Local Heat/Mass Transfer Measurements in a Rectangular Duct With Discrete Ribs

1999 ◽  
Vol 122 (3) ◽  
pp. 579-586 ◽  
Author(s):  
H. H. Cho ◽  
S. J. Wu ◽  
H. J. Kwon
Keyword(s):  
Mass Transfer ◽  
Heat Mass Transfer ◽  
Local Heat ◽  
Friction Loss ◽  
Combined Effects ◽  
Rectangular Duct ◽  
Transfer Enhancement ◽  
Transfer Coefficients ◽  
Mass Transfer Enhancement ◽  
Discrete Ribs

The influence of arrangement and length of discrete ribs on heat/mass transfer and friction loss is investigated. Mass transfer experiments are conducted to obtain the detailed local heat/mass transfer information on the ribbed wall. The aspect ratio (width/height) of the duct is 2.04 and the rib height is one tenth of the duct height, such that the ratio of the rib height to hydraulic diameter is 0.0743. The ratio of rib-to-rib distance to rib height is 10. The discrete ribs were made by dividing each continuous rib into two, three, or five pieces, which were attached periodically to the top and bottom walls of the duct with a parallel orientation. The combined effects of rib angle and length of the discrete ribs on heat/mass transfer are considered for the rib angles (α) of 90 and 45 deg. As the number of the discrete ribs increases, the uniformity of the heat/mass transfer distributions increases. For α=90 deg, the heat/mass transfer enhancement with the discrete ribs is remarkable, while the heat/mass transfer performances are slightly higher than that of the transverse continuous ribs due to the accompanied high friction loss penalty. For α=45 deg, the average heat/mass transfer coefficients and the heat/mass transfer performances decrease slightly with the discrete ribs compared to the case of the angled continuous ribs.[S0889-504X(00)00103-3]

Local Heat/Mass Transfer Measurements in a Rectangular Duct With Discrete Ribs

1999 ◽  
Author(s):  
H. H. Cho ◽  
S. J. Wu ◽  
H. J. Kwon
Keyword(s):  
Mass Transfer ◽  
Heat Mass Transfer ◽  
Local Heat ◽  
Friction Loss ◽  
Transfer Enhancement ◽  
Transfer Coefficients ◽  
Mass Transfer Coefficients ◽  
Average Heat ◽  
Duct Wall ◽  
Discrete Ribs

The influence of the arrangement and the length of discrete ribs on heat/mass transfer and friction loss is investigated. The mass transfer experiments are conducted to obtain detailed local heat/mass transfer coefficients on the duct wall. The aspect ratio (width/height) of the duct is 2.04 and the rib height is one tenth of the duct height, such that the ratio of rib height to hydraulic diameter is 0.0743. The ratio of rib-to-rib distance to rib height is 10. The discrete ribs are made by dividing continuous ribs into 2, 3 and 5 pieces and attached periodically to the top and bottom surfaces of the duct with a parallel orientation. After examining the effects of rib angle of attack (α) for continuous ribs, the combined effects of the rib angle and the length of discrete ribs on heat/mass transfer on the duct wall are investigated for α = 90° and 45°. As the number of broken pieces of a rib increases, the more disturbed flows affect greatly heat/mass transfer and increase the uniformity of heat/mass transfer distributions. For α = 90°, the heat/mass transfer enhancement with the discrete ribs is remarkable, so that the discrete ribs augment up to 27% of the average heat/mass transfer coefficients compared with the transverse continuous rib. However, the heat/mass transfer performances of the discrete ribs are slightly higher than that of the transverse continuous rib due to the accompanied high friction loss penalty. For α = 45°, the average heat/mass transfer coefficients are decreased slightly with the discrete ribs, and the heat/mass transfer performances of the angled discrete ribs are also decreased even though the friction losses are lower.

