Anroşman tabanlı kanallarda çözünmüş oksijen artırımının deneysel olarak incelenmesi

Experimental investigation of dissolved oxygen enhancement by paved with boulders

Tez No: 963234
Yazar: AYŞEGÜL ÜÇÜNCÜ
Danışmanlar: PROF. DR. ŞEVKET ÇOKGÖR
Tez Türü: Yüksek Lisans
Konular: İnşaat Mühendisliği, Civil Engineering
Anahtar Kelimeler: Çözünmüş Oksijen, Oksijen Transferi, Kaya, Akım, Oksijen Verimliliği, Açık Kanal
Yıl: 2025
Dil: Türkçe
Üniversite: İstanbul Teknik Üniversitesi
Enstitü: Lisansüstü Eğitim Enstitüsü
Ana Bilim Dalı: İnşaat Mühendisliği Ana Bilim Dalı
Bilim Dalı: Hidrolik ve Su Kaynakları Mühendisliği Bilim Dalı
Sayfa Sayısı: 115

Özet

Çözünmüş Oksijen (ÇO) konsantrasyonu, çevresel faktörler göz önüne bulundurulduğunda özellikle suyun kalitesi ve canlı yaşamı konusunda akarsulardaki en kritik parametrelerden biridir. Doğada ve insan eliyle gerçekleşen birçok kimyasal, fiziksel ve biyolojik olaylar sudaki oksijeni farklı şekillerde tüketmekte ve bu sebeple sudaki çözünmüş oksijen miktarı azalmaktadır. ÇO azalması halinde akarsu kalitesi çok düşmekte ve biyolojik yaşamı tehdit eden bir konuma gelmektedir. Akarsulardaki çözünmüş oksijen seviyesindeki artış sucul habitatın iyileştirilmesiyle birlikte dolaylı olarak bitki ve insan yaşamını da olumlu yönde etkilemektedir. Bu çalışmada, bir akarsuyun çözünmüş oksijen (ÇO) seviyesinin artırılması amaçlanmıştır. Bu amaçla laboratuvar ortamında doğada bulunan farklı yüksekliklerdeki taşlar kullanılarak eğimli, pürüzlü doğal bir basamaklı yapı oluşturulmuştur. Çalışmanın amacı, açık kanallarda doğa ile bütünleşmeyeni suni bir yapıya ihtiyaç duyulmadan, halihazırda var olan taş düzenekleriyle sudaki havalandırmanın sağlanmasıdır. Deneylerde basamaklı taş düzeneğinin, su derinliğinin, hava ve su sıcaklığının ve skimming ile nap akım koşullarının sudaki çözünmüş oksijen transferine olan etkisi incelenmiştir. Bu çalışma iki ana noktaya odaklanmıştır; birinci olarak, oluşturulan deney düzeneğinin çeşitli kısımlarında oksijen seviyelerindeki değişim ölçülmüş ve kaydedilmiştir. İkinci olarak ise açık hava girişi olabilmesi için gerekli olan türbülans kinetik enerjisi ve türbülansın düşey doğrultuda hızın çalkantı bileşeni hesaplanmıştır. Söz konusu deneyler genişliği 0,5m, yüksekliği 0,45m, boyu 12,5m olan yatay tabanlı, taşlardan oluşturulmuş düzeneğin orta kısmına sabitlendiği bir kanalda gerçekleştirilmiştir. Suyun oksijen miktarının belirlenmesinde, OXI 330i membran tipi oksijen ölçerler kullanılmıştır. Kanalda yapılan ölçümler akımın taşlardan etkilenmediği uzaklığında kalacak şekilde membada, taşlardan oluşan düzeneğin bulunduğu kısımda ve oluşan hidrolik sıçramanın öncesinde olacak şekilde 7 farklı noktasında alınmıştır. Bu ölçümler skimming ve nap akım koşullarında çokça kez tekrarlanmıştır. Akım koşullarının değişiminin ve taş düzeneğinin ÇO miktarına nasıl bir etkisi olduğu gözlemlenmiştir. Sonrasında ise oksijen seviyesindeki değişimin daha net görülebilmesi amaçlanarak su içerisine kimyasal madde eklenerek su içerisindeki çözünmüş oksijen seviyesi düşürülmüş ve bu koşullarda deneyler tekrarlanmıştır. Elde edilen sonuçlar neticesinde deney düzeneği üzerinde PIV ölçümü gerçekleştirilmiştir. Elde edilen datalar Matlab aracılığı ile bir analize sokulmuş, elde edilen sonuçlar ile ÇO seviyesinin TKE ile olan ilişkisi değerlendirilmiştir. Bu çalışmada, deneylerden elde edilen sonuçlar yorumlanmıştır. Sonuç olarak, farklı akım koşulları altında kaydedilen deney verileri kullanılarak elde edilen sonuçlar, oluşturulan doğal taş düzeneğinin akarsuyun daha zengin oksijen içeriğine sahip olmasını sağladığını ve benzer nitelikteki yapılara göre habitata daha uyumlu olduğunu göstermiştir. Yapılan bu çalışma, akarsulardaki oksijen seviyesinin doğal ortam korunarak arttırılmasına yönelik yapılabilecek çalışmalar hususunda değerli bilgiler sağlayacaktır.

