Augmentation of Stagnation Region Heat Transfer Due to Turbulence From an Advanced Dual-Annular Combustor

2002 ◽  
Author(s):  
G. James Van Fossen ◽  
Ronald S. Bunker
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Turbulence Intensity ◽  
Leading Edge ◽  
Radius Of Curvature ◽  
Length Scale ◽  
Hot Wire ◽  
Stagnation Region ◽  
Heat Transfer Augmentation ◽  
Annular Combustor

Heat transfer measurements have been made in the stagnation region of a flat plate with an elliptical leading edge. The radius of curvature at the stagnation point was similar to that of a first stage turbine vane airfoil used in a large commercial high-bypass turbofan engine. The airfoil was mounted downstream of an arc segment of a dual-annular combustor similar to the type used in an advanced turbine engine. Testing was done in air at atmospheric temperature and at pressures up to 376 kPa to simulate the vane leading edge Reynolds number seen in the engine. Spanwise average stagnation region heat transfer was measured with an electrically heated aluminum strip. Turbulence intensity, length scale and isotropy were measured using standard 2-wire hot wire probes. The combustor contained two annular rows of fuel-air swirlers which were aligned in the radial direction. Both heat transfer and hot wire data were taken at two circumferential positions; one directly downstream of a pair of swirlers and one half way between two pairs of swirlers. Reynolds number based on vane leading edge diameter was varied from 51000 to 160000. The maximum Reynolds number for turbulence measurements was limited to 87000. Turbulence intensity averaged over all test conditions was found to be 31.6%. Average axial, integral length scale was 1.29 cm, which gave a length scale-to-leading edge diameter ratio of 1.08. The turbulence was found to be nearly isotropic with the average ratio of axial to circumferential fluctuating components of 1.15. Heat transfer augmentation above laminar levels was found to vary from 34 to almost 59% depending on the Reynolds number. No effect of circumferential position was found. The heat transfer augmentation was found to be well predicted by a correlation derived from grid generated turbulence.

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.

An Investigation of the Physical Mechanism of Heat Transfer Augmentation in Stagnating Flows Subject to Freestream Turbulence

2008 ◽  
Author(s):  
Andrew R. Gifford ◽  
Thomas E. Diller ◽  
Pavlos P. Vlachos
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Turbulence Intensity ◽  
Physical Mechanism ◽  
Length Scale ◽  
Freestream Turbulence ◽  
Stagnation Region ◽  
Heat Transfer Augmentation

Experiments have been performed in a water tunnel facility to examine the physical mechanism of heat transfer augmentation by freestream turbulence in classical Hiemenz flow. A unique experimental approach to studying the problem is developed and demonstrated herein. Time-Resolved Digital Particle Image Velocimetry (TRDPIV) and a new variety of thin film heat flux sensor called the Heat Flux Array (HFA) are used simultaneously to measure the spatio-temporal influence of coherent structures on the heat transfer coefficient as they approach and interact with the stagnation region. Velocity measurements of grid generated freestream turbulence are first performed to quantify the turbulence intensity, integral length scale, and isotropy of the flow. Laminar flow and heat transfer at low levels of freestream turbulence (Tux ≅ 0.5–1.0%) are then examined to provide baseline flow characteristics and heat transfer coefficient. Similar experiments using the turbulence grid are then performed to examine the effects of turbulence with mean turbulence intensity, Tux ≅ 5.5%, and integral length scale, Λx ∼ 3.25 cm. At a mean Reynolds number of ReD = 21,000 an average increase in the mean heat transfer coefficient of 43% above the laminar level was observed. To better understand the mechanism of this augmentation, flow structures in the stagnation region are identified using a coherent structure identification scheme and tracked in time using a customized tracking algorithm. Tracking these structures reveals a complex flow field in the vicinity of the stagnation region. Filaments of vorticity from the freestream are amplified near the plate surface leading to the formation of counterrotating vortex pairs and single sweeping vortex structures. By comparing the transient heat flux measurements with the tracked vortex structures it is clear that heat transfer augmentation is due primarily to amplification of stream-wise vorticity and subsequent vortex formation near the surface. The vortex strength, length scale, and distance from the stagnation plate are key parameters affecting augmentation. Finally, a mechanistic model is examined which captures the physical interaction near the wall. Model results agree well with measured heat transfer augmentation.

Experiments on the Physical Mechanism of Heat Transfer Augmentation by Freestream Turbulence at a Cylinder Stagnation Point

2005 ◽  
Author(s):  
A. C. Nix ◽  
T. E. Diller
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Stagnation Point ◽  
Turbulence Intensity ◽  
Integral Length Scale ◽  
Length Scale ◽  
Freestream Turbulence ◽  
Hot Wire ◽  
Velocity Signal ◽  
Heat Transfer Augmentation

Detailed time records of velocity and heat flux were measured near the stagnation point of a cylinder in low-speed air flow. The freestream turbulence was controlled using five different grids positioned to match the characteristics from previous heat flux experiments at NASA Glenn using the same wind tunnel. A hot wire was used to measure the cross-flow velocity at a range of positions in front of the stagnation point. This gave the average velocity and fluctuating component including the turbulence intensity and integral length scale. The heat flux was measured with a Heat Flux Microsensor located on the stagnation line underneath the hot-wire probe. This gave the average heat flux and the fluctuating component simultaneous with the velocity signal, including the heat flux turbulence intensity and the coherence with the velocity. The coherence between the signals allowed identification of the crucial positions for measurement of the integral length scale and turbulence intensity for prediction of the time average surface heat flux. The frequencies corresponded to the most energetic frequencies of the turbulence, indicating the importance of the penetration of the turbulent eddies from the freestream through the boundary layer to the surface. The distance from the surface was slightly less than the local value of length scale, indicating the crucial role of the turbulence in augmenting the heat flux. The resulting predictions of the analytical model matched well with the measured heat transfer augmentation.

