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.

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.

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.

The Cavity Width Effect on Immersion Cooling of Discrete Flush-Heaters on One Vertical Wall of an Enclosure Cooled from the Top

1989 ◽  
Vol 111 (4) ◽  
pp. 268-276 ◽  
Author(s):  
R. Carmona ◽  
M. Keyhani
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Aspect Ratio ◽  
Ethylene Glycol ◽  
Transfer Coefficient ◽  
Length Scale ◽  
Core Flow ◽  
Width Effect ◽  
Cavity Width

The width effect on natural convection heat transfer due to discrete flush-heated sections of equal height in an enclosure cooled from the top is experimentally investigated. Five heated sections are uniformly distributed along a vertical side wall, where the height of the unheated sections is equal to that of the heated sections. All other vertical surfaces and the bottom plate are insulated. The experiments are conducted for six values of cavity width resulting in a variation in the cavity height-to-width ratio (aspect ratio) of 3.67 to 12.22. Ethylene glycol and FC-75 (a dielectric fluid) are used as the convective media. The flow visualization results with ethylene glycol reveal a fairly inactive core flow at low power inputs and small cavity width. Higher values of the power input and cavity width transforms a rather well structured core flow into a time dependent one with higher horizontal velocities toward the “hot” wall. For a given cavity width, it is found that the heat transfer results for all the heated sections can be unified and presented by a single correlation through use of a local height as the length scale. The local height of a given heated section is measured from the bottom of the cavity to the mid-height of that section. Based on the local height length scale the data for all the cavity widths are correlated and an explicit relation for the aspect ratio effect on local Nusselt number is reported. The data indicate that an increase in the cavity width results in an increase in the heat transfer coefficients of the heated sections. A comparison between ethylene glycol and FC-75 revealed no appreciable Prandtl number effect, which is in agreement with a previously reported prediction. A heat transfer coefficient for the top surface is defined based on the total convective heat flux at this surface and an average temperature difference between the heated sections and the top plate. The results show that for a given heat flux at the top, the highest heat transfer coefficient on this surface is obtained with the lowest cavity width.

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.

A Transient Infrared Technique for Measuring Surface and Endwall Heat Transfer in a Transonic Turbine Cascade

2010 ◽  
Author(s):  
C. Reagle ◽  
A. Newman ◽  
S. Xue ◽  
W. Ng ◽  
S. Ekkad ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Heat Transfer Coefficient ◽  
Mach Number ◽  
Transfer Coefficient ◽  
Infrared Thermography ◽  
Turbulence Intensity ◽  
Measurement Techniques ◽  
Freestream Turbulence ◽  
Exit Mach Number

This paper describes a method for obtaining surface and endwall heat transfer in an uncooled transonic cascade facility using infrared thermography measurements. Midspan heat transfer coefficient results are first presented for an engine representative first stage nozzle guide vane at exit Mach number of 0.77, Reynolds number of 1.05×106 and freestream turbulence intensity of 16%. The results obtained from infrared thermography are compared with previously published results using thin film gauges in the same facility on the same geometry. There is generally good agreement between the two measurement techniques in both trend and overall level of heat transfer coefficient over the vane surface. Stanton number contours are then presented for a blade endwall at exit Mach number of 0.88, Reynolds number of 1.70×106 and freestream turbulence intensity of 8%. Infrared thermography results are qualitatively compared with results from a published work obtained with liquid crystals at similar flow conditions. Results are qualitatively in agreement.

scholarly journals Experimental Determination of the Heat Transfer Coefficient of Real Cooled Geometry Using Linear Regression Method

Energies ◽  
2020 ◽  
Vol 14 (1) ◽  
pp. 180
Author(s):  
Asif Ali ◽  
Lorenzo Cocchi ◽  
Alessio Picchi ◽  
Bruno Facchini
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Linear Regression ◽  
Surface Temperature ◽  
Transfer Coefficient ◽  
External Surface ◽  
Internal Surface ◽  
Regression Method ◽  
Thermal Transient

The scope of this work was to develop a technique based on the regression method and apply it on a real cooled geometry for measuring its internal heat transfer distribution. The proposed methodology is based upon an already available literature approach. For implementation of the methodology, the geometry is initially heated to a known steady temperature, followed by thermal transient, induced by injection of ambient air to its internal cooling system. During the thermal transient, external surface temperature of the geometry is recorded with the help of infrared camera. Then, a numerical procedure based upon a series of transient finite element analyses of the geometry is applied by using the obtained experimental data. The total test duration is divided into time steps, during which the heat flux on the internal surface is iteratively updated to target the measured external surface temperature. The final procured heat flux and internal surface temperature data of each time step is used to find the convective heat transfer coefficient via linear regression. This methodology is successfully implemented on three geometries: a circular duct, a blade with U-bend internal channel, and a cooled high pressure vane of real engine, with the help of a test rig developed at the University of Florence, Italy. The results are compared with the ones retrieved with similar approach available in the open literature, and the pros and cons of both methodologies are discussed in detail for each geometry.

Sign in / Sign up

Export Citation Format

Share Document