Tersine tasarım yönteminin düşük hızlı eksenel fanlara uygulanması
Application of inverse design method to low speed axial flow fans
Yazar: HIDIR MARAL
Danışman: YRD. DOÇ. DR. LEVENT KAVURMACIOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Isı-Akışkan Bilim Dalı
Derece: Yüksek Lisans, 2012
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Eksenel akışlı fan = Axial flow fan ; Fan tasarımı = Fan design ; Tersine mühendislik = Reverse engineering ; Turbo makineler = Turbomachinery

Özet:
Bu çalışmada türbomakinalarda kanat geometrisinin tanımlanan basınç dağılımınabağlı olarak elde edildiği tersine tasarım yöntemi üzerinde durulacaktır.Teknolojinin gelişmesiyle birlikte sayısal akışkanlar dinamiğinde(SAD) de önemligelişmeler meydana gelmiştir. Akış denklemlerinin bilgisayarlar kullanılarakçözülmesi türbomakinalarda kanat tasarımında önemli bir adım olarak görülmektedir.SAD, türbomakinalarda hidrodinamik ve aerodinamik analiz; sonrasındaoptimizasyon için oldukça önemli bir konuma gelmiştir. Bugün çeşitli yöntemlerletürbomakinalarda oldukça karmaşık kabul edilen 3-Boyutlu akış incelenebilmekte;istenmeyen akış ayrılmaları, ters basınç gradyeni durumunda akışkan davranışı,viskoz etkilerin türbomakina performansı üzerine etkileri gibi akışa vetürbomakinaya ait detaylı bilgiler elde edilebilmektedir.Günümüzde türbomakina tasarımı pek çok durumda tasarımcının deneyimine bağlıolarak yapılmaktadır. Tasarım, var olan geometrinin ardışık yaklaşımlarladeğiştirilmesi ve sonrasında akış analizi ve(veya) deneylerle doğrulanması şeklindeyapılmaktadır. Bu yöntem, tasarımcıya bağlı olarak hem zaman hem de maliyetsorununa neden olabilir.Son yıllarda gittikçe önemli bir konu olan tersine mühendisliğin türbomakinalarauygulanması kanat tasarımında büyük kolaylıkları beraberinde getirmiştir. Tersinetasarım yaklaşımında, akış analizi sonucunda elde edilen veriler giriş verileri olarakkabul edilir. Akış analizleri sonucunda elde edilen kanat üzerindeki yük dağılımı(basınç dağılımı) başta olmak üzere diğer parametreler tersine tasarım yöntemindegiriş verisi olmaktadır. Tersine tasarım yöntemi, farklı bir ifadeyle ?istenilen akışözelliklerini sağlayan kanat tasarımı? olarak tanımlanabilir. Kanat üzerindeki yükdağılımı bu yaklaşımda kullanılan en önemli tasarım parametresidir. Bu yaklaşımbelirtilen yük dağılımına (gerekli basınç dağılımı) uygun kanat tasarımına olanaksağlamaktadır.Tersine tasarım yöntemi kullanılarak var olan bir fan geometrisinin yeniden tasarımıyapılmıştır. Meridyenel geometri sabit tutup yük dağılımı ve diğer parametrelerindeğiştirilmesiyle farklı kanat geometrileri elde edilmiştir. Elde edilen fangeometrileri hızlı prototipleme yöntemiyle imal edilmiştir. Daha sonra bugeometrilere performans deneyleri uygulanmıştır. Orijinal fan ile benzer performansgösteren model/modellere ANSYS CFX ile SAD analizi uygulanmarak sayısal vedeneysel sonuçlar karşılaştırılmıştır.Yapılan çalışmalar sonucunda merideyenel geometri sabit tutularak yüksek verimesahip ve yüksek basınç artışı sağlayan fan geometrisi elde edilmiştir.

