Je-Chin Han

An investigation of heat transfer and friction for rib-roughened surfaces

Abstract An investigation of rib-roughened surface was undertaken to determine the effects of rib shape, angle of attack and pitch to height ratio on friction factor and heat-transfer results. A parallel plate geometry was used. Based on the law of the wall similarity and the application of the heat-momentum analogy developed by Dipprey and Sabersky, a general correlation for friction factor and heat transfer was developed to account for rib shape, spacing and angle of attack. Ribs at a 45° angle of attack were found to have superior heat transfer performance at a given friction power when compared to ribs at a 90° angle of attack or when compared to sand-grain roughness.

Developing heat transfer in rectangular channels with rib turbulators

Abstract The combined effects of the rib angle-of-attack and the channel aspect ratio on the distributions of the local heat transfer coefficient for developing flow in short rectangular channels (L/D = 10 and 15) with a pair of opposite rib-roughened walls were determined for Reynolds numbers from 10 000 to 60 000. The rib angle-of-attack was varied from 90° to 60°, to 45°, and to 30°, whereas the corresponding channel width-to-height ratio was varied from 1 to 2 and to 4, respectively. Semi-empirical heat transfer and friction correlations were obtained to account for rib angle, rib spacing, channel aspect ratio, rib height and Reynolds number. The results can be used in the design of turbine blade cooling channels.

Augmented Heat Transfer in Square Channels With Parallel, Crossed, and V-Shaped Angled Ribs

The effect of the rib angle orientation on the local heat transfer distributions and pressure drop in a square channel with two opposite in-line ribbed walls was investigated for Reynolds numbers from 15,000 to 90,000. The square channel composed of ten isolated copper sections has a length-to-hydraulic diameter ratio of 20; the rib height-to-hydraulic diameter ratio is 0.0625; the rib pitch-to-height ratio equals 10. Nine rib configurations were studied: 90 deg rib, 60 and 45 deg parallel ribs, 60 and 45 deg crossed ribs, 60 and 45 deg V-shaped ribs, and 60 and 45 deg {Lambda}-shaped ribs. The results show that the 60 deg (or 45 deg) V-shaped rib performs better than the 60 deg (or 45 deg) parallel rib and, subsequently, better than the 60 deg (or 45 deg) crossed rib and the 90 deg rib. The V-shaped rib produces the highest heat transfer augmentation, while the {Lambda}-shaped rib generates the greatest pressure drop. The crossed rib has the lowest heat transfer enhancement and the smallest pressure drop penalty.

Heat transfer and friction characteristics in rectangular channels with rib turbulators

The effect of the channel aspect ratio on the distribution of the local heat transfer coefficient in rectangular channels with two opposite ribbed walls (to simulate turbine airfoil cooling passages) was determined for a Reynolds number range of 10,000 to 60,000. The channel width-to-height ratios (W/H, ribs on side W) were 1/4, 1/2, 1, 2, and 4. The test channels were heated by passing current through thin, stainless steel foils instrumented with thermocouples. The local heat transfer coefficients on the ribbed side wall and on the smooth side wall of each test channel from the channel entrance to the fully developed regions were measured for two rib spacings (P/e = 10 and 20). The rib angle-of-attack was kept at 90 deg. The local data in the fully developed region were averaged and correlated, based on the heat transfer and friction similarity laws developed for ribbed channels, to cover the ranges of channel aspect ratio, rib spacing, rib height, and Reynolds number. The results compare well with the published data for flow in a square channel with two opposite ribbed walls. The correlations can be used in the design of turbine airfoil cooling passages.

Heat transfer enhancement in channels with turbulence promoters

Repeated rib-roughness elements have been used in advanced turbine cooling designs to enhance the internal heat transfer. Often the ribs are perpendicular to the main flow direction so that they have an angle-of-attack of 90 degrees. The objective of this investigation was to determine the effect of rib angle-of-attack on the pressure drop and the average heat-transfer coefficients in the fully developed turbulent air flow in a square duct with two opposite rib-roughened walls for Reynolds numbers varied from 7,000 to 90,000. The rib height-to-equivalent diameter ratio was kept at a constant value of 0.063, the rib pitch-to-height ratio was varied from 10 to 20, and the rib angle-of-attack (alpha) was varied from 90 to 60 deg to 45 to 30 deg, respectively. The thermal-performance comparison indicated that the increased heat conductance for the rib with an oblique angle to the flow (alpha = 45 deg - 30 deg) was about 10-20 percent higher than the rib with a 90 deg angle to the flow, and the pumping power requirement for the angled rib was about 20-50 percent lower than the transverse rib. Semi-empirical correlations for friction factor and heat-transfer coefficients were developed to account for rib spacing and rib angle. The correlations can be used in the design of turbine-blade cooling passages.

