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.

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.

Turbulence and Vorticity Effects on Stagnation Region Heat Transfer

2001 ◽  
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 ◽  
Correlation Models ◽  
Low Speed Wind Tunnel

An experimental study has been performed to investigate the influences of freestream turbulence, including the vorticity field, on the stagnation region heat transfer. A heat transfer model with a cylindrical leading edge was tested in a low speed 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 used to generate the freestream turbulence with well-defined primary vortex lines. The grids were placed at several locations upstream of the heat transfer model in orientations where the rods were perpendicular and parallel to the stagnation line. The turbulence intensity and the ratio of integral length scale to leading edge diameter were in the range of 3.93 to 11.78 % and 0.07 to 0.7, respectively. The measured heat transfer data and freestream turbulence characteristics are compared with existing correlation models. A new correlation for the stagnation line heat transfer has been developed that includes the spanwise fluctuating vorticity components.

Influence of Freestream Vortical Structures on Stagnation Region Heat Transfer

2000 ◽  
Author(s):  
Aung N. Oo ◽  
Chan Y. Ching
Keyword(s):  
Heat Transfer ◽  
Leading Edge ◽  
Heat Transfer Model ◽  
Transfer Model ◽  
Vertical Orientation ◽  
Stagnation Region ◽  
Stagnation Line ◽  
Vortical Structures ◽  
Horizontal Orientation ◽  
The Difference

Abstract An experimental study was performed to investigate the influence of freestream vortical structures on stagnation region heat transfer. A heat transfer model with a cylindrical leading edge was tested in a low speed wind tunnel at Reynolds numbers ranging from 67,750 to 142,250 based on leading edge diameter of the model. Turbulence generating grids of parallel rods of diameter, 2.86 cm, 1.59 cm and 0.95 cm, were placed upstream of the heat transfer model in horizontal and vertical orientations to generate freestream turbulence with different orientations of vortical structures. The rods in horizontal orientation were perpendicular to the stagnation line and those in vertical orientation were parallel to the stagnation line of the heat transfer model. The distance between the grid and heat transfer model was varied from 25 to 125 rod diameters. The grids with rods in the horizontal orientation, where the primary vortical structures are expected to be perpendicular to the stagnation line, result in higher heat transfer than with the grids where the rods are in the vertical orientation. The difference in heat transfer with the two grid orientations decreases with increasing grid-to-model distance for a given rod-grid. The difference also decreases with decreasing rod size for a given normalized grid-to-model distance. For the 2.86 cm rod-grid, the difference in heat transfer augmentation between horizontal and vertical grid-orientations is highest at the stagnation line and decreases with streamwise distance. This difference, however, remains fairly constant over the whole stagnation region for the 1.59 cm and 0.95 cm rod-grids.

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.

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.

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.

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.

scholarly journals Preliminary Results of a Study of the Relationship Between Free Stream Turbulence and Stagnation Region Heat Transfer

1985 ◽  
Author(s):  
G. James VanFossen ◽  
Robert J. Simoneau
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Free Stream ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Nasa Lewis Research ◽  
Hot Wire ◽  
Stagnation Region ◽  
Stagnation Line ◽  
Free Stream Turbulence

A study is being 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 is being conducted in a wind tunnel at atmospheric conditions to facilitate measurements of turbulence and heat transfer. The model size is 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 the boundary layer remains laminar in character even in the presence of free stream turbulence at the Reynolds numbers tested. If roughness is added the boundary layer becomes 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. Circumferential traverses of a hot wire probe very near the surface of the cylinder have shown the fluctuating component of velocity changes in character depending on free stream turbulence and Reynolds number. Finally smoke wire flow visualization and liquid crystal surface heat transfer visualization have been combined to show that, in the wake of an array of parallel wires, heat transfer is 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 is toward the cylinder surface.

scholarly journals Direct numerical simulation of heat transfer from the stagnation region of a heated cylinder affected by an impinging wake

