Opposite Rib-roughened Walls Research Articles

Contrastive experimental study on heat transfer and friction characteristics in steam cooled and air cooled rectangular channels with rib turbulators

The present experiment compares the heat transfer and friction characteristics in steam cooled and air cooled rectangular channels (simulating a gas turbine blade cooling passage) with two opposite rib-roughened walls. The Reynolds number (Re) whose length scale is the hydraulic diameter of the passage is set within the range of 10000–60000. The channel length is 1000 mm. The pitch-to-rib height ratio, the channel aspect ratio and the channel blockage ratio is 10, 0.5 and 0.047, respectively. It is found that the average Nu, the average friction coefficient, and the heat transfer performance of both steam and air in the ribbed channels show almost the same change trend with the increase of Re. Under the same test conditions, the average Nu of steam is 30.2% higher than that of air, the average friction coefficient is 18.4% higher, and the heat transfer performances of steam on the ribbed and the smooth walls are 8.4% and 7.3% higher than those of air, respectively. In addition, semi-empirical correlations for the two test channels are developed, which can predict the Nu under the given test condition. The correlations can be used in the design of the internal cooling passage of new generation steamcooled gas turbine blade/vane.

Heat transfer characteristics in steam-cooled rectangular channels with two opposite rib-roughened walls

An experimental study of heat transfer characteristics in steam-cooled rectangular channels with two opposite rib-roughened walls for Reynolds number (Re) in the range of 10,000–80,000 was conducted. To simulate the actual geometry and heat transfer structure of turbine blade/vane internal cooling passage, each of the test channels was made by welding four stainless steel plates. The pitch-to-rib height ratio p/e was kept at 10 and the channel length L was kept at 1000 mm. The channel aspect ratios (W/H) were 0.25, 0.5 and 1, respectively. The channel blockage ratios were 0.047 for W/H = 1, 0.5 and 0.078 for W/H = 0.25. We have found that the average Nusselt number (Nu) for the channel with α = 45° is about 15–25 percent higher than that for the channel with α = 60°. For the channels with W/H = 0.5 or 1, the average Nu decreases along the rib axes because of the rib-induced secondary flow that moves from the left-hand side to the right-hand side. In addition, based on the heat transfer results, we have developed the semi-empirical correlations for the two test channels with the highest and the lowest heat transfer. The correlations can be used in the design of the internal cooling passage of new-generation steam-cooled gas turbine blade/vane.

Influence of duct aspect ratio on heat/mass transfer in coolant passages with rotation

The combined effects of duct aspect ratio and rotation on heat/mass transfer characteristics are investigated. Mass transfer experiments are conducted to obtain detailed local heat/mass transfer coefficients on the leading and trailing surfaces in rotating two-pass ducts with a pair of opposite rib-roughened walls. The ducts of three different aspect ratios ( W/ H = 0.5, 1.0, and 2.0) are employed with a fixed hydraulic diameter ( D h) of 26.7 mm. In all duct cases, the rib height-to-hydraulic diameter ratio ( e/ D h) is 0.056 and the rib pitch-to-rib height ( p/ e) is 10. The rotation number ranges from 0.0 to 0.20 while the Reynolds number is fixed at 10,000. To verify the heat/mass transfer augmentation, internal flow structures are calculated for a smooth two-pass square duct using numerical simulations. The results show that Sherwood number ratios are approximately 2.5 times higher than the fully developed value in a stationary smooth pipe due to the flow reattachment near ribbed surfaces. The overall heat/mass transfer coefficient increases as the duct aspect ratio increases. It is because the core flow is highly disturbed and accelerated in the midsections of the ribs, as the rib height-to-duct height ratio ( e/ H) increases. Dean vortices generated due to 180°-turn augment heat/mass transfer in the turning region and in the upstream region of the second pass. The rotation of duct produces heat/mass transfer discrepancy between leading and trailing surfaces, having higher Sherwood number on the trailing surface in the first pass and on the leading surface in the second pass. However, the effects of duct turning curvature and rotation on heat/mass transfer become less significant for the higher aspect ratio.

