Surface Heat Transfer Augmentation Research Articles

Numerical Prediction of 45° Angled Ribs Effects on U-shaped Channels Heat Transfer and Flow under Multi Conditions

AbstractRibs effects on the heat transfer performance and cooling air flow characteristics in various aspect ratios (AR) U-shaped channels under different working conditions are numerically investigated. The ribs angle and channel orientation are 45° and 90°, respectively, and the aspect ratios are 1:2, 1:1, 2:1. The inlet Reynolds number changes from 1e4 to 4e4 and rotational speeds include 0, 550 rpm, 1,100 rpm. Local heat transfer coefficient, endwall surface heat transfer coefficient ratio and augmentation factor are the three primary criteria to measure channel heat transfer. Ribs increase the heat transfer area and improve heat transfer coefficient of ribbed surfaces significantly, especially in the 1st pass, while the endwall surface contributes more to channel heat transfer because of the larger area and relatively smaller heat transfer coefficient. The wide channel (AR =2:1) owns the better augmentation factor than the narrow channel (AR =1:2) and ribs heat transfer weight increases with an increase of the inlet Reynolds number. Rotating slightly reduces the ribs heat transfer weight in channel and the trailing surface in 1st pass is the main influence object of rotating.

Effects of hole shape on impingement jet array heat transfer with small-scale, target surface triangle roughness

Considered is the addition of special surface roughness patterns to impingement target surfaces to improve the effectiveness and surface heat transfer augmentation levels of impingement cooling. Three different impingement hole shapes are employed within a jet array: circle, racetrack, and triangle. These are designed so that all three hole shapes have the same hydraulic diameter. Different sizes, different shapes, and different distributions of surface roughness elements are utilized, including arrays of small triangle roughness, with different roughness heights, employed with and without the addition of large pin roughness elements. Data are presented for impingement jet Reynolds numbers of 900, 1500, 5000, and 11,000. Overall, the racetrack hole configuration generally provides approximately the same heat transfer augmentation as the circle hole configuration, with slightly better performance, under some conditions. The triangle hole configuration provides lower heat transfer augmentation, compared to both the circle hole and racetrack hole configurations. The addition of small triangle roughness to the target plates shows an increase in heat transfer augmentation, compared to the baseline target plate, for both the circle and racetrack hole configurations. For the triangle hole configuration, the addition of small triangle roughness generally shows negligible or negative improvement, compared to the baseline target plate. Also illustrated is significantly different Reynolds number dependence for impingement cooling, as hole shape changes from circle to racetrack, and triangle.

Influences of target surface small-scale rectangle roughness on impingement jet array heat transfer

The present investigation considers the effects of special roughness patterns on impingement target surfaces to improve the effectiveness and surface heat transfer augmentation levels of impingement jet array cooling. This investigation utilizes various sizes, distributions, shapes, and patterns of surface roughness elements for impingement cooling augmentation. The surface roughness shape considered here is rectangle, in combination with larger rectangular pins. Combinations of small rectangle roughness and large pins are considered together, along with arrays of small rectangular roughness alone. Tests are performed at impingement jet Reynolds numbers of 900, 1500, 5000, and 11,000. Local and overall impingement cooling performance depends upon the pattern, distribution, arrangement, and height of the roughness elements, as well as upon the jet Reynolds number. Depending upon the magnitude of jet Reynolds number, different behavior and trends are observed for the small rectangle roughness and large pins together, compared with arrays of small rectangular roughness alone. Overall, results demonstrate the remarkable ability of target surface roughness to produce increased surface heat transfer augmentation levels of impingement jet array cooling, relative to target surfaces which are smooth.

