Deg Ribs Research Articles

Effect of the number of ribbed walls on the thermal performance in rectangular channels with 45-deg parallel or criss-cross ribs

Regionally averaged heat transfer distributions and friction characteristics are experimentally investigated for fully developed, turbulent flow in a rectangular channel with a width-to-height aspect ratio (AR=3). 45-deg angled rib turbulators are placed on one, two, and four sides of the channel in parallel and criss-cross configurations with a pitch-to-height ratio(P/e=10) and a rib blockage ratio(eDh=4.1%). For a roughness Reynolds number range(e+=25−500), results show that increasing the number of ribbed walls improves the overall channel heat transfer for the 45-deg parallel ribs. Results reveal that four-sided ribbed channels provided the greatest thermal performance followed by the two-sided and one-sided ribbed channels, respectively, for both angled rib designs. On the basis of the law of wall similarity, friction and heat transfer roughness functions are derived using the four-sided data reported in this work. Furthmore, friction factors and heat transfer coefficients of similar angled rib designs were obtained from open literature and similarly correlated to derive more robust friction and heat transfer correlations over the roughness Reynolds number range. Thermal designers can use these proposed correlations to predict friction and heat transfer in rectangular cooling channels (AR=1-5) with various numbers of ribbed walls. The measured friction factor and heat transfer coefficient of this study can be predicted using these semi-empirical correlations with maximum errors of 9% and 7%, respectively.

Heat Transfer in a Rotating, Blade-Shaped, Two-Pass Cooling Channel With Various 45-Deg Rib Turbulators and a Tip Turning Vane

Abstract This work experimentally studied a rotating, blade-shaped, two-pass cooling channel with various 45-deg rib turbulators and a tip-turning vane. The first passage of the cooling channel has an aspect ratio (AR) = 4:1 with the coolant moving radially outward. After a 180-deg blade-shaped tip turn, the coolant flows radially inward into the second passage with an AR = 2:1. The first and second passages are oriented at 50 deg and 105 deg from the rotation direction, respectively. In the tip-turn portion, a tip-turning vane with an oval cross section connects the mid-lines of the two passes. Two orientations of 45 deg angled ribs were considered in this work: unusual and criss-cross. The ribs have a profiled cross section and were placed in-line (P/e = 10, e/H = 0.16) on the leading and trailing surfaces of the channel. Five inlet Reynolds numbers (10,000–45,000) and five rotational speeds (0−400 rpm) were considered in this study, with a maximum rotation number of Ro = 0.38 achieved in the first pass. The results showed that the effect of the turning vane on heat transfer and pressure loss for the 45-deg unusual rib case was insignificant. The overall heat transfer for the criss-cross rib case was slightly increased by the turning vane. For pressure loss, the turning vane provided approximately an 8% reduction in the 45-deg criss-cross rib case, which was the highest in the rib cases. The 45-deg criss-cross ribs presented the highest thermal performance with the turning vane.

Heat Transfer and Friction in a Rectangular Channel With Varying Numbers of Walls Roughened With 90-Deg Transverse or 45-Deg V-Shaped Ribs

Abstract An experimental investigation was performed to measure the regionally averaged heat transfer distributions and friction factors for fully developed turbulent flow in a rectangular channel with a width-to-height aspect ratio (AR = 3). Data for transverse 90-deg and 45-deg V-shaped, carbon steel ribs placed on one, two, or four sides of the channel were reported for a Reynolds number range (Re = 5,000–60,000). The pitch-to-rib height ratio (P/e) was set at 10 while the rib height-to-channel hydraulic diameter (e/Dh) is 0.041. This corresponds to a roughness Reynolds number range (e+=25−500). For both rib configurations, results show the highest thermal performance was achieved in the channels with four-ribbed wall followed by the two and one-ribbed wall channels, respectively. Using the law of the wall similarity, friction roughness functions (R) and heat transfer roughness functions (G) were developed based on four-sided friction and heat transfer data. Moreover, friction and heat transfer data of similar rib designs were obtained from literature and correlated to the corresponding friction and heat transfer roughness functions. Covering a wide range of roughness Reynolds numbers, more robust heat transfer and friction correlations were developed which can be used by designers to accurately predict both heat transfer and friction in ribbed channels with different numbers of ribbed walls. Using these correlations, one can predict the measured friction and heat transfer data of this study with maximum errors of 9% and 7%, respectively.

