Free-Stream Turbulence Effects on Film Cooling With Shaped Holes

2002 ◽  
Author(s):  
Christian Saumweber ◽  
Achmed Schulz ◽  
Sigmar Wittig
Keyword(s):  
Turbulence Intensity ◽  
Film Cooling ◽  
Free Stream ◽  
Operating Conditions ◽  
Integral Length Scale ◽  
Length Scale ◽  
Free Stream Turbulence ◽  
Blowing Ratio ◽  
Cylindrical Holes ◽  
The Impact

A comprehensive set of generic experiments has been conducted to investigate the effect of elevated free-stream turbulence on film cooling performance of shaped holes. A row of three cylindrical holes as a reference case, and two rows of holes with expanded exits, a fanshaped (expanded in lateral direction), and a laidback fanshaped hole (expanded in lateral and streamwise direction) have been employed. With an external (hot gas) Mach number of Mam = 0.3 operating conditions are varied in terms of free-stream turbulence intensity (up to 11%), integral length scale at constant turbulence intensity (up to 3.5 hole inlet diameters), and blowing ratio. The temperature ratio is fixed at 0.59 leading to an engine-like density ratio of 1.7. The results indicate that shaped and cylindrical holes exhibit very different reactions to elevated free-stream turbulence levels. For cylindrical holes film cooling effectiveness is reduced with increased turbulence level at low blowing ratios whereas a small gain in effectiveness can be observed at high blowing ratios. For shaped holes, increased turbulence intensity is detrimental even for the largest blowing ratio (M = 2.5). In comparison to the impact of turbulence intensity the effect of varying the integral length scale is found to be of minor importance. Finally the effect of elevated free-stream turbulence in terms of heat transfer coefficients was found to be much more pronounced for the shaped holes.

Free-Stream Turbulence Effects on Film Cooling With Shaped Holes

2003 ◽  
Vol 125 (1) ◽  
pp. 65-73 ◽  
Author(s):  
Christian Saumweber ◽  
Achmed Schulz ◽  
Sigmar Wittig
Keyword(s):  
Turbulence Intensity ◽  
Film Cooling ◽  
Free Stream ◽  
Operating Conditions ◽  
Integral Length Scale ◽  
Length Scale ◽  
Free Stream Turbulence ◽  
Blowing Ratio ◽  
Cylindrical Holes ◽  
The Impact

A comprehensive set of generic experiments has been conducted to investigate the effect of elevated free-stream turbulence on film cooling performance of shaped holes. A row of three cylindrical holes as a reference case, and two rows of holes with expanded exits, a fanshaped (expanded in lateral direction), and a laidback fanshaped hole (expanded in lateral and streamwise direction) have been employed. With an external (hot gas) Mach number of Mam=0.3 operating conditions are varied in terms of free-stream turbulence intensity (up to 11%), integral length scale at constant turbulence intensity (up to 3.5 hole inlet diameters), and blowing ratio. The temperature ratio is fixed at 0.59 leading to an enginelike density ratio of 1.7. The results indicate that shaped and cylindrical holes exhibit very different reactions to elevated free-stream turbulence levels. For cylindrical holes film cooling effectiveness is reduced with increased turbulence level at low blowing ratios whereas a small gain in effectiveness can be observed at high blowing ratios. For shaped holes, increased turbulence intensity is detrimental even for the largest blowing ratio M=2.5. In comparison to the impact of turbulence intensity the effect of varying the integral length scale is found to be of minor importance. Finally, the effect of elevated free-stream turbulence in terms of heat transfer coefficients was found to be much more pronounced for the shaped holes.

Evaluating the Impact of Free-Stream Turbulence on Convective Cooling of Overhead Conductors Using Large Eddy Simulations

2019 ◽  
Vol 141 (6) ◽  
Author(s):  
Mohamed Abdelhady ◽  
David H. Wood
Keyword(s):  
Wind Speed ◽  
Real Time ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Diameter Ratio ◽  
Weather Data ◽  
Length Scale ◽  
Free Stream Turbulence ◽  
Large Eddy ◽  
The Impact

The international trend of using renewable energy sources for generating electricity is increasing, partly through harvesting energy from wind turbines. Increasing electric power transmission efficiency is achievable through using real-time weather data for power line rating, known as real-time thermal rating (RTTR), instead of using the worst case scenario weather data, known as static rating. RTTR is particularly important for wind turbine connections to the grid, as wind power output and overhead conductor rating both increase with increasing wind speed, which should significantly increase real-time rated conductor from that of statically rated. Part of the real-time weather data is the effect of free-stream turbulence, which is not considered by the commonly used overhead conductor codes, Institute of Electrical and Electronics Engineers (IEEE) 738 and International Council on Large Electric Systems (CIGRÉ) 207. This study aims to assess the effect free-stream turbulence on IEEE 738 and CIGRÉ 207 forced cooling term. The study uses large eddy simulation (LES) in the ANSYS fluent software. The analysis is done for low wind speed, corresponding to Reynolds number of 3000. The primary goal is to calculate Nusselt number for cylindrical conductors with free-stream turbulence. Calculations showed an increase in convective heat transfer from the low turbulence value by ∼30% at turbulence intensity of 21% and length scale to diameter ratio of 0.4; an increase of ∼19% at turbulence intensity of 8% and length scale to diameter ratio of 0.4; and an increase of ∼15% at turbulence intensity of 6% and length scale to diameter ratio of 0.6.