Influence of channel height on heat transfer augmentation in rectangular channels with two opposite rib-roughened walls

Influence of channel height on heat transfer augmentation in rectangular channels with two opposite rib-roughened walls

Effect of Rib Profiles on Turbulent Channel Flow Heat Transfer

Introduction R EPEATED ribs have been used as the promoters of turbulence to enhance the heat transfer to the  ow of coolants in channels. Several publications have addressed the comprehensive review of turbine blade cooling and the analysis of heat transfer and friction characteristics of  ow in channels with two opposite rib-roughened walls. The effects of  ow Reynolds number and rib geometry (rib height e, rib spacing P, rib angle of attack a, and rib orientation) on heat transfer and pressure drop in the fully developed region of square, rectangular, and triangular channels have been investigated. 4 Semiempirical correlations over a wide range of rib geometry for the friction and heat transfer design calculations are derived from the law-of-the-wall similarity for  ow over rough surfaces. A detailed analysis of the application of the similarity laws in the case of rectangular channels is presented by Han. Also, studies of the effect of a number of channel-ribbed walls on heat and friction characteristics have been reported. Experiments with some nonrectangular rib shapes have been reported. The previously mentioned studies were limited in the number of rib shapes and were for different conditions (rib height to channel hydraulic diameter ratio e/Dh, and Reynolds number Re). This study focuses on the effect of different rib proŽ les on turbulent channel  ow heat transfer and friction characteristics.

Calculation of fully developed turbulent flow in rectangular ducts with two opposite roughened walls

The predictive capabilities of three transport-type turbulence models are analyzed by comparing predictions with experimental data obtained for fully developed turbulent flow in a square duct (Case 1) and in a 2:1 aspect ratio rectangular duct (Case 2), each with two opposite rib-roughened walls. The models include the Demuren-Rodi (DR) k-ϵ model, the Sugiyama et al. (S) k-ϵ model, and the differential stress (DS) model proposed recently by the authors. For Case 1, the S model is unable to predict experimentally observed axial mean velocity distortion induced by secondary flow in the cross plane. For both test cases, the DR and DS models exhibit similar levels of performance, except in the vicinity of a corner formed by the juncture of adjacent smooth and rib-roughened walls, where the DS model outperforms the DR model. The results are analyzed to explain why the DR model leads to the formation of a spurious secondary flow cell near this corner that is not present in the experimental flow.

Calculation of fully developed turbulent flow in rectangular ducts with two opposite roughened walls

The predictive capabilities of three transport-type turbulence models are analyzed by comparing predictions with experimental data obtained for fully developed turbulent flow in a square duct (Case 1) and in a 2:1 aspect ratio rectangular duct (Case 2), each with two opposite rib-roughened walls. The models include the Demuren-Rodi (DR) k- ϵ model, the Sugiyama et al. (S) k- ϵ model, and the differential stress (DS) model proposed recently by the authors. For Case 1, the S model is unable to predict experimentally observed axial mean velocity distortion induced by secondary flow in the cross plane. For both test cases, the DR and DS models exhibit similar levels of performance, except in the vicinity of a corner formed by the juncture of adjacent smooth and rib-roughened walls, where the DS model outperforms the DR model. The results are analyzed to explain why the DR model leads to the formation of a spurious secondary flow cell near this corner that is not present in the experimental flow.

Heat Transfer Coefficients in an Orthogonally Rotating Duct With Turbulators

Experiments were conducted to determine the influence of rotation on local heat transfer coefficient for the turbulent flow in a short square duct (L/DH = 15) with a pair of opposite rib-roughened walls. The ribs are configured in an in-line arrangement with an attack angle of 90 deg to the main flow. The coolant used was air with the flow direction in the radially outward direction. The Reynolds numbers ranged from 5000 to 25,000; the rib pitch-to-height ratio was 5; and the rib height-to-hydraulic diameter ratio was kept at a value of 0.20. The rotation number range was 0 to 0.5. Local Nusselt number variations along the duct were determined over the trailing and leading surfaces. In addition, local heat transfer measurements on all sides of a typical rib as well as on a typical exposed base surface between two consecutive ribs in a fully developed region were conducted at various rotational speeds. It is shown that the Coriolis acceleration tends to improve the heat transfer due to the presence of strong secondary flow. Centripetal buoyancy is shown to influence the heat transfer response with heat transfer being suppressed on both leading and trailing surfaces as the wall-to-coolant temperature difference is increased with other controlling parameters hold constant. Results are also compared with previous investigations. It was found that the results agree very well with those reported by other works in this field.

Enhanced heat transfer in rectangular channels with full and discrete, transverse and angled protuberances

An experimental study of fully developed turbulent air flow in rectangular channels of different aspect ratios (W/H = 3:1, 1:3, 2:1, and 1:2) with two opposite rib-roughened walls was performed to determine the effects of configuration of full and discrete ribs on the heat transfer coefficient and friction factor. Transverse and angled rib configurations were studied. The tests were performed for Reynolds numbers from 10,000 to 80,000. Discrete angled ribs with higher aspect ratio channel improved the heat transfer performance. On the basis of pumping power consumption, the performance of discrete 45° ribs was better than other rib configurations. Overall, discrete angled ribs with lower aspect ratio performed better.

