On flow structure, heat transfer and pressure drop in varying aspect ratio two-pass rectangular channel with ribs at 45°

To increase the thermal efficiency of gas turbines, inlet temperature of gas is increased. This results in the requirement of cooling of gas turbine blades and vanes. Internal cooling of gas turbine blades and vanes is one of several options. Two-pass channels are provided with ribs to enhance heat transfer at the expense of an increased pressure drop. The space in the blade is limited and requires channels with small aspect ratios. Numerical simulations have been performed to investigate heat transfer, flow field and pressure loss in a two-pass channel equipped with 45° ribs with aspect ratio (W_in/H) equal to 1:3 in the inlet pass and 1:1 in the outlet pass with both connected together with a 180° bend. The results are compared with a higher aspect ratio channel (W_in/H = 1:2, inlet pass). In the ribbed channel, a decrease in pressure drop was observed with a decrease in the aspect ratio of the channel. The smaller aspect ratio channel not only allows using more cooling channels in the blade, but also results in more heat transfer enhancement. The divider-to-tip wall distance (W_el) has influence on the pressure drop, as well as on the heat transfer enhancement at the bend and outlet pass. Heat transfer decreases with decrease in aspect ratio of the inlet pass of the two-pass channel. With increase in divider-to-tip wall distance, heat transfer tries to attain a constant value.     

Detailed heat transfer measurements inside straight and tapered two-pass channels with rib turbulators

Most of the studies on gas turbine blade internal channels have focused on constant cross-sectional areas from entrance to turn. Gas turbine blades are typically tapered from hub to tip to reduce thermal loading. These channels exist inside high-performance turbine blades for providing effective cooling to the blade external surface, which is exposed to high-temperature gas flow. Heat transfer measurements are presented for both the straight and tapered square channels including the turn region with and without rib turbulators. The straight channels will have a uniform square cross-section area of 5.08×5.08 cm². For the tapered channels, the square cross-sectional area reduces from entrance into the first pass (5.08×5.08 cm²) to the 180° turn (2.54×2.54 cm²) and then expands from turn to exit in the second pass (5.08×5.08 cm²). The heat transfer results for tapered channels are compared with results for straight channels. Results show that heat transfer in tapered smooth channels is enhanced significantly due to flow acceleration in the first pass, a combination of taper and turn and flow deceleration in the second pass. Overall, the tapered channels significantly produce higher heat transfer enhancements compared to the Dittus–Boelter correlation for fully developed flow especially in the after-turn region. Based on the results from this study, the heat transfer inside tapered channels in the after-turn region cannot be predicted by calculating local Reynolds numbers and using straight channel heat transfer correlations. However, the first pass Nusselt number enhancement distributions are similar for both straight and tapered channels when normalized using the local Nusselt number based on local Reynolds number. The difference in the after-turn region between the straight and tapered channels is reduced with the addition of rib turbulators.     

Heat/Mass transfer in a two-pass rotating rectangular duct with and without 70°-angled ribs

The present study investigates convective heat/mass transfer and flow characteristics inside a cooling passage of rotating gas-turbine blades. The rotating duct with and without rib turbulators are used. The ribs of 70° attack angle are attached on leading and trailing surfaces in a staggered arrangement. A naphthalene sublimation technique is employed to determine detailed local heat transfer coefficients using the heat and mass transfer analogy. Additional numerical calculations are conducted to analyze the flow patterns in the cooling. The local heat/mass transfer and the flow pattern in the passage are changed significantly according to rib configurations, duct turning geometry and duct rotation speed. The results show that the duct rotation generates the heat transfer discrepancy between the leading and trailing walls due to the secondary flows induced by the Coriolis force. The heat/mass transfer on the ribbed duct shows 80% higher than the smooth duct because the ribs attached on the walls disturb the mainflow resulting in recirculation and secondary flows near the rib with the secondary flow generated by rotation. The overall heat transfer pattern on the leading and trailing walls for the first and second passes depend on the rotating speed and the turning geometry, but the local heat transfer trend is affected mainly by the rib arrangeements.     

