Pressure drop and thermal performance in rotating two-pass ducts with various cross rib arrangements

The present study investigates the pressure drop characteristics of rotating two-pass ducts. The duct has an aspect ratio (W/H) of 0.5 and a hydraulic diameter (D _h ) of 26.67 mm. Rib turbulators are attached in the four different cross arrangements on the leading and trailing surfaces of the test ducts. The ribs have a rectangular cross section of 2 mm (e) × 3 mm (w) and a rib angle-of-attack of 70°. The pitch-to-rib-height ratio (p/e) is 7.5 and the rib-height-to-hydraulic-diameter ratio (e/D _h ) is 0.075. The measured results for each region show that the highest pressure drop appears in the turning region in the stationary case, but appears in the upstream region of the second pass in the rotating case. The heat transfer and the pressure coefficients in the first pass are similar for the stationary and rotating cases in all the tested rib arrangements. After the turning region, however, the heat transfer and pressure drop are high in the cases with the cross NN- and PP-type ribs in the stationary ducts. In the rotating ducts, they are high in the cases with the cross NP- and PP-type ribs.     

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.     

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.     

Detailed measurement of heat/mass transfer and pressure drop in a rotating two-pass duct with transverse ribs

The present study investigates the convective heat/mass transfer and pressure drop characteristics in a rotating two-pass duct with and without transverse ribs. The Reynolds number based on the hydraulic diameter is kept constant at 10,000 and the rotation number is varied from 0.0 to 0.2. When rib turbulators are installed, heat/mass transfer and friction loss are respectively augmented 2.5 times and 5.8 times higher than those of the smooth duct since the main flow is turbulated by reattaching and separating on the vicinity of the duct surfaces. Differences of heat/mass transfer and pressure coefficient between leading and trailing surfaces result from the rotation of duct, so that Sherwood number ratios and pressure coefficients are high on the trailing surface in the first-pass and on the leading surface in the second-pass. In the turning region, a pair of Dean vortices shown in the stationary case transform into one large asymmetric vortex cell, and subsequently heat/mass transfer and pressure drop characteristics also change. As the rotation number increases, the discrepancies of the heat/mass transfer and the pressure coefficient enlarge between the leading and trailing surfaces.     

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.     

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.     

Forced convection and friction in triangular duct with uniformly spaced square ribs on inner surfaces

Experimental investigation had been conducted to study the steady-state forced convection heat transfer and pressure drop characteristics of the hydrodynamic fully-developed turbulent flow in the air-cooled horizontal equilateral triangular ducts, which were fabricated with the same length and hydraulic diameter. Inner surfaces of the ducts were fixed with square ribs with different side lengths of 6.35, 9.525 and 12.7 mm, respectively, and the uniform separation between the centre lines of two adjacent ribs was kept constant at 57.15 mm. Both the triangular ducts and the ribs were fabricated with duralumin. The experiments were performed with the hydraulic diameter based Reynolds number ranged from 3100 to 11300. The entire inner wall of the duct was heated uniformly, while the outer surface was thermally insulated. It was found that the Darcy friction factor of the duct was increasing rather linearly with the rib size, and forced convection could be enhanced by an internally ribbed surface. However, the heat transfer enhancement was not proportional to the rib size but a maximum forced convection heat transfer augmentation was obtained at the smallest rib of 6.35 mm. Non-dimensional expressions for the determination of the steady-state heat transfer coefficient and Darcy friction factor of the equilateral triangular ducts, which were internally fabricated with uniformly spaced square ribs of different sizes, were also developed. Received on 25 May 1999     

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.     

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 and flow temperature measurements in a rotating triangular channel with various rib arrangements

In the present study, the regionally-averaged heat transfer coefficients and flow temperature distributions were measured in an equilateral triangular channel with three different rib arrangements (α = 45, 90 and 135°). To measure regionally-averaged heat transfer coefficients in the channel, two rows of copper blocks and a single heater were installed on two ribbed walls. The fluid temperature distributions were obtained using a thermocouple-array. The rotation number ranged from 0.0 to 0.1 with a fixed Reynolds number of 10,000. For the 90° ribs, the heat transfer coefficients on the pressure side surface were increased significantly with rotation, while the suction side surface had lower heat transfer coefficients than the stationary channel. For the angled ribs, rib-induced secondary flow dominated the heat transfer characteristics and high heat transfer rates were observed on the regions near the inner wall for the 45° angled ribs and near the leading edge for the 135° angled ribs.     

Experimental investigation of local heat transfer in a square duct with various-shaped ribs

In the present study, experimental studies are carried out to investigate the heat transfer and friction characteristics in a square duct roughened by various-shaped ribs on one wall. The ribs are oriented transversely to the main stream in a periodic arrangement. Liquid crystal thermography is employed to measure the local and average heat transfer coefficient on the ribbed surface. The rib height-to-duct hydraulic diameter ratio is fixed at 0.1; the rib pitch-to-height ratio varies from 8 to 15 and the test Reynolds number spans from 8,000 to 20,000. The results show that the trapezoidal-shaped ribs with decreasing height in the flow direction (case C) provide the highest heat transfer enhancement factor and are likely to be used to suppress the local hot spot which usually occurs in the region just behind the ribs.     