Özet (Çeviri)

Dissolved Oxygen (DO) concentration is one of the most critical parameters in rivers, especially concerning water quality and aquatic life when considering environmental factors. In nature, various chemical, physical, and biological processes consume oxygen in water in different ways, leading to a decrease in dissolved oxygen levels. A reduction in dissolved oxygen levels significantly affects river quality and poses a threat to biological life. Increasing dissolved oxygen levels in rivers provides indirect benefits to both plant and human life by improving aquatic habitats. The literature suggests various structural solutions to increase DO levels in streams, including weirs (Kim and Walters, 2001), hydraulic jumps (Chanson and Qiao, 1994), spillways (Chanson, 1990), and cascades (Toombes and Chanson, 2000). One of the most commonly used structures in terms of oxygen reclamation in rivers is the transfer of the flow over a stepped structure. These structures have been used very often throughout history. The characteristic of the flow over the cascade channel is a high level of turbulence and a large amount of air intake. There are two different flow regimes in cascading channels. These are skimming flow regime and nap flow regime. In skimming flow, the flow moves in the direction of the slope of the structure and no hydraulic jump occurs as the flow passes over the steps. Translations occur on the steps. In the nap flow, the flow hits each step like a water jet and a hydraulic jump occurs at each step. Cokgor and Kucukali (2004) analyzed the effects of rocks on DO levels in streams in their experimental study and showed that the height of rocks relative to water depth is the most significant parameter affecting DO gain. Although structures built to increase DO in streams improve the concentration of dissolved oxygen, they may have adverse environmental consequences by disrupting interactions between upstream and downstream sections of streams. This disruption can lead to interruptions in particle transport and block fish passage. Considering these factors, more natural and environmentally compatible alternatives are preferred in river rehabilitation projects and at spillway outlets. The amount of dissolved oxygen in streams decreases due to the oxygen consumption by the existing biological diversity along the stream and the discharge of water with low O2 concentration into the stream. Nowadays, the increase in such discharges plays a negative role in the DO levels in streams. The rise in settlements within river basins leads to an increase in fine (suspended) solids in the stream, which affects flow conditions by reducing the vertical turbulence component and, especially, the DO entry into the lower layers of the stream. Consequently, the biological diversity in streams is gradually declining, and some species are disappearing. Various regulations are being implemented today to sustain the biological diversity of river systems. The impact of structures such as dams and weirs, built for other purposes, on DO levels is being examined, and the potential contribution to DO levels is being considered as a parameter in their design (Kucukali, S., 2002). This study also aims to provide re-aeration in open channels using naturally occurring rock arrangements without the need for artificial hydraulic structures. Experiments have been conducted to investigate the effects of stepwise rock arrangements on the transfer of dissolved oxygen in water, considering factors such as water depth, air and water temperature, and the presence of skimming and plunging flows. Air – Water Gas Transfer Equations The oxygen transfer occurring around hydraulic structures, such as weirs and cascades, can be determined by using the following expression defined by Gameson (1957)“ r=”(“C”_“S”-“C”_“U”)/(“C”_“S”-“C”_“D”) (1) where CS, CU, and CD are the saturation, upstream, and downstream oxygen concentrations(mg/L), respectively, and r denotes the deficit ratio, which varies from 1 for no transfer of oxygen to infinity for a complete transfer. On the other hand, the efficiency of this transfer can be obtained from the following equation