Two-pass Rectangular Channel Research Articles

Flow analysis of a rectangular channel with triangular and semi-spherical protrusions

Thermo-flow analysis of an internal cooling channel with turbulators is carried out pertinent to gas turbine blade cooling. Two-pass rectangular channel with 180° bend is adopted to assess the effect of geometric configurations of turbulators on heat transfer enhancement and pressure drop. Two-pass rectangular channel features triangular straight turbulators having ratio of 1/8 rib height to hydraulic diameter, while semi-spherical protrusion has radius of 1/8 hydraulic diameter. Protrusions are introduced along the flow path to promote mixing of flow. Influence of Reynolds number on heat transfer rates and pressure drop is also incorporated. The Reynolds number ranges in between 19,500 and 69,000, which produce same effect of the land-based air breathing engines which operate at high air speeds. To simulate flow and thermal fields in the channel, 3-dimensional configuration of channel is adopted in numerical computations. Predictions are validated with the experimental work. It is found that by introducing triangular and semi-spherical prostrations at channel wall, heat transfer rates improves significantly while pressure drop reduces. Influence of turbulators on heat transfer enhancement becomes more apparent as the heating performance is compared to smooth channel.

Effects of Aspect Ratio on Heat Transfer and Pressure Drop in Stationary and Rotating Two-Pass Rectangular Channels with Smooth Wall

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Experimental Investigation of Heat Transfer on the Internal Tip Wall in a Rotating Two-Pass Rectangular Channel

Abstract The tip turn region within the gas turbine blade experienced severe thermal issues related to temperature variations and temperature gradients. The current study experimentally measured the heat transfer distribution of the internal blade tip wall in a rotating cooling channel. The aspect ratio of this rectangular channel was 1:4, and the hydraulic diameter was 25.6 mm. Due to the impact of the 180 deg turn, complex three-dimensional flow significantly affected the heat transfer on the internal tip surface. The steady-state liquid crystal method was used to obtain a detailed distribution of heat transfer on the internal tip surface. In this study, the leading and trailing surfaces of the channel wall were either smooth or roughened with 45 deg angled ribs. The Reynolds number inside the pressurized cooling channel ranged from 10,000 to 30,000, and the rotation number was up to 0.53. Furthermore, two-channel orientations (90 deg and 135 deg) with respect to the rotation direction were tested. The tip heat transfer from the smooth channel wall was more sensitive to rotation, and the largest heat transfer enhancement caused by rotation was 68%. Channel orientation of 90 deg produced higher heat transfer compared to the orientation of 135 deg.

Effect of 45-deg Rib Orientations on Heat Transfer in a Rotating Two-Pass Channel With Aspect Ratio from 4:1 to 2:1

Abstract The internal cooling passages of gas turbine blades mostly have varying aspect ratios from one passage to another. However, there are limited data available in the open literature that used a reduced cross section and aspect ratio (AR), after the tip turn. Therefore, the current study presents heat transfer and pressure drop of three different α = 45 deg profiled rib orientations, typical parallel (usual), reversed parallel (unusual), and crisscross patterns in a rotating two-pass rectangular channel with AR = 4:1 and 2:1 in the first radially outward flow and second radially inward flow passages, respectively. For each rib orientation, regional averaged heat transfer results are obtained for both the flow passages with the Reynolds number ranging from 10,000 to 70,000 for the first passage and 16,000 to 114,000 for the second passage with a rotational speed range of 0–400 rpm. This results in the highest rotation number of 0.39 and 0.16 for the first and second passage respectively. The effects of rib orientation, aspect ratio variation, 180-deg tip turn, and rotation number on the heat transfer and pressure drop will be addressed. According to the results, for usual, unusual and crisscross rib patterns, increasing rotation number causes the heat transfer to decrease on the leading surface and increase on the trailing surface for the first passage and vice versa for the second passage. The overall heat transfer enhancement of the usual and unusual rib patterns is higher than the crisscross one. In terms of the pressure losses, the crisscross rib pattern has the lowest and the usual rib pattern has the highest-pressure loss coefficients. When pressure loss and heat transfer enhancement are both taken into account together, the crisscross or unusual rib pattern might be an option to use in the internal cooling method. Therefore, the results can be useful for the turbine blade internal cooling design and heat transfer analysis.

