High Rotation Numbers Research Articles

Rotating Heat Transfer Measurements on Realistic Multi-pass Geometry

In this contribution, a novel rig was used to assess the heat transfer coefficients on a full internal multi pass cooling scheme.Transient liquid crystal technique was used for the measurement of the heat transfer coefficient (HTC) on channel's internal surfaces.A first set of experiments were performed at engine similar conditions of Re=21000 and Ro=0.074. In order to assess the reliability of the measurement methodology and to explore the thermal behavior at higher rotation numbers, tests were also carried out at Re=17000 and Ro=0.074-0.11. From the spatially resolved HTC maps made available, it is possible to characterize the thermal performances of the whole passage and to highlight the effect of rotation.

Heat transfer in a two-inlet rotating wedge-shaped channel with various locations of the second inlet

The heat transfer characteristics of a two-inlet wedge-shaped channel with lateral fluid extraction are experimentally investigated under both rotating and non-rotating conditions. Three different locations of the second inlet are tested. The effective inlet Reynolds number is fixed at 15,000 whereas the mass flow rate ratio (MR, second inlet mass flow rate/major inlet mass flow rate) ranges from 0 to 1.0. The highest rotation number is around 0.23. The results show that injecting the secondary coolant at the top of the channel should be the best option in the view of optimizing the bulk temperature and the heat transfer of the high-radius corner of the channel in both rotating and non-rotating cases. Two critical MR can be identified in the non-rotating channels. Higher the radius of the coolant injection, lower the critical MR. The magnitude of the second critical MR is double of the first one in all three cases. However, the local critical mass flow ratio (MRx) is almost independent of the location of the second inlet. In both rotating and non-rotating cases, operating at high MR over the critical point is not recommended because the cooling air is not used efficiently. The influence of the second coolant injection is limited in the vicinity of the major inlet, which brings the convenience to optimize the local heat transfers without disturbing the heat transfer at the upstream locations.

Determination of the rotational population of H2 and D2 including high-N states in low temperature plasmas via the Fulcher-α transition

Vibrational and rotational excitation of the hydrogen molecule can significantly affect molecular reaction rates in low pressure low temperature plasmas, for example for the creation of H−/D− ions via the dissociative attachment process. In general, the rotational population in these discharges is known to be non-thermal with an overpopulation of states with high rotational quantum number N. In contrast to a sophisticated direct measurement of the rotational distribution in the XΣg+1,v=0 state, it is demonstrated that the determination can also be carried out up to high-N levels rather easily via optical emission spectroscopy utilizing the Fulcher-α transition of H2 and D2. The measured rotational populations can be described with a two-temperature distribution where the cold part reflects the population according to the gas temperature of the discharge. This has been verified by using the emission of the second positive system of nitrogen as independent gas temperature diagnostic. The hot part where the rotational temperature reaches several thousand Kelvin arises most probably from recombinative desorption of hydrogen at the discharge vessel wall where parts of the binding energy are converted into rotational excitation. Neglecting the hot population – what is often done when using the Fulcher-α transition as gas temperature diagnostic – can lead to a strong overestimation of Tgas. No fundamental differences in the rotational distributions between hydrogen and deuterium have been found, only the hot rotational temperature is smaller for D2 indicating an isotope-dependency of the recombinative desorption process.

Direct numerical simulation of turbulent flow in a spanwise rotating square duct at high rotation numbers

In this paper, direct numerical simulations have been performed to study the effects of Coriolis force on the turbulent flow field confined within a square duct subjected to spanwise system rotations at high rotation numbers. In response to the system rotation, secondary flows appear as large streamwise counter-rotating vortices, which interact intensely with the four boundary layers and have a significant impact on flow statistics, velocity spectra and coherent structures. It is observed that at sufficiently high rotation numbers, a Taylor–Proudman region appears and complete laminarization is almost reached near the top and side walls. The influence of large organized secondary flows on the production rate and re-distribution of turbulent kinetic energy has been investigated through a spectral analysis. It is observed that the Coriolis force dominates the transport of Reynolds stresses and turbulent kinetic energy, and forces the spectra of streamwise and vertical velocities to synchronize within a wide range of scales.