Effects of cross ribs on heat/mass transfer in a two-pass rotating duct

Two-pass internal cooling passage with rib turbulators has been investigated for convective heat/mass transfer under rotating conditions. The flow and heat transfer characteristics in the cooling passage are very complicated so that it is required the detail analysis to design more efficient gas turbine blades. A naphthalene sublimation technique is employed to determine detailed local heat transfer coefficients using the heat and mass transfer analogy. The local heat/mass transfer and flow pattern in the cooling passage are changed significantly according to rib configurations, duct turning geometries and duct rotation speeds. Four different rib configurations are investigated to obtain the combined effects of the angled rib, duct turning and rotation. The results show that the duct rotation generates the heat/mass transfer discrepancy between the leading and trailing walls due to the secondary flows induced by the Coriolis force. The angled ribs generate a single rotating secondary flow with the cross-rib arrangement and the duct turning makes a strong Dean-type vortex. These vortices affect significantly the heat/mass transfer on the duct wall. The overall heat transfer pattern on the leading and trailing surfaces for the first and second passes are dependent on the duct rotation, but the local heat transfer trend is affected mainly by the rib arrangements. In addition, the present study observes the rotating effect in the two-pass smooth duct to obtain the baseline data in comparison with the ribbed duct for various rib arrangements.     

Flow and Heat Transfer in a High-Aspect-Ratio Rib-Roughed Cooling Channel with Longitudinal Intersecting Ribs

This paper describes a detailed numerical investigation of a stationary high-aspect-ratio rib-roughed rectangular cooling channel with longitudinal intersecting ribs near the gas turbine blade trailing edge region. In order to overcome the heat transfer performance degeneration in the highaspect- ratio channel, longitudinal intersecting ribs are arranged on the channel bottom surface. The effect of the number of longitudinal intersecting ribs on the flow and heat transfer is systematically studied in the Reynolds number range Re = 10 000–30 000. The results show that a heat transfer augmentation region exists just downstream the junction between the longitudinal rib and the angled rib due to additional secondary flows. With more longitudinal intersecting ribs, the heat transfer distributions on the channel surfaces are more uniform. Though the pressure loss is also enlarged with an increase in the number of longitudinal intersecting ribs, the overall thermal efficiency increases in the entire range of Reynolds numbers investigated. The configuration with two sets of longitudinal intersecting ribs shows the best overall thermal efficiency.     

Heat transfer performance comparison of steam and air in gas turbine cooling channels with different rib angles

Using steam as working fluid to replace compressed air is a promising cooling technology for internal cooling passages of blades and vanes. The local heat transfer characteristics and the thermal performance of steam flow in wide aspect ratio channels (W/H = 2) with different angled ribs on two opposite walls have been experimentally investigated in this paper. The averaged Nusselt number ratios and the friction factor ratios of steam and air in four ribbed channels were also measured under the same test conditions for comparison. The Reynolds number range is 6,000–70,000. The rib angles are 90°, 60°, 45°, and 30°, respectively. The rib height to hydraulic diameter ratio is 0.047. The pitch-to-rib height ratio is 10. The results show that the Nusselt number ratios of steam are 1.19–1.32 times greater than those of air over the range of Reynolds numbers studied. For wide aspect ratio channels using steam as the coolant, the 60° angled ribs has the best heat transfer performance and is recommended for cooling design.     

Heat transfer and fluid flow analysis of artificially roughened ducts having rib and groove roughness

Artificially roughness is one of the well known methods of enhancing heat transfer from the heat transfer surface in the form of repeated ribs, grooves or combination of ribs and groove (compound turbulators). The artificial roughness produced on the heat transferring surface is used in cooling of gas turbine blades, nuclear reactor, solar air heating systems etc. Solar air heaters have wide applications in low to moderate temperature range, namely, drying of foods, agricultural crops, seasoning of wood and space heating etc. Solar air heaters have low value of convective heat transfer coefficient between the working fluid (air) and the heat transferring surface, due to the formation of thin laminar viscous sub-layer on its surface. The heat transfer from the surface can be increased by breaking this laminar viscous sub layer. Hence, in the present work compound turbulators in the form of integral wedge shaped ribs with grooves are used on the heat transfer surface, to study its effect on the heat transfer coefficient (Nusselt number) and friction factor in the range of Reynolds number 3,000–18,000. The roughness produced on the absorber plate forms the wetted side of upper broad wall of the rectangular duct of solar air heater. The relative groove position (g/p) was varied from 0.4 to 0.8 and the wedge angle (Φ) was varied from 10° to 25°, relative roughness pitch (p/e) and relative roughness height (e/D) was maintained as 8.0 and 0.033 respectively. The aspect ratio of the rectangular duct was maintained as 8. The Nusselt number and friction factor of the artificially roughened ducts were determined experimentally and the corresponding values were compared with that of smooth surface duct. It is observed that wedge-groove roughened surface shows more enhancement in heat transfer compared to only rib roughened surface arrangement. The investigation revealed that Nusselt number increases 1.5–3 times, while the friction factor increases two to three folds that of the smooth surface duct in the range of operating parameters. It is also observed that in rib–groove roughness arrangement with relative groove position of 0.65 shows the maximum enhancement in the heat transfer compared to the other rib-groove roughness arrangements. Statistical correlations for the Nusselt number and friction factor have been developed by the regression method in terms of the operating and roughness parameters. A program was also developed in MATLAB for the calculation of thermal efficiency and thermal effectiveness. It was observed that the thermal efficiency is more for wedge angle of 15° and relative groove position of 0.65 and its value ranges from 42 to 73 %. The uncertainties in the measurements due to various instruments for the Reynolds number, Nusselt number, and friction factor have been estimated as ±3.8, ±4.54 and ±7.6 % respectively in the range of investigation made.     