Evaluating the Impact of Free-Stream Turbulence on Convective Cooling of Overhead Conductors Using Large Eddy Simulations (LES)

2018 ◽  
Author(s):  
Mohamed Abdelhady ◽  
David H. Wood
Keyword(s):  
Turbulence Intensity ◽  
Wind Farm ◽  
Free Stream ◽  
Diameter Ratio ◽  
Length Scale ◽  
Ansys Fluent ◽  
Free Stream Turbulence ◽  
Low Wind Speed ◽  
Large Eddy ◽  
The Impact

This study uses Large Eddy Simulation in the ANSYS Fluent software to assess the accuracy of the forced cooling term for the overhead conductor codes, IEEE 738 [1] and CIGRÉ 207 [2], for Real Time Thermal Rating of a wind farm power line. The analysis is done for low wind speed, corresponding to Reynolds Number of 3,000. The primary goal is to calculate Nusselt Number for cylindrical conductors with free-stream turbulence. Calculations showed an increase in convective heat transfer from the low turbulence value by ∼ 30 % at turbulence intensity of 21% and length scale to diameter ratio of 0.4; and an increase of ∼ 19 % at turbulence intensity of 8% and length scale to diameter ratio of 0.4.

Film Cooling With Forward and Backward Injection for Cylindrical and Fan-Shaped Holes Using PSP Measurement Technique

2014 ◽  
Author(s):  
Andrew F. Chen ◽  
Shiou-Jiuan Li ◽  
Je-Chin Han
Keyword(s):  
Film Cooling ◽  
Free Stream ◽  
Density Ratio ◽  
Cooling Effectiveness ◽  
Film Cooling Effectiveness ◽  
Free Stream Turbulence ◽  
Blowing Ratio ◽  
Cylindrical Holes ◽  
Density Ratios ◽  
Better Than

A systematic study was performed to investigate the combined effects of hole geometry, blowing ratio, density ratio and free-stream turbulence intensity on flat plate film cooling with forward and backward injection. Detailed film cooling effectiveness distributions were obtained using the steady state pressure sensitive paint (PSP) technique. Four common film-hole geometries with forward injection were used in this study: simple angled cylindrical holes and fan-shaped holes, and compound angled (β = 45°) cylindrical holes and fan-shaped holes. Additional four film-hole geometries with backward injection were tested by reversing the injection direction from forward to backward to the mainstream. There are seven holes in a row on each plate and each hole is 4 mm in diameter. The hole length to diameter ratio is 7.5. The blowing ratio effect was studied at 10 different blowing ratios ranging from M = 0.3 to M = 2.0. The coolant to main stream density ratio (DR) effect was studied by using foreign gases with DR = 1 (N2), 1.5 (CO2), and 2 (15% SF6 + 85% Ar). The free stream turbulence intensity effect was tested at 0.5% and 6%. The results show higher density coolant provides higher effectiveness than lower density coolant, fan-shaped holes perform better than cylindrical holes, and compound angled holes are better than simple angled holes. In general, the results show the film cooling effectiveness with backward injection is greatly reduced for shaped holes as compared with the forward injection. However, significant improvements can be seen in both simple angled and compound angled cylindrical holes at higher blowing ratios and density ratio (DR = 2). Comparison was made between experimental data and empirical correlations for simple angled fan-shaped holes at engine representative density ratios. An improved correlation which covers a wider range of density ratios (DR = 1.0 to DR = 2.0) is proposed.

scholarly journals Measurements of the Influence of Integral Length Scale on Stagnation Region Heat Transfer

1997 ◽  
Vol 3 (2) ◽  
pp. 117-132 ◽  
Author(s):  
G. James Van Fossen ◽  
Chan Y. Ching
Keyword(s):  
Heat Transfer ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Leading Edge ◽  
Integral Length Scale ◽  
Width Ratio ◽  
Length Scale ◽  
Grid Data ◽  
Stagnation Region ◽  
Free Stream Turbulence

The purpose of the present work was twofold: first, to determine if a length scale existed that would cause the greatest augmentation in stagnation region heat transfer for a given turbulence intensity and second, to develop a prediction tool for stagnation heat transfer in the presence of free stream turbulence. Toward this end, a model with a circular leading edge was fabricated with heat transfer gages in the stagnation region. The model was qualified in a low turbulence wind tunnel by comparing measurements with Frossling's solution for stagnation region heat transfer in a laminar free stream. Five turbulence generating grids were fabricated; four were square mesh, biplane grids made from square bars. Each had identical mesh to bar width ratio but different bar widths. The fifth grid was an array of fine parallel wires that were perpendicular to the axis of the cylindrical leading edge. Turbulence intensity and integral length scale were measured as a function of distance from the grids. Stagnation region heat transfer was measured at various distances downstream of each grid. Data were taken at cylinder Reynolds numbers ranging from 42,000 to 193,000. Turbulence intensities were in the range 1.1 to 15.9 percent while the ratio of integral length scale to cylinder diameter ranged from 0.05 to 0.30. Stagnation region heat transfer augmentation increased with decreasing length scale. An optimum scale was not found. A correlation was developed that fit heat transfer data for the square bar grids to within ±4%. The data from the array of wires were not predicted by the correlation; augmentation was higher for this case indicating that the degree of isotropy in the turbulent flow field has a large effect on stagnation heat transfer. The data of other researchers are also compared with the correlation.

