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.

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.

Experimental Measurements and Modeling of the Effects of Large-Scale Freestream Turbulence on Heat Transfer

2004 ◽  
Author(s):  
A. C. Nix ◽  
T. E. Diller ◽  
W. F. Ng
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Turbine Blade ◽  
Large Scale ◽  
Surface Heat ◽  
Integral Length Scale ◽  
Length Scale ◽  
Freestream Turbulence ◽  
Suction Surface ◽  
Surface Heat Transfer

The influence of freestream turbulence representative of the flow downstream of a modern gas turbine combustor and first stage vane on turbine blade heat transfer has been measured and analytically modeled in a linear, transonic turbine cascade. High intensity, large length-scale freestream turbulence was generated using a passive turbulence-generating grid to simulate the turbulence generated in modern combustors after passing through the first stage vane row. The grid produced freestream turbulence with intensity of approximately 10–12% and an integral length scale of 2 cm (Λx/c = 0.15) near the entrance of the cascade passages. Mean heat transfer results with high turbulence showed an increase in heat transfer coefficient over the baseline low turbulence case of approximately 8% on the suction surface of the blade, with increases on the pressure surface of approximately 17%. Time-resolved surface heat transfer and passage velocity measurements demonstrate strong coherence in velocity and heat flux at a frequency correlating with the most energetic eddies in the turbulence flow field (the integral length-scale). An analytical model was developed to predict increases in surface heat transfer due to freestream turbulence based on local measurements of turbulent velocity fluctuations and length-scale. The model was shown to predict measured increases in heat flux on both blade surfaces in the current data. The model also successfully predicted the increases in heat transfer measured in other work in the literature, encompassing different geometries (flat plate, cylinder, turbine vane and turbine blade) and boundary layer conditions.

The Physical Mechanism of Heat Transfer Augmentation in Stagnating Flows Subject to Freestream Turbulence

2010 ◽  
Vol 133 (2) ◽  
Author(s):  
Andrew R. Gifford ◽  
Thomas E. Diller ◽  
Pavlos P. Vlachos
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Coherent Structures ◽  
Physical Mechanism ◽  
Length Scale ◽  
Freestream Turbulence ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
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 spatiotemporal influence of coherent structures on the heat transfer coefficient as they approach and interact with the stagnation surface. Laminar flow and heat transfer at low levels of freestream turbulence (Tux¯=0.5–1.0%) are examined to provide baseline flow characteristics and heat transfer coefficients. Similar experiments using a turbulence grid are performed to examine the effects of turbulence with mean streamwise turbulence intensity of Tux¯=5.0% and an integral length scale of Λx¯=3.25 cm. At a Reynolds number of ReD¯=U∞¯D/υ=21,000, an average increase in the mean heat transfer coefficient of 64% above the laminar level was observed. Experimental studies confirm that coherent structures play a dominant role in the augmentation of heat transfer in the stagnation region. Calculation and examination of the transient physical properties for coherent structures (i.e., circulation, area averaged vorticity, integral length scale, and proximity to the surface) shows that freestream turbulence is stretched and vorticity is amplified as it is convected toward the stagnation surface. The resulting stagnation flow is dominated by dynamic, counter-rotating vortex pairs. Heat transfer augmentation occurs when the rotational motion of coherent structures sweeps cooler freestream fluid into the laminar momentum and thermal boundary layers into close proximity of the heated stagnation surface. Evidence in support of this mechanism is provided through validation of a new mechanistic model, which incorporates the transient physical properties of tracked coherent structures. The model performs well in capturing the essential dynamics of the interaction and in the prediction of the experimentally measured transient and time-averaged turbulent heat transfer coefficients.

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.