Heat Transfer in a Rotating Two-Pass Rectangular Channel Featuring a Converging Tip Turn With Various 45 deg Rib Coverage Designs

Varying aspect ratio (AR) channels are found in modern gas turbine airfoils for internal cooling purposes. Corresponding experimental data are needed in understanding and assisting the design of advanced cooling systems. The present study features a two-pass rectangular channel with an AR = 4:1 in the first pass with the radial outward flow and an AR = 2:1 in the second pass with the radial inward flow after a 180 deg tip turn. Effects of rib coverage near the tip region are investigated using profiled 45 deg ribs (P/e = 10, e/Dh ≈ 0.11, parallel and in-line) with three different configurations: less coverage, medium coverage, and full coverage. The Reynolds number (Re) ranges from 10,000 to 70,000 in the first passage. The highest rotation number achieved was Ro = 0.39 in the first passage and 0.16 in the second passage. Heat transfer coefficients on the internal surfaces were obtained by the regionally averaged copper plate method. The results showed that the rotation effects on both heat transfer and pressure loss coefficient are reduced with an increased rib coverage in the tip turn region. Different rib coverage upstream of the tip turn significantly changes the heat transfer in the turn portion. Heat transfer reduction (up to −27%) on the tip wall was seen at lower Ro. Dependence on the Reynolds number can be seen for this particular design. The combined geometric, rib coverage, and rotation effects should be taken into consideration in the internal cooling design.

Heat Transfer in a Rotating Two-Pass Rectangular Channel Featuring Reduced Cross-Sectional Area After Tip Turn (Aspect Ratio = 4:1 to 2:1) With Profiled 60 deg Angled Ribs

The present study features a two-pass rectangular channel with an aspect ratio (AR) = 4:1 in the first pass and an AR = 2:1 in the second pass after a 180-deg tip turn. In addition to the smooth-wall case, ribs with a profiled cross section are placed at 60 deg to the flow direction on both the leading and trailing surfaces in both passages (P/e = 10, e/Dh ∼ 0.11, parallel and in-line). Regionally averaged heat transfer measurement method was used to obtain the heat transfer coefficients on all internal surfaces. The Reynolds number (Re) ranges from 10,000 to 70,000 in the first passage, and the rotational speed ranges from 0 to 400 rpm. Under pressurized condition (570 kPa), the highest rotation number achieved was Ro = 0.39 in the first passage and 0.16 in the second passage. The results showed that the turn-induced secondary flows are reduced in an accelerating flow. The effects of rotation on heat transfer are generally weakened in the ribbed case than the smooth case. Significant heat transfer reduction (∼30%) on the tip wall was seen in both the smooth and ribbed cases under rotating condition. Overall pressure penalty was reduced for the ribbed case under rotation. Reynolds number effect was found noticeable in the current study. The heat transfer and pressure drop characteristics are sensitive to the geometrical design of the channel and should be taken into account in the design process.

Numerical study of the influence of rib orientation on heat transfer enhancement in two-pass ribbed rectangular channel

ABSTRACTStrong secondary flow generated by ribbed channel and U-shaped bend is the key for forced convection performance and energy dissipation in U-shaped cooling passage. This investigation studies the coupling of nine different rib orientation and the 180-degree bend on overall friction loss and forced convection in the U-shaped passage by ANSYS CFX commercial CFD package when Re = 30000. The comprehensive evolution of secondary flow is visualized by vortex core method and colored by turbulence kinetic energy. The qualitative results show that the Nu ratio and overall pressure loss in the downstream passage (Passage 2) is highly affected by the upstream geometry. The N-type rib orientation in Passage 1 delivers more disturbance energy into Passage 2 where P-type rib orientation can reduce the momentum loss of the upstream secondary flows and pressure loss. Based on the understanding of interaction of secondary flow near the bend, modified bend geometry is proposed with 9% thermal performance gain over the existent optimized rib orientation. This investigation suggests vortex core method is a promising visualization tool for the flow control and optimization in U-shaped cooling channel with angled ribs.

Effect of droplet characteristics and rotation speed on the flow and heat transfer characteristics of mist/air cooling in a rotating ribbed two-pass rectangular channel

The flow and heat transfer characteristics of mist/air cooling in the rotating ribbed two-pass rectangular channel are numerically investigated using the CFD software ANSYS-CFX. In this article, a comparison in heat transfer performance between the mist/air cooling and the air-only cooling is performed. Additionally, the effect of the initial mist diameter, temperature, velocity and the channel rotation speed on the mist/air cooling performance is analysed. The results show that the droplet flow distance and Nusselt number of the mist/air cooling increase as the initial mist temperature decreases. In addition, as the initial mist diameter decreases, the diameter of mist on the whole channel decreases, resulting in the higher heat transfer, whilst the mist concentration also decreases, leading to the lower heat transfer. Therefore, there is an optimal initial mist diameter which makes the heat transfer performance best. Nevertheless, the droplet movement and heat transfer performance of mist/air cooling are nearly insensitive to the initial mist velocity. It is also noted that the Coriolis force increases with the channel rotation speed, causing the flow deflection becomes more obvious. Consequently, as the channel rotation speed increases, in the first passage the averaged Nusselt number on the trailing wall increases, and that on the leading wall decreases, while the trend in the second passage is reversed.

