Dikey dalan sıvı jeti için farklı jet çıkış geometrilerinin havalandırma miktarına etkisinin sayısal olarak incelenmesi
Investigation of the different nozzle geometries for vertical plunging water jet air entrainment with numerical methods
Yazar: BURAK ALP KARAN
Danışman: DOÇ. DR. YAKUP ERHAN BÖKE ; DR. ZAFER GEMİCİ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Maden Mühendisliği Ana Bilim Dalı / Isı-Akışkan Bilim Dalı
Derece: Yüksek Lisans, 2017
Konu: Kimya Mühendisliği = Chemical Engineering ; Makine Mühendisliği = Mechanical Engineering ; Nükleer Mühendislik = Nuclear Engineering
Dizin: Makine mühendisliği = Mechanical engineering

Özet:
Sıvıların havalandırılması durumu, doğada ve sanayide sıklıkla karşılaşılan bir olaydır. Bazı kontrol hacimlerinin bulunduğu yerlerde havalandırma, sürekliliği sağlanabilmesi için mecburi olarak yapılmak zorundadır. Dalan sıvı jeti ile havalandırma yöntemi şu anda bilinen mevcut havalandırma yöntemleri arasında kurulum olarak en basit ve ucuz olan yöntemdir ve çoğu süreçte sıklıkla kullanılmaktadır. Fakat bu yöntem ile kontrol sağlanması diğer yöntemlere göre biraz daha zordur. Bazı kısıtlayıcı şartlar altında havalandırma miktarlarını arttırmak adına jet püskürtücülerinin tasarımlarını değiştirme yoluna başvurulabilmektedir.
Bu çalışma kapsamında, su-hava ikilisi için farklı püskürtücü ağız formlarının oluşturmuş olduğu jetlerin durgun su havuzu üzerindeki etkileri sayısal yöntemler ile incelenmiştir. Esas amaç olarak, sabit debi ve giriş çapına sahip püskürtücüler için hava sürükleme oranları arttırılmak istenmiştir. Bunun için düz jetlere alternatif olabilecek halkasal ağız yapılarına sahip iki adet püskürtücü incelenmiştir. Öncelikli olarak giriş ve çıkşında sabit çapa sahip olan düz bir püskürtücü modellenerek durgun havuz üzerindeki etkileri incelenmiştir. Ardından çıkış alanları eşit olan farklı halkasal çıkışlara sahip püskürtücülerin oluşturduğu etkiler araştırılmıştır. Bu araştırma sırasında literatürde daha önceden sıklıkla hem deneysel hemde sayısal olarak çalışılmış olan düz jetlerin, havuz üzerinde oluşturdukları etkilerin evreleri incelenmiş ardından bu evrelere göre çalışma kapsamında tasarlanan püskürtücülerin oluşturduğu jetlerin zaman bağlı şekilde oluşan havuz üzerindeki etkileşimleri Hesaplamalı Akışkanlar Dinamiği yöntemleriyle çözülmeye çalışılmıştır. 
Çalışma kapsamında 0,1167 kg/s su debisine ve 0,014 m giriş çapına sahip olan jetler, akış alanında türbülans oluşturmaktadırlar. Daha önceki deneysel çalışmalara bakıldığında, elde edilen bilgilere göre havuz içerisinde oluşacak en küçük hava kabarcığı çapının 0,5 mm mertebelerinde olmasından dolayı oluşturulan çözüm ağlarının köşegen uzunlukları, dalma noktası etrafında maksimum 0,5 mm olacak şekilde tasarlanmıştır. Hava sürüklenmesinin gerçekleştiği dalma noktası etrafında yüksek oranda türbülans yoğunluğu yaşanmasından ötürü buradaki akışı çözebilecek olan Large Eddy Simulation yöntemi dinamik Lilly SGS modeli ile birlikte seçilmiştir. İki fazlı akışı modellemek adına ise Volume of Fluid yöntemi, arayüzey oluşum algoritması olarak Geo-Reconstruction yöntemi ile birlikte kullanımıştır. 
LES-VOF ikilisinin sonuçlarına göre püskürtücülerin sürüklediği çok farklı çaplardaki hava kabarcıkları yakalanmıştır. Bu sayede daha gerçekci hava sürüklenme debileri elde edilmiştir. Sayısal çözümün sonuçlarına göre, püskürtücü çıkışındaki türbülans yoğunluğu daha fazla olan halkasal çıkışlı püskürtücülerin oluşturduğu jetler düz jete kıyasla iki kattan daha fazla hava sürükleyebilmektedir.