Thermal and friction characteristics of turbulent flow through rectangular and square ducts with transverse ribs and wire-coil inserts

The heat transfer and the pressure drop characteristics of turbulent flow of air through rectangular and square ducts with internal transverse rib turbulators on two opposite surfaces of the ducts and with wire-coil inserts have been studied experimentally. Circular duct has also been used. The transverse ribs in combination with wire-coil inserts have been found to perform better than either ribs or wire-coil inserts acting alone. The flow friction and thermal characteristics are governed by duct aspect ratio, coil helix angle and wire diameter of the coil, rib height and rib spacing, Reynolds number and Prandtl number. Correlations developed for friction factor and Nusselt number have predicted the experimental data satisfactorily. It has been found that on the basis of constant pumping power, up to 35% heat duty increase occurs for the combined ribs and wire-coil inserts case compared to the individual ribs and wire-coil inserts cases in the measured experimental parameters space. On the constant heat duty basis, the pumping power has been reduced up to 20% for the combined enhancement geometry than the individual enhancement geometries.     

Heat transfer investigation of an array of jets impinging on a target plate with detached ribs

This combined experimental and numerical study focuses on impingement jet cooling in combination with detached rib turbulators on a flat target for turbomachinery applications. The investigated impingement array consists of an impingement plate with 9 × 9 jet holes with diameter D and a target plate with detached ribs installed beneath the jet hole. The effects of different separation distances (H/D=3-5), jet Reynolds numbers (15,000-35,000) and rib clearances (0.3D and 0.08D) are investigated. The heat transfer is investigated experimentally by the transient liquid crystal (TLC) method. A computational fluid dynamics (CFD) model is carried out within the software package ANSYS CFX. This model uses a steady-state three-dimensional Reynolds-Averaged Navier-Stokes (RANS) approach with the Shear Stress Transport (SST) turbulence model. Numerical simulations allow detailed insight into the fluid mechanics of the complex flow field and complement experimental measurements. Detached ribs in the impingement channel have a strong influence on the flow field and can increase the global Nusselt number by up to 4% if the ribs have adiabatic boundary conditions. The usage of the detached rib reduces the relative discharge coefficient by up to 11% compared to a smooth target.     

Convective heat transfer in ribbed channels with a 180° turn

Detailed quantitative maps of the heat transfer distribution near a 180° sharp turn of a square channel with rib turbulators are measured by means of infrared thermography associated with the heated-thin-foil technique. Air flows into the channel where ribs are mounted on two opposite walls and placed at 60° with respect to its axis. Two rib pitches, two different rib arrangements and two heating conditions are investigated. Results are presented in terms of local and averaged Nusselt numbers which are normalized with the classical Dittus and Boelter correlation for three different Reynolds numbers.     

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.     

Parametric study on heat transfer enhancement and pressure drop of an internal blade tip-wall with pin-fin arrays

One way to cool gas turbine tips is to design serpentine passages with 180° turns inside the blades to fully utilize the coolant potential. It is therefore a desire to improve the cooling of the blade tips to ensure a long durability and safe operation. In the present work, a two-pass channel with a 180° turn and various arrays of pin-fins mounted internally on the tip-cap is considered. The effects of pin-fin height, diameter and pitches on the heat transfer enhancement and pressure drop are investigated numerically. The nominal ratio of height to diameter (H/D) of the pin-fins is 2, and the ratio of tip clearance to pin-fin height is about 10. The inlet Reynolds numbers based on hydraulic diameter are ranging from 100,000 to 600,000. Details of the three dimensional fluid flow and heat transfer over the pin-finned tips are presented. The overall performances of various tips are compared. It is found that due to the combination of turning, impingement and pin-fin crossflow, the heat transfer coefficient of the pin-finned tips is up to a factor of 2.1 higher than that of the smooth tip. This augmentation is achieved at the expense of a penalty of pressure drop around 30%. Results show that the magnitude of the heat transfer enhancement depends upon pin-fin configuration and arrangement. It is suggested that pin-fins are suitable to enhance the blade tip heat transfer and thus to improve the tip cooling.     

