بررسی عددی اثر ابعاد حوضچه رسوب‌گیر شهری بر روی راندمان رسوب‌گیری

نویسندگان

چکیده

در این مقاله اثر تغییرات طول،‏ عرض و ارتفاع حوضچه روی راندمان حوضچه‌های رسوب‌گیر شهری مستطیلی بررسی شده است. برای تعیین ابعاد اولیه حوضچه،‏ از حوضچه‌های رسوب‌گیر شهری تهران به‌عنوان الگو استفاده شده است. بر همین اساس سیزده حوضچه با ابعاد مختلف و با حجم یکسان با مدل عددی فلوئنت شبیه‌سازی شدند. برای حل میدان‌های سرعت و غلظت در حوضچه از یک مدل دو فازی پیچیده و دقیق به شکل اولری- اولری و کوپل شده با مدل آشفتگی RNG k-?،‏ استفاده شد و سپس راندمان حوضچه‌ها محاسبه شده است. بر همین اساس،‏ نمودار تغییرات زمانی راندمان حوضچه،‏ برای حوضچه‌های با طول،‏ عرض و ارتفاع گوناگون به دست آمد. نتایج حاصل از این نمودارها نشان داد که به‌دلیل درشت دانه بودن ذرات و همچنین شکل هندسی خاص حوضچه‌های شهری،‏ عرض حوضچه تأثیر بیشتری در افزایش راندمان حوضچه داشته است.

کلیدواژه‌ها


عنوان مقاله [English]

Numerical study on the effects of the dimensions of sedimentation basin on the -- oval efficiency

نویسندگان [English]

  • Farhad Golriz
  • Seyed Ali Akbar Salehi Neyshabouri
چکیده [English]

Urban developments and increase in construction are associated with an increase in impermeable areas and, however, increasing impermeable areas may lead to an increase in surface runoff of these areas. Today, these increases in surface runoff and damages which are arising from extreme rainfalls have been emerged as big challenge urban areas. Urban surface water collection systems are a series of hydraulic structures which are being used as a mean for controlling and conveying storm waters in urban areas. Improving urban runoff quality is one of the important factors in proper performance of the urban runoff collection system. Sedimentation basins are one of the first components in urban drainage and are employed to improve runoff quality before reaching runoff to the storm water collection system by settling the coarse to medium-sized sediments. Design of these basins is usually based on uniform and non-turbulent flows without considering the presence of sediments and their effects. In reality, however, the incoming flow to an Urban Sedimentation Basin (USB) is syllabic that is naturally turbulent. Although, the removal of suspended solids in a USB will be performed by gravity, various inflow conditions, coarse solid particles and different geometry of the USB led to form a complicated hydrodynamic in the basin. The proper design of these structures by considering the real conditions may guarantees the other parts of the drainage network. Thus, understanding the hydrodynamics of a USB, in addition to determining the turbulent flow and concentration fields, the basin efficiency can be also calculated more accurately.
In general, the flow in Sedimentation basins (SBs) is a two-phase turbulent flow. Many experimental and numerical studies have been carried out to understand the hydrodynamics of this turbulent flow in SBs. In previous years, most of the researches in the first place has been allocated to a better understanding of the hydrodynamics of flow and sediment particles by taking into account various parameters such as turbulence, the three-dimensionality of the flow and so on. In the second place, researchers have tried to improve the efficiency of SBs in various methods including changes in geometry and hydraulic conditions. In this paper, at the first stage, an accurate, complicated, Euler-Euler coupled two-phase model which uses similarity between solid particles with gas molecules were used to simulate the sedimentation process in a USB. In comparison with previously used methods, the advantage of this method is its more accuracy to calculate the properties of the solid phase. In other words, in this method for the solid phase, a separate momentum equation will be solved in which the characteristics of the solid phase, will be calculated using the kinetic theory. The model is able to simulate flows which contain a high proportion of sediment particles, with different flow pattern, through solving a separated momentum equation for solid phase. In this way, by using this model, first, the laboratory basin of Van Rijn (1986) was simulated, and the results were compared with their experimental data, and so, a well-accommodation was achieved. Then, however, the urban sedimentation basins (USBs) of Tehran, which have a relatively high sediment concentration and coarse solid particles, as a sample were studied. In the second section, parametric studies were performed on the basin. So, length, width and height of a USB with a constant volume were changed and their effects on the efficiency were investigated. Accordingly thirteen basins with different dimensions, by keeping the same volume, using the FLUENT model were simulated
Results in the first part showed that one-hour average efficiency of the USB, is less than 30%. It has been resulted that the main factors in reducing the efficiency of the urban SBs are the two outlets at the bottom of the basin which are being used as drainage flow at low flow rates. In this part, the dynamic distribution of sediment in the basin is also obtained. In the second part, time variation of the basin efficiency diagram by different length, width and height was obtained. The efficiency calculations showed that, against other researcher’s studies that have considered length as the most effective parameter in the removal of the solid particles; width changes have the greatest impact on increasing the removal efficiency. The main reasons are related to coarse particles and the specific geometry of the USB.