Experimental Measurements and Modeling of the Effects of Large-Scale Freestream Turbulence on Heat Transfer

2006 ◽  
Vol 129 (3) ◽  
pp. 542-550 ◽  
Author(s):  
A. C. Nix ◽  
T. E. Diller ◽  
W. F. Ng
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Turbine Blade ◽  
Large Scale ◽  
Surface Heat ◽  
Integral Length Scale ◽  
Length Scale ◽  
Freestream Turbulence ◽  
Suction Surface ◽  
Surface Heat Transfer

The influence of freestream turbulence representative of the flow downstream of a modern gas turbine combustor and first stage vane on turbine blade heat transfer has been measured and analytically modeled in a linear, transonic turbine cascade. High-intensity, large length-scale freestream turbulence was generated using a passive turbulence-generating grid to simulate the turbulence generated in modern combustors after passing through the first stage vane row. The grid produced freestream turbulence with intensity of approximately 10–12% and an integral length scale of 2cm(Λx∕c=0.15) near the entrance of the cascade passages. Mean heat transfer results with high turbulence showed an increase in heat transfer coefficient over the baseline low turbulence case of approximately 8% on the suction surface of the blade, with increases on the pressure surface of approximately 17%. Time-resolved surface heat transfer and passage velocity measurements demonstrate strong coherence in velocity and heat flux at a frequency correlating with the most energetic eddies in the turbulence flow field (the integral length scale). An analytical model was developed to predict increases in surface heat transfer due to freestream turbulence based on local measurements of turbulent velocity fluctuations and length scale. The model was shown to predict measured increases in heat flux on both blade surfaces in the current data. The model also successfully predicted the increases in heat transfer measured in other work in the literature, encompassing different geometries (flat plate, cylinder, turbine vane, and turbine blade) and boundary layer conditions.

Effects of Freestream Turbulence Intensity, Turbulence Length Scale, and Exit Reynolds Number on Vane Endwall Secondary Flow and Heat Transfer in a Transonic Turbine Cascade

2020 ◽  
Author(s):  
Zhigang Li ◽  
Bo Bai ◽  
Luxuan Liu ◽  
Jun Li ◽  
Shuo Mao ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Secondary Flow ◽  
Turbulence Intensity ◽  
Thermal Load ◽  
Secondary Flows ◽  
Length Scale ◽  
Freestream Turbulence ◽  
Turbulence Length ◽  
Turbulence Length Scale

Abstract In gas turbine engines, the first-stage vanes usually suffer harsh incoming flow conditions from the combustor with high pressure, high temperature and high turbulence. The combustor-generated high freestream turbulence and strong secondary flows in a gas turbine vane passage have been reported to augment the endwall thermal load significantly. This paper presents a detailed numerical study on the effects of high freestream turbulence intensity, turbulence length scale, and exit Reynolds number on the endwall secondary flow pattern and heat transfer distribution of a transonic linear turbine vane passage at realistic engine Mach numbers, with a flat endwall no cooling. Numerical simulations were conducted at a range of different operation conditions: six freestream turbulence intensities (Tu = 1%, 5%, 10%, 13%, 16% and 20%), six turbulence length scales (normalized by the vane pitch of Λ/P = 0.01, 0.04, 0.07, 0.12, 0.24, 0.36), and three exit isentropic Mach number (Maex = 0.6, 0.85 and 1.02 corresponding exit Reynolds number Reex = 1.1 × 106, 1.7 × 106 and 2.2 × 106, respectively, based on the vane chord). Detailed comparisons were presented for endwall heat transfer coefficient distribution, endwall secondary flow field at different operation conditions, while paying special attention to the link between endwall thermal load patterns and the secondary flow structures. Results show that the freestream turbulence intensity and length scale have a significant influence on the endwall secondary flow field, but the influence of the exit Reynolds number is very weak. The Nusselt number patterns for the higher turbulence intensities (Tu = 16%, 20%) appear to be less affected by the endwall secondary flows than the lower turbulence cases. The thermal load distribution in the arc region around the vane leading edge and the banded region along the vane pressure side are influenced most strongly by the freestream turbulence intensity. In general, the higher freestream turbulence intensities make the vane endwall thermal load more uniform. The Nusselt number distribution is only weakly affected by the turbulence length scale when Λ/P is larger than 0.04. The heat transfer level appears to have a significant uniform augmentation over the whole endwall region with the increasing Maex. The endwall thermal load distribution is classified into four typical regions, and the effects of freestream turbulence, exit Reynolds number in each region were discussed in detail.

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