The Influence of Leading Edge Diameter on Stagnation Region Heat Transfer Augmentation Including Effects of Turbulence Level, Scale, and Reynolds Number

2012 ◽  
Vol 135 (1) ◽  
Author(s):  
Preethi Gandavarapu ◽  
Forrest E. Ames
Keyword(s):  
Heat Transfer ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Test Surface ◽  
Bypass Transition ◽  
Stagnation Region ◽  
Heat Transfer Augmentation ◽  
Wide Range ◽  
Turbulence Condition ◽  
Level Scale

Stagnation region heat transfer measurements have been acquired on two large cylindrical leading edge test surfaces having a four to one range in leading edge diameter. Heat transfer measurements have been acquired for six turbulence conditions including three grid conditions, two aero-combustor conditions, and a low turbulence condition. The data have been run over an eight to one range in Reynolds numbers for each test surface with Reynolds numbers ranging from 62,500 to 500,000 for the large leading edge and 15,625 to 125,000 for the smaller leading edge. The data show augmentation levels of up to 110% in the stagnation region for the large leading edge. However, the heat transfer results for the large cylindrical leading edge do not appear to infer a significant level of turbulence intensification in the stagnation region. The smaller cylindrical leading edge shows more consistency with earlier stagnation region heat transfer results correlated on the TRL parameter. These results indicate that the intensification of approaching turbulence is more prevalent with the more rapid straining of the smaller leading edge. The downstream regions of both test surfaces continue to accelerate the flow but at a much lower rate than the leading edge. Bypass transition occurs in these regions providing a useful set of data to ground the prediction of transition onset and length over a wide range of Reynolds numbers and turbulence intensity and scales.

The Influence of Leading Edge Diameter on Stagnation Region Heat Transfer Augmentation Including Effects of Turbulence Level, Scale, and Reynolds Number

2011 ◽  
Author(s):  
P. Gandavarapu ◽  
F. E. Ames
Keyword(s):  
Heat Transfer ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Test Surface ◽  
Bypass Transition ◽  
Stagnation Region ◽  
Heat Transfer Augmentation ◽  
Wide Range ◽  
Turbulence Condition ◽  
Level Scale

Stagnation region heat transfer measurements have been acquired on two large cylindrical leading edge test surfaces having a four to one range in leading edge diameter. Heat transfer measurements have been acquired for six turbulence conditions including three grid conditions, two aero-combustor conditions, and a low turbulence condition. The data have been run over an eight to one range in Reynolds numbers for each test surface with Reynolds numbers ranging from 62,500 to 500,000 for the large leading edge and 15,625 to 125,000 for the smaller leading edge. The data show augmentation levels of up to 110% in the stagnation region for the large leading edge. However, the heat transfer results for the large cylindrical leading edge do not appear to infer a significant level of turbulence intensification in the stagnation region. The smaller cylindrical leading edge shows more consistency with earlier stagnation region heat transfer results correlated on the TRL parameter. These results indicate that the intensification of approaching turbulence is more prevalent with the more rapid straining of the smaller leading edge. The downstream regions of both test surfaces continue to accelerate the flow but at a much lower rate than the leading edge. Bypass transition occurs in these regions providing a useful set of data to ground the prediction of transition onset and length over a wide range of Reynolds numbers and turbulence intensity and scales.

scholarly journals Stagnation Region Heat Transfer Augmentation at Very High Turbulence Levels

2015 ◽  
Author(s):  
J. E. Kingery ◽  
F. E. Ames
Keyword(s):  
Heat Transfer ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Bypass Transition ◽  
Stagnation Region ◽  
Heat Transfer Augmentation ◽  
Wide Range ◽  
High Turbulence ◽  
Very High ◽  
Turbulence Generator

A database for stagnation region heat transfer has been extended to include heat transfer measurements acquired downstream from a new high intensity turbulence generator. This work was motivated by gas turbine industry heat transfer designers who deal with heat transfer environments with increasing Reynolds numbers and very high turbulence levels. The new mock aero-combustor turbulence generator produces turbulence levels which average 17.4%, which is 37% higher than the older turbulence generator. The increased level of turbulence is caused by the reduced contraction ratio from the liner to the exit. Heat transfer measurements were acquired on two large cylindrical leading edge test surfaces having a four to one range in leading edge diameter (40.64 cm and 10.16 cm). Gandvarapu and Ames [1] previously acquired heat transfer measurements for six turbulence conditions including three grid conditions, two lower turbulence aero-combustor conditions, and a low turbulence condition. The data are documented and tabulated for an eight to one range in Reynolds numbers for each test surface with Reynolds numbers ranging from 62,500 to 500,000 for the large leading edge and 15,625 to 125,000 for the smaller leading edge. The data show augmentation levels of up to 136% in the stagnation region for the large leading edge. This heat transfer rate is an increase over the previous aero-combustor turbulence generator which had augmentation levels up to 110%. Note, the rate of increase in heat transfer augmentation decreases for the large cylindrical leading edge inferring only a limited level of turbulence intensification in the stagnation region. The smaller cylindrical leading edge shows more consistency with earlier stagnation region heat transfer results correlated on the TRL (Turbulence, Reynolds number, Length scale) parameter. The downstream regions of both test surfaces continue to accelerate the flow but at a much lower rate than the leading edge. Bypass transition occurs in these regions providing a useful set of data to ground the prediction of transition onset and length over a wide range of Reynolds numbers and turbulence intensity and scales.

