Tersine Tasarım Yönteminin Düşük Hızlı Eksenel Fanlara Uygulanması

Abstract

Bu çalışmada türbomakinalarda kanat geometrisinin tanımlanan basınç dağılımına bağlı olarak elde edildiği tersine tasarım yöntemi üzerinde durulacaktır. Teknolojinin gelişmesiyle birlikte sayısal akışkanlar dinamiğinde(SAD) de önemli gelişmeler meydana gelmiştir. Akış denklemlerinin bilgisayarlar kullanılarak çözülmesi türbomakinalarda kanat tasarımında önemli bir adım olarak görülmektedir. SAD, türbomakinalarda hidrodinamik ve aerodinamik analiz; sonrasında optimizasyon için oldukça önemli bir konuma gelmiştir. Bugün çeşitli yöntemlerle türbomakinalarda 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 ve türbomakinaya ait detaylı bilgiler elde edilebilmektedir. Günümüzde türbomakina tasarımı pek çok durumda tasarımcının deneyimine bağlı olarak yapılmaktadır. Tasarım, var olan geometrinin ardışık yaklaşımlarla değiştirilmesi ve sonrasında akış analizi ve(veya) deneylerle doğrulanması şeklinde yapılmaktadır. Bu yöntem, tasarımcıya bağlı olarak hem zaman hem de maliyet sorununa neden olabilir. Son yıllarda gittikçe önemli bir konu olan tersine mühendisliğin türbomakinalara uygulanması kanat tasarımında büyük kolaylıkları beraberinde getirmiştir. Tersine tasarım yaklaşımında, akış analizi sonucunda elde edilen veriler giriş verileri olarak kabul edilir. Akış analizleri sonucunda elde edilen kanat üzerindeki yük dağılımı (basınç dağılımı) başta olmak üzere diğer parametreler tersine tasarım yönteminde giriş verisi olmaktadır. Tersine tasarım yöntemi, farklı bir ifadeyle “istenilen akış özelliklerini sağlayan kanat tasarımı” olarak tanımlanabilir. Kanat üzerindeki yük dağılımı bu yaklaşımda kullanılan en önemli tasarım parametresidir. Bu yaklaşım belirtilen yük dağılımına (gerekli basınç dağılımı) uygun kanat tasarımına olanak sağlamaktadır. Tersine tasarım yöntemi kullanılarak var olan bir fan geometrisinin yeniden tasarımı yapılmıştır. Meridyenel geometri sabit tutup yük dağılımı ve diğer parametrelerin değiştirilmesiyle farklı kanat geometrileri elde edilmiştir. Elde edilen fan geometrileri hızlı prototipleme yöntemiyle imal edilmiştir. Daha sonra bu geometrilere performans deneyleri uygulanmıştır. Orijinal fan ile benzer performans gösteren model/modellere ANSYS CFX ile SAD analizi uygulanmarak sayısal ve deneysel sonuçlar karşılaştırılmıştır. Yapılan çalışmalar sonucunda merideyenel geometri sabit tutularak yüksek verime sahip ve yüksek basınç artışı sağlayan fan geometrisi elde edilmiştir.