Abstract:
In this thesis a 3-dimensional inverse design method in which the blade geometry inturbomachinery is calculated for specified pressure distribution is described.There have been significant improvements in Computational Fluid Dynamics (CFD)due to the improvements in technology. It is seen as an important step inturbomachinery blade design to use computers in order to solve flow equations.Computational fluid dynamics has been become a considerably important way inturbomachinery for both hydrodynamically and aerodynamically analysis and thenfor optimization process. Today using various methods 3-dimensional flow which isassumed complicated in turbomachines can be investigated and a detailedinformation on both turbomachinery and flow such as undesired flow seperations,behaviour of fluid in case adverse pressure gradient, viscous effects onturbomachinery performance can be obtained.In the field of turbomachinery, there are two main approaches for aerodynamicdesign. The first and basic method is defined as direct method which means that flowfield through an impeller is tried to be determined for a given blade shape. Thesecond and respectively newer method is inverse design method which means thatthe optimum blade shape is tried to be obtained depending on the input data.Using direct method, blade design process is achieved by applying flow analysis andmaking changes at blade geometry using results obtained from numericalsimulations. Today using various methods 3-dimensional flow which is assumedcomplicated in turbomachines can be investigated and as a result detailedinformation on both turbomachinery and flow through the impeller such as undesiredflow separations, behaviour of fluid in case adverse pressure gradient, viscous effectson turbomachinery performance can be obtained. Today in most casesturbomachinery design depends on the designer experince. Blade design is done bysuccessice alterations in the geometry and flow analysis and/or verifications by theexperiments. This method can consume more time and result high cost.Application of inverse engineering which has been become important recently toturbomachinery brings facilities together. In inverse design approach, data obtainedfrom flow analysis is assumed as input data. Particularly blade loading distributionand other data obtained from flow analysis are specified as input parameters ininverse design method. In other words, inverse design method can be defined as theblade design method which meets the desired flow conditions. Blade loadingdistribution is the most important design parameter in this method. This methodallows the blade design with respect to specified blade loading distribution.Axial flow fans are used in wide variety of areas including air conditioning,automotive applications, home appliances and electronics applications. CFD analysisof flow through an axial flow impellers help to understand the main features thatxxaffect the fan performance. CFD does put forth any modifications to improve fanperformance. In this study main mechanism affecting the fan performance are tried tobe figured out using inverse desing method.In this study, an axial flow fan was redesigned by using inverse design method, acommercial code Turbodesign-1. Turbodesign-1 developed for 3D inviscid fluidflow is based on potential flow theory. Main inputs of the Turbodesign-1 are bladeloading distribution along spanwise and streamwise direction, stacking condition. Inthis turbomachinery design method blades are represented by sheet of vorticitywhose strength is related with the bound circulation (2?rV?). The prescribedcirculation distribution is directly related to the blade loading, pressure differenceacross the blade surfaces and by the specification of the circulation and bladethickness distribution blade shape is tried to be determined iteratively. In order todetermine the circulation, angular momentum per unit mass distribution is used,since it is directly related to the circulation. Inverse design method is a successivemethod in which the blade shape is obtained from velocity field whereas velocityfield is calculated using vortex vector. And vortex vector depends on the blade shape.Thus, in inverse design method blade shape is calculated iteratively. After calculatingthe velocity field, blade shape can be obtained by applying inviscid slip conditionwhich means that blade must be aligned with the local velocity vector.Different blade geometries are obtained by keeping the meridional geometryunchanged whereas blade loading distribution and other parameters are modified.The most important parameter in the inverse design code is the blade loading alongboth spanwise and streamwise directions. Variation in blade loading in spanwisedirection gives an idea about vortex pattern of the axial flow fan whereas variation instreamwise direction is directly related to the pressure difference between thepressure side and suction side of the blade. In this study both spanwise andstreamwise variation is considered while designing new models. Different vortexpatterns are used in order to improve the blade geometries. By having variable vortexpattern