The Combined Effects of Rib Arrangements and Discrete Ribs on Local Heat/Mass Transfer in a Square Duct

2001 ◽  
Author(s):  
H. H. Cho ◽  
S. Y. Lee ◽  
S. J. Wu
Keyword(s):  
Mass Transfer ◽  
Heat Mass Transfer ◽  
Local Heat ◽  
Secondary Flows ◽  
Friction Loss ◽  
Combined Effects ◽  
Height Ratio ◽  
Square Duct ◽  
Flow Mixing ◽  
Discrete Ribs

Local heat/mass transfer and friction loss in a square duct roughened with various types of continuous and discrete rib tabulators are investigated. The combined effects of the gap flows of the discrete ribs and the secondary flows are examined for the purpose of the reduction of thermally weak regions and the promotion of the uniformity of heat/mass transfer distributions as well as the augmentation of average heat/mass transfer. The rib-to-rib pitch to the rib height ratio (p/e) of 8 and the rib angles of 90 and 60 deg are selected with e/Dh=0.08. The vortical structure of the secondary flows induced by the parallel angled arrays are quite distinct from that induced by the cross angled arrays. This distinction influences on heat/mass transfer and friction loss in all the tested cases. The gap flows of the discrete ribs reduce the strength of the secondary flows but promote local turbulence and flow mixing. As a result, the fairly uniform heat/mass transfer distributions are obtained with two row gaps.

Effects of Duct Aspect Ratios on Heat/Mass Transfer With Discrete V-Shaped Ribs

2003 ◽  
Author(s):  
Dong Ho Rhee ◽  
Dong Hyun Lee ◽  
Hyung Hee Cho ◽  
Hee Koo Moon
Keyword(s):  
Mass Transfer ◽  
Heat Mass Transfer ◽  
Local Heat ◽  
Attack Angle ◽  
Geometric Feature ◽  
Rectangular Duct ◽  
Transfer Coefficients ◽  
Aspect Ratios ◽  
Sublimation Method ◽  
Cooling Passage

The present study investigates the effects of rib arrangements and aspect ratios of a rectangular duct simulating the cooling passage of a gas turbine blade. Two different V-shaped rib configurations are tested in a rectangular duct with the aspect ratios (W/H) of 3 to 6.82. One is the continuous V-shaped rib configuration with 60° attack angle, and the other is the discrete V-shaped rib configuration with 45° attack angle. The designed aspect ratio of the duct is obtained by changing the height with a fixed width of 150 mm. The square ribs (3 mm) with the pitch to height ratio of 10.0 are installed on the test section in a parallel arrangement for both rib configurations. Reynolds numbers based on the hydraulic diameter are changed from 10,000 to 30,000 in this study to investigate the variation of duct Reynolds number. A naphthalene sublimation method is used to measure local heat/mass transfer coefficients. For the continuous V-shaped rib configuration, two pairs of counter-rotating vortices are generated in a duct, and high transfer region is formed at the center of the ribbed walls of the duct. However, for the discrete V-shaped rib configuration with 45° attack angle, complex secondary flow patterns are generated in the duct due to its geometric feature, and more uniform heat/mass transfer distributions are obtained for all tested cases.

Effects of Bleed Flow on Heat/Mass Transfer in a Rotating Rib-Roughened Channel

2006 ◽  
Vol 129 (3) ◽  
pp. 636-642 ◽  
Author(s):  
Yun Heung Jeon ◽  
Suk Hwan Park ◽  
Kyung Min Kim ◽  
Dong Hyun Lee ◽  
Hyung Hee Cho
Keyword(s):  
Mass Transfer ◽  
Heat Mass Transfer ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Main Flow ◽  
Hydraulic Diameter ◽  
Transfer Coefficients ◽  
Square Channel ◽  
Rib Turbulators ◽  
Sublimation Method