Investigation of Gas Turbine Internal Cooling Using Supercritical CO2—Effect of Surface Roughness and Channel Aspect Ratio

Abstract In this paper, an experimental and numerical investigation of internal cooling channels with rib turbulators is presented with sCO2 as the working fluid at process conditions (pressure-20.7 MPa and temperature up to 150 °C). The effect of channel aspect ratio up to 2:1 on thermal-hydraulic performance is explored in additively manufactured rectangular channels and square channels, both with and without 60 deg ribs on the top and bottom sides. The Wilson-plot method is employed to experimentally measure channel-averaged Nusselt number over a Reynolds number range up to 370,000. The friction factor is calculated from pressure drop and mass flow rate and additionally, the overall thermal performance factor (TPF) is reported. A companion computational fluid dynamics (CFD) simulation is performed for the rib turbulated cooling configurations reported in the experiments using the Reynolds average Navier–Stokes-based turbulence model. The objective of the numerical study is to gain insight into the local heat transfer augmentation in the ribbed channels as a result of varying the aspect ratio, channel configuration (square versus rectangular), operating conditions (Reynolds number) and the surface roughness, an inherent outcome of the additive manufacturing process. Surface roughness is simulated using sand grain roughness height (KS) calculated from the experimental data, and a comparison is presented with the corresponding channel configuration with varying surface roughness heights starting from smooth surfaces (KS = 0). Experimental results indicate that the heat transfer augmentation is negligible in the rectangular channels with ribs on the long side compared to the square channel. However, it is enhanced by 60% in comparison to placing ribs on the shorter side. The TPF remains constant at around 1 for the entire range of Reynolds numbers consistent with prior work at the National Energy Technology Laboratory (NETL). The simulation results highlight that increased surface roughness can have a favorable considerable influence on Nusselt number and overall thermal performance enhancement.

Heat Transfer in a Rotating, Blade-Shaped, Two-Pass Cooling Channel With Various 45-Deg Rib Orientations

Abstract This study features a rotating, blade-shaped, two-pass cooling channel, which follows the contour and shape of realistic airfoils. Effects of two different 45-deg rib orientations on the heat transfer and pressure loss were investigated and discussed. The cross section and orientation with respect to rotation vary in the serpentine cooling channel. The first passage of the channel is oriented at 50 deg from the direction of rotation, and the second passage is angled at 105 deg. The radially outward first passage has an aspect ratio (AR) = 4:1. After a 180-deg blade-shaped tip turn, the coolant flows radially inward into the AR = 2:1 s passage. The copper plate method was applied to calculate average heat transfer coefficients in each region of the cooling channel. The 45 deg angled ribs with a profiled cross section are placed on the leading and trailing surfaces in two ways: unusual and criss-cross. The rib spacing is P/e = 10, and rib height is e/H = 0.16. In this study, the Reynolds number ranges from 10,000 to 45,000 in the first passage, resulting in Re = 16,000 to 73,000 in the second passage. The rotational speed varies from 0 to 400 rpm, corresponding to maximum rotation numbers of 0.38 and 0.15 in the first and second passages, respectively. From the results, the 45-deg usual and unusual ribs generally have higher heat transfer than the criss-cross ribs. However, the criss-cross ribs have the lowest pressure loss penalty among the three cases. Taking both heat transfer and pressure loss into account, the 45-deg unusual ribs have a higher thermal performance under the stationary condition while the criss-cross ribs are better than other rib orientations under rotating conditions. Thus, the 45-deg unusual and criss-cross ribs can be considered for applications in real engines. The heat transfer and pressure loss data in this study provide important information for internal cooling of gas turbine blades.

