دانلود رایگان مقاله مدل صیقلی از لحاظ فیزیکی مدرن و ارتقاء یافته برای افزودنی حباب جت هوا

Abstract

Hydropower is a clean, renewable, and environmentally friendly source of energy. It produces 3930 (TW.h).a−1, and yields 16% of the world’s generated electricity and about 78% of renewable electricity generation (in 2015). Hydropower and climate change show a double relationship. On the one hand, as an important renewable energy resource, hydropower contributes significantly to the avoidance of greenhouse gas (GHG) emissions and to the mitigation of global warming. On the other hand, climate change is likely to alter river discharge, impacting water availability and hydropower generation. Hydropower contributes significantly to the reduction of GHG emissions and to energy supply security. Compared with conventional coal power plants, hydropower prevents the emission of about 3 GT CO2 per year, which represents about 9% of global annual CO2 emissions. Hydropower projects may also have an enabling role beyond the electricity sector, as a financing instrument for multipurpose reservoirs and as an adaptive measure regarding the impacts of climate change on water resources, because regulated basins with large reservoir capacities are more resilient to water resource changes, less vulnerable to climate change, and act as a storage buffer against climate change. At the global level, the overall impact of climate change on existing hydropower generation may be expected to be small, or even slightly positive. However, there is the possibility of substantial variations across regions and even within countries. In conclusion, the general verdict on hydropower is that it is a cheap and mature technology that contributes significantly to climate change mitigation, and could play an important role in the climate change adaptation of water resource availability. However, careful attention is necessary to mitigate the substantial environmental and social costs. Roughly more than a terawatt of capacity could be added in upcoming decades.

4. Conclusions

The proposed enhancements to a physically based scour model originally developed by Bollaert and Schleiss [2,3] take into consideration the influence of jet air entrainment by the use of systematic experiments with near-prototype velocity jets. With the proposed modifications, the CSM is the only engineering method to evaluate the erosion of rock downstream of jets issued from hydraulic schemes that is fully based on the physical-mechanical processes involving the three phases; namely, water, rock, and air. The adaptations involve the time-averaged pressures attaining the water-rock interface as a result of the dissipation in the pool, which is greatly influenced by air entrainment. These pressures are represented by the aerated time-averaged pressure coefficient, which considers the lower density of the air-water mixture as well as the lower dissipation of the aerated jet flow in the pool, thus taking into consideration the recent findings of Duarte et al. [7]. Moreover, adaptations were proposed to the representation of the dynamic impulse applied on a dislodged block at the pool bottom. This feature is represented by the maximum dynamic impulsion coefficient, which was found to be a value of approximately 0.2, especially for high-velocity jets impinging into deep ools. Finally, the impulsion acting on a rock block is the combined influence of the aerated time-averaged pressure coefficient and the maximum dynamic impulsion coefficient. A case study of the Kariba Dam scour hole was presented. The results are close to the bottom elevation obtained in the 1981 and 2001 surveys. Specifically, if the failure criterion is that a block is ejected whenever one quarter of its height leaves the cavity, the computed ultimate scour depth coincides with the measured elevation of the scour hole bottom (306 m a.s.l.). However, it was pointed out that the DI method only considers the erosion capacity of the impinging jet, and does not account for other scour mechanisms, such as the wall jet created by the deflection of the impinging jet on the water-rock interface. Additional developments to the model may include the influence of the block dimensions relative to the jet dimensions and their influence on parameters such as the wave celerity and the maximum dynamic impulsion. Additional case studies will also help to validate the adapted model for engineering practice.