Heat transfer study in a rotating ribbed two-pass channel with engine-similar cross section at high rotation number

The heat transfer in a rotating ribbed two-pass channel with engine-similar cross section has been experimentally investigated in the current study. The Reynolds number ranges from 10,000 to 50,000 for the inlet pass and 12,000 to 62,000 for outlet pass. The highest rotation number is 1.88 for the inlet pass and 0.972 for outlet pass. For channels, the leading and trailing sides are roughed with ribs (P/e=9.8) placed at 90° angle to the mainstream flow, and three channel orientations are considered (0°, −22.5°, and −45°). Results show that placing ribs strengthen heat transfer for both stationary and rotating cases, and the ribs weaken the entrance effect. In the inlet pass, the most remarkable effect of Ro on Nu/Nus is obtained on the trailing side, and a critical Ro is formed on leading and outer side. The heat transfer is enhanced by the rotation more severely at a larger Ro (Ro1>1, Ro2>0.5). Moreover, the β=0° channel experiences the strongest heat transfer enhanced by rotation with the least in the β=−45° channel. At last, the average Nu/Nus correlations considering Ro and Re are developed, a relatively pleasant accuracy (⩽10%) has achieved.

Aerothermal Characterization of a Rotating Ribbed Channel at Engine Representative Conditions—Part II: Detailed Liquid Crystal Thermography Measurements

The present two-part work deals with a detailed characterization of the flow field and heat transfer distribution in a model of a rotating ribbed channel performed in a novel setup which allows test conditions at high rotation numbers (Ro). The tested model is mounted on a rotating frame with all the required instrumentation, resulting in a high spatial resolution and accuracy. The channel has a cross section with an aspect ratio of 0.9 and a ribbed wall with eight ribs perpendicular to the main flow direction. The blockage of the ribs is 10% of the channel cross section, whereas the rib pitch-to-height ratio is 10. In this second part of the study, the heat transfer distribution over the wall region between the sixth and seventh ribs is obtained by means of liquid crystal thermography (LCT). Tests were first carried out at a Reynolds number of 15,000 and a maximum Ro of 1.00 to evaluate the evolution of the heat transfer with increasing rotation. On the trailing side (TS), the overall Nusselt number increases with rotation until a limit value of 50% higher than in the static case, which is achieved after a value of the rotation number of about 0.3. On the leading side (LS), the overall Nusselt number decreases with increasing rotation speed to reach a minimum which is approximately 50% of the one found in static conditions. The velocity measurements at Re = 15,000 and Ro = 0.77 provided in Part I of this paper are finally merged to provide a consistent explanation of the heat transfer distribution in this model. Moreover, heat transfer measurements were performed at Reynolds numbers of 30,000 and 55,000, showing approximately the same trend.

Aerothermal Characterization of a Rotating Ribbed Channel at Engine Representative Conditions—Part I: High-Resolution Particle Image Velocimetry Measurements

The present work is part of a detailed aerothermal investigation in a model of a rotating internal cooling channel performed in a novel facility setup which allows test conditions at high rotation numbers (Ro). The test section is mounted on a rotating frame with all the required instrumentation, resulting in a high spatial resolution and accuracy. The channel has a cross section with an aspect ratio of 0.9 and a ribbed wall with eight ribs perpendicular to the main flow direction. The blockage of the ribs is 10% of the channel cross section, whereas the rib pitch-to-height ratio is 10. In this first part of the paper, the flow over the wall region between the sixth and seventh ribs in the symmetry plane is investigated by means of two-dimensional particle image velocimetry (PIV). Tests were carried out at a Reynolds number (Re) of 15,000 in static and rotating conditions, with a maximum Ro of 0.77. Results are in good agreement with the data present in literature at the same Reynolds number and with rotation numbers of 0 (static conditions) and 0.38 in a channel with the same geometry as in the present work. When Ro is increased from 0.38 to 0.77, the main velocity and turbulence fields show important changes. At a rotation number of 0.77, although the extension of the recirculation bubble after the sixth rib on the trailing side does not vary significantly, it covers the full inter-rib area on the leading side in the streamwise direction. The turbulence intensity on the leading side shows a low value with respect to the static case but roughly at the same level as in the lower Ro case. On the trailing side, the maximum value of the turbulence intensity slightly decreases from Ro = 0.38 to Ro = 0.77, the wall shear layer is restabilized along the second half of the pitch due to the high rotation, and the secondary flows are redistributed causing spanwise vortex compression. The observed result is the rapid decay of turbulent fluctuations in the second half of the inter-rib area.