In this thesis a 3-dimensional inverse design method in which the blade geometry in turbomachinery 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 in turbomachinery blade design to use computers in order to solve flow equations. Computational fluid dynamics has been become a considerably important way in turbomachinery for both hydrodynamically and aerodynamically analysis and then for optimization process. Today using various methods 3-dimensional flow which is assumed complicated in turbomachines can be investigated and a detailed information on both turbomachinery and flow such as undesired flow seperations, behaviour of fluid in case adverse pressure gradient, viscous effects on turbomachinery performance can be obtained. In the field of turbomachinery, there are two main approaches for aerodynamic design. The first and basic method is defined as direct method which means that flow field through an impeller is tried to be determined for a given blade shape. The second and respectively newer method is inverse design method which means that the 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 and making changes at blade geometry using results obtained from numerical simulations. Today using various methods 3-dimensional flow which is assumed complicated in turbomachines can be investigated and as a result detailed information on both turbomachinery and flow through the impeller such as undesired flow separations, behaviour of fluid in case adverse pressure gradient, viscous effects on turbomachinery performance can be obtained. Today in most cases turbomachinery design depends on the designer experince. Blade design is done by successice alterations in the geometry and flow analysis and/or verifications by the experiments. This method can consume more time and result high cost. Application of inverse engineering which has been become important recently to turbomachinery brings facilities together. In inverse design approach, data obtained from flow analysis is assumed as input data. Particularly blade loading distribution and other data obtained from flow analysis are specified as input parameters in inverse design method. In other words, inverse design method can be defined as the blade design method which meets the desired flow conditions. Blade loading distribution is the most important design parameter in this method. This method allows 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 analysis of flow through an axial flow impellers help to understand the main features that xx affect the fan performance. CFD does put forth any modifications to improve fan performance. In this study main mechanism affecting the fan performance are tried to be figured out using inverse desing method. In this study, an axial flow fan was redesigned by using inverse design method, a commercial code Turbodesign-1. Turbodesign-1 developed for 3D inviscid fluid flow is based on potential flow theory. Main inputs of the Turbodesign-1 are blade loading distribution along spanwise and streamwise direction, stacking condition. In this turbomachinery design method blades are represented by sheet of vorticity whose strength is related with the bound circulation (2πrVθ). The prescribed circulation distribution is directly related to the blade loading, pressure difference across the blade surfaces and by the specification of the circulation and blade thickness distribution blade shape is tried to be determined iteratively. In order to determine the circulation, angular momentum per unit mass distribution is used, since it is directly related to the circulation. Inverse design method is a successive method in which the blade shape is obtained from velocity field whereas velocity field is calculated using vortex vector. And vortex vector depends on the blade shape. Thus, in inverse design method blade shape is calculated iteratively. After calculating the velocity field, blade shape can be obtained by applying inviscid slip condition which means that blade must be aligned with the local velocity vector. Different blade geometries are obtained by keeping the meridional geometry unchanged whereas blade loading distribution and other parameters are modified. The most important parameter in the inverse design code is the blade loading along both spanwise and streamwise directions. Variation in blade loading in spanwise direction gives an idea about vortex pattern of the axial flow fan whereas variation in streamwise direction is directly related to the pressure difference between the pressure side and suction side of the blade. In this study both spanwise and streamwise variation is considered while designing new models. Different vortex patterns are used in order to improve the blade geometries. By having variable vortex pattern 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 from blade loading is called stacking condition which implies the wrap angle distribution on leading edge or trailing edge of the blade. It is obtained from both experimental and computational studies in the past that blade loading parameters have important effects on fan efficiency, fan performance; stacking condition has significant effects on noise level of the axial fan. Differet stacking conditions are introduced in order to get details about effect of this parameter. In this study, using variable blade loadings by changing the rVθ distribution along streamwise and spanwise, it is aimed to be able to determine the optimum blade geometry. Effects of blade loading parameters are concerned in order to find out the optimum blade loading distribution. For axial flow fans the optimum blade loading is not known, therefore it is aimed to figure out the effect of the blade loading parameters, thus optimum blade loading. Optimum blade loading for axial flow fan becomes an important issue. To obtain optimum blade geometries, lots of tries were carried out. After improving new models, using rapid prototype techniques these models are manufactured. Then performance tests are applied to each model with respect to fan startands. Pressure rise characteristics are determined experimentally for each model. Also, efficiency test was introduced to some models, both original fan and inverse xxi designed geometry. In addition, in order to find out the effect of the tip clearance on fan performance, some additional tests were carried out using different casings. In order to verify the experimental results, CFD analysis is applied to models which have experimentally similar performance characteristics with base fan using ANSYS CFX commercial version. Mesh was generated using ANSYS ICEM CFD 12.0. In order to avoid time problem depending on the number of mesh, rotational periodicity was used for analysis. Fluid domain involves three different parts; inlet, outlet, and rotor. Of these models, inlet and outlet parts are stationary whereas rotor composes the rotational part. Hexahedral mesh was used for all parts and all models. By using hexahedral mesh, number of mesh required to provide convergence was reduced compared to tetrahedral mesh. For numerical simulations, to avoid convergence problems inlet and outlet volumes of the flow domain were extended to longer length. And it was confirmed that extension of the inlet and outlet domains achieved the convergence problems. The numerical simulations were carried out with ANSYS CFX. Steady type flow analysis was used in simulations. Turbulence model used in the simulation was SST (Shear Stress Transport). Convergence criterion was 10-8 for all simulations. For each model, performance characteristics were obtained. After numerical simulations were completed, experimental and numerical results are compared. Not all models using Turbodesign-1There have been differences between the experimental and numerical characteristics, since the effect of the electric motor is ignored in numerical simulations. Nevertheless, both results provide detailed information on flow through axial flow fans. Numerical results reveal that undesired flow mechanisms such as tip vortex and hub vortex are reduced. Strength of the tip vortex is reduced considerably. Tip leakage flow is one of the most important flow mechanisms which increase the losses and noise level. Within the scope of this thesis, effect of the tip clearance was able to be examined. Effect of tip clearance on fan performance was obtained both experimentally and numerically. Also, hub vortex was reduced into a small area with respect to original fan. When the performances of both original fan and inverse designed fan compared experimentally, they show differences in different flow rates. Performance of the both fans beyond and above the design flow rate differs from each other. For instance, while at low flow rates original fan has higher performance; at high flow rates inverse designed fan has better performance. Applying the inverse desing approach results axial flow fan geometries which have higher efficiency and provides higher pressure rise with respect to base fan. Concequently, experimental and numerical results reveal that inverse design method is an effective way of turbomachinery design. Differing from direct method, inverse design method enables the user to be able to control the blade loading. Therefore, it facilitates to design blade geometries under prescribed conditions. Also, this approach consumes less than time direct method. The main drawback of this method is that optimum blade loading is not known for the axial flow fans. However, a parametric study can be carried out in order to determine the optimum blade loading for axial flow fans. Inverse design method is considerably effective to improve the fan performance under prescribed pressure distribution, since the information xxii obtained from numerical methods help to gain insight about turbomachinery design. And this method should be improved to remove its drawbacks.