Flow and Heat Transfer Performance of Channels with ‎‎45 Degree Ribs in Staggered Array‎

The cooling efficiency of blade is growing demand with increasing turbine inlet temperature in gas ‎turbine development. Ribs used in cooling channels is a common cooling structure, therefore, many ‎configurations were studied by previous literatures, including angle, spacing, shape etc. However, there ‎are less research about the dislocation ribs structure. In this paper, the 45-deg parallel ribs, crossed ribs ‎and dislocation ribs were investigated by numerical simulation, in order to reveal the heat transfer ‎performance and flow mechanism. Refer to the experiment, SST k-ω model was applied in steady ‎simulation, at Re from 20000 to 50000. Due to the angled ribs can induce the secondary flow and ‎generate small helical vortices at front corner, heat transfer performance was elevated. The large rotating ‎vortices influenced by the ribs arrangement occupy the center channel, thence the dislocation caused ‎different flow and heat transfer results. The results shown that parallel rib has higher heat transfer ‎enhancement than crossed ribs, but pressure loss possess considerable level. At Re=21587, the averaged ‎turbulent kinetic energy of Case2.2 with dislocation ribs is 22.4% lower than parallel ribs. The all 45-deg ‎crossed ribs present higher level of overall thermal performance, and Case2.2 is optimal for the range of ‎Re investigated‎.

Impact of Sharp Versus Rounded Edge of Turbulator on Transport Enhancement

Considering the constraints of manufacturing sharp ribs, a systematic study of the aerothermal behavior of the sharp and rounded-edge ribs has been performed. The investigation was conducted in a square channel with 45 deg ribs over a wide range of Reynolds numbers ( to 135,000). The rounded-edge ribs are known to reduce heat transfer and friction compared to the sharp ribs. However, the studies at high blockage ratio show that rounded edges of the ribs have a smaller effect on the heat transfer but the substantial effect on the pressure drop, which is favorable for better cooling effectiveness. This new study with the lower blockage ratio (i.e., 0.0625) rounded-edge ribs offers an interesting insight. The result confirms that at the lower blockage ratio the rounded-edge ribs reduce the heat transfer capacity to a greater extent but have minimal effect on the pressure drop, especially at high Reynolds number. Numerical studies were also conducted using realizable , , and shear stress transport turbulence models. The results are compared with the experimental data to shed light on the prediction capability of these models for both kinds of the ribs at such a wide range of Reynolds numbers. The effect of the rounded-edge ribs on the flow behavior and the consequent impact on the local heat transfer distribution is also studied.

Experimental Study on the Influence of Film Cooling Hole Extraction on Heat Transfer and Flow Field in Internal Ribbed Cooling Channels of Turbine Blades

Abstract Film cooling and internal passage cooling play a fundamental role in turbine blade cooling. As the cooling holes are fed by the internal crossflow, interaction of these both cooling strategies is implicated. The influence of film hole extraction on the internal flow field and heat transfer of a ribbed cooling channel is investigated in this study. Therefore, a rectangular cooling channel (AR = 2:1) is equipped with parallel ribs of four different geometries (90 deg ribs, 60 deg ribs, 60 deg V-shaped ribs, and 60 deg Λ-shaped ribs) and also with bleed holes at varying positions between the ribs. The different geometrical configurations are examined using 2D-particle image velocimetry (PIV) for flow measurements and transient thermochromic liquid crystal (TLC) technique for heat transfer measurements. Depending on the rib-induced heat transfer pattern, cooling hole positions in the rib segments are found, which can enhance passage internal heat transfer. 90 deg and 60 deg ribs show the best results for upstream hole positions regardless of the lateral positioning. V and Λ ribs reveal a benefit for lateral positioned cooling holes near the upstream rib.

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.

Turning Guide Vane Effect on Internal Cooling of Two-Passage Channel With Parallel Ribs

Abstract The present work investigates the effects of various guide vane designs on the heat transfer enhancement of rotating U-duct configuration with parallel 45-deg ribs. The ribs were installed on the bottom wall of the channel, which has a constant heat flux boundary condition. The channel has a square cross section with a 5.08 cm hydraulic diameter. The first and second passes are 514 mm and 460 mm, respectively. The range of Reynolds number for turbulent flow is up to 35,000. The channel rotates at various speeds up to 600 rpm, which brings the maximum rotation number of 0.75. Several computational fluid dynamics simulations are carried out for this study to understand the effect of guide vanes on flow and heat transfer in serpentine channels under various operating conditions.