The present study investigates the effects of bleed flow on heat/mass transfer and pressure drop in a rotating channel with transverse rib turbulators. The hydraulic diameter (Dh) of the square channel is 40.0mm. 20 bleed holes are midway between the rib turburators on the leading surface and the hole diameter (d) is 4.5mm. The square rib turbulators are installed on both leading and trailing surfaces. The rib-to-rib pitch (p) is 10.0 times of the rib height (e) and the rib height-to-hydraulic diameter ratio (e∕Dh) is 0.055. The tests were conducted at various rotation numbers (0, 0.2, 0.4), while the Reynolds number and the rate of bleed flow to main flow were fixed at 10,000 and 10%, respectively. A naphthalene sublimation method was employed to determine the detailed local heat transfer coefficients using the heat/mass transfer analogy. The results suggest that for a rotating ribbed passage with the bleed flow of BR=0.1, the heat/mass transfer on the leading surface is dominantly affected by rib turbulators and the secondary flow induced by rotation rather than bleed flow. The heat/mass transfer on the trailing surface decreases due to the diminution of main flow. The results also show that the friction factor decreases with bleed flow.

Local Heat/Mass Transfer Characteristics on a Rotating Blade With Flat Tip in a Low-Speed Annular Cascade—Part II: Tip and Shroud

2005 ◽  
Vol 128 (1) ◽  
pp. 110-119 ◽  
Author(s):  
Dong-Ho Rhee ◽  
Hyung Hee Cho
Keyword(s):  
Heat Transfer ◽  
Mass Transfer ◽  
Flow Velocity ◽  
Rotational Speed ◽  
Heat Mass Transfer ◽  
Local Heat ◽  
Incoming Flow ◽  
Transfer Coefficients ◽  
Low Speed ◽  
Annular Cascade

The local heat/mass transfer characteristics on the tip and shroud were investigated using a low speed rotating turbine annular cascade. Time-averaged mass transfer coefficients on the tip and shroud were measured using a naphthalene sublimation technique. A low speed wind tunnel with a single stage turbine annular cascade was used. The turbine stage is composed of sixteen guide plates and blades. The chord length of blade is 150 mm and the mean tip clearance is about 2.5% of the blade chord. The tested Reynolds number based on inlet flow velocity and blade chord is 1.5×105 and the rotational speed of the blade is 255.8 rpm at design condition. The results were compared with the results for a stationary blade and the effects of incidence angle of incoming flow were examined for incidence angles ranging from −15 to +7deg. The off-design test conditions are obtained by changing the rotational speed with a fixed incoming flow velocity. Flow reattachment on the tip near the pressure side edge dominates the heat transfer on the tip surface. Consequently, the heat/mass transfer coefficients on the blade tip are about 1.7 times as high as those on the blade surface and the shroud. However, the heat transfer on the tip is about 10% lower than that for the stationary case due to reduced leakage flow with the relative motion. The peak regions due to the flow reattachment are reduced and shifted toward the trailing edge and additional peaks are formed near the leading edge region with decreasing incidence angles. But, quite uniform and high values are observed on the tip with positive incidence angles. The time-averaged heat/mass transfer on the shroud surface has a level similar to that of the stationary cases.

Effects of Fin Shapes and Arrangements on Heat Transfer for Impingement/Effusion Cooling With Crossflow

2005 ◽  
Author(s):  
Sung Kook Hong ◽  
Dong-Ho Rhee ◽  
Hyung Hee Cho
Keyword(s):  
Mass Transfer ◽  
Heat Mass Transfer ◽  
Flow Characteristics ◽  
Local Heat ◽  
Wall Jet ◽  
Transfer Coefficients ◽  
Mass Transfer Coefficients ◽  
Blowing Ratio ◽  
Transfer Characteristics ◽  
Sublimation Method