Impingement jet array heat transfer with small-scale cylinder target surface roughness arrays

The present investigation considers special small-scale roughness patterns on impingement target surfaces to increase surface heat transfer augmentation levels of impingement jet array cooling. Utilized are three different heights of cylinder small roughness elements, which are mounted on three different test surfaces. The cylinder-shaped roughness is small-scale because diameter is 7.5 percent and maximum height is 25 percent of impingement hole diameter. Associated results are compared with spatially-averaged Nusselt number ratio distributions measured on target surfaces with triangle small roughness, or rectangle small roughness. Data are obtained for impingement jet Reynolds numbers of 900, 1500, 5000, and 11,000. Nusselt number variations for the small cylinder roughness show different trends with streamwise development and changing roughness height, compared to target plates with small rectangle roughness and small triangle roughness. In general, this is because roughness elements which contain surface shapes with sharp edges generate increased magnitudes of vorticity with length scales of the order of the roughness element diameter. Such generation is not always present in an abundant fashion with the small cylinder roughness because of the smooth contours around each roughness element periphery. Such effects are illustrated by several data sets, including Nusselt numbers associated with the small cylinder roughness with a height of 0.250D at a turbulent Reynolds number of 11,000.

Measurement of flow structures and heat transfer behind a wall-proximity square rib using TSP, PIV and split-fiber film

In the present study, complementary measurement techniques—temperature sensitive paint (TSP), planar particle image velocimetry (planar PIV) and a split-fiber film probe—were used to investigate the effects of a “wall-proximity square rib” on flow structure and surface heat transfer augmentation. TSP was used to measure the time-averaged wall temperature field at three different Reynolds numbers ( $$ Re_{d} $$ = 3800, 7600 and 11,400) based on the rib height d and the mainstream velocity $$ U_{o} $$ , and wall-proximity configurations with four different gap ratios (gap size $$ G $$ over rib height d), G/d = 0, 0.25, 0.50 and 0.75. The two-dimensional distribution of the normalized Nusselt number convincingly demonstrated the existence of a hot spot immediately behind the rib in the attached rib configuration (G/d = 0) and surface heat transfer augmentation in the reattachment zone. Among the three wall-proximity configurations, G/d = 0.25 resulted in maximum heat transfer augmentation immediately behind the rib and overall improvement in surface heat removal. However, no distinctly different spatial patterns of the normalized Nusselt number distribution were found at the three different Reynolds numbers. A subsequent experiment examined the flow pattern and flow structures at $$ Re_{d} $$ = 7600 and three wall-proximity configurations (G/d = 0, 0.25 and 0.50). Velocity field measurements using PIV, along with complementary measurements using split-fiber film, gave a clear view of the flow pattern behind the rib; a very slender separation bubble with highly unsteady flow reversal was found close to the surface when G/d = 0.25. For configurations with G/d = 0.25 and 0.50, the high-speed jet issuing from the gap had a complicated influence on the interaction between the upper free shear layer and lower strong shear layer, resulting in slanted movement of the coupled wake flow. Proper orthogonal decomposition was used to identify the spatial characteristics of the superimposed flow structures. In the configuration G/d = 0.25, coherent structures were found throughout most of the wake region, beginning roughly 1.2d behind the rib. Fluid flow in the region near the wall was strongly influenced by the dominant coherent structures, which were evidently the main mechanism for surface heat removal. In the configuration with G/d = 0.50, the area with large velocity fluctuation intensity shifted away from the wall toward the mainstream due to rapid expansion of the wall jet. This, in combination with large-scale coherent structures from the surface, meant a much reduced influence on the fluid near the wall and a corresponding deterioration of surface heat transfer beyond the station x/d = 4 (x being the downstream distance from the rib’s trailing edge).

Impingement Jet Array Heat Transfer: Target Surface Roughness Shape, Reynolds Number Effects