Experimental Investigation on Effect of Cross-Flow Reynolds Number on Film Cooling Effectiveness

Film cooling effectiveness and discharge coefficient of cylindrical holes fed by different internal cross-flow channels with varying cross-flow Reynolds numbers are investigated. Three cross-flow cases—a smooth case and ribbed cases with 135-deg ribs and 45-deg ribs—are studied at three blowing ratios and two . Film cooling effectiveness contours are obtained by the transient liquid-crystal measurement technique. For the smooth case and 45-deg case, the asymmetrical vortex is more pronounced with increasing cross-flow direction velocity, resulting in more skewed film coverage at and . Helical motion within the film hole for the 45-deg case is more violent. On the contrary, laterally symmetric film-coverage contours are displayed for the 135-deg case, and the film cooling effectiveness at decreases slowly as flowing downstream. The influence of on lateral distribution is greater at . The lateral spread of film coverage at is wider than that at . At , there is almost no film coverage in the downstream region at . The jet is relatively attached greater to the surface, thus producing positive influence on the film cooling effectiveness at . The hole-exit velocity contour on the side mainly affects film coverage. The discharge coefficient increases as increases. The 45-deg case is the lowest, whereas the 135-deg case is the highest.

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 Enhancement and Turbulent Flow in a Rectangular Channel Using Perforated Ribs With Inclined Holes

In internal cooling passages in a turbine blade, rib structures are widely applied to augment convective heat transfer by the coolant passing through over the ribbed surfaces. This study concentrates on perforated 90 deg ribs with inclined holes in a cooling duct with rectangular cross section, aiming at improving the perforated holes with additional secondary flows caused by inclined hole arrangements. Two sets of perforated ribs are used in the experiments with the inclined angle of the holes changing from 0 deg to 45 deg and the cross section are, respectively, circular and square. Steady-state liquid crystal thermography (LCT) is applied to measure the ribbed surface temperature and obtain corresponding convective heat transfer coefficients (HTCs). Two turbulence models, i.e., the k–ω shear stress transportation (SST) model and the detached eddy simulation (DES) model, are used in the numerical studies to simulate the flow fields. All the inclined cases have slightly larger overall averaged Nusselt number (Nu) than with straight cases. The enhancement ratio is approximately 1.85–4.94%. The averaged Nu in the half portion against the inclined direction is enlarged for the inclined hole cases. The inclined hole cases usually have smaller averaged Nu in the half portion along the inclined direction. For the straight hole case and small inclined angle case, the penetrated flows mix with the mainstream flows at the perforated regions. When the inclined angle is larger, the penetrated flows are pushed to the inclined direction and mixing with the approaching flows occurs just at the side of the inclined direction.

Numerical Simulation of the Effect of Channel Orientation on Fluid Flow and Heat Transfer at High Buoyancy Number in a Rotating Two-Pass Channel With Angled Ribs

Convective heat transfer in a rotating two-pass square channel with 45 deg ribs is numerically investigated to simulate turbine blade cooling operation under extreme design cooling conditions (high rotation number, high density ratio, and high buoyancy number). Two channel orientations are examined β = 0 deg and β = 45 deg in order to determine the effects of passage orientation on flow and heat transfer. For a reference pressure of 10-atm and a Reynolds number of 25,000, the results show that at low buoyancy number and for both channel orientations, the combined effect of Coriolis and centrifugal buoyancy forces generates an important thermal gradient between low- and high-pressure surfaces of the first passage, while the second passage remains almost unchanged compared to the stationary cases. At high buoyancy number, and unlike low buoyancy number, the interaction of Coriolis-driven cells, rib-induced vortices, and buoyancy-driven cells are destructive, which degrade the heat transfer rate on trailing and leading surfaces in the first passage for β = 0 deg. In contrast, for β = 45 deg, this interaction is constructive, which enhances the heat transfer rate on co-trailing and co-leading surfaces. In the second passage, the interaction of rib-induced vortices and buoyancy-driven cells deteriorates significantly the heat transfer rate in case of β = 0 deg than in case of β = 45 deg compared to low buoyancy number. The computations are performed using the second-moment closure turbulence model and the numerical results are in fair agreement with available experimental data.

Local Heat Transfer Coefficient Measurements on an Engine-Representative Internal Cooling Passage

Spatially resolved heat transfer measurements of turbomachinery components can lead to more accurate metal temperature distributions, leading to better predictions of thermal stresses in the blade and, ultimately, blade life. The increased fidelity of the spatially resolved data also provides a data set, which can delineate the complex fluid and heat transfer processes for the purpose of numerical validation. This research applies transient liquid crystal experiments on stationary engine-representative internal cooling passages with a filleted rectangular cross section and staggered 45 deg ribs on two opposing walls. The ribs have a rounded profile and span half the width of the passage. Pressure loss measurements and spatially resolved heat transfer coefficients have been collected over the full surface of the test section over a range of Reynolds numbers (18,000–105,000) for three aspect ratios (, , and ). The experimental results were compared to widely used empirical correlations for ribbed passages with full-width ribs in sharp-cornered passages. The average Nusselt number and friction factor differed from industry-standard correlations by a maximum of 45 and 10%, respectively.