Predicting Adiabatic Film Effectiveness of a Turbine Vane by Two-Equation Turbulence Models

2015 ◽  
Author(s):  
Prasert Prapamonthon ◽  
Huazhao Xu ◽  
Jianhua Wang ◽  
Ge Li
Keyword(s):  
Turbulence Intensity ◽  
Film Cooling ◽  
Free Stream ◽  
Turbulence Models ◽  
Leading Edge ◽  
Suction Side ◽  
Operating Conditions ◽  
Pressure Side ◽  
Free Stream Turbulence ◽  
Turbine Vane

The thermal efficiency of gas turbine engines increases with turbine inlet temperature (TIT) directly. However, the TIT is limited by the allowable temperature of current blade materials. Film cooling technique is an effective method to maintain turbine vane working smoothly under high TIT conditions. The adiabatic film effectiveness has been widely employed to understand film cooling mechanism. Therefore, the prediction of the adiabatic effectiveness of gas turbine engines under real operating conditions is essential. The showerhead film cooled turbine vane reported by L. P. Timko (NASA CR-168289) is adopted in the present study. There are two rows of film holes on the leading edge, three rows on the pressure side, and two rows on the suction side. All holes are cylindrical, which are placed at an angle of 45 degrees to the vane surface in the span-wise direction. This numerical investigation discusses the influences of free stream turbulence intensity on the adiabatic film effectiveness in the vane leading edge region and its vicinity. Five two-equation turbulence models based on Reynolds Averaged Navier-Stokes (RANS) are employed to predict the adiabatic film effectiveness under real operating conditions at a blowing ratio (BR) of 1.41 and three free stream turbulence intensities (Tu=3.3, 10, and 20%). The adiabatic film effectiveness on the vane surface at 8, 52.5, and 89% span in an x/C range between −0.4 and 0.4 is presented. Obviously, the numerical results predicted by all five models show that on the suction side, the increasing free stream turbulence intensity can reduce film effectiveness except at 8% span. On the pressure side, the RNG k-ε, Realizable k-ε and SST k-ω models predict the same trend of the adiabatic film effectiveness, especially the RNG k-ε and SST k-ω models. Those three models predict that the locally adiabatic film effectiveness (especially near film holes) can be improved when turbulence intensity increases. However, at a span of 89% within the x/C range between −0.4 and −0.2, all k-ε models and SST k-ω model predict that the increase of turbulence intensity can reduce the adiabatic film effectiveness. In addition, the film effectiveness contours show a significant variation of film effectiveness predicted by the five turbulence models on the leading edge when turbulence intensity increases. For the near-pressure side, all models except the Standard k-ω model predict that the high turbulence intensity can reduce the film spreading from film holes dramatically.

A Parametric Numerical Study of the Effects of Freestream Turbulence Intensity and Length Scale on Anti-Vortex Film Cooling Design at High Blowing Ratio

2013 ◽  
Author(s):  
Timothy W. Repko ◽  
Andrew C. Nix ◽  
James D. Heidmann
Keyword(s):  
Turbulence Intensity ◽  
Film Cooling ◽  
Numerical Study ◽  
Length Scale ◽  
Freestream Turbulence ◽  
Cooling Effectiveness ◽  
Length Scales ◽  
Film Cooling Effectiveness ◽  
Blowing Ratio ◽  
Cooling Design

An advanced, high-effectiveness film-cooling design, the anti-vortex hole (AVH) has been investigated by several research groups and shown to mitigate or counter the vorticity generated by conventional holes and increase film effectiveness at high blowing ratios and low freestream turbulence levels. [1, 2] The effects of increased turbulence on the AVH geometry were previously investigated and presented by researchers at West Virginia University (WVU), in collaboration with NASA, in a preliminary CFD study [3] on the film effectiveness and net heat flux reduction (NHFR) at high blowing ratio and elevated freestream turbulence levels for the adjacent AVH. The current paper presents the results of an extended numerical parametric study, which attempts to separate the effects of turbulence intensity and length-scale on film cooling effectiveness of the AVH. In the extended study, higher freestream turbulence intensity and larger scale cases were investigated with turbulence intensities of 5, 10 and 20% and length scales based on cooling hole diameter of Λx/dm = 1, 3 and 6. Increasing turbulence intensity was shown to increase the centerline, span-averaged and area-averaged adiabatic film cooling effectiveness. Larger turbulent length scales were shown to have little to no effect on the centerline, span-averaged and area-averaged adiabatic film-cooling effectiveness at lower turbulence levels, but slightly increased effect at the highest turbulence levels investigated.

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