scholarly journals Stagnation Region Heat Transfer Augmentation at Very High Turbulence Levels

2016 ◽  
Vol 138 (8) ◽  
Author(s):  
J. E. Kingery ◽  
F. E. Ames
Keyword(s):  
Heat Transfer ◽  
High Intensity ◽  
Gas Turbines ◽  
Large Scale ◽  
Large Diameter ◽  
Leading Edge ◽  
Test Surface ◽  
Bypass Transition ◽  
Stagnation Region ◽  
Heat Transfer Augmentation

Current land-based gas turbines are growing in size producing higher approach flow Reynolds numbers at the leading edge of turbine nozzles. These vanes are subjected to high intensity large scale turbulence. This present paper reports on the research which significantly expands the parameter range for stagnation region heat transfer augmentation due to high intensity turbulence. Heat transfer measurements were acquired over two constant heat flux test surfaces with large diameter leading edges (10.16 cm and 40.64 cm). The test surfaces were placed downstream from a new high intensity (17.4%) mock combustor and tested over an eight to one range in approach flow Reynolds number for each test surface. Stagnation region heat transfer augmentation for the smaller (ReD = 15,625–125,000) and larger (ReD = 62,500–500,000) leading edge regions ranged from 45% to 81% and 80% to 136%, respectively. These data also include heat transfer distributions over the full test surface compared with the earlier data acquired at six additional inlet turbulence conditions. These surfaces exhibit continued but more moderate acceleration downstream from the stagnation regions and these data are expected to be useful in testing bypass transition predictive approaches. This database will be useful to gas turbine heat transfer design engineers.

Stagnation Line Heat Transfer Augmentation Due to Freestream Vortical Structures and Vorticity

2002 ◽  
Vol 124 (3) ◽  
pp. 583-587 ◽  
Author(s):  
Aung N. Oo ◽  
Chan Y. Ching
Keyword(s):  
Heat Transfer ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Heat Transfer Model ◽  
Transfer Model ◽  
Freestream Turbulence ◽  
Stagnation Region ◽  
Stagnation Line ◽  
Vortical Structures ◽  
Heat Transfer Augmentation

An experimental study has been performed to investigate the effect of freestream vortical structures and vorticity on stagnation region heat transfer. A heat transfer model with a cylindrical leading edge was tested in a wind tunnel at Reynolds numbers ranging from 67,750 to 142,250 based on leading edge diameter of the model. Grids of parallel rods were placed at several locations upstream of the heat transfer model in orientations where the rods were perpendicular and parallel to the stagnation line to generate freestream turbulence with distinct vortical structures. All three components of turbulence intensity, integral length scale and the spanwise and transverse vorticity were measured to characterize the freestream turbulence. The measured heat transfer data and freestream turbulence characteristics were compared with existing empirical models for the stagnation line heat transfer. A new correlation for the stagnation line heat transfer has been developed that includes the spanwise fluctuating vorticity components.

scholarly journals Experimental Heat Transfer Distributions Over an Aft Loaded Vane With a Large Leading Edge at Very High Turbulence Levels

2016 ◽  
Author(s):  
Justin W. Varty ◽  
Forrest E. Ames
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Constant Heat Flux ◽  
Test Surface ◽  
Stagnation Region ◽  
Experimental Heat ◽  
High Turbulence ◽  
Very High