Effects of coolant mass flow rate ratio on heat transfer in a two-inlet rotating wedge-shaped channel

Heat transfer characteristics of a novel turbine blade trailing edge internal cooling concept, two-inlet wedge-shaped channel with lateral fluid extraction, are experimentally investigated under both rotating and non-rotating conditions. In order to improve the heat transfer of the channel, a second coolant inlet is introduced in the vicinity of the low heat transfer region that previously observed in single-inlet configuration. The mass flow rate ratio (MR, second inlet mass flowrate/major inlet mass flow rate) ranges from 0 to 1. The major inlet Reynolds number and rotation number are varied from 6000 to 24,000 and 0 to 0.23, respectively. The results show that the second inlet coolant injection decreases local bulk temperature and increases local heat transfer at the high-radius half channel. In rotating cases, the second inlet notably improves the heat transfer at the top of the channel and compensates the negative effects induced by the rotation. The effect of the additional inlet is only significant in the vicinity of the coolant injection in both non-rotating and rotating cases. Moreover, with a MR of under 0.3, the second coolant injection benefits overall wall heat transfer at high rotation numbers.

Heat transfer investigation in a rotating U-turn smooth channel with irregular cross-section

In the current study, heat transfer performance in a rotating U-turn smooth channel with engine-similar irregular cross-section was investigated experimentally. The channel consists of an inlet pass with irregular cross section (d1=24.5mm), a sharp 180-degree turn, and an outlet pass with nearly rectangular cross section (d2=19.6mm). In the experiments, the Reynolds number ranges from 25,000 to 50,000 for the inlet pass and 31,000 to 62,000 for the outlet pass, respectively. In addition, the highest rotation number is 0.72 for the inlet pass and 0.37 for the outlet pass. The mean density ratio maintains around 0.138 in all working conditions. The results indicate that the cross-section affects the heat transfer in both stationary and rotating conditions; but the strengths of effect are different. In the stationary case, the Nu/Nu0 ratio on the trailing side is up to 1.5 times of that on the leading side in the inlet pass. In the rotating case, the Nu/Nu0 ratio is up to 4.3 on the trailing side in the inlet pass, but not changing remarkable on the leading side and all sides in the outlet pass. Due to the shrinking hydraulic diameter of the outlet pass, the Nu/Nu0 ratio in the outlet pass is approximately 1.5 times of that in the inlet pass. In the outlet pass, the Nu/Nus ratio shows different trendency with Ro at different Reynolds numbers, which means that the Nu/Nus–Ro graph cannot eliminate the influence of Re for the current work.

Three-dimensional natural convection of molten Lithium in a differentially heated rotating cubic cavity about a vertical ridge

Laminar natural convection heat transfer in three-dimensional molten Lithium filled differentially heated enclosure, rotating about the vertical ridge is studied numerically. Computations are performed for a wide range of dimensionless parameters including the rotational Rayleigh number, and Taylor number while the values of Rayleigh number and Prandtl numbers are maintained constant at Ra=2×104 and Pr=0.0321, respectively. The results indicate that Coriolis forces seem to have a dominant role to suppress the thermal buoyancy driven effects. Transverse velocity isovaleurs show three-dimensional characteristics of the flow. By increasing, the Taylor number, the maxima of heat transfer gradually reduced about 55%. In the contrast, high rotational Rayleigh number values are found to significantly alter the flow patterns by increasing mixing which turn enhances the heat transfer.