Thermal and friction characteristics of laminar flow through rectangular and square ducts with transverse ribs and wire coil inserts

The heat transfer and the pressure drop characteristics of laminar flow of viscous oil (195 < Pr < 525) through rectangular and square ducts with internal transverse rib turbulators on two opposite surfaces of the ducts and with wire coil inserts have been studied experimentally. Circular duct has also been used. The transverse ribs in combination with wire coil inserts have been found to perform better than either ribs or wire coil inserts acting alone. The heat transfer and the pressure drop measurements have been taken in separate test sections. Heat transfer tests were carried out in electrically heated stainless steel ducts incorporating uniform wall heat flux boundary conditions. Pressure drop tests were carried out in acrylic ducts. The flow friction and thermal characteristics are governed by duct aspect ratio, coil helix angle and wire diameter of the coil, rib height and rib spacing, Reynolds number and Prandtl number. Correlations developed for friction factor and Nusselt number have predicted the experimental data satisfactorily. The performance of the geometry under investigation has been evaluated. It has been found that on the basis of constant pumping power, up to fifty per cent heat duty increase occurs for the combined ribs and wire coil inserts case compared to the individual ribs and wire coil inserts cases in the measured experimental parameters space. On the constant heat duty basis, the pumping power has been reduced up to forty per cent for the combined enhancement geometry than the individual enhancement geometries.     

Convective heat transfer in a square channel with angled ribs on two opposite walls

Detailed quantitative maps of the heat transfer distribution in a square channel with angled rib turbulators are measured by means of infrared (IR) thermography associated with the heated-thin-foil technique. Air flows in the channel where square ribs are mounted on two opposite walls at an angle of either 30° or 45° with respect to the duct axis. Two rib pitches, two different rib arrangements and two heating conditions are investigated. Results are presented in terms of local and averaged Nusselt numbers, which are normalised with the classical Dittus and Boelter correlation, for three different Reynolds numbers.     

Heat transfer and pressure drop measurements in rib-roughened rectangular ducts

In the present study, the thermal and hydraulic performance of three rib-roughened rectangular ducts is investigated. The aspect ratio of the ducts was 1 to 8, and the ribs were arranged staggered on the two wide walls. Three rib configurations were tested: parallel ribs and V-shaped ribs pointing upstream or downstream of the main flow direction. For all cases, the rib height-to-hydraulic diameter ratio was 0.06, with an attack angle of 60° and a pitch-to-height ratio of 10. The Reynolds number range was from 1000 to 6000. Liquid crystal thermography was employed in the heat transfer experiment to demonstrate detailed temperature distribution between a pair of ribs on the ribbed surfaces. The secondary flows caused by the inclined ribs create a significant spanwise variation of the heat transfer coefficients on the rib-roughened wall with high heat transfer coefficient at one end of the rib and low value at the other. In the streamwise direction between two consecutive ribs, the temperature distribution shows a sawtooth fashion because of flow reattachment. Based on the local heat transfer coefficients, the average Nusselt numbers were estimated as weighted mean values. Isothermal pressure drop data were taken and presented as Fanning friction factors. The ducts are compared to each other by considering both heat transfer and friction factor performance.     

Flow field analysis inside a gas turbine trailing edge cooling channel under static and rotating conditions: Effect of ribs

The present work is part of a wider research program which concerns the aero-thermal characterization of cooling channels for the trailing edge of gas turbine blades. The selected passage model is characterized by a trapezoidal cross-section of high aspect-ratio and coolant discharge at the blade tip and along the wedge-shaped trailing edge, where seven elongated pedestals are also installed. In this contribution, a new channel configuration provided with inclined ribs installed inside the radial development region is analyzed, extending the previous results and completing the already available data base, thus providing an overall review of the aero-thermal performance of the considered passage. The velocity field inside the channel was measured by means of 2D and Stereo-PIV techniques in multiple flow planes under static and rotating conditions. The tests were performed under engine similar conditions with respect to both Reynolds (Re = 20,000) and Rotation (Ro = 0, 0.23) numbers. Time averaged flow fields and velocity fluctuation data inside the stationary and rotating channels are analyzed and also critically compared with the data acquired without ribs. In this way the effects on the flow field induced by both rotation and ribs are clearly described. In particular, the ribs modify substantially both the flow field on the channel walls where they are installed and the 3D separation structures that surround the pedestals. If also rotation is taken into account, the relative flow field is characterized by a considerable guiding effect of the ribs coupled with a stronger flow separation on the obstacles that further enhances the heat transfer performances. This behavior was confirmed exploiting the wide thermal data base already available, obtaining a direct link between the observed flow features and the heat transfer performances.     