Vane heat transfer distributions have been acquired on an aft loaded vane with a large leading edge over a range of turbulence conditions and across a range of Reynolds numbers. The large leading edge was designed to reduce heat transfer levels around the vane stagnation region and provide an opportunity to internally cool the region using a double wall cooling method. Heat transfer measurements were acquired in a linear cascade using a constant heat flux technique. The cascade was designed in a four vane, three full passage configuration with inlet bleeds flows and exit tailboards shaped along streamlines. Heat transfer measurements were acquired at exit chord Reynolds numbers of 500,000, 1,000,000, and 2,000,000 over seven turbulence conditions. The turbulence conditions included a low turbulence condition (Tu ≈ 0.7%), a small grid (M = 3.175 cm) at far and near locations (Tu ≈ 3.5% & 7.9%), a larger grid (Tu ≈ 8.0%), an aero-combustor closely coupled to the cascade and with a decay spool in between (Tu ≈ 13.5% and 9.3%) as well as with a new very high turbulence generator (Tu ≈ 17.4%). Heat transfer levels in the stagnation region are correlated in terms of approach flow Reynolds number and turbulence conditions and compared with recent large cylindrical leading edge test surface data using the TRL parameter. The surface heat transfer measurements are presented at different Reynolds numbers in terms of Stanton number based on exit conditions. These comparisons provide useful information on the level of turbulence augmentation in laminar regions of the flow as well as the onset location and length of transition. Midspan surface static pressure distributions were acquired at all the conditions and were used as a basis to determine experimental isentropic Mach number distributions. These data are reported in part but were also used to help generate the free-stream boundary condition for a boundary layer calculation. Predictive comparisons generated from boundary layer calculations (STAN7) using an algebraic turbulence model (ATM) and a well-known transition model (Mayle) are provided. At low turbulence levels the close comparisons provide confidence in the experimental technique. At higher turbulence levels the comparisons may provide a better indication of the physics of response of vane heat transfer to the external turbulence. These data are expected to help clarify the physics of vane heat transfer at very high turbulence levels.

Slot Film Cooling in an Accelerating Boundary Layer With High Free-Stream Turbulence

2014 ◽  
Author(s):  
Mitch L. Busche ◽  
Joseph E. Kingery ◽  
Forrest E. Ames
Keyword(s):  
Film Cooling ◽  
Free Stream ◽  
Surface Film ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Test Surface ◽  
Turbulence Level ◽  
Stagnation Region ◽  
Turbulence Condition ◽  
Slot Film Cooling

Slot film cooling and downstream heat transfer measurements have been acquired in the accelerating flows over two cylindrical leading edge test surfaces. Measurements were conducted at four blowing ratios, two Reynolds numbers and six well documented turbulence conditions for each test surface. Film cooling measurements were acquired over a four to one range in blowing ratio at the lower Reynolds number and at the two lower blowing ratios for the higher Reynolds numbers. The film cooling measurements were acquired at a coolant to free-stream density ratio of approximately 1.04. The flows were subjected to a low turbulence condition (Tu = 0.7%), two levels of turbulence for a smaller sized grid (Tu = 3.5%, and 7.9%), one turbulence level for a larger grid (8.1%), and two levels of turbulence generated using a mock aero-combustor (Tu = 9.3% and 13.7%). Turbulence level is shown to have a significant influence in mixing away film cooling coverage progressively as the flow develops in the streamwise direction. Effectiveness levels for the aero-combustor turbulence condition are reduced to as low as 20% of low turbulence values by the furthest downstream region. The slot in each case is located close to the stagnation region of the leading edge and the upstream boundary layers are very thin and accelerating. The slot is angled at 30° to the surface. Film cooling data, from the larger cylindrical stagnation region test surface, show that transitional flows have significantly improved effectiveness levels compared with turbulent flows. These data are expected to be very useful in grounding computational predictions of slot film cooling with elevated turbulence levels and acceleration.

Wake-Induced Unsteady Stagnation-Region Heat-Transfer Measurements

1992 ◽  
Author(s):  
P. J. Magari ◽  
L. E. LaGraff
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Rate ◽  
Transfer Rate ◽  
Reynolds Numbers ◽  
Edge Region ◽  
Isentropic Compression ◽  
Stagnation Region ◽  
Stagnation Line ◽  
Wide Range ◽  
Mach Numbers

An experimental investigation of wake-induced unsteady heat transfer in the stagnation region of a cylinder was conducted. The objective of the study was to create a quasi-steady representation of the stator/rotor interaction in a gas turbine using two stationary cylinders in crossflow. In this simulation, a larger cylinder, representing the leading-edge region of a rotor blade, was immersed in the wake of a smaller cylinder, represenung the trailing-edge region of a stator vane. Time-averaged and time-resolved heat-transfer results were obtained over a wide range of Reynolds numbers at two Mach numbers: one incompressible and one transonic. The tests were conducted at Reynolds numbers, Mach numbers and gas-to-wall temperature ratios characteristic of turbine engine conditions in an isentropic compression-heated transient wind tunnel (LICH tube). The augmentation of the heat transfer in the stagnation region due to wake unsteadiness was documented by comparison with isolated cylinder tests. It was found that the time-averaged heat-transfer rate at the stagnation line, expressed in terms of the Frossling number (Nu/√Re), reached a maximum independent of the Reynolds number. The power spectra and cross correlation of the heat-transfer signals in the stagnation region revealed the importance of large vortical structures shed from the upstream wake generator. These structures caused large positive and negative excursions about the mean heat-transfer rate in the stagnation region.