enables the designer to figure out the effect of free vortex, compound vortex,and forced vortex in design phase. One of the important parameters except fromblade loading is called stacking condition which implies the wrap angle distributionon leading edge or trailing edge of the blade. It is obtained from both experimentaland computational studies in the past that blade loading parameters have importanteffects on fan efficiency, fan performance; stacking condition has significant effectson noise level of the axial fan. Differet stacking conditions are introduced in order toget details about effect of this parameter.In this study, using variable blade loadings by changing the rV? distribution alongstreamwise and spanwise, it is aimed to be able to determine the optimum bladegeometry. Effects of blade loading parameters are concerned in order to find out theoptimum blade loading distribution. For axial flow fans the optimum blade loading isnot known, therefore it is aimed to figure out the effect of the blade loadingparameters, thus optimum blade loading. Optimum blade loading for axial flow fanbecomes an important issue. To obtain optimum blade geometries, lots of tries werecarried out.After improving new models, using rapid prototype techniques these models aremanufactured. Then performance tests are applied to each model with respect to fanstartands. Pressure rise characteristics are determined experimentally for each model.Also, efficiency test was introduced to some models, both original fan and inversexxidesigned geometry. In addition, in order to find out the effect of the tip clearance onfan performance, some additional tests were carried out using different casings.In order to verify the experimental results, CFD analysis is applied to models whichhave experimentally similar performance characteristics with base fan using ANSYSCFX commercial version. Mesh was generated using ANSYS ICEM CFD 12.0. Inorder to avoid time problem depending on the number of mesh, rotational periodicitywas used for analysis. Fluid domain involves three different parts; inlet, outlet, androtor. Of these models, inlet and outlet parts are stationary whereas rotor composesthe rotational part. Hexahedral mesh was used for all parts and all models. By usinghexahedral mesh, number of mesh required to provide convergence was reducedcompared to tetrahedral mesh. For numerical simulations, to avoid convergenceproblems inlet and outlet volumes of the flow domain were extended to longerlength. And it was confirmed that extension of the inlet and outlet domains achievedthe convergence problems.The numerical simulations were carried out with ANSYS CFX. Steady type flowanalysis was used in simulations. Turbulence model used in the simulation was SST(Shear Stress Transport). Convergence criterion was 10-8 for all simulations. For eachmodel, performance characteristics were obtained.After numerical simulations were completed, experimental and numerical results arecompared. Not all models using Turbodesign-1There have been differences betweenthe experimental and numerical characteristics, since the effect of the electric motoris ignored in numerical simulations. Nevertheless, both results provide detailedinformation on flow through axial flow fans.Numerical results reveal that undesired flow mechanisms such as tip vortex and hubvortex are reduced. Strength of the tip vortex is reduced considerably. Tip leakageflow is one of the most important flow mechanisms which increase the losses andnoise level. Within the scope of this thesis, effect of the tip clearance was able to beexamined. Effect of tip clearance on fan performance was obtained bothexperimentally and numerically. Also, hub vortex was reduced into a small area withrespect to original fan.When the performances of both original fan and inverse designed fan comparedexperimentally, they show differences in different flow rates. Performance of theboth fans beyond and above the design flow rate differs from each other. Forinstance, while at low flow rates original fan has higher performance; at high flowrates inverse designed fan has better performance.Applying the inverse desing approach results axial flow fan geometries which havehigher efficiency and provides higher pressure rise with respect to base fan.Concequently, experimental and numerical results reveal that inverse design methodis an effective way of turbomachinery design. Differing from direct method, inversedesign method enables the user to be able to control the blade loading. Therefore, itfacilitates to design blade geometries under prescribed conditions. Also, thisapproach consumes less than time direct method. The main drawback of this methodis that optimum blade loading is not known for the axial flow fans. However, aparametric study can be carried out in order to determine the optimum blade loadingfor axial flow fans. Inverse design method is considerably effective to improve thefan performance under prescribed pressure distribution, since the informationxxiiobtained from numerical methods help to gain insight about turbomachinery design.And this method should be improved to remove its drawbacks.