Influence of Rotating Directions on Hydrothermal Characteristics of a Two-Pass Parallelogram Channel With Detached Transverse Ribs

The detailed Nusselt number distributions on leading and trailing endwalls together with the Fanning friction factors of a rotating two-pass parallelogram ribbed channel are simultaneously measured under forward and backward rotations. The tested Reynolds number, rotation number, density ratio, and buoyancy number are respectively in the ranges of 5000 < Re < 15,000, 0 < Ro < 0.3, 0.044<Δρ/ρ < 0.2, and 0 < Bu < 0.142. The area-averaged leading and trailing Nusselt numbers at forward rotations are 0.69–1.77 and 0.85–1.98 relative to the static-channel Nusselt number references, respectively. With backward rotations, the ratios of regionally averaged Nusselt numbers between rotating and static channels for leading and trailing endwalls fall in the respective range to 0.86–2 and 0.91–1.76. At both forward and backward rotations, all the f factors over leading endwall (LE) and trailing endwall (TE) are elevated from the static-channel levels and increased by increasing Ro. Channel averaged f/f0 ratios are respectively raised to 1.21–2.21 and 1.21–2.1 at forward and backward rotations. As the heat transfer enhancements (HTE) attributed to the presence of detached transverse ribs taking precedence of the accompanying f augmentations, all the thermal performance factors are above unity in the range of 1.26–2.94. Relative to the similar rotating two-pass parallelogram channel with attached 90 deg ribs, the detached ribs generate the higher degrees of heat transfer enhancements with the larger extents of f augmentations.

Effect of rib orientation on thermal and fluid-flow features in a two-pass parallelogram channel with abrupt entrance

Detailed secondary flow patterns, Nusselt number (Nu) distributions, Fanning friction factor (f), and thermal performance factor (TPF) are compared between three different rib orientations (45-deg, −45-deg, and 90-deg) in a stationary two-pass parallelogram channel with an asymmetrically and suddenly contracted inlet condition. Velocity and temperature measurements are performed with Particle Image Velocimetry (PIV) and Infrared Thermometry (IT). Temperature and pressure drop measurements are conducted under Reynolds number (Re) ranging from 5000 to 20,000, while the velocity measurements are carried out at fixed Re=10,000. A cross-sectional equal length, 45.5mm, of adjacent sides is selected and two pairs of opposite angles are 45-deg and 135-deg. The rib-height to channel-height ratio and rib pitch-to-height ratio are 0.1 and 10, respectively. It is found that the combined effect of inlet condition and rib orientation extends to whole parallelogram channel rather than the first passage only. Among the rib orientations investigated, the 45-deg ribs strengthen the effects of parallelogram slant sidewalls and entrance and further widen the Nu difference between top and bottom walls. In contrast, the other two rib configurations, 90-deg ribs especially, weaken these effects and narrow the corresponding differences. Based on the ability to elevate the surface-averaged Nu0‾/Nu∞ ratio, the orders (45-deg, −45-deg, 90-deg) and (90-deg, 45-deg, −45-deg) respectively for present top and bottom walls are significantly distinct from those in square and rectangular channels. The variations of channel-averaged Nu0‾/Nu∞ and f0‾/f∞ with the associated TPF values against Re are plotted and compared with previous results obtained from literature. Overall speaking, three tested angled ribs all provide the TPF values above than unity; however, only the TPF values in 45-deg case are always higher than those in corresponding smooth parallelogram channel (ASI case). Divided by the TPF values in ASI case, the normalized TPF values are respectively 1.09–1.04, 1.07–0.87, and 0.83–0.99 for 45-deg, 90-deg, −45-deg ribbed cases. It is thus important to take the entrance effect into account when applying the angled ribs in a two-pass parallelogram channel.