High performance heat transfer ducts with parallel broken and V-shaped broken ribs

Abstract The effect of the broken rib orientation on the local heat transfer distributions and pressure drop in a square channel with two opposite in-line ribbed walls is investigated for Reynolds numbers from 15 000 to 90 000. The square channel is composed of ten isolated copper sections and has a length-to-hydraulic diameter ratio of 20. The rib height-to-hydraulic diameter ratio is 0.0625, and the rib pitch-to-height ratio equals 10. The results show that the 60° parallel broken rib or 60° V-shaped broken rib provides a higher heat transfer augmentation than the 45° parallel broken rib or 45° V-shaped broken rib and, subsequently, higher than the 90° broken rib. The parallel ‘broken rib’ or V-shaped ‘broken rib’ has 2.5–4 times heat transfer augmentation compared with the previous parallel ‘continuous rib’ or V-shaped ‘continuous rib’ with 2–3 times heat transfer augmentation for the same amount of 7–8 times pressure drop penalty.

Detailed heat transfer distributions in two-pass square channels with rib turbulators

Abstract Detailed Nusselt number distributions are presented for a two-pass square channel with one ribbed wall. This wall of the channel is sprayed with thermochromic liquid crystals, and a transient test is run to obtain the local heat transfer coefficients. Results are presented for Reynolds numbers ranging from 6000 to 60 000. The rib height-to-hydraulic diameter ratio is 0.125; the rib pitch-to-height ratio is 10; and the rib configurations are 90° parallel, 60° parallel, 60° V, and 60° broken V. Detailed measurements are presented in the first pass, before the 180° turn, in the turn region, after the turn, and further downstream in the second pass. The detailed distributions provide a clear understanding of the secondary flows induced by the 180° turn and the rib turbulators.

Heat transfer performance comparisons of five different rectangular channels with parallel angled ribs

This paper systematically presents the results of heat transfer and friction factor data measured in five short rectangular channels with turbulence promoters. The project investigated the combined effects of the channel aspect ratio, rib angle-of-attack, and flow Reynolds number on heat transfer and pressure drop in rectangular channels with two opposite ribbed walls. The channel aspect ratio (width-to-height, W/H, ribs on side W) varied from 14 to 12, to 1, 2. and 4, while the corresponding rib angles-of-attack a were 90°, 60°, 45°, and 30°, respectively. The Reynolds number range was 10,000–60,000. The results suggest that the narrow aspect ratio channels (W/H 1). For the square channel (W/H = 1), the 60°45° angled ribs provide the best heat transfer performance. For the narrow aspect ratio channel (W/H = 14 or 12), the 45°60° angled ribs are recommended while the 30°45° angled ribs are better for wide aspect ratio channels (W/H = 4 or 2).

Augmented heat transfer in rectangular channels of narrow aspect ratios with rib turbulators

Abstract The effects of the rib angle-of-attack on the distributions of the local heat transfer coefficient and on the friction factors in short rectangular channels of narrow aspect ratios with a pair of opposite rib-roughened walls are determined for Reynolds numbers from 10 000 to 60 000. The channel width-to-height ratios are 2 4 and 1 4 ; the corresponding rib angles-of-attack are 90°, 60°, 45°, and 30°, respectively. The results indicate that the narrow-aspect-ratio channels give better heat transfer performance than the wide-aspect-ratio channels for a constant pumping power. Semi-empirical friction and heat transfer correlations are obtained. The results can be used in the design of turbine cooling channels of narrow aspect ratios.

Heat transfer and friction behaviors in rectangular channels with varying number of ribbed walls

Abstract An experimental study of surface heat transfer and friction characteristics of a fully developed turbulent air flow in a square channel with transverse ribs on one, two, three, and four walls is reported. Tests were performed for Reynolds numbers ranging from 10,000 to 80,000. The pitch-to-rib height ratio, P / e , was kept at 8 and rib-height-to-channel hydraulic diameter ratio, e / D h was kept at 0.0625. The channel length-to-hydraulic diameter ratio, L / D h , was 20. The heat transfer coefficient and friction factor results were enhanced with the increase in the number of ribbed walls. The friction roughness function, R ( e + ), was almost constant over the entire range of tests performed and was within comparable limits of the previously published data. The heat transfer roughness function, G ( e + ), increased with roughness Reynolds number and compared well with previous work in this area. Both correlations could be used to predict the friction factor and heat transfer coefficient in a rectangular channel with varying number of ribbed walls. The results of this investigation could be used in various applications of turbulent internal channel flows involving different number of rib roughened walls.