2011 ◽  
Vol 669 ◽  
pp. 64-89 ◽  
Author(s):  
JAN G. WISSINK ◽  
WOLFGANG RODI
Keyword(s):  
Heat Transfer ◽  
Free Stream ◽  
Three Dimensional ◽  
Stream Velocity ◽  
Stagnation Region ◽  
Stagnation Line ◽  
Vortical Structures ◽  
Free Stream Turbulence ◽  
Heated Cylinder ◽  
Three Dimensional Simulations

The effect of an incoming wake on the flow around and heat transfer from the stagnation region of a circular cylinder was studied using direct numerical simulations (DNSs). Four simulations were carried out at a Reynolds number (based on free-stream velocity and cylinder diameterD) ofReD= 13200: one two-dimensional (baseline) simulation and three three-dimensional simulations. The three-dimensional simulations comprised a baseline simulation with a uniform incoming velocity field, a simulation in which realistic wake data – generated in a separate precursor DNS – were introduced at the inflow plane and, finally, a simulation in which the turbulent fluctuations were removed from the incoming wake in order to study the effect of the mean velocity deficit on the heat transfer in the stagnation region. In the simulation with realistic wake data, the incoming wake still exhibited the characteristic meandering behaviour of a near-wake. When approaching the regions immediately above and below the stagnation line of the cylinder, the vortical structures from the wake were found to be significantly stretched by the strongly accelerating wall-parallel (circumferential) flow into elongated vortex tubes that became increasingly aligned with the direction of flow. As the elongated streamwise vortical structures impinge on the stagnation region, on one side they transport cool fluid towards the heated cylinder, while on the other side hot fluid is transported away from the cylinder towards the free stream, thereby increasing the heat transfer. The DNS results are compared with various semi-empirical correlations for predicting the augmentation of heat transfer due to free-stream turbulence.

Large Eddy Simulation of the Laminar Heat Transfer Augmentation on the Pressure Side of a Turbine Vane Under Freestream Turbulence

2019 ◽  
Vol 141 (4) ◽  
Author(s):  
Yousef Kanani ◽  
Sumanta Acharya ◽  
Forrest Ames
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Leading Edge ◽  
Transition To Turbulence ◽  
Experimental Measurements ◽  
Pressure Side ◽  
Freestream Turbulence ◽  
Heat Transfer Augmentation ◽  
Eddy Simulation ◽  
Large Eddy

Vane pressure side heat transfer is studied numerically using large eddy simulation (LES) on an aft-loaded vane with a large leading edge over a range of turbulence conditions. Numerical simulations are performed in a linear cascade at exit chord Reynolds number of Re = 5.1 × 105 at low (Tu ≈ 0.7%), moderate (Tu ≈ 7.9%), and high (Tu ≈ 12.4%) freestream turbulence with varying length scales as prescribed by the experimental measurements of Varty and Ames (2016, “Experimental Heat Transfer Distributions Over an Aft Loaded Vane With a Large Leading Edge at Very High Turbulence Levels,” ASME Paper No. IMECE2016-67029). Heat transfer predictions on the vane pressure side are in a very good agreement with the experimental measurements and the heat transfer augmentation due to the freestream turbulence is well captured. At Tu ≈ 12.4%, freestream turbulence enhances the Stanton number on the pressure surface without boundary layer transition to turbulence by a maximum of about 50% relative to the low freestream turbulence case. Higher freestream turbulence generates elongated structures and high-velocity streaks wrapped around the leading edge that contain significant energy. Amplification of the velocity streaks is observed further downstream with max rms of 0.3 near the trailing edge but no transition to turbulence or formation of turbulence spots is observed on the pressure side. The heat transfer augmentation at the higher freestream turbulence is primarily due to the initial amplification of the low-frequency velocity perturbations inside the boundary layer that persist along the entire chord of the airfoil. Stanton numbers appear to scale with the streamwise velocity fluctuations inside the boundary layer.

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