Crossover Jet Impingement in a Rib-Roughened Trailing-Edge Cooling Channel

Airfoil trailing-edge cooling is the main focus of this study. The test section was made up of two adjacent trapezoidal channels, simulating the trailing-edge cooling cavity of a gas turbine airfoil and its neighboring cavity. Eleven racetrack-shaped holes were drilled on the partition wall between the two channels to produce 11 cross-over jets that impinged on the rib-roughened wall of the trailing-edge channel. The jets, after impinging on their respective target surface, turned toward the trailing-edge channel exit. Smooth target wall, as a baseline case, as well as four rib angles with the flow of 0 deg, 45 deg, 90 deg, and 135 deg are investigated. Cross-over holes axes were on the trailing-edge channel center plane, i.e., no tilting of the cross-over jets. Steady-state liquid crystal thermography technique was used in this study for a range of jet Reynolds number of 10,000–35,000. The test results are compared with the numerical results obtained from the Reynolds-averaged Navier–Stokes and energy equation. Closure was attained by k–ω with shear stress transport (SST) turbulence model. The entire test rig (supply and trailing-edge channels) was meshed with variable density hexagonal meshes. The numerical work was performed for boundary conditions identical to those of the tests. In addition to the impingement heat transfer coefficients, the numerical results provided the mass flow rates through individual cross-over holes. This study concluded that: (a) the local Nusselt numbers correlate well with the local jet Reynolds numbers, (b) 90 deg rib arrangement, that is, when the cross-over jet axis was parallel to the rib longitudinal axis, produced higher heat transfer coefficients, compared to other rib angles, and (c) numerical heat transfer results were generally in good agreement with the test results. The overall difference between the computational fluid dynamics (CFD) and test results was about 10%.

Pressure drop and heat transfer performance in a rotating two-pass channel with staggered 45-deg ribs

A rotating two-pass ribbed square channel is applied to perform the pressure drop and heat transfer study. The inlet Reynolds number and rotation number vary from 20,000 to 60,000, and 0 to 1.02, respectively. The regional heat transfer coefficients are obtained by heated cooper plate technique. The rotational pressure drops are measured by the rotation-to-stationary pressure measurement system. The ribs with attack angle of 45-deg are staggered roughened on leading and trailing walls. Due to only one wall-temperature ratio was performed in current study, the isolated buoyancy effect was not able to be disclosed.The results show that the 45-deg ribs bring 40–80% higher heat transfer for ribbed walls and 15–65% for smooth walls. The rib-to-smooth Nu ratio declines with Reynolds number and is more sensible in the second passage. Meanwhile, 150–210% higher pressure drop in ribbed channel is observed compared to smooth one, but this penalty decreases with Reynolds number. Rotation presents more dominant influences on pressure drop in heated channel than unheated one. But rotation-to-stationary friction factors decrease with rotation number for both heated and unheated channel as Ro<0.55, after which an increase trend is observed. The presence of buoyancy effect in heated channel helps to promotes rotational pressure drop variation. Also, ribbed-to-smooth thermal performance is significantly elevated by rotation when rotation number reaches over 0.25. The surface averaged heat transfer coefficients are developed with respect to buoyancy parameters.

Numerical Simulation of the Effect of Rib Orientation on Fluid Flow and Heat Transfer in Rotating Serpentine Passages

The effect of rib orientation on flow and heat transfer in a four-pass square channel with skewed ribs in nonorthogonal-mode rotation was numerically studied by using omega-based Reynolds stress model (SMC−ω). Two cases are examined: in first case, the ribs are oriented with respect to the main flow direction at an angle of −45 deg in the first and third passage and at an angle of +45 deg in the second passage. The second case is identical to the first case with the ribs oriented at angle of +45 deg in the three passages. The calculations are carried out for a Reynolds number of 25,000, a rotation number of 0.24, and a density ratio of 0.13. The results show that the secondary flows induced by −45 deg ribs and by rotation combine partially destructively in the first and third passage of first case. In contrast, for second case, the secondary flows induced by +45 deg ribs and by rotation combine constructively in the first passage, while the flow is dominated by the vortices induced by +45 deg ribs in the third passage. In first case, a significant degradation of the heat transfer rate is observed on the coleading side of the first passage and on both cotrailing and coleading sides of the third as compared to second case. Consequently, the rib orientations at +45 deg are preferred in the radial outward flowing passage with an acceptable pressure drop. The numerical results are in agreement with the available experimental data.