Heat Transfer and Friction Factor Measurements in Ducts With Staggered and In-Line Ribs

The combined effects of rib alignment and channel aspect ratio on the distributions of the local heat transfer coefficient and on the friction factors for developing and fully developed flow in short square and rectangular channels (L/DH = 13.5–18) with a pair of opposite rib-roughened walls were determined for Reynolds numbers ranging from 13,000 to 130,000. The channel aspect ratios are 1/2 and 1 and the rib alignment configurations are arranged as staggered and in-line types, respectively. The pitch to rib height ratio is 5.31 for all test channels. The local heat transfer distributions on the bottom rib-roughened wall from the channel entrance to the downstream region are presented and discussed. Semi-empirical heat transfer and friction correlations are developed, and the results are compared with those of previous investigations for similarly configured channels, which were roughened by regularly spaced transverse ribs.

An experimental investigation of heat transfer characteristics for turbulent flow over staggered ribs in a square duct

An experimental study of developing and fully developed turbulent air flow in a square duct with two opposite rib-roughened walls in which the ribs are attached in a staggered fashion was conducted to determine the heat transfer characteristics. The rib height-to-hydraulic diameter ratio ( e/ D H ) was 0.19, the rib pitch-to-height ratio ( p/ e) was 5.31. The streamwise temperature distribution was measured, and a law of the wall for the thermal boundary layer at each free-stream turbulence level was obtained. The effects of free-stream turbulence intensity with variations of 4–11% on heat transfer coefficients were also examined. Finally, the relationship between Nusselt number and Reynolds number was correlated. The results might be used in the design of turbine blade cooling channels.

An Experimental Investigation of Heat Transfer Coefficients in a Spanwise Rotating Channel With Two Opposite Rib-Roughened Walls

Liquid crystals are used in this experimental investigation to measure the heat transfer coefficient in a spanwise rotating channel with two opposite rib-roughened walls. The ribs (also called turbulence promoters or turbulators) are configured in a staggered arrangement with an angle of attack to the mainstream flow, α, of 90 deg for all cases. Results are presented for the three values of turbulator blockage ratio e/Dh (0.1333, 0.25, 0.333) and for a range of Reynolds numbers from 15,000 to 50,000 while the test section is rotated at different speeds to give rotational Reynolds numbers between 450 and 1800. The Rossby number range is 10 to 100 (rotation number of 0.1 to 0.01). The effect of turbulator blockage ratios on heat transfer enhancement is also investigated. Comparisons are made between the results of geometrically identical stationary and rotating passage of otherwise similar operating conditions. The results indicate that a significant enhancement in heat transfer is achieved in both the stationary and rotating cases, when the surfaces are roughened with turbulators. For the rotating case, a maximum increase over that of the stationary case of about 45 percent in the heat transfer coefficient is seen for a blockage ratio of 0.133 on the trailing surface in the direction of rotation and the minimum is a decrease of about 6 percent for a blockage ratio of 0.333 on the leading surface, for the range of rotation numbers tested. The technique of using liquid crystals to determine heat transfer coefficients in this investigation proved to be an effective and accurate method especially for nonstationary test sections.

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

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.

Pressure drop and mass transfer in two-pass ribbed channels

The combined effects of the sharp 180-deg turn and of the rib configuration on the pressure drop and mass transfer characteristics in a two-pass square channel with a pair of opposite rib-roughened walls (to simulate turbine airfoil cooling passages) were determined for a Reynolds number range of 10,000-60,000. Heat transfer enhancements were compared for the first pass and for the two-pass channel with the sharp 180-deg turn. Correlations for the fully-developed friction factors and loss coefficients were obtained.

Developing heat transfer in rectangular channels with rib turbulators

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.

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 deg. 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 number varied from 7000 to 90,000. The rib height-to-equivalent diameter ratio (e/D) was kept at a constant value of 0.063, the rib pitch-to-height ratio (P/e) was varied from 10 to 20, and the rib angle of attack (α) was varied from 90 to 60 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 (α = 45–30 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.

Heat Transfer and Friction in Channels With Two Opposite Rib-Roughened Walls

An experimental study of fully developed turbulent air flow in square ducts with two opposite rib-roughened walls was performed to determine the effects of the rib pitch-to-height and rib height-to-equivalent diameter ratios on friction factor and heat transfer coefficients with Reynolds number varied between 7000 and 90,000. Based on the four-sided, smooth-duct correlation and the four-sided, ribbed-duct similarity law, a general prediction method for average friction factor and average Stanton number in rectangular ducts with two smooth and two opposite ribbed walls was developed. There was good agreement between prediction and measurements. The results may be useful for gas turbine blade internal cooling design.