The effect of a hub turning vane on turbulent flow and heat transfer in a four-pass channel at high rotation numbers

This paper investigates effects of rotation and a turning vane on flow field and heat transfer of a 4-pass smooth channel through CFD (Computational Fluid Dynamics) simulations. An 180° hub U-bend connecting the middle two passages (AR=2:1) is the focus of the study. Reynolds number varies from 10,000 to 40,000 and rotation number changes from 0 to 0.4. In stationary conditions, effect of the turning vane is to reduce recirculation and non-uniformity of mainstream. In addition, pressure drop and secondary flow kinetic energy (SKE) are largely reduced by the turning vane. Heat transfer deterioration due to flow separation and recirculation is suppressed by the turning vane in Hub Turn. In rotating conditions, size of recirculation is reduced and uniformity of mainstream is increased comparing to stationary case. Velocity distribution and heat transfer exhibit different profiles on LE and TE considering the flow direction due to Coriolis. Effects of turning vane to suppress recirculation and to reduce pressure drop are not obvious. In Hub Turn and downstream 3rd pass, heat transfer enhancement due to turning vane is very weak.

Heat Transfer in Rotating Serpentine Coolant Passage With Ribbed Walls at Low Mach Numbers

This paper experimentally investigates the effect of rotation on heat transfer in typical turbine blade serpentine coolant passage with ribbed walls at low Mach numbers. To achieve the low Mach number (around 0.01) condition, pressurized Freon R-134a vapor is utilized as the working fluid. The flow in the first passage is radial outward, after the 180 deg tip turn the flow is radial inward to the second passage, and after the 180 deg hub turn the flow is radial outward to the third passage. The effects of rotation on the heat transfer coefficients were investigated at rotation numbers up to 0.6 and Reynolds numbers from 30,000 to 70,000. Heat transfer coefficients were measured using the thermocouples-copper-plate-heater regional average method. Heat transfer results are obtained over a wide range of Reynolds numbers and rotation numbers. An increase in heat transfer rates due to rotation is observed in radially outward passes; a reduction in heat transfer rate is observed in the radially inward pass. Regional heat transfer coefficients are correlated with Reynolds numbers for nonrotation and with rotation numbers for rotating condition, respectively. The results can be useful for understanding real rotor blade coolant passage heat transfer under low Mach number, medium–high Reynolds number, and high rotation number conditions.

Buoyancy effect on heat transfer in rotating smooth square U-duct at high rotation number

The buoyancy effect on heat transfer in a rotating, two-pass, square channel is experimentally investigated in current work. The classical copper plate technique is performed to measure the regional averaged heat transfer coefficients. In order to perform a fundamental research, all turbulators are removed away. Two approaches of altering Buoyancy numbers are selected: varying rotation number from 0 to 2.08 at Reynolds number ranges of 10000 to 70000, and varying inlet density ratio from 0.07 to 0.16 at Reynolds number of 10000. And thus, Buoyancy numbers range from 0 to 12.9 for both cases. According to the experimental results, the relationships between heat transfer and Buoyancy numbers are in accord with those obtained under different rotation numbers. For both leading and trailing surface, a critical Buoyancy number exists for each X/D location. Before the critical point, the effect of Buoyancy number on heat transfer is limited; but after that, the Nusselt number ratios show different increase rate. Given the same rotation number, higher wall temperature ratios with its corresponding higher Buoyancy numbers substantially enhance heat transfer on both passages. And the critical exceed-point that heat transfer from trailing surface higher than leading surface happens at the same Buoyancy number for different wall temperature ratios in the second passage. Thus, the stronger buoyancy effect promotes heat transfer enhancement at high rotation number condition.

