مطالعه اثرات صفحات ضد گرداب بر استهلاک جریان گردابی، ضریب آبگذری و ضریب اُفت ورودی در آبگیر نیروگاه‌ها

پدیده هیدرولیکی مهمی که معمولاً در آبگیری از سد‌ها به‌وقوع می‌پیوندد و باعث بروز مشکلاتی نظیر ایجاد اُفت انرژی و کاهش ضریب آبگذری آبگیر می‌گردد، چرخش آب و ایجاد گرداب در دهانه آبگیر و ورود هوا به‌داخل مجرای آن می‌باشد. از میان انواع آبگیرهای در معرض پدیده گرداب، آبگیر‌های نیروگاهی که به‌منظور تامین آب مورد نیاز برای توربین‌ها و نهایتاً تولید برق به‌کارمی‌روند، از اهمیت ویژه‌ای برخوردارند. این آبگیر‌ها عمدتاً از نوع اُفقی می‌باشند. برای از بین بردن گرداب می‌توان از صفحات افقی مشبک بر روی پیشانی آبگیر استفاده نمود. در این تحقیق برای بررسی عملکرد صفحات مشبک، از یک مدل فیزیکی استفاده شده است. این مدل طوری طراحی شده که بتواند قوی‌ترین نوع گرداب با هسته هوا و با قدرت‌های مختلف را تولید کند. با ایجاد 36 نوع گرداب قوی، عملکرد 10 نوع صفحه مشبک با ابعاد، ضخامت‌ها و بازشدگی‌های مختلف مورد آزمایش قرار گرفت و نهایتاً با انجام 360 آزمایش مشخص گردید که تاثیر میزان بازشدگی صفحات مشبک در استهلاک قدرت گرداب، بیش از اثر ابعاد و ضخامت تیغه‌های صفحات می‌باشد. همچنین تاثیر استفاده از صفحه ضد گرداب بر ضریب آبگذری و ضریب اُفت ورودی آبگیر مورد بررسی قرار گرفت و مشخص شد استفاده از صفحه ضد گرداب مشبک با بازشدگی‌های 70% ، 58% و 50% به ترتیب به میزان 5.9 درصد، 10.5 درصد و 13.4 درصد از میزان ضریب آبگذری آبگیر می‌کاهد و بترتیب موجب افزایش 12.9 درصد، 24.7 درصد و 33.5 درصد اُفت ورودی آبگیر می‌گردد.

The Effect of Anti-Vortex Plates on Vortex Dissipation, Discharge Coefficient and Inlet Loss Coefficient in Hydropower Intakes

Introduction The formation of vortices at the intake and the air entertainment into the intake duct is an important hydraulic phenomenon that usually occurs in the dam intakes and causes such problems as energy loss and reduction of intake discharge coefficient. Among different types of intakes exposed to the vortex phenomenon are hydropower intakes used to supply water to turbines and generate electricity. These intakes are mainly horizontal. To prevent the formation of strong surface vortices, their strength must be physically controlled. A practical solution for this is to use antivortex structures. These structures mainly eliminate the vortex by reducing the flow velocity near the intake, lengthening the flow path between the free water surface and the mouth of the intake, as well as energy dissipation. Some studies on the structural methods of vortex dissipation have been done by Amiri et all (2011), Tahershamsi et all (2012), Monshizadeh et all (2018), Taghvaei et all (2012). In this study, the effect of horizontal perforated plates on the dissipation of the strong vortices, the intake discharge coefficient and inlet loss coefficient of the intake is studied, so far no special studies have been done in this area. MethodologyIn the present study, a physical model was used to investigate the performance of horizontal perforated plates. This model was designed to produce the strongest type of vortex with air core and different strengths. The main components of the experimental setup are: reservoir, intake duct, pump and electromotor speed controller device. The dimensions of reservoir is 1.3 m in wide, 3.5 m long and 2 m high. The mouth of intake extends 20 cm into the reservoir and is positioned so that the side walls and the bottom of the reservoir do not affect the flow conditions. The length of the intake pipe is 4.5 m and its diameter is 16 cm. At a distance of 2 m upstream of the intake in the reservoir, some blades are installed vertically that by changing their angle relative to the intake axis, the angle of inflow to the intake can be changed. This makes it possible to strengthen the upstream vorticity to reach stronger vortices. For modeling the perforated antivortex plates, some plastic mesh with different openings and different thicknesses were used. For each plate, the corresponding mesh was placed in a metal coil and this coil is screwed to the reservoir wall so that the perforated plate be placed on the mouth of the intake. By creating 36 types of strong vortices, the performance of 10 types of perforated plates with different dimensions, thicknesses and openings was tested.Results and DiscussionCalibration tests showed that in the range of 1.5D to 2D (D is the diameter of the intake pipe) for submergence depth, flow discharges of 15 to 30 lit/s and blade angles of 0 to 20 degrees, the stable strong vortices are formed. A total of 36 strong vortices (three relative submergence depths, four flow discharges and three blade angles) were formed with different strengths in the model. In order to consider the appropriate confidence limit in this study, the performance of each of the antivortex plates in the model was considered so that it is able to dissipate vortex typesix or decrease to typetwo vortices. Therefore, the conditions in which the strength of a typesix vortex was reduced by the relevant antivortex plate to a typethree (or higher) vortex are known as critical conditions. It should be noted that the type of vortex is determined based on its appearance. Finally with 360 tests it was concluded that the effect of opening of the plates to eliminate the vortex strength is more than the dimensions and the thickness of the plates. In addition, the effect of using perforated horizontal plates on discharge coefficient and inlet loss coefficient of the intake was investigated. It was concluded that the use of perforated antivortex plate with openings of 70%, 58% and 50% reduces the intake discharge coefficient by 5.9%, 10.5%, and 13.4%, respectively. It is also caused 12.9%, 24.7% and 33.5% for inlet loss coefficient of the intake, respectively.ConclusionThe effect of submergence depth on the vortex strength is greater than the flow discharge and it is also greater than the geometric asymmetry. Dimensions of the plate have little effect on the vortex dissipation. The thickness of the plates has little effect on the vortex strength. The opening rate of the plates has a great effect on the vortex and a plate with 50% opening, was able to dissipate all strong vortices. The vortex strength has a direct relationship with the inflow angle and the flow discharge and is inversely proportional to the submergence depth. As the flow discharge increases, the discharge coefficient decreases and the inlet loss coefficient increases.