Eksenel akışlı fan tasarımı
Axial flow fan design
Yazar: MESUT ÇALIŞKAN
Danışman: PROF. DR. FIRAT OĞUZ EDİS
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Uçak ve Uzay Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Uçak Mühendisliği = Aircraft Engineering
Dizin: Eksenel akışlı fan = Axial flow fan ; Fan tasarımı = Fan design ; Sayısal akışkanlar dinamiği = Computational fluids dynamic

Özet:
Fanlar endüstride ve ticari hayatta birçok uygulamalarda kullanılmaktadır. Havalandırma sistemlerinden rüzgar tünellerine ve uçak motorlarına kadar birçok alanda birçok uygulaması mevcuttur. Yaygın olarak kullanılan iki farklı çeşidi bulunmaktadır. Bunlar; eksenel ve radyal fanlardır. Radyal akışlı fanlar, akışkanı dönme eksenine dik şekilde atarlarken, eksenel akışlı fanlar ise dönme eksenine paralel akış oluşturmaktadırlar. Çalıştıkları ortamda basınç farkı oluşturmaktadırlar.
Bu çalışma kapsamında eksenel akışlı fan tasarımı ele alınmıştır. Eksenel akışlı fanların da kendi içerisinde çeşitleri bulunmaktadır. Ancak, gerçekleştirilecek eksenel akışlı fan kovan tipli olacaktır. Tasarımın uygunluğu olarak rüzgar tünelinde kullanılabilir aerodinamiğe, yeterli debi ve verim sahip olması amaçlanmıştır. Benzer şekilde belirli bir seviye gücün altında çalıştırılabilir olması da tasarım amaçları arasında yer almaktadır. Tasarım, ihtiyaç duyulan asgari 15 m3/s hacimsel debiyi üretecek şekilde, uygun sayıda ve uygun aerodinamik yapıda pallere sahip olacak şekilde gerçekleştirilmiştir.
Tasarımda ilk adım olarak var olan bilgiler ve isterler ile birlikte, temel analitik aerodinamik denklemlerin uygulanmasıyla bir boyutlu olarak nitelendirilebilecek sonuçlar oluşturulmuştur. Elde edilen sonuçlar ile, literatürde var olan parametrelerin uygunluğu ve makul değerlerde olması kontrol edilerek tasarım ilerlemesi sağlanmıştır. Sonuçların uygunluğu doğrultusunda, iki boyutta tek boyutlu sonuçlar irdelenerek, tasarım bir sonraki aşamaya taşınmıştır. Bu aşamada, akışkan hareketini yöneten denklemlerin çözümüne olanak sağlayan hesaplamalı akışkanlar dinamiği methodundan faydalanılmıştır. Çözüm için gerekli kontrol hacimleri kaba ve nitelikli ağ örgüsü şeklinde oluşturularak analizler gerçekleştirilmiştir. İki boyutta hızlı çözümler elde edilerek tasarım hakkında fikir sahibi olunmuş, profilin etkisi incelenmiş ve bir boyutlu sonuçlar elde edilmeye çalışılmıştır. Bu işlem fan palinin radyal doğrultusunda üç farklı noktada gerçekleştirilerek incelenmiştir. Palin kökünde, ucunda ve çevresel olarak alan ortalamalı olacak şekilde %50 sine denk gelen profil esas alınarak araştırmalar gerçekleştirilmiştir. Nihai etkileri ve sonuçları anlamak için son aşama olarak gösterilebilecek üç boyutlu analiz uygulaması yapılmıştır. Bu uygulama çözüm süresi açısından uzun süreç oluşturduğu için tasarım adımında son sırada yer almaktadır. Üç boyutlu hesaplamalı akışkanlar dinamiği analizi uygulanarak, gerçeğe daha yakın bir model simulasyonu oluşturulmuş ve üç boyutta etkileri inceleme olanağı sağlanmıştır.
Tasarım sırasında aerodinamik analitik denklemlerinin çözümünde hızlı sonuç alınması ve çok fazla iterasyona olanak sağlayan turbomakina yazılımı kullanılmıştır. Akışkan hareketini yöneten denklemlerin çözümünde kullanılan sonlu hacimler programı için gerekli ağ örgüsünün de hızlı bir şekilde kullanımı için turbomakina yazılımından faydalanılmış ve zaman olarak çalışmaya önemli ölçüde katkısı olmuştur.

Abstract:
The term fan is used for machines imparting only a small increase in pressure to a flowing gas. Fans are widely used in industrial and commercial applications. From shop ventilation to material handling to boiler and computer applications, fans are critical for process support and human health. The purpose of a fan is to raise the pressure of a gas flow and to deliver a given mass or volume flow rate. The first of the requirements thus classes fans as diffusing machines whose aim is to convert fluid kinetic energy into pressure.
There are two primary types of fans: axial and centrifugal. An axial flow fan moves air or gas parallel to the axis of rotation. By comparison, a centrifugal or radial flow fan moves air perpendicular to the axis of rotation. Axial flow fans, while incapable of developing high pressures, they are well suitable for handling large volumes of air at relatively low pressures. Axial flow fans are better suited for low-resistance, high-flow applications, whereas centrifugal flow fans apply to high-pressure resistance, low-flow conditions. Axial fans can have widely varied operating characteristics depending on blade width and shape, number of blades and tip speed. Axial fans are used for creating a pressure difference. This difference creates a reduction of flow rate of fan so the similar reduction can be seen in the power of fan. This study will try to design a new axial flow fan design.