E“=”(“C”_“D”-“C”_“U”)/(“C”_“S”-“C”_“U”)=“1”-1/r (2) where the change in concentration is expressed as a fraction of the initial deficit. In this latter equation, the parameter E denotes the aeration efficiency which ranges between 0 and 1, for conditions of no aeration and total downstream saturation, respectively. These two dimensionless quantities, i.e. the deficit ratio and the aeration efficiency, constitute a basis in order to compare the effects of various aeration structures or different configurations on the oxygen transfer. However, as the oxygen transfer is sensitive to water temperature, a temperature correction factor should be employed in the determination of the aeration efficiency. Within the context of the present study, the following relationship, developed by Gulliver et al. (1990), was used.“ 1”〖-“E”〗_“20”“=”(“1”-“E”_“T”)^(“1”⁄“f”_“T”) (3) In Equation 3, the parameter E20 denotes the aeration efficiency at 20C, ET is the efficiency at the water temperature of measurement, and fT is a coefficient given by the relation“f”_“T”“=1+0.02103∙”(“T”_“w”-“20”)“+8.261∙”〖“10”〗^“-5”“∙”(“T”_“w”-“20”)^“2”(4) where Tw is the water temperature. Experimental Setup The experiments were conducted in a laboratory circulation channel with dimensions of 0.5 m in width, 0.45 m in height, and 12.5 m in length, constructed from rocks and featuring a horizontal base with a fixed slope of 1/4 in the middle section. The flow was maintained by recirculating water through this channel. To determine the oxygen content of the water, OXI 330i membrane-type oxygen meters were used. These oxygen meters not only measure dissolved oxygen but also include temperature sensors to measure water temperature, which is crucial since temperature affects dissolved oxygen levels. The oxygen measurement device is calibrated in air before each measurement, allowing for the determination of air and water temperatures beforehand, which is essential for the formulation process. The experimental setup designed to increase oxygen levels in the circulation channel consisted of an inclined ramp constructed with angular rock pieces averaging 10 cm in diameter, measuring 110 cm in length and 28 cm in height. The upstream section of the ramp was supported by similarly sized rocks piled at a reverse slope. This setup was positioned and fixed at the midpoint of the laboratory channel. Measurements were taken at seven different points: upstream at a distance unaffected by the rock arrangement, within the section containing the rock setup, and before the resulting hydraulic jump. Calculations for the distances between these points indicated that they should be marked starting 20 cm from the beginning and highest part of the setup, with intervals of 36 cm. Rocks at each marked measurement point were arranged to allow proper positioning of the membrane-type oxygen meter. For accurate measurement of the dissolved oxygen concentration in the flowing water, the oxygen meter needed to remain stationary at each measurement point for at least 60 seconds. Therefore, the measurement device was mounted on a profile to remain fixed under different flow conditions. All measurements mentioned were adjusted for flow rates under skimming and nap flow conditions, and a total of 40 sets of data were collected. Each repeated experiment involved a different flow rate, and the results were tabulated for analysis. It was observed that variable flow conditions and the rock setup influenced DO levels at all marked points. Subsequently, to clearly observe changes in oxygen levels, two chemical substances, cobalt (III) chloride hexahydrate and sodium sulfite (97%), were added to the water to reduce the dissolved oxygen levels, and the experiments were repeated for five measurement sets using the aforementioned methods. Results And Discussion The effects of the constructed rock arrangement on the distribution of dissolved oxygen concentration are the primary focus of this study. Additionally, the study examines the impacts of different flow conditions, the state of turbulence, and the situation resulting from the chemical addition of dissolved oxygen to the water. The turbulent kinetic energy and the vertical velocity's turbulent