wall Interaction (FWI) for methane/air flames diluted by hot combustion products was investigated using Direct Numerical Simulation (DNS). One-Dimensional (1D) Head-On Quenching (HOQ) was first simulated to examine operating parameter effects on CO emissions from transient quenching processes. Average CO within the quenching region was used to evaluate these effects, and the species transport budget was used to investigate the dominant terms. At higher dilution levels, the peak average near-wall CO decreases, and the rate of near-wall CO reduction also decreases. At higher wall temperatures, the peak average near-wall CO and its reduction rate increases. The near-wall CO may be modelled under some conditions using only the integrated diffusion term. Then, a two-Dimensional (2D) laminar V-flame was simulated in both steady and forced conditions. The changes in peak near-wall CO due to varying dilution level and wall temperature show similar trends to the 1D results. The exhaust CO is linked directly to the oxidation residence time, which is determined by the flame length. Due to the role of the flame length, the contribution of near-wall CO to the exhaust CO increases as dilution level or the wall temperature is reduced. Premixed flames can extinguish inside the cold-wall thermal boundary layer, which can leave high near-wall CO. This results in disproportionate levels of CO mass flux in the near-wall regions. The near-wall CO features large variations when the local Damkohler number is greater than 0.1. Analysis of the CO transport budget shows that unlike 1D simulation, both convection and diffusion dominate the CO transport in the near-wall region, except for the case with autoignition at the wall. Finally, a three-Dimensional (3D) turbulent V-flame in a channel was simulated with hot and cold walls. A main reaction zone in the central region supported by periodic bulk ignition events changes the position of volumetric reaction zones where CO is formed. Consistent with the 2D results, a lower wall temperature leads to a longer flame, thereby having more contribution to the exhaust CO. Near-wall turbulence-flame interaction creates wrinkled and streaky flame surfaces, and localizes the near-wall CO distribution. The high mean of CO mass fraction locates in the free-stream where the free-stream autoignition happens, while the high RMS of CO mass fraction is present closer to the wall. 1D flame solutions might be sufficient for modelling CO in the free-stream region and some parts of the near-wall region but not closer to be adjacent to the wall. Turbulent mixing and diffusion contribute to this deviation. These results set a benchmark for future near-wall CO modelling.