Physical hydraulic model study of the throttle trap concept for branch flow control
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Sediment induced wear of hydro-turbines has been a major problem in utilizing the water resources in sediment loaded rivers around the world. The problems are more sever in Himalayan Rivers, which favor high head plants but are highly loaded with sediment. During wet season, there is ample of water available in these rivers carrying high concentration of sediment. The generation efficiency of the turbines decreases drastically when subjected to high sediment loads of monsoon season and maintenance costs are increased. Thus, besides the potential for high power generation with respect to the availability of water, the generation capability is reduced due to the decrease in the efficiency of the turbines. A considerable amount of research has been carried out to minimize these sediment related problems in power plants. A research on using hydro-cyclones for sediment handling in Himalayan River has been carried out by Hari Pandit (Pandit, 2009) in his Ph.D work. He worked on improving the performance of the hydro-cyclone by improving the inlet and outlet arrangements for the hydro-cyclones. However, headloss over the efficient hydro-cyclone is quite significant which is in general not affordable by the existing power plants, hence making it impractical for application. A new concept called “Throttle Trap concept” has been developed by Professor Haakon Støle to overcome issues of application of a Hydro-cyclone based on its head requirement. It involves increasing velocity in a section of a waterway to such level that the increase in the velocity head and simultaneously decrease in pressure head is high enough to generate the head required in operating the hydro-cyclone. The pressure head is then gradually regained with decrease in the velocity and the kinetic energy is transformed back to potential energy in the waterway downstream. Thus the “Throttled Trap concept” involves splitting of these diment rich bottom flow from a waterway, process it in the hydro-cyclones and then finally return the processed flow to the waterway. The Throttled trap system can be placed anywhere in a pressurized waterway of a plant where pressure line above it is sufficiently higher than the head required by the hydro-cyclones to operate. In this thesis work a theoretical study on the hydraulic features of the concept has been carried out. Based on an in operation reference project, the Jhimruk Hydropower Plant of Nepal, an excel spreadsheet model has been prepared for preliminary dimensioning of the concept. Two different layout of the Throttle Trap concept has been prepared where it is applied to penstock and to tunnel setup. A model study on the concept has been carried out for the throttled trap in a penstock setup.The concept is functional based on the observations made in the model. The theoretically expected pattern of pressure drop in the pipe was achieved as expected. However, the headloss over the system was relatively high. The headloss over the branch flow was substantially high to achieve the head required by the cyclone to operate. Moreover, thepressure drop generated over the throttled trap system was not found to be fully utilized overthe hydro-cyclone. Based on these observations, modifications were made in improving the hydraulics of the system and optimizing the geometry for better utilization of the generated pressure drop under modification I and II respectively. The modifications improved the performance of the system generating an overall efficiency of around 19% and the delivery efficiency of the branch flow arrangement to be about 78%. However, there is still room for improvement and modifications have been suggested for further studies. The concept of locating Throttle Trap concept in a tunnel setup also seems to be very promising in regards to safety working at low pressure and flexibility in working on it relative to the existing embedded penstocks subjected to high pressure. However, the geological considerations are to be dealt with in detail. The selection of the best location of the concept is based on the available head in the waterway and the flexibility in working on it. Further studies are therefore recommended on the concept in a tunnel setup. This study involves only the steady state of flow in the waterway, hence further studies on the realistic scenarios are recommended.