Effects of coolants on the flow and heat transfer characteristics in a non-rotating and rotating two-pass rectangular channel

The effects of coolants on the flow and heat transfer characteristics in a non-rotating and rotating two-pass rectangular channel with 45-deg angled rib turbulators are numerically investigated using the CFD software ANSYS-CFX. The three-dimensional Reynolds-Averaged Navier–Stokes (RANS) solution with SST turbulence model is applied in this paper. Four kinds of coolants, such as air, steam, mist/air and mist/steam, are used to analyze the effects of coolant types on the heat transfer performance in the channel at six different Reynolds number flow conditions. The channel averaged Nusselt number for air-only are numerically studied and the results are in good agreement with the experimental data. The internal flow pattern and heat transfer characteristics in the channel with air-only are conducted at two kinds of temperature difference. The results indicate that the 39K temperature difference provides a slightly better heat transfer performance than that of the 60K temperature difference. In addition, the obtained results show that the mist/steam gives the best heat transfer enhancement and thermal performance in the channel. The detailed flow pattern and Nusselt number distribution for four coolants are also illustrated and discussed.

Optimization of Rotating Cooling Channel with Pin Fins Downstream of Turning Region

The heat transfer performance of a rotating two-pass rectangular channel with staggered pin-fin arrays is optimized through Reynolds-averaged Navier–Stokes analysis, a surrogate method, and a multi-objective evolutionary algorithm. As the Pareto-optimal front produces a set of optimal solutions, the trends in the objective functions in relation to the design variables are predicted by a hybrid multi-objective evolutionary algorithm. Two objective functions related to heat transfer and friction loss, respectively, have been considered to estimate the performance of the cooling channel in turbine blades. Twodesign variables are selected, viz., the ratio of the diameter to height of the pin fins and the ratio of the streamwise spacing between the pin fins to height of the pin fins, and 20 designs are generated by Latin hypercube sampling. A response surface approximation model as a surrogate model is constructed for each objective function, and a hybrid multi-objective evolutionary algorithm is applied to obtain the Pareto-optimal front. The Reynolds number that is based on the hydraulic diameter of the channel is fixed at 10,000, while the rotation number is held constant at 0.15. The turning region upstream of the pin fins enhances the heat transfer through the channel, since a strong vortical flow structure is created in this region. At some optimal points on the Pareto-optimal front, it is found that the present multi-objective optimization enhances both the heat transfer performance and the pressure-loss characteristics. The effects of the rotation number on the heat transfer and pressure loss are also discussed.

High Rotation Number Effects on Heat Transfer in a Rectangular (AR=2:1) Two-Pass Channel

This paper experimentally investigated the rotational effects on heat transfer in a smooth two-pass rectangular channel (AR=2:1), which is applicable to the cooling passages in the midportion of the gas turbine blade. The test channel has radially outward flow in the first passage and radially inward flow in the second passage after a 180 deg sharp turn. In the first passage, the flow is developing and heat transfer is increased compared with the fully developed case. Rotation slightly reduces the heat transfer on the leading surface and increases heat transfer on the trailing surface in the first pass. Heat transfer is highly increased by rotation in the turn portion of the first pass on both leading and trailing surfaces. Rotation increased heat transfer enhancement in the tip region up to a maximum Nu ratio (Nu/Nus) of 1.83. In the second passage, under rotating conditions, the leading surface experienced heat transfer enhancements above the stationary case while the trailing surface decreased. The current study has more than four times the range of the rotation number previously achieved for the 2:1 aspect ratio channel. The increased range of the rotation number and buoyancy parameter reached in this study are 0–0.45 and 0–0.8, respectively. The higher rotation number and buoyancy parameter have been correlated very well to predict the rotational heat transfer in the two-pass, 2:1 aspect ratio flow channel.