The present paper has investigated the effects of fin on the flow and heat/mass transfer characteristics for the impingement/effusion cooling with crossflow. The fins of circular or rectangular shape are installed between two perforated plates and the crossflow passes between these two plates. The blowing ratio is changed from 0.5 to 1.5 for a fixed jet Reynolds number of 10,000. A naphthalene sublimation method is used to obtain the local heat/mass transfer coefficients on the effusion plate. A numerical calculation is also performed to investigate the flow characteristics. Flow and heat/mass transfer characteristics are changed significantly due to installation of fins. In the injection region, wall jet spreads more widely than the case without fins because fin prevents the wall jet from being swept away by the crossflow. In the effusion region, higher heat/mass transfer coefficient is obtained due to the flow disturbance and acceleration by the fin. As the blowing ratio increases, the effects of fin against the crossflow become more significant and then the higher average heat/mass transfer coefficients are obtained. Especially, the cases with rectangular fins have about 40%∼45% enhancement at the high blowing ratio of M = 1.5. However, the increase of blockage effect gives more pressure loss in the channel.

Local Heat/Mass Transfer Measurement on the Effusion Plate in Impingment/Effusion Cooling System

2000 ◽  
Author(s):  
Hyung Hee Cho ◽  
Dong Ho Rhee
Keyword(s):  
Mass Transfer ◽  
Heat Mass Transfer ◽  
Cooling System ◽  
Perforated Plate ◽  
Local Heat ◽  
Impinging Jets ◽  
Transfer Coefficients ◽  
Stagnation Region ◽  
Perforated Plates ◽  
Flow Acceleration

The present study is conducted to investigate the local heat/mass transfer characteristics for flow through perforated plates. A naphthalene sublimation method is employed to determine the local heat/mass transfer coefficients on the effusion plate. Two parallel perforated plates are arranged in two different configurations: staggered and shifted in one direction. The experiments are conducted for hole pitch-to-diameter ratios of 6.0, for gap distance between the perforated plates of 0.33 to 10 hole diameters, and for Reynolds numbers of 5,000 to 12,000. The result shows that the high transfer region is formed at stagnation region and at the mid-line of the adjacent impinging jets due to secondary vortices and flow acceleration to the effusion hole. For flows through the perforated plates, the mass transfer rates on the surface of the effusion plate are about six to ten times higher than for effusion cooling alone (single perforated plate). In general, higher heat/mass transfer is obtained with smaller gap distance between two perforated plates.

Effect of Trailing-Edge Ejection on Local Heat (Mass) Transfer in Pin Fin Cooling Channels in Turbine Blades

1994 ◽  
Vol 116 (1) ◽  
pp. 159-168 ◽  
Author(s):  
R. D. McMillin ◽  
S. C. Lau
Keyword(s):  
Mass Transfer ◽  
Gas Turbine ◽  
Heat Mass Transfer ◽  
Turbine Blades ◽  
Local Heat ◽  
Transfer Coefficients ◽  
Pin Fin ◽  
Flow Through ◽  
Straight Flow ◽  
Cooling Air

Experiments are conducted to study the local heat transfer distribution and pressure drop in a pin fin channel that models the cooling passages in modern gas turbine blades. The detailed heat/mass transfer distribution is determined via the naphthalene sublimation technique for flow through a channel with a 16-row, staggered 3 × 2 array of short pin fins (with a height-to-diameter ratio of 1.0, and streamwise and spanwise spacing-to-diameter ratios of 2.5) and with flow ejection through holes in one of the side walls and at the straight flow exit (to simulate ejection through holes along the trailing edges and through tip bleed holes of turbine blades). The pin fin heat/mass transfer and the channel wall heat/mass transfer are obtained for the straight-flow-only and the ejection-flow cases. The results show that the regional pin heat/mass transfer coefficients are generally higher than the corresponding regional wall heat/mass transfer coefficients in both cases. When there is side wall flow ejection, a portion of the flow turns to exit through the ejection holes and the rate of heat/mass transfer decreases in the straight flow direction as a result of the reducing mass flow rate along the channel. The rate of cooling air flow through a pin fin channel in a gas turbine blade must be increased to compensate for the “loss” of the cooling air through trailing edge ejection holes, so that the blade tip is cooled sufficiently.

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