Experiments on the Physical Mechanism of Heat Transfer Augmentation by Freestream Turbulence at a Cylinder Stagnation Point

2009 ◽  
Vol 131 (2) ◽  
Author(s):  
A. C. Nix ◽  
T. E. Diller
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Stagnation Point ◽  
Turbulence Intensity ◽  
Integral Length Scale ◽  
Length Scale ◽  
Freestream Turbulence ◽  
Hot Wire ◽  
Velocity Signal ◽  
Heat Transfer Augmentation

Detailed time records of velocity and heat flux were measured near the stagnation point of a cylinder in low-speed airflow. The freestream turbulence was controlled using five different grids positioned to match the characteristics from previous heat flux experiments at NASA Glenn using the same wind tunnel. A hot wire was used to measure the cross-flow velocity at a range of positions in front of the stagnation point. This gave the average velocity and fluctuating component including the turbulence intensity and integral length scale. The heat flux was measured with a heat flux microsensor located on the stagnation line underneath the hot-wire probe. This gave the average heat flux and the fluctuating component simultaneous with the velocity signal, including the heat flux turbulence intensity and the coherence with the velocity. The coherence between the signals allowed identification of the crucial positions for measurement of the integral length scale and turbulence intensity for prediction of the time-averaged surface heat flux. The frequencies corresponded to the most energetic frequencies of the turbulence, indicating the importance of the penetration of the turbulent eddies from the freestream through the boundary layer to the surface. The distance from the surface was slightly less than the local value of length scale, indicating the crucial role of the turbulence in augmenting the heat flux. The resulting predictions of the analytical model matched well with the measured heat transfer augmentation.

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.

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.

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 The Influence of Large Scale High Intensity Turbulence on Vane Heat Transfer

1995 ◽  
Author(s):  
Forrest E. Ames
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
High Intensity ◽  
Large Scale ◽  
Grid Turbulence ◽  
Absolute Level ◽  
Relative Level ◽  
Stagnation Region ◽  
Pressure Surface ◽  
Heat Transfer Augmentation

An experimental research program was undertaken to examine the influence of large scale high intensity turbulence on vane heat transfer. The experiment was conducted in a four vane linear cascade at exit Reynolds numbers of 500,000 and 800,000 based on chord length corresponding to exit Mach numbers of 0.17 and 0.27. Heat transfer measurements were made for four inlet turbulence conditions including a low turbulence case (Tu ≅ 1%), a grid turbulence case (Tu ≅ 7.5%), and two levels of large scale turbulence generated with a mock combustor at two upstream locations (Tu ≅ 12% & Tu ≅ 8%). The heat transfer data demonstrated that the length scale, Lu, has a significant effect on stagnation region and pressure surface heat transfer. The average heat transfer augmentation over the pressure surface was found to scale reasonably well on the relative level of dissipation. The stagnation region heat transfer correlated well on the {Tu ReD5/12 (Lu/D)−1/3} parameter of Ames and Moffat (1990). The dependence of heat transfer augmentation on Reynolds number was estimated to scale on the 1/3 power for the pressure surface. The absolute level of heat transfer augmentation was found to be highest near the stagnation region. The combustor closely coupled to the cascade produced an average augmentation on the pressure surface of 56 percent at a Reynolds number of 800,000.

Separate and Combined Effects of Surface Roughness and Turbulence Intensity on Vane Heat Transfer

1997 ◽  
Author(s):  
Ronald S. Bunker
Keyword(s):  
Heat Transfer ◽  
Surface Roughness ◽  
Reynolds Number ◽  
Turbulence Intensity ◽  
Pressure Ratio ◽  
Development Program ◽  
Leading Edge ◽  
Thin Foil ◽  
Combined Effects ◽  
Average Roughness

A research and development program has been undertaken to ascertain the effects of surface roughness levels on vane external heat transfer, with varied conditions of inlet freestream turbulence intensity and vane Reynolds number. A transonic linear vane cascade was constructed to operate at a nominal overall pressure ratio of 1.86. Airfoil heat transfer distributions are measured using a thin-walled stainless steel airfoil having imbedded thermocouples. The methodology incorporates a thin-foil surface heater to provide a known heat flux condition, with room temperature mainstream air at approximately 5 atm pressure. Heat transfer is characterized for uniform surface average roughness levels of 0.4, 1.85, and 4.5 micrometers, with inlet turbulence intensity levels from 4 to 13%. Airfoil Reynolds number based on axial chord length and exit velocity ranges from 2.2 to 4.8 · 106. Results show consistent individual and combined effects of Reynolds number, turbulence, and surface roughness changes. Higher roughness levels tend to dominate turbulence effects in most regions except the leading edge.

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