Heat Transfer in a Two-Pass Rectangular Channel (AR=1:4) Under High Rotation Numbers

This paper experimentally investigated the rotational effects on heat transfer in a two-pass rectangular channel (AR=1:4), which is applicable to the channel near the leading edge of the gas turbine blade. The test channel has radially outward flow in the first passage through a redirected sharp-bend entrance and radially inward flow in the second passage after a 180deg sharp turn. In the first passage, rotation effects on heat transfer are reduced by the redirected sharp-bend entrance. In the second passage, under rotating conditions, both leading and trailing surfaces experienced heat transfer enhancements above the stationary case. Rotation greatly increased heat transfer enhancement in the tip region up to a maximum Nu ratio (Nu∕Nus) of 2.4. The objective of the current study is to perform an extended parametric study of the low rotation number (0–0.3) and low buoyancy parameter (0–0.2) achieved previously. By varying the Reynolds numbers (10,000–40,000), the rotational speeds (0–400rpm), and the density ratios (inlet density ratio=0.10–0.16), the increased range of the rotation number and buoyancy parameter reached in this study are 0–0.67 and 0–2.0, respectively. The higher rotation number and buoyancy parameter have been correlated very well to predict the rotational heat transfer in the two-pass, 1:4 aspect ratio flow channel.

Computation of flow and heat transfer in rotating two-pass rectangular channels (AR = 1:1, 1:2, and 1:4) with smooth walls by a Reynolds stress turbulence model

Numerical predictions of three-dimensional flow and heat transfer are presented for rotating two-pass smooth channels with three aspect ratios (AR = 1:1; 1:2; 1:4). Detailed predictions of mean velocity, mean temperature and Nusselt number for two Reynolds numbers ( Re = 10,000 and 100,000) were carried out. A total of fifteen calculations have been performed with various combinations of rotation number, Reynolds number, and coolant-to-wall density ratio. The rotation number and inlet coolant-to-wall density ratio varied from 0.0 to 0.28 and from 0.13 to 0.40, respectively. The focus of this study is to investigate the effect of the channel aspect ratio, the Reynolds number, and the coolant-to-wall density ratio on the nature of the flow and heat transfer. A multi-block Reynolds-averaged Navier–Stokes (RANS) method was employed in conjunction with a near-wall second-moment turbulence closure. In the present method, the convective transport equations for momentum, energy, and turbulence quantities are solved in curvilinear, body-fitted coordinates using the finite-analytic method.

Heat Transfer in Two-Pass Rotating Rectangular Channels (AR=2) With Five Different Orientations of 45 Deg V-Shaped Rib Turbulators

An experimental study was made to obtain heat transfer data for a two-pass rectangular channel (aspect ratio=2:1) with smooth and ribbed surfaces for two channel orientations (90 deg and 135 deg with respect to the plane of rotation). The V-shaped ribs are placed on the leading and trailing surfaces. Five different arrangements of 45 deg V-shaped ribs are studied. The Reynolds number and rotation number ranges are 5000–40000, and 0.0–0.21, respectively. The rib height to hydraulic diameter ratio (e/D) is 0.094; the rib pitch-to-height ratio (P/e) is 10; and the inlet coolant-to-wall density ratio (Δρ/ρ) is maintained around 0.115 for every test. The results show that the rotation-induced secondary flow enhances the heat transfer of the first pass trailing surface and second pass leading surface. However, the first pass leading and the second pass trailing surfaces show a decrease in heat transfer with rotation. The results also show that parallel 45 deg V-shaped rib arrangements produce better heat transfer augmentation than inverted 45 deg V-shaped ribs and crossed 45 deg V-shaped ribs, and a 90 deg channel orientation produces greater rotating effect on heat transfer than a 135 deg orientation.

Heat Transfer in a Two-Pass Rectangular Rotating Channel With 45-deg Angled Rib Turbulators

Abstract Experimental heat transfer results are presented in a two-pass rectangular channel (aspect ratio=2:1) with smooth and ribbed surfaces for two channel orientations (90 and 135 deg to the direction of rotational plane). The rib turbulators are placed on the leading and trailing sides at an angle 45 deg to the main stream flow. Both 45-deg parallel and cross rib orientations are studied. The results are presented for stationary and rotating cases at three different Reynolds numbers of 5000, 10,000, and 25,000, the corresponding rotation numbers are 0.21, 0.11, and 0.04. The rib height to hydraulic diameter ratio (e/D) is 0.094; the rib pitch-to-height ratio (P/e) is 10 and the inlet wall-to-coolant density ratio (Δρ/ρ) is maintained at 0.115 for all surfaces in the channel. Results show that the rotating ribbed wall heat transfer coefficients increase by a factor of 2 to 3 over the rotating smooth wall results. The heat transfer from the first pass trailing and second pass leading surfaces are enhanced by rotation. However, the first pass leading and the second pass trailing sides show a decrease in heat transfer with rotation. The result show that 45-deg parallel ribs produce a better heat transfer augmentation than 45-deg cross ribs, and a 90-deg channel orientation produces higher heat transfer effect over a 135-deg orientation.