The present investigation considers the effects of special roughness patterns on impingement target surfaces to improve the effectiveness and surface heat transfer augmentation levels of impingement jet array cooling. This investigation uses various sizes, distributions, shapes, and patterns of surface roughness elements for impingement cooling augmentation. The surface roughness shapes considered here are the rectangle and triangle, in combination with larger rectangle roughness. The configurations considered include 1) arrays of small rectangle roughness, 2) arrays of small triangle roughness, 3) combinations of small rectangle roughness and large roughness together, and 4) combinations of small triangle roughness and large roughness together. Tests are performed at impingement jet Reynolds numbers of 900, 1500, 5000, and 11,000. Local and overall impingement cooling performance depends upon the shape of the roughness elements, as well as upon the jet Reynolds number. Depending upon the magnitude of the jet Reynolds number, different behaviors and trends are observed for the arrays of small rectangle roughness when compared with arrays of small triangle roughness. These differences are related to the abilities of the two different roughness shapes to generate different distributions of local mixing and vorticity at different length scales and timescales. Overall, the results demonstrate the remarkable ability of target surface roughness to produce increased surface heat transfer augmentation levels of impingement jet array cooling, relative to target surfaces that are smooth.

Experimental Study of Heat Transfer Augmentation Near the Entrance to a Film Cooling Hole in a Turbine Blade Cooling Passage

Film cooling is extensively used by modern gas turbine blade designers as a means of limiting the blade temperature when exposed to extreme combustor outlet temperatures. The following paper describes an experimental study of heat transfer near the entrance to a film cooling hole in a turbine blade cooling passage. Steady state heat transfer results were acquired by using a transient measurement technique in a 40 times actual rectangular channel, representative of an internal cooling channel of a turbine blade. Platinum thin film gauges were used to measure the inner surface heat transfer augmentation as a result of thermal boundary layer renewal and impingement near the entrance of a film cooling hole. Measurements were taken at various suction ratios, extraction angles, and wall temperature ratios with a main duct Reynolds number of 25,000. A numerical technique based on the resolution of the unsteady conduction equation, using a Crank–Nicholson scheme, is used to obtain the surface heat flux from the measured surface temperature history. Computational fluid dynamics predictions were also made to provide better understanding of the near-hole flow. The results show extensive heat transfer enhancement as a function of extraction angle and suction ratio in the near-hole region and demonstrate good agreement with a corresponding study. Furthermore it was shown that the effect of a wall-to-coolant ratio is of a second order and can therefore be considered negligible compared with the primary variables such as the suction ratio and extraction angle.o

Comparison of Heat Transfer Augmentation Techniques

Dr. Phil Ligrani is currently of Mechanical Engineering and Director of the Convective Heat Transfer Laboratory at the University of Utah and a Fellow of the American Society of Mechanical Engineers. He has beenworking on convection heat transfer and fluid mechanics research problems since he received his Ph.D. degree from the Department of Mechanical Engineering at Stanford University in 1980. From 1979 to 1982, he was an Assistant in the Turbomachinery Department of the von Karman Institute for Fluid Dynamics, Rhode-Saint-Genese, Belgium. From 1982 to 1984, he worked in the Department of Aeronautics of the Imperial College of Science and Technology, University of London. From 1984 to 1992, he was an Associate in the Department of Mechanical Engineering of the U.S. Naval Postgraduate School. In his research, he has investigated the ultra-small-scale motions that exist near walls in turbulent boundary layers, the effects of surface roughness on turbulent boundary layers, transitional phenomena in curved channels including the development and structure of Dean vortex pairs, and innovative schemes for internal cooling and surface heat transfer augmentation, such as dimpled surfaces and swirl chambers, as well as a variety of gas turbine heat transfer and blade cooling problems. He served as Guest Editor for the journal Measurement Science and Technology from 1998 to 2000, and he will serve as Associate Technical Editor for the Journal of Heat Transfer from 2003 to 2006. He has published approximately 150 journal papers, conference papers, and book chapters. In 1995, he was presented with the Professor of the Year award at the University of Utah for outstanding classroom teaching. Some of his other activities and recognitions include a Guest Professorship in 2000 at the Institut fur Thermische Stroemungs-maschinen-Universitaet Karlsruhe, a Visiting Senior Research Fellowship from 1982 to 1983 at the Imperial College of Science and Technology-University of London, a NASA Space Act Tech Brief Award in 1991 for Development of Subminiature Multi-Sensor Hot-Wire Probes, and the Carl E. and Jessie W. Menneken Faculty Award in 1990 for Excellence in Scientific Research. E-mail: ligrani@mech.utah.edu.