scholarly journals Influence of Turbulence Parameters, Reynolds Number, and Body Shape on Stagnation-Region Heat Transfer

1995 ◽  
Vol 117 (3) ◽  
pp. 597-603 ◽  
Author(s):  
G. J. Van Fossen ◽  
R. J. Simoneau ◽  
C. Y. Ching
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Velocity Gradient ◽  
Isotropic Turbulence ◽  
Leading Edge ◽  
Length Scale ◽  
Stagnation Region ◽  
Free Stream Turbulence ◽  
Heat Transfer Augmentation ◽  
Optimum Scale

This experiment investigated the effects of free-stream turbulence intensity, length scale, Reynolds number, and leading-edge velocity gradient on stagnation-region heat transfer. Heat transfer was measured in the stagnation region of four models with elliptical leading edges downstream of five turbulence-generating grids. Stagnation-region heat transfer augmentation increased with decreasing length scale but ann optimum scale was not found. A correlation was developed that fit heat transfer data for isotropic turbulence to within ±4 percent but did not predict data for anisotropic turbulence. Stagnation heat transfer augmentation caused by turbulence was unaffected by the velocity gradient. The data of other researchers compared well with the correlation. A method of predicting heat transfer downstream of the stagnation point was developed.

A Study of the Relationship Between Free-Stream Turbulence and Stagnation Region Heat Transfer

1987 ◽  
Vol 109 (1) ◽  
pp. 10-15 ◽  
Author(s):  
G. J. VanFossen ◽  
R. J. Simoneau
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Free Stream ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Cylinder Surface ◽  
Nasa Lewis Research ◽  
Stagnation Region ◽  
Stagnation Line ◽  
Free Stream Turbulence

A study has been conducted at the NASA Lewis Research Center to investigate the mechanism that causes free-stream turbulence to increase heat transfer in the stagnation region of turbine vanes and blades. The work was conducted in a wind tunnel at atmospheric conditions to facilitate measurements of turbulence and heat transfer. The model size was scaled up to simulate Reynolds numbers (based on leading edge diameter) that are to be expected on a turbine blade leading edge. Reynolds numbers from 13,000 to 177,000 were run in the present tests. Spanwise averaged heat transfer measurements with high and low turbulence have been made with “rough” and smooth surface stagnation regions. Results of these measurements show that, at the Reynolds numbers tested, the boundary layer remained laminar in character even in the presence of free-stream turbulence. If roughness was added the boundary layer became transitional as evidenced by the heat transfer increase with increasing distance from the stagnation line. Hot-wire measurements near the stagnation region downstream of an array of parallel wires has shown that vorticity in the form of mean velocity gradients is amplified as flow approaches the stagnation region. Finally smoke wire flow visualization and liquid crystal surface heat transfer visualization were combined to show that, in the wake of an array of parallel wires, heat transfer was a minimum in the wire wakes where the fluctuating component of velocity (local turbulence) was the highest. Heat transfer was found to be the highest between pairs of vortices where the induced velocity was toward the cylinder surface.

Effect of Turbulence With Different Vortical Structures on Stagnation Region Heat Transfer

2001 ◽  
Vol 123 (4) ◽  
pp. 665-674 ◽  
Author(s):  
Aung N. Oo ◽  
Chan Y. Ching
Keyword(s):  
Heat Transfer ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Heat Transfer Model ◽  
Transfer Model ◽  
Stagnation Region ◽  
Stagnation Line ◽  
Vortical Structures ◽  
Transfer Data ◽  
Correlation Models

The effect of freestream turbulence with different vortical structures on the stagnation region heat transfer was experimentally studied. Reynolds numbers, based on leading edge diameter of the heat transfer model with a cylindrical leading edge, ranged from 67,750 to 142,250. Turbulence generating grids of parallel rods were placed at several positions upstream of the heat transfer model in orientations where the rods were perpendicular and parallel to the stagnation line. The turbulence intensity and ratio of integral length scale to leading edge diameter were in the range 3.93 to 11.78 percent and 0.07 to 0.70, respectively. The grids with rods perpendicular to the stagnation line, where the primary vortical structures are expected to be perpendicular to the stagnation line, result in higher heat transfer than those with rods parallel to the stagnation line. The measured heat transfer data and turbulence characteristics are compared with existing correlation models.

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