The process of finding the optimum blade loading in many problems can start by considering the results of a CFD computation of an equivalent conventionally designed blade row. By detailed study of the flow field in the blade one can understand the main flow mechanism which generate most of the loss. For example it could be flow separation, secondary flows, shock waves, tip clearance etc. Appropriate choice of blade loading can then help to minimize losses. For example to control tip clearance in centrifugal compressor an aftloaded distribution can be beneficial as it is important to reduce tip clearance in the inducer section of the impeller.. One important aspect of the design is that once the optimum blade loading for a particular type of machine if found the results are quite general and can be applied with ease to machine of similar type.
In the past many types of design specifications have been employed. Many of the early 2D methods used blade surface pressure on both surfaces . However, such an approach has server limitation as the blade thickness can not specified and in many cases it is impossible to ensure the closure of the blade. Alternative approaches to get round this problem has been based on the specification of the blade pressure distribution on one surface, normally the suction surface, and the blade thickness. However, both of these approaches can not be readily applied in 3D as the pressure distribution at each streamline is dependent on the streamline patterns and the distribution of pressure at other streamwise sections. This so called compatability condition means that methods based on specification of surface pressure on either one or two blade surfaces may not always provide a solution for a specified pressure distribution. In TURBOdesign1 and Method 1 of TURBOdesign2, the blade loading or meridional derivative of rVθ is specified together with blade thickness. This provides a number of advantages. Firstly the method can be applied in a robust and consistent manner in 3D. Also the blade loading is directly related to pressure jump across the blade and hence one can have control over the 3D pressure field. In addition the area under the blade loading diagram for each streamwise section is directly related to the specific work and hence once can implicitly ensure that the blade is designed for a given specific work.
The main input parameters are Rotating Speed, Inlet Flow Distribution (which are specified), the Meridional Profile, Blade Number and Blade Thickness (which are assumed based on experience and structural strength) with the most important parameters being the Blade Loading and the Stacking Condition, (which are specified). Any assumed data may be optimised separately once other parameters are optimised.
Existing version of TURBOdesign2 is limited to design of compressible axial machines. The existing version os also limited to the design of a single blade row at a time.
- Inviscid theory.
- Boundary layer growth and separation are not modeled. User needs to check the viscous effects by using CFD analysis.
- Design code for single blade row.
- Thin blade theory : In the case of the thick blade such as axial turbine blades, accurate control of pressure distribution on the blade can prove difficult, but in many success have already been accomplished with the assessment of the CFD code.
- Poor convergence of the 3D version of the code for low specific speed pump applications. However, the Actuator Duct or throughflow version of the code can be used for the design of these types of machines.
- Geometry is optimized from a fluid dynamic point of view : Manufacturing and stress considerations must be taken into account separately and in some cases manufacturing requirements may require a compromise on performance.
The current version of TURBOdesign-1 does not allow users to directly specify geometric constaints. However, there are a number of ways in which the blade geometry can be simplified by the choice of the blade loading specification. ADT has developed considerable know-how in this area which it shares with its customers.
In TURBOdesign1 it is possible to specify the normal thickness which in most cases should help to ensure the structural integrity of the design. However in some cases TURBOdesign1 and TURBOdesign2 can create complicated 3D blade geometries. Care should be taken in the choice of blade loading and stacking conditions in order to minimize bending stresses. ADT has developed considerable amount of know-how in this area which is made available to customers of TURBOdeisgn1 and TURBOdesign2. Also the blade geometry can be easily exported to FEA codes through the IGES and STL file formats for detailed analysis of the stress and vibration characteristic of the designed blade.
Yes, it is. TURBOdesign2 uses 3D Euler solver including viscous effects to solve flow field in the blade passage.
TURBOdesign1 is a design software for turbomachines, and it doesn't have the functionality for the performance prediction and CFD analysis capability. However a Q3D inviscid flow solver will be implemented in TURBOdesign1 in the near future to enable the computation of blade loading on existing blade rows. This code will help to convert conventional design database into one based on the loading distribution. Also in early 2005 ADT will release an accurate and fast 3D CFD code as an optional package to be used with TURBOdesign1 .
TURBOdesign2 essentially has the capability to solve 3D flow field using 3D Euler solver including viscous effects as other CFD codes. Users can analyze the flow field and performance of the existing blade. However, the most important feature of TURBOdesign2 is its 3D inverse design capability to design the blade geometry to realize the specified loading distribution. Existing CFD code can analyze the flow filed in the turbomachinery but never suggest the direction of the blade geometry modification from the results.
TURBOdesign1 is quite different from CFD. TURBOdesign1 is an inverse design software for turbomachines in which the 3D blade geometry will be calculated inversely from the specified loading distribution, and it doesn't have the functionality for the performance prediction and CFD analysis capability. However TURBOdesign1 not only computes the blade geometry but it also provides very accurate predictions of the 3D inviscid pressure field, which compares quite well with the corresponding predictions of the blade surface pressure distribution.
TURBOdesign1 has the functionality to convert the geometry data from inverse design to other CFD, structural analysis or CAD software. The following file conversions are supported:
- Fluent G/Turbo
- CFX BladeGen
- CFX TurboGrid
- Es Turbo (STARCD)
Existing database should be the good start point for the inverse design. User can use existing meridional geometry, thickness distribution and design work requirement from the existing experimental or analysis results. Also the loading distribution of existing designs can be obtained from CFD computations and used as a starting point in the choice of blade loading when using TURBOdesign-1. In the next release of TURBOdesign-1 (version 2.3) planned for Q2 of 2005 a quasi-3D analysis code will be provided which enable users to obtain the loading distribution on any existing blades with ease and then import that into TURBOdesign-1 as a starting point of their redesign process.