Mechanical Engineering - Theses
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ItemUnderstanding loss mechanisms in turbomachinery to increase efficiency( 2016)The improvement in the design of any mechanical device is always carried out to increase its efficiency. Turbomachinery, more specifically, the low pressure turbine (LPT) in aircraft engines, is no exception. In order to enhance its efficiency, the specific fuel consumption has to be reduced, which implies different loss mechanisms (mechanisms of entropy generation) in the LPT have to be studied. Conventional loss calculations consider a control volume around the blade and find the total loss in that region. However, there are a number of sources of losses around LPT blades, prominently the mixing out of incident wakes from the stator to the rotor. In order to design more efficient blades, knowing the total loss in the blade control volume is not enough. It is important to quantify the various sources of loss due to wake mixing. Denton has derived an analytical expression for losses due to mixing, which consists of 3 terms - losses from the trailing edge region, losses due to boundary layer effects, and losses due to blade blockage effects. This equation, however, is built upon a number of assumptions such as steady, incompressible flow conditions which are not realised for real flows. Denton does not define a distinctive trailing edge region, methods to calculate boundary layer thickness and base pressures. In order to enhance the applicability of Denton’s mixed out loss equation, it is important to identify the dependence of Denton’s equation on these parameters; which is an important objective of this work. This work aims to improve the robustness of Denton’s equation by analysing the effect of not defining the trailing edge region properly on the total loss; and then proposes 4 criteria to define the trailing edge region. This analysis was conducted on a LPT blade with steady flow conditions and it was found that the losses calculated with the aid of the trailing edge criteria lie within 5% of the loss in the blade control volume. Sensitivity analyses on the total loss from Denton’s equation has been conducted using boundary layer thickness criteria, base pressure and non-uniform flow averaging techniques as input parameters. It was found that there are large variances in the total loss, if the boundary layer thickness is not defined correctly, and therefore researchers need to be very careful and consistent in their selection of boundary layer thickness. Visual observation, along with the boundary layer thickness criteria identified in this work, will serve as a good method to determine the boundary layer thickness. Denton’s equation does not deal with unsteady dissipation effects. In order to analyse those effects, an analysis has been conducted on an unsteady flow field consisting of incoming wakes; which has been broken up into 20 quasi-steady phases. Denton’s losses were calculated for each of these phases in an attempt to understand the effects of unsteady flow on Denton’s equation. Based on the analysis conducted, it was concluded that better averaging techniques are required to average the losses from the 20 phases to quantify the losses due to unsteady effects. Overall, Denton’s analysis, which is dependent on a number of criteria, has the potential to give a good rough estimate of the loss source. Due to the absence of more accurate methods, it is helpful for designers; and if conducted by the same user, allows for good qualitative comparison of different flow configurations. It is however difficult to quantify the exact amounts of losses based on this method. For a quantitative comparison with total losses in the control volume, it is probably necessary to come up with new ways of quantifying the different sources. The report provides ideas for future analyses to further improve the understanding of loss mechanisms via Denton’s equation and other methods.
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ItemInstabilities of canopy flows( 2018)Canopy flow is theoretically studied using three approaches from the literature that use empirically obtained mean flows. The first approach neglects the presence of the canopy and studies the stability of the mean flow. The second approach adds a canopy drag source term to the models in the first approach. The third approach treats the canopy as a porous medium by solving Darcy’s equation within the canopy, solving Rayleigh’s equation outside the canopy and coupling these solutions at the region the canopy loses its influence on the mean flow. The resulting dispersion relations show that the drag source term approach performs worse than when neglecting canopy drag, if the results are compared with experimental correlations. The dispersion relations also show that the porous medium approach is the best predictor of the dominant wavenumber. The perturbations in the vertical and streamwise directions as well as the vertical-streamwise perturbation correlations are obtained analytically and numerically. A PIV experiment currently in progress is aimed to verify the relevant model that agrees with experimental perturbation results or improve the canopy drag models currently in use.