Abstract:
The air entrainment of liquids is a frequent occurrence in nature and in the industry. Where certain control volumes are present, ventilation must be done compulsorily to ensure continuity. Where such needs exist, it may be necessary to control the flow and ventilation pipes. Plunging liquid jet aeration is the simplest and cheapest method of installation among the currently available ventilation methods and is often used in most processes. But, this method is more difficult to provide with some control over the other methods. Under some restrictive conditions such as constant flow rate, limited number of injectors, adaptation to existing geometry, the way to change the designs of jet sprayers can be applied to increase the ventilation amounts.
 Within the scope of this study, the effects on the static water pool of the jets in which different sprayer mouth forms were formed for the water-air mixture were investigated by numerical methods. As a main objective, it is desirable to increase the air dragging rates for the nozzles having constant flow rate and inlet diameter. Two nozzles having annular nozzle structures designed to increase turbulence at the nozzle outlet, which may be an alternative to straight nozzles have been investigated for this. Firstly, the effects on the still pool were examined by modeling a flat sprayer with constant diameter at the inlet and outlet. Then the effects of the sprayers with different annular outputs with equal outlet areas were investigated. During this research, the literature has examined the effects of the flat jets, which have been studied both experimentally and numerically in the literature frequently, and tried to solve the time-dependent interaction of the jets formed by the sprayers designed in this study with respect to these phases by means of Computational Fluid Dynamics methods. Indeed, although 3B which and depending on the time of branch can change position in radial direction of the jet and the free surface under air bubble distribution, instant first interaction of the first moment of impact called as jet pool surface and due to the axial symmetry around the plunge from steady-state later in this transient, entrained air estimating the bubble problem behalf 2B was dissolved in an axial symmetrical manner.
In order to evaluate the performance of the sprayers, a geometric adjustment was made for a plunge height of 0.12 m, each with a fixed water flow rate of 0.167 kg / s, with an inlet diameter of 14 mm. For this, the still water pool is an open cylinder with a diameter of 0,3 m and a height of 0,4 m. It is considered that the jets emerging from the water sprayers in this cylinder have plunged from the top of the pool. In order to carry out an analysis as described, a free water surface pressure profile was created to give a constant water height by giving annular water outlets to the bottom edges of the cylinder, allowing the water to flow as much as the water entering the flow area.
 Large Eddy Simulation method was chosen together with the dynamic Lilly SGS model in order to solve the flow there due to the high turbulence density around the plunging point of air entrainment. The LES method has the ability to effectively solve the turbulent flow by forming the correct sub-filter scales. Different filtering options are available for the LES method. Numerical solutions were made using the FLUENT module of the package program ANSYS. FLUENT structure as finite volume method LES Because filtering to be formed by a so-called Box-Filtering filtering method using mesh sizes were used. First of all the amount of entrained air will be taken as a priority, the Volume of Fluid method in the Euler method class and working with a single executive equation set is used in the modeling of the two-phase flow. Different interface estimation algorithms are used together with VOF method. Due to submerged bubbles that occur during the problem and tend to dissociate and coalesce, too many interface occurrences occur. İn order to capture interface, comprising using a successful PLIC algorithm as estimation algorithm Geo-reconstruction method is preferred.The surface tensions resulting from the molecular interaction of the two phases in the water-air interface, was incorporated into the solution with Continuum Surface Force method.
Many factors have been taken into account in the creation of the solution network. Due to the propellants the turbulences are concentrated around the plunge point in the flow field. Considering the previous experimental works, the diagonal lengths of the solution nets formed are designed to be 0.5 mm maximum around the plunging point, since the smallest air bubble diameter in the pool is 0.5 mm in diameter. The resolution capability of the LES method varies depending on the solution network directly created due to the box-filtering used. But the use of extremely small dimensions to the method of calculation LES cell converts the DNS method. In this way scale studies that did not fall below 0.5 mm because it will bring a lot of computational load in calculating cells. Same thing in the capture small bubbles of VOF method is also in question. In the calculation cells that the interfacial formation algorithm works, it is necessary for the fluid entering the cell not to skip over the cell dimensions within the determined time step. In order to satisfy this condition expressed by the Courant number, very small time steps must be given in the small size calculation cells, which causes an increase in the calculation time. All of these reasons, approximately 250,000 calculations in solving computational cell in the direction given by the infrastructure facilities were created. The time step varies from 10-5 to 5.10-5s, so that the total number of Courant throughout the flow field does not exceed FLUENT, VOF method Courant for accuracy before starting a certain number of tabs defined willingly solution divides the time step in the absence of steps took time meeting this requirement. Thus in regions where there are high speed interface formation is estimated more accurately.