Based on to the study,  a new axial flow fan will be designed. What is more, there are different types of axial flow fans. These axial fans are; propeller fans, tubeaxial fans, vaneaxial fans and 2-stage axial flow fans. However, tubeaxial fan will be discussed and designed in this study. The tubeaxial fan is a propeller fan mounted in a cylindrical tube or duct and is often called a duct fan. Since, the design will have a purpose for the wind tunnel, it will have some special requirements. Fan will have been adequate aerodynamic characteristics when the design finished. It will have enough aerodynamic blade shapes to get best efficiency and volumetric flow. It is only comprised by the rotor. Moreover, it needs a shaftpower to supply sufficient volumetric flow but the power will not exceed a certain number. Since, there is a restriction for the power and it has an upper limit, 10kw for the power.
While doing this project's aim, some methods will be applied to the study during the analysis. Solving such a problem can be complex and too long but there is enough time to deal with it. The work packages start with researching about the subject which can be named as literature review. The literature knowledge is very important such these studies. The design parameters, dimensions, efficiencies can be defined with searching the similar articles, studies.
The first step of the design is using the basic aerodynamic relationships. The requirements and the initial parameters are known. If analytical equations solved using with these parameters, it will give an initial idea about the design. This step is called meanline design. Meanline design results give the dimensions of the blades, angles of the blades and inlet and exit parameters. Therefore, the results obtained from the meanline design have to be checked for reasonable values.
The design moves to the next stage so the next step comes up with two dimensional analysis. At this stage, the governing equations of fluid motion (Reynolds averaged Navier-Stokes equations) solution that enables computational fluid dynamics method is utilized. The computational fluid dynamics investigate the flow field, volumetric flow rate and chemical reactions. The method solves the mathematical equations and gives an idea of the flow.
In this study, CFD will be used to calculate the performance of an axial flow fan. A logical control volume is necessary for the solution of fluid motion. That's why coarse and fine mesh is created. However, this section takes too much time because it is repeated several times and this is the reason why two dimensional analysis is used for this step. Quick solutions in two dimensions have been obtained and the results give an idea about the design and the impact of the profile. This step is called blade to blade computational fluid dynamic analysis. The object of a blade to blade method is to calculate the blade surface velocity distribution and the outlet angle for a given three dimensional cascade. The interaction between the airfoils and the fluid's motion can be understand using this method.
The specification of the blades for an axial flow fan is at the important place of fan design. Blade to blade flow determines the airfoil shape, volumetric flow, exit static pressure etc. For the blade to blade surface at a particular spanwise position the local blade shape does not fix the outlet flow direction or pressure rise. These are determined by the whole blade and not just the spanwise section being considered. There is  wide agreement on the general equations of continuity, momentum, energy, etc., governing the flow and the methods differ in the numerical techniques used for solving the equations and applying the boundary conditions.
In this study, blade to blade analysis of the fan blade profiles apply three different points in the radial direction. The three airfoils are investigated for the fan performance. These two points are located in the root and tip of the blade. The third point is the mid point but not in the mid point of the radial direction. It is located in circumferentially %50 area averaged point. This is a case formed of the turbomachinery software. 
When the results of the balde to blade analysis does not match with the requirements, the design has some modifications like blade inlet and exit angles or etc. These modifications are done to get the requirements of the study, such as mass flow or the efficiency. All of the modifications are called optimization of the design. This is done for the most of the designs in turbomachinery system but fans are not too complicated systems according to the compressors or turbines. That's why it can need less modifications compare to them and this is back to the project as the gain of time.
To understand the ultimate impact and results can be dispalyed as the final step is applied to three dimensional analysis. This application takes long time to design a new axial fan so this design step takes place in the last rank of the process.  Three dimensional computational fluid dynamics analysis, applying a model closer to a real simulation. In three dimensional analysis, it has been created and provided the opportunity to review the impact more realistic. Three dimensional CFD analysis can determine the losses mostly because of the boundary layer effect, incidence losses, airfoil losses etc. All of these parameters effect the efficiency of the fan. Therefore, it can give closer efficiency to real performance of the fan.
All in all, meanline design is the one dimensional analytical solution. It gives exactly the results which must have requirements at the end of the design. One dimensional analysis is just the beginning point of the design. Then, two dimensional analysis, blade to blade analysis shows the airfoil effect. When such a project is decided to design, throughflow and blade to blade CFD analysis apply to the project. Nevertheless, the throughflow analysis does not need to do for this study and this is the reason of throughflow analysis that this project does not include the throughflow.
As a result, the design continues with the three dimesional CFD to be sure that it supplies the volumetric flow rate. When the design finished, the study does not encompass any test chance. Thus, the reliable results are depend on the CFD results, especially three dimensional analysis. As a consequence, this study shows a new axial flow fan design steps and the interactions of the results. The steps are done with a turbomachinery software to get quick results and there are too many softwares doing the same job. Especially, the companies which are related with turbomachinery or aircraft engine develop their own softwares. This project was done by one of them.