Prediction of Flow and Heat Transfer in Rotating Two-Pass Rectangular Channels With 45-deg Rib Turbulators

Abstract Numerical predictions of three-dimensional flow and heat transfer are presented for a rotating two-pass rectangular channel with 45-deg rib turbulators and channel aspect ratio of 2:1. The rib height-to-hydraulic diameter ratio e/Dh is 0.094 and the rib-pitch-to-height ratio P/e is 10. Two channel orientations are studied: β=90deg and 135 deg, corresponding to the mid-portion and the trailing edge regions of a turbine blade, respectively. The focus of this study is twofold; namely, to investigate the effect of the channel aspect ratio and the channel orientation on the nature of the flow and heat transfer enhancement. A multi-block Reynolds-averaged Navier-Stokes (RANS) method was employed in conjunction with a near-wall second-moment turbulence closure. In the present method, the convective transport equations for momentum, energy, and turbulence quantities are solved in curvilinear, body-fitted coordinates using the finite-analytic method. The numerical results compare reasonably well with experimental data for both stationary and rotating rectangular channels with rib turbulators at Reynolds number (Re) of 10,000, rotation number (Ro) of 0.11 and inlet coolant-to-wall density ratio (Δρ/ρ) of 0.115.

Flow and heat transfer in rotating two-pass rectangular channels (AR=2) by Reynolds stress turbulence model

Numerical predictions of three-dimensional turbulent flow and heat transfer are presented for a rotating two-pass smooth rectangular channel with channel aspect ratio of 2:1. The focus of this study is to investigate the effect of rotation, channel orientation and the sharp 180° turn on the flow and heat transfer distributions. Two channel orientations are studied: β=90° and β=135°. A multi-block Reynolds-averaged Navier–Stokes (RANS) method was employed in conjunction with a near-wall second-moment turbulence closure. The Reynolds number ( Re) is fixed at 10,000 while the rotation number ( Ro) is varied from 0 to 0.22. Two inlet coolant-to-wall density ratios Δρ/ ρ are studied (0.115 and 0.22). The numerical results are compared with the experimental data for both stationary and rotating two-pass rectangular channels.

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729 傾斜内壁をもつ矩形断面往復流路内流動のPIV計測(オーガナイズドセッション : 乱流せん断流2)

Impingement Cooling in Rotating Two-Pass Rectangular Channels with Ribbed Walls

Impingementcooling wasstudiedonribbed wallsinrotatingtwo-passrectangularchannelswith asharp 180-deg turn. Two rows of circular jets impinged opposite to the direction of rotation in one channel and in the same direction of rotation in the other channel. Square cross-sectioned ribs were periodically attached to the target walls and inclined at a 45-deg angle to the direction of spent aire ow. Rib pitch to height and rib height to channel hydraulicdiameterratiosweree xed at10 and0.125,respectively. Afterimpingement,spent air (crosseow)traveled radially outward and inward in the e rst and the second impingement channel, respectively. Jet Reynolds numbers and rotation numbers varied from 4 £ 10 3 to 1 £ 10 4 and 0 to 0.0133, respectively. As the jet Reynolds number increased from 4 ££ 10 3 to 1 £ 10 4 , the ratios of channel-averaged ribbed to smooth surface Nusselt numbers increased from 13 to 47% for the nonrotating test. For the rotating test, however, these ratios changed from 9 to 44% as the jet Reynolds number increased. Rotation-induced Coriolis and centrifugal forces altered the pressure distribution, but the jet velocity distribution was not signie cantly affected. However, those rotation-induced forces decreased the Nusselt number values up to 20% in the ribbed impingement channels.

Heat transfer in a rotating two-pass smooth passage with a 180° rectangular turn

Measurements are presented of the distributions of heat transfer coefficients along the leading and trailing walls of a rotating two-pass rectangular channel with through flow. The channel has a 180° rectangular turn and a cross-sectional aspect ratio of 1.25. The Reynolds number and rotational number were varied from 5.0×10 3 to 5.0×10 4 and 0 to 0.44, respectively. Complementary flow visualization and laser-Dopper velocimetry measurements were also performed to facilitate interpretation of the measured heat transfer distributions. The results show that the secondary flow induced by the curvature and turbulence enhancement associated with unsteadiness of separation bubble downstream of the sharp turn are responsible for the significant heat transfer augmentation attained in the first part of the second pass. Moreover, the quantitative effects of the Coriolis force on the skewness of streamwise mean velocity profiles and in turn different local and average heat transfer features in the radially outward and inward flows are demonstrated and addressed.