Shape optimization of staggered ribs in a rotating equilateral triangular cooling channel

A rotating equilateral triangular cooling channel with staggered square ribs inside the leading edge of a turbine blade has been optimized in this work based on surrogate modeling. The fluid flow and heat transfer in the channel have been analyzed using three-dimensional Reynolds-averaged Navier–Stokes (RANS) equations under uniform heat flux condition. Shear stress transport turbulence model has been used as a turbulence closure. Computational results for area-averaged Nusselt number have been validated compared to the experimental data. The objectives related to the heat transfer rate and pressure drop has been linearly combined with a weighting factor to define the objective function. The angle of the rib, the rib pitch-to-hydraulic diameter ratio, and the rib width-to-hydraulic diameter ratio have been selected as the design variables. Twenty-two design points have been generated by Latin Hypercube sampling, and the values of the objective function have been calculated by the RANS analysis at these points. The surrogate model for the objective function has been constructed using the radial basis neural network method. Through the optimization, the objective function value has been improved by 21.5 % compared to that of the reference geometry.     

Heat (mass) transfer and friction loss in two-pass ducts with various parallel rib arrangements

The present study investigated the effects of four parallel rib arrangements on heat/mass transfer and pressure drop characteristics in rotating two-pass ducts. The experiments of mass transfer and pressure measurement are performed to obtain heat transfer distributions and friction losses. The highest averaged heat (mass) transfer and friction loss in each region appeared in the turning region in the stationary cases, but appeared in the upstream region of the second pass in the rotating cases. Among the tested rib arrangements, the averaged heat (mass) transfer and the friction factor in the second pass in the stationary ducts were high in the cases with the NN- and PP-type ribs; however, in the rotating ducts, they were high in the cases with the NN- and PN-type ribs. The thermal performance in the four different rib arrangements was almost the same.     

Investigation of flow separation in a transonic-fan linear cascade using visualization methods

An extensive experimental study into the nature of the separated flows on the blade suction surface of modern transonic fans is described in this paper. The study was a subtask of a larger experimental effort focused on blade flutter excited by flow separation in the blade tip region. The tip sections of airfoils on transonic fan blades are designed for precompression and consequently they differ from sections on the rest of the blade. The blade tip section was modeled by a low aspect ratio blade and therefore most of the blade tested was exposed to the secondary flow effects. The aim of this work was to supply reliable data on flow separation on transonic fan blades for validation of future analytical studies. The experimental study focused on two visualization techniques: surface flow visualization using dye oils and schlieren (and shadowgraph) flow visualization. The following key observations were made during the study. For subsonic inlet flow, the flow on the suction surface of the blade was separated over a large portion of the blade, and the separated area increased with increasing inlet Mach number. For the supersonic inlet flow condition, the flow was attached from the leading edge up to the point where a bow shock from the upper neighboring blade imposed on the blade surface. Downstream, there was a separated flow region in which air flowed in the direction opposite the inlet flow. Finally, past the separated flow region, the flow reattached to the blade surface. For subsonic inlet flow, the low cascade solidity resulted in an increased area of separated flow. For supersonic flow conditions, the low solidity resulted in an improvement in flow over the suction surface.     

Convective heat transfer and fluid flow physics in some ribbed ducts using liquid crystal thermography and PIV measuring techniques

Enhancement of forced convection is important in several engineering applications. Surface modifications like rib-roughening are commonly used in applications such as compact heat exchangers and internal cooling of gas turbine blades and vanes. This paper gives a brief summary of convective heat transfer and fluid flow in some ribbed ducts using liquid crystal thermography and PIV measuring techniques. Details of the flow pattern and the influence of rib configuration and arrangement on the heat transfer are presented. Nevertheless, the understanding of the flow and thermal physics in ribbed ducts is not yet complete and further studies are needed.