Effect of a Turning Vane on Heat Transfer in Rotating Multipass Rectangular Smooth Channel

This paper experimentally investigated the effect of a turning vane on hub region heat transfer in a multipass rectangular smooth channel at high rotation numbers. The experimental data were taken in the second and the third passages () connected by a 180 deg U bend. The flow was radial inward in the second passage and was radial outward after the 180 deg U bend in the third passage. Results showed that rotation increased heat transfer on the leading surface but decreased it on the trailing surface in the second passage. In the third passage, rotation decreased heat transfer on the leading surface but increased it on the trailing surface. Without a turning vane, rotation reduced heat transfer substantially on all surfaces in the hub 180 deg turn region. After adding a half-circle-shaped turning vane, heat transfer coefficients did not change in the second passage before turn, while they were quite different in the turn region and after-turn region in the third passage. Regional heat transfer coefficients were correlated with rotation numbers for a multipass rectangular smooth channel with and without a turning vane.

Heat transfer in two-pass smooth square channel under large rotation numbers

The rotation number range was expanded significantly through increasing the absolute pressure of the channel to about 5 atm. Experiments were carried out to investigate heat transfers in a smooth square two-pass channel under large rotation numbers. In current study,Reynolds and rotation number range from 10 000 to 70 000 and zero to 2. 08 respectively,which covered the real engine parameters. It is found that when the rotation number is large enough,the effect of rotation dominates the heat transfer at the inlet and the turn portion,instead of the entrance effect and the turn effect. As the channel rotated,the Nusselt number of the first pass was found to increase monotonically on the trailing wall,however to increase on the middle and downstream of leading wall exceeding certain critical rotation numbers. And a new phenomena is found in the downstream of the second pass,which is that the heat transfer on trailing surface exceeds the leading surface at high rotation numbers.

Numerical Calculations on Flow and Heat Transfer in Smooth and Ribbed Two-Pass Square Channels under Rotational Effects

U-shaped channel, which is also called two-pass channel, commonly exists in gas turbine internal coolant passages. Ribbed walls are frequently adopted in internal passage to enhance the heat transfer. Considering the rotational condition of gas turbine blade on operation, the effect of rotation is also investigated for the coolant channel which is close to real operation condition. Thus, the objective of this study is to discuss the effect of rotation on fluid flow and heat transfer performance of U-shaped channel with ribbed walls under high rotational numbers. Investigated Reynolds number is Re=12500and the rotation numbers areRob=0.4and 0.6. In the results, the spatially heat transfer coefficient distributions are exhibited to discuss the effect of rotation and roughened walls. It is found that ribbed walls enhance the heat transfer rate significantly. Under the rotational condition, theNuin the first pass with outward flow is increased while that in the second pass is decreased. Finally, averageNuratio, friction ratio, and thermal performance are all presented to discuss the thermal characteristics.

Effect of Rotation on Detailed Heat Transfer Distribution for Various Rib Geometries in Developing Channel Flow

The effects of Coriolis force and centrifugal buoyancy have a significant impact on heat transfer behavior inside rotating internal serpentine coolant channels for turbine blades. Due to the complexity of added rotation inside such channels, detailed knowledge of the heat transfer will greatly enhance the blade designer's ability to predict hot spots so coolant may be distributed more effectively. The effects of high rotation numbers are investigated on the heat transfer distributions for different rib types in near entrance and entrance region of the channels. It is important to determine the actual enhancement derived from turbulating channel entrances where heat transfer is already high due to entrance effects and boundary layer growth. A transient liquid crystal technique is used to measure detailed heat transfer coefficients (htc) for a rotating, short length, radially outward coolant channel with rib turbulators. Different rib types such as 90 deg, W, and M-shaped ribs are used to roughen the walls to enhance heat transfer. The channel Reynolds number is held constant at 12,000 while the rotation number is increased up to 0.5. Results show that in the near entrance region, the high performance W and M-shaped ribs are just as effective as the simple 90 deg ribs in enhancing heat transfer. The entrance effect in the developing region causes significantly high baseline heat transfer coefficients thus reducing the effective of the ribs to further enhance heat transfer. Rotation causes increase in heat transfer on the trailing side, while the leading side remains relatively constant limiting the decrement in leading side heat transfer. For all rotational cases, the W and M-shaped ribs show significant effect of rotation with large differences between leading and trailing side heat transfer.

