COMPARATIVE NUMERICAL ANALYSIS OF FLOW DEVELOPMENT IN THE PRESENCE OF ENDWALL FENCES UPSTREAM OF A VERTICAL CYLINDER IN CROSS FLOW
Open Access
- Author:
- Gokce, Zeki Ozgur
- Graduate Program:
- Aerospace Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- None
- Committee Members:
- Cengiz Camci, Thesis Advisor/Co-Advisor
Cengiz Camci, Thesis Advisor/Co-Advisor - Keywords:
- horseshoe vortex formation around a vertical cylin
upstream endwall fences - Abstract:
- ABSTRACT Secondary flow properties and related total pressure losses have negative effects on the performance of turbines. Horseshoe vortices which complicate turbine flow are particularly adverse determinants of turbine function. The development of horseshoe vortices within turbines could possibly be decreased by modifying the environment through which the flow passes. However, there are a large number of modifications that could be designed for this purpose. Thus, studying each one experimentally under real time conditions would be very difficult. Studying computerized simulations of relevant modifications in modeled turbine systems, on the other hand, could provide a fast, numerical and inexpensive method of evaluating the effects of such modifications on flow characteristics and thereby contribute to improved turbine design by highlighting the options most likely to be beneficial, as well as preventing loss of man power and economical resources for the study of unfavorable configurations. This view comprises the starting point of the current study which focuses on comparative analysis of flow characteristics in a domain containing a vertical cylinder subjected to a relatively uniform duct flow and endwall fences placed upstream of the cylinder. The vertical cylinder in cross flow was chosen as the best virtual representative of the leading edge diameter of a turbine nozzle guide vane (NGV) blade. The main postulations of this research were that analyzing the flow around a turbine NGV blade could be simplified by modeling it as a vertical cylinder with a diameter equal to the leading edge diameter of the NGV blade and that adding upstream endwall fences could, completely or partly, attenuate the effects of the horseshoe vortex that naturally forms around the cylinder in cross flow and thus reduce total pressure loss within the horseshoe vortex region. The goal in mention was tested by inserting three fences of various shapes at a location considered critical to the formation of the horseshoe vortex. To our knowledge, the current study is the first comparative numerical study of flow characteristics around a system consisting of a vertical cylinder subjected to cross flow and endwall fences placed upstream of the cylinder. A commercial computational fluid dynamics (CFD) software package, Fluent, was used to conduct the numerical analysis. The flow within the computational domain was assumed to be fully turbulent, which would be the best representation of the flow within a turbine, just downstream of a combustor. Numerical results derived for different fence configurations were compared with each other and the baseline configuration which models the flow around the cylinder only. For further validation of the modeling strategy, conventional cylinder in cross flow data available in the literature were reobtained under virtual conditions using the parameters provided by the authors of the experiments. The current study showed that, placing an endwall fence 1 mm high, 1 mm deep and either 4 mm or 2 mm long upstream of the cylinder, results in a 7% decrease in the lateral distance between the two legs of the horseshoe vortex when compared to the baseline case, downstream of the cylinder. A 1 mm high, 1 mm deep and 4 mm long fence having oblique lateral sides placed upstream of the cylinder creates a 14% decrease in the lateral distance between the two legs of the horseshoe vortex, downstream of the cylinder. These findings indicate that an endwall fence can result in decreased interaction between the horseshoe vortices created by consecutive blades in a row of NGV blades, which would result in improved flow conditions within turbine passages. The oblique fence provided an additional benefit: Secondary vortices were completely removed, both in front of and behind the fence. All fence types effectively changed the location of the main horseshoe vortex roll-up. Furthermore, it was observed that the height of the fence was more influential than the length of the fence, in modifying flow characteristics. When compared to the baseline case, the existence of the fences slightly increased the mass averaged total pressure loss far downstream of the cylinder; however, beneficial near-fence flow characteristics were observed for all cases containing a fence. Investigating the effect of adding upstream endwall fences on the flow attributes of physical models of turbine systems is warranted. Therefore, detailed wind tunnel tests and other experiments must be conducted to further understand the effects of using endwall fences to reduce the detrimental effects of the horseshoe vortex.