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.     

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.     

Experimental study of developing turbulent flow and heat transfer in ribbed convergent/divergent square ducts

The local heat transfer and pressure drop characteristics of developing turbulent flows of air in three stationary ribbed square ducts have been investigated experimentally. These are: ribbed square duct with constant cross-section (straight duct), ribbed divergent square duct and ribbed convergent square duct. The convergent/divergent duct has an inclination angle of 1°. The measurement was conducted within the range of Reynolds numbers from 10 000 to 77 000. The heat transfer performance of the divergent/convergent ducts is compared with the ribbed straight duct under three constraints: identical mass flow rate, identical pumping power and identical pressure drop. Because of the streamwise flow acceleration or deceleration, the local heat transfer characteristics of the divergent and convergent ducts are quite different from those of the straight duct. In the straight duct, the fluid flow and heat transfer become fully developed after 2–3 ribs, while in the divergent and convergent ducts there is no such trend. The comparison shows that among the three ducts, the divergent duct has the highest heat transfer performance, the convergent duct has the lowest, while the straight duct locates somewhere in between.     

Experimental investigation of the effect of variously-shaped ribs on local heat transfer on the outer wall of the turning portion of a U-channel inside solar air heater

In the present work, an experimental investigation of convective heat transfer and pressure drop was carried out for the turning portion of a U-channel where the outer wall was equipped with ribs. The shape of the ribs was varied. The investigation aims to give guidelines for improving the thermo-hydraulic performance of a solar air heater at the turning portion of a U-channel. Both the U-channel and the ribs were made in acrylic material to allow optical access for measuring the surface temperature by using a high-resolution technique based on narrow band thermochromic liquid crystals (TLC R35C5 W) and a CCD camera placed to face the turning portion of the U-channel. The uncertainties were estimated to 5 and 7 % for the Nusselt number and friction factor, respectively. The pressure drop was approximately the same for all the considered shapes of the ribs while the dimpled rib case gave the highest heat transfer coefficient while the grooved rib presented the highest performance index.     

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.     

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.     

Developing and fully developed turbulent flow in ribbed channels

Wall-mounted roughness features, such as ribs, are often placed along the walls of a channel to increase the convective surface area and to augment heat transfer and mixing by increasing turbulence. Depending on the relative roughness size and orientation, the ribs also have varying degrees of increased pressure losses. Designs that use ribs to promote heat transfer encompass the full range of having only a few streamwise ribs, which do not allow fully developed flow conditions, to multiple streamwise ribs, which do allow the flow to become fully developed. The majority of previous studies have focused on perturbing the geometry of the rib with little attention to the spatially and temporally varying flow characteristics and their dependence on the Reynolds number. A staggered rib-roughened channel study was performed using time-resolved digital particle image velocimetry (TRDPIV). Both the developing (entry region) and a fully developed region were interrogated for three Reynolds numbers of 2,500, 10,000, and 20,000. The results indicate that the flow was more sensitive to Reynolds number at the inlet than within the fully developed region. Despite having a similar mean-averaged flowfield structure over the full Reynolds number range investigated, the population and distribution of coherent structures and turbulent dissipation within the fully developed region were also found to be Reynolds number dependent. Exploring the time-accurate flow characteristics revealed that in addition to vortices shed from the rib shear layer, the region of the rib wake was governed by a periodic process of bursting of the wake vortices resulting in the intermittent ejection of the inter-rib recirculation region into the core flow. This periodic process was the driving mechanism resulting in mixing and heat transfer augmentation. A quadrant-splitting burst analysis was also performed to determine the characteristic frequency and duration of inter-rib bursting as well as the wake shedding frequency, both of which were determined to be Reynolds number dependent.     

Heat/mass transfer and pressure drop in a triangular-rib-roughened rectangular channel

In this paper, the heat/mass transfer analogy was used to investigate the heat transfer and pressure drop in a square channel with triangular ribs on its two opposite walls. Reynolds number varied from 1 × 10⁴ to 7 × 10⁴; the dimensionless heights of the triangular ribs H/W were 0.04, 0.07, and 0.1; and their dimensionless pitches S/W were 0.45, 0.63, 1.0, 1.37, 1.55, and 2.1. Experimental results showed that the heat transfer coefficients of the wall with triangular rib were about 1 to 2.3 times larger than those of a smooth-channel wall, and the pressure drops along this roughened channel were about 1 to 10 times larger than those for a smooth channel. Correlations of heat transfer and pressure drop were obtained, which are useful for practical designs.     

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.