fluctuation component required for open-air entry have been calculated to allow for vertical turbulence. The dissolved oxygen gain at specific points upstream, downstream, and on the rock arrangement itself has been analyzed. A common observation across all experimental sets was an increase in oxygen concentrations at measurement points up to 0.72 m. This increase was more pronounced in experiments 1 and 2 compared to experiments 3 and 4. This difference is attributed to the regime difference between skimming flow and nap flow. In the skimming flow regime, the turbulence kinetic energy associated with the drop between the rocks results in greater aeration of the water. At each measurement point on the setup, a deficit ratio r and an oxygen transfer efficiency E value were calculated. These values were determined using Equations 1-4. By applying these equations and substituting the maximum downstream oxygen concentration CD observed with the maximum CD value among the points in the downstream section, the maximum values of the deficit ratio rmax and the oxygen transfer efficiency Emax were obtained. In the laboratory flume, we observe hydraulic jumping. The upstream and downstream heights of the water entering the flume were measured according to the given flow rates. The Froude Number was calculated based on these heights. Equation 5 shows the calculation of the Froude number. Fr=v/√(g⋅h) (5) If the Froude number is greater than one, it is called the flood regime, and if it is less than one, it is called the river regime. A hydraulic jump is an event that occurs when the flow transitions from the flood regime (supercritical flow) to the river regime (subcritical flow), creating a sudden rise in the water surface. When the value ΔE=h_k= E_1- E_2 is greater than zero, a hydraulic jump occurs. When it is less than zero, energy enhancement is not possible. When it is equal to zero, it is referred to as a critical flow. In all our experimental sets, these ΔE=h_kvalues range from 0.22 to 0.26. Hydraulic jumps create an effective physical mixing in the water by forming a two-phase mixture with air being absorbed into the water through the bubbles created (Breitenöder and Dorer, 1967). The aeration efficiency increases, leading to a rise in dissolved oxygen levels. The h_2,V_2, and 〖Fr〗_2 values are measured and calculated immediately before the hydraulic jump. After the primary experiments without chemicals, the addition of cobalt (III) chloride hexahydrate and sodium sulfite (97%) to the experimental setup showed no specific difference compared to the experiments without chemicals. The reason for adding chemicals was to reduce the dissolved oxygen concentration of the water from an average of 10 mg/L to 5 mg/L, allowing for the observation of oxygen increases with larger increments. The dissolved oxygen values increased much more rapidly. The final increases proportionally matched those observed in the experiments without chemicals. As a result of the results obtained with oxygen measurements, PIV measurement was performed on the experimental setup. The data obtained were analyzed by Matlab and the relationship between the DO level and TKE was evaluated with the results obtained. CONCLUSIONS The effects of a sloped rock arrangement on the dissolved oxygen (DO) levels in streams were investigated in a laboratory open-channel setup. The results can be summarized as follows: Changes in velocity, driven by turbulent kinetic energy, were observed at points where water flowed over the rock arrangement. This turbulence-induced aeration led to an increase in the dissolved oxygen levels in the water. The results obtained from experimental data recorded under different flow conditions demonstrated that the constructed natural rock arrangement contributed to higher oxygen content in the river, making the habitat more suitable compared to similar structures. The natural stone arrangement increased the amount of dissolved oxygen between the upstream and downstream, enriching the DO levels of the stream. Therefore, the application of this method is crucial for river biodiversity and environmental health.

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