High Rotation Number Effect on Heat Transfer in a Leading Edge Cooling Channel of Gas Turbine Blades With Three Channel Orientations

Heat transfer in a leading edge, triangular-shaped cooling channel with three channel orientations under high rotation numbers is investigated in this study. Continuous ribs and V-shaped ribs (P/e = 9, e/Dh = 0.085), both placed at an angle (α = 45 deg) to the mainstream flow, are applied on the leading and trailing surfaces. The Reynolds number range is 15,000–25,000 and the rotation number range is 0–0.65. Effects of high rotation number on heat transfer with three angles of rotation (90 deg, 67.5 deg, and 45 deg) are tested. Results show that heat transfer is influenced by the combined effects of rib and channel orientation. When the rotation number is smaller than 0.4, rotation causes a decrease in the average Nusselt number ratios on the leading surface at a channel orientation of 90 deg. Heat transfer is enhanced gradually on the leading surface when the channel orientation varies from 90 deg to 45 deg for both ribbed cases. The highest heat transfer enhancement due to rotation is found at the highest rotation number of 0.65.

Heat transfer study in rotating smooth square U-duct at high rotation numbers

Rotation effects on heat transfer in a rotating smooth square U-duct at high rotation numbers have been experimentally investigated via classical copper plate technique. In order to obtain convincing correlations, an extensive heat transfer data are measured. The Reynolds number ranges from 10,000 to 70,000, and the highest rotation number reaches to 2.08. Besides, the mean density ratio maintains around 0.14 in all working conditions.Due to these experimental data, two interesting phenomena are observed in current work. Firstly, in consistent with the previous study, the critical rotation number phenomenon on leading wall (heat transfer on a specific location is weakened by rotation at first, then after a critical Ro, the descending trend is reversed) in radial outward channel is observed. Moreover, we found that the critical Ro varies with dimensionless location parameter X/D. Interestingly, the product of critical Ro and X/D is a constant. Secondly, in radial inward passage, the heat transfer enhancement on leading wall is lower than trailing wall at high rotation numbers, which differs from low Ro scenario.Possible explanations for these two phenomena are proposed. It seems that centrifugal buoyancy force plays an important role in influencing the flow and heat transfer in this channel at high rotation numbers. The interactions of buoyancy and Coriolis force act in different ways in radial outward and inward passages, and are responsible to these phenomena mentioned above. Finally, comprehensive correlations on leading and trailing surfaces in both legs are fitted. The effects of Ro and X/D are included in these correlations simultaneously.

Pressure drop and heat transfer in rotating smooth square U-duct under high rotation numbers

Rotation effects on heat transfer, pressure drop, and thermal performance in a smooth square U-duct have been experimentally investigated. The Reynolds number ranges from 10,000 to 70,000, and the highest Rotation number reaches to 2.08. The density ratio maintains around 0.14 in all working conditions. In current study, a new rotating pressure measurements system is designed and validated. And the Nusselt numbers, friction factors and thermal performances in current study agree well with that in open literature.The experimental results show that the rotating-to-stationary ratios of these three parameters correlate with Rotation number in a very good way. In general, Nusselt number ratio increases linearly with Rotation number, but the friction ratio is oscillating when Rotation number increases. More detailed analysis of rotation effects on heat transfer shows that rotation has most dominate effects on averaged Nusselt number at the sharp turn compared with two straight passages by reinforcing the impingements over there. Due to the influence of sharp turn, rotation effects in second passage become dominant at high Rotation numbers. The friction ratio data suggest that the rotation has more apparent effects on friction ratio in heated channel than unheated channel. The Buoyancy force plays a very important role in channel’s friction factor. Generally, it reduces the friction resistance in a smooth U-duct especially at high Rotation numbers.