Suchergebnisse

Hydropower sites with very low head are recently getting renewed interest with governments providing subsidies to meet the renewable energy targets. This study deals with the investigation of the Dethridge wheel, which was originally meant for flow measurement purposes, for developing power from very low head sites in open channel flow. Two different approaches are taken to assess the potential of the Dethridge wheel for electricity generation. The first is the experimental approach with a physical model of the wheel built and tested in the laboratory environment. The second approach is the three dimensional numerical modelling of the Dethridge wheel using a commercial Computational Fluid Dynamics (CFD) code Flow-3D. The physical model tests show that at constant water levels the rotational speed of the wheel holds a linear relationship with torque as well as with flow rate. Using these linear regression models, full performance curves of the wheel are developed. Uncertainty analysis is carried out for all the measured data. The wheel housing, wheel to channel width ratio, blade numbers, channel transition shapes and the bottom clearance gap were experimentally investigated for their impact on the wheel performance. Original wheel housing is modified to allow side filling and emptying of the blade cells, which led to gain in efficiency. Performance of the wheel is observed to drop with increased number of blades. Wheel to channel width ratio plays vital role on the performance of the wheel. Two times wider channel is desired to allow for the side filling and emptying of the blade cells and thus improved efficiency. Effect of gradual transition is minimal on wheel performance suggesting no requirement of special channel transition profiles. Bottom clearance gap is identified to be important in controlling leakage losses and consequently wheel performance. A three dimensional CFD model of Dethridge wheel is developed. The Renormalization Group (RNG) model is employed for modelling the turbulence. The wheel motion is modelled using the General Moving Object (GMO) model. Numerical model results are assessed for numerical and model uncertainties. Effect of surface roughness, turbulence model, advection scheme, blade thickness and domain symmetry is studied and validated against the physical model results. A rigorous approach of numerical uncertainty analysis called Grid Convergence Index (GCI) method is employed. Impact of blade shape and shroud shape modification is investigated on the numerical model. Modification of both blade and shroud shape led to the improvement in performance of the wheel. The computed results are in good agreement with the measured results. The development of a CFD model enhances our understandings of the complex hydrodynamics of the wheel. The research findings has revealed that Dethridge wheel is a potential device for energy extraction from open channel flows. An efficiency of around 60% is achieved on the physical model tests. Given the simple and robust design of the wheel, this wheel could be implemented for decentralised pico-scale hydropower generation from very low head sites.

Lowering taxes and raising government spending are both forms of expansionary fiscal policy. Often students mistakenly believe that their relative impact on GDP is equivalent, i.e. a dollar increase in government expenditure increases GDP by the same amount as a dollar decrease in taxes. This paper describes a classroom exercise that demonstrates a) how both fiscal stimuli create greater aggregate output and b) which fiscal stimulus spending is more effective. In this exercise, six students come to the front of class and act as business owners, the instructor acts as the government with a stimulus plan, and the remaining students are workers/consumers. Going through a sequence of financial decisions of how much to save and spend, students learn how the initial fiscal shock sets off a chain reaction, leading to successive rounds of changes in spending and income. This simple framework can then be used to evaluate the effect of relaxing standard assumptions, and to demonstrate the impact of automatic stabilizers such as taxes and transfer payments. Finally, this exercise can be used to facilitate a discussion on how the theoretical predictions differ from the actual real-world results due to credit liquidity restrictions, unemployment benefits, inflation, and governmental inefficiency.