The boundary conditions of the flow field are given so as to realize the conditions under analysis. Flat velocity profiles were created for each model by defining the velocity input to the injector inlet defined at the top of the flow field. At the inlet of the injector there is no air bubbles in front of the flow, so only the water is conditioned at the inlet of the speed. The top of the pool, which will remain open for air inlet and outlet, is defined along the conditioned boundary of the atmospheric pressure, defined as the pressure input. Water is not allowed to pass in the event of any reverse flow at the pressure inlet. The hydrostatic pressure distribution of the water at 28 cm height is given by using the boundary conditions defined by FLUENT for the open channel flow at the outlet of the pool located at the bottom of the pool. The pressure output is conditioned so that there is no air outflow to avoid any adverse effect. The reference pressure to be used during the analysis is set to the atmospheric pressure conditions defined at the pressure inlet. 
Before starting the analysis under the initial conditions, a volume of 28 cm height in the flow area and a volume ratio of water inside the sprayer were set as one. The area with a height of 12 cm between the water pool and the sprayer is given in order to represent the air mass there as a volumetric ratio of air. The open channel initial conditions defined from the pressure outlet are defined along the flow field to form the hydrostatic pressure of the water in the pool.
 When the analysis results are analyzed, it is seen that the solution made by the LES-VOF pair provides close results for all the sprayer models. All the occurrences of the first impact mentioned earlier in the literature have been obtained at different times in all models. The flat jet model impacts the surface of the water 0.030 seconds later than the others due to its larger area than the others as outlet section. At the moment of impact, the characteristics of all the jets are compared and the results of the annular model show that the turbulence intensities at the ejector outlets are twice as high as those of the plain jets. The velocity fluctuations resulting from turbulence are more than two quarts in the annular jet because the velocity of the straight jet is slower than the others. Jet outlet velocities are close to 1.22 m / s in annular jet. Plain jet, the output speed remains the same as the input speed. Due to the plunging height, the velocity of the jet from the single annular outlet is 1,75 m / s, the velocity of the jet with double annular outlet is 1,821 m / s, and the jet velocity of the jet is 1,473 m / s at the first impact at the moment of acceleration. When the flat jet collides with the free surface at the moment of impact, it contracts and appears to have a flat surface structure. On the other hand, other jets have relatively wavy surfaces, and it seems that the jet formed by a single hallowed jet had a bulged-tip compared to others. It is seen that the jets formed from the annular discharges also have bubbles entraining in the jet due to their mouth forms before they hit the free surface. Steady state as time dependent distributions of the volumetric ratio in flow area quickly since the features herein have been substituted examined only at certain times. Since the volumetric rate distributions formed in the continuous flow regime change rapidly with time, these properties have only been studied at certain times. These results were found to vary in accordance with the literature of air entrainment mechanisms are examined. Since the flat jet crosses a certain critical Weber number, it has been seen that only a small amount of air bubbles have been dragged from the end point of the slot along the plunging axis. In wave jets, the mechanism of air entrainment, which is defined as pinching in the literature, occurs due to the continuous opening and closing of the free surface. For this reason a larger diameter bubbles entrained towards the bottom of the free surface, and higher flow rates obtained. Furthermore, under the free surface, the interaction between the dissociation and association of large diameter bubbles could be detected. When looking at the air entrainment rates, an apparent increase of entrained air quantity was seen at the annular jets due to the difference between the ventilation mechanisms.
According to the results obtained, the annular jet jets dragged two quarts of air under free surface compared to the straight jet for the same flow rate. The results were compared with the entrainment ratios proposed by some researchers in the literature and it was found that Bin's and Chanson's suggestions for straight jets had a reasonable error rate.