Aerodynamic Influence of Endwall Fences Located in the Vicinity of the Leading Edge-Endwall Junction of Nozzle Guide Vanes

Abstract

Secondary flow characteristics like horseshoe vortices and related total pressure losses decrease turbine efficiency. Computerized simulations of potentially favorable modifications in turbine systems could provide a fast, numerical and inexpensive method of evaluating their effects on flow properties: This paper consists of a comparative numerical study of the flow characteristics of a domain containing a vertical cylinder subjected to cross flow and upstream endwall modifications. Analyzing the flow around a turbine nozzle guide vane (NGV) could be simplified by modeling it as a vertical cylinder with a diameter proportional to the leading edge diameter of the blade, and adding upstream endwall fences of varying dimensions and alignments could attenuate the development of a horseshoe vortex. A commercial computational fluid dynamics (CFD) software package, Fluent, was used for the numerical analysis. To validate the modeling strategy, experimental data previously reported in the literature for conventional cylinders in cross flow were compared to the current predictions. A grid independence study was also performed. The lateral distance between the two legs of the horseshoe vortex downstream of the cylinder was decreased by 7% to 14%. All fence types effectively changed the location of the main horseshoe vortex roll-up. The height of the fence was more influential than the length of the fence in modifying flow characteristics. 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 in all cases. Also, it was noted that an endwall fence could possibly result in decreased interaction between the horseshoe vortices created by consecutive blades in a row of NGV blades, which would be expected to result in improved flow conditions within actual turbine passages.

Unsteady Flow Characteristics and Viscous Flow Losses Behind Multiple Rows of Circular Pin Fins in Coolant Channels

“…The static pressure coefficient calculated over a line on the cylinder surface, according to Equation 4, is an example to such a parameter. A grid independence study conducted in this fashion was realized in our previous publications [19], [20] which led to the current paper: The grid independence study indicated that the flow modeling technique used in our studies (the meshing done in Gambit and the flow solution conducted in Fluent) accurately captured the static pressure coefficient, and generated identical results with three different mesh sizes [19], [20] . Hence, a mesh size which will be on the same order of magnitude for all cases was selected for use.…”

“…Please refer to Gokce [19] and Gokce and Camci [20] for further methodology details, verification methods and geometrical information which are not presented here for the sake of brevity.…”

“…Keeping in mind the points mentioned about the flow around a NGV, and after having studied the previously published work, we conducted 2 CFD based studies [19], [20] . Throughout these studies, we had decided to model the NGV blade as a circular cylinder, where the diameter of the cylinder is proportional to the leading edge diameter of the blade.…”

“…This main goal was desired to be achieved without affecting the freestream region. The results indicated that these goals were attained [19], [20] .…”

“…Considering the important aspects of cooling flow within NGV and rotor blades, the published research on the topic, and following the results obtained in our previous works [19], [20] , we decided to analyze the heat transfer and cooling aspects of such flows, along with the heat transfer aspect of flow around a NGV. The circular cylinder is a good representative of the circular pin fins located within the cooling channels of turbine blades as well, therefore the circular cylinder model we use in our CFD calculations is perfectly suitable for investigating cooling flow within cooling channels, along with the flow in the vicinity of a NGV.…”