Change search
ReferencesLink to record
Permanent link

Direct link
Investigation of CFD models for compressor calculations
2004 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

CFD (computational fluid dynamics) is growing in popularity when performing analysis of compressors. The idea of the method is to approximate the exact differential equations governing the flow into algebraic equations that can be solved on a computer. Some approximations have to be made through a model, such as turbulence models, which are models for the turbulence in the flow. The method has though the ability to predict both accurate and inaccurate results and an investigation of different models available in CFD codes are made. The investigation will contain the effect upon the choice of turbulence model, the accuracy of the discretization required to achieve an accurate result and the influence of the levels of turbulent properties applied to inlet and outlet boundaries. The required accuracy of the discretization has been evaluated through Richardson extrapolation, which provides an effective order of the scheme, an extrapolated value (result for a very fine grid) and a relative gird convergence error. The investigation shows that the number of cells required for each rotor and stator domain, for a relative grid convergence error of 10% in total pressure loss is about 250.000 for wall function grids and 450.000 for low Reynolds grids. For low Reynolds grids, the correct level of y+ for the cells closest to the walls are in the solver manual told to be between 0.2 and 1.0. This is not always easy to achieve for complex flows. To check the effect of bad y+ values for low Reynolds grids, two 2D calculations have been made, one with good values and one with bad values according to the manual. The calculations shows that it is important that the y+ values for low Reynolds grids are in the correct range, in order to resolve the boundary layers in an accurate manner. Incorrect values might predict separation, while correct values do not. The influence of the levels of turbulent properties (intensity and length scale) applied to inlet and outlet boundaries are investigated through a factorial design. The investigation shows that it is the intensity level that affects the results the most. An increase in intensity level form 2% to 15% increases the total pressure loss for one rotor only with about 15-20% depending on turbulence model. For 3½ stage compressor calculations the low Reynolds turbulence models predicts efficiencies that are about 2-3 percentage units lower than experimental data. The standard k-e turbulence model with law of the wall applied to no slip boundaries however predicts efficiencies much closer the same.

Place, publisher, year, edition, pages
2004.
Keyword [en]
Technology, CFD, Compressor, Factorial design, Richardson extrapolation
Keyword [sv]
Teknik
Identifiers
URN: urn:nbn:se:ltu:diva-54277ISRN: LTU-EX--04/319--SELocal ID: b3ae3e0f-9f49-4366-9a15-a88535f46b90OAI: oai:DiVA.org:ltu-54277DiVA: diva2:1027658
Subject / course
Student thesis, at least 30 credits
Educational program
Mechanical Engineering, master's level
Examiners
Note
Validerat; 20101217 (root)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

Open Access in DiVA

No full text

Search outside of DiVA

GoogleGoogle Scholar

Total: 3 hits
ReferencesLink to record
Permanent link

Direct link