Effect Of Channel Orientation Research Articles

TURNING REGION HEAT TRANSFER IN FIVE, STATIONARY AND ROTATING, MULTI-PASS CHANNELS WITH VARIOUS ASPECT RATIOS

Abstract This work summarizes studies that experimentally investigated the effect of rotation on heat transfer in the 180-deg tip and hub turns of cooling channels with various aspect ratios (ARs). The studied AR ranges from 1:4 to 4:1, which is the typical range within turbine blades. In addition to the smooth surface case, the cases with 45-deg angled ribs and turning vanes are also included. For several designs, the effect of channel orientation with respect to the angle of rotation is also investigated. This work covers a wide range of Reynolds and buoyancy numbers. The rib turbulators are found to have higher heat transfer enhancement and larger disturbance on the flow impingement on the tip and hub walls in channels with a wider aspect ratio. The rotational effect is reduced by the presence of ribs and turning vanes, and it is also reduced in the blade-shaped channels with the angled rotation feature. The tip wall heat transfer is increased by rotation due to the pumping effect from the centrifugal force; however, the hub wall heat transfer is reduced. The effect of rotation is most prominent in the first pass of the channels and is gradually mitigated in the following pass after the turns. This work provides stationary and rotational heat transfer coefficients in the tip and hub turning regions in blades, and heat transfer correlations for a variety of cooling channel designs can be generated, which is of benefit to the gas turbine community.

β-Ga2O3 FinFETs with ultra-low hysteresis by plasma-free metal-assisted chemical etching

In this work, β-Ga2O3 fin field-effect transistors (FinFETs) with metalorganic chemical vapor deposition grown epitaxial Si-doped channel layer on (010) semi-insulating β-Ga2O3 substrates are demonstrated. β-Ga2O3 fin channels with smooth sidewalls are produced by the plasma-free metal-assisted chemical etching (MacEtch) method. A specific on-resistance (Ron,sp) of 6.5 mΩ·cm2 and a 370 V breakdown voltage are achieved. In addition, these MacEtch-formed FinFETs demonstrate DC transfer characteristics with near zero (9.7 mV) hysteresis. The effect of channel orientation on threshold voltage, subthreshold swing, hysteresis, and breakdown voltages is also characterized. The FinFET with channel perpendicular to the [102] direction is found to exhibit the lowest subthreshold swing and hysteresis.

Effect of channel orientation on heat transfer in a rotating impingement cooling channel

The jet impingement cooling technology is usually applied in the turbine blade leading edge which suffers high thermal loads. In a high-speed rotating turbine blade, however, the channel orientation (β, the angle between the direction of the jet and the rotating plane) could alter the jets and heat transfer. Therefore, the effects of channel orientation (from 90∘ to 180∘) on heat transfer of confined jet impingement cooling are experimentally studied under an iso-thermal boundary. Both the jet-to-target surface distance and the jet-to-jet spacing are 3 times of the jet diameter. The jet Reynolds number varies from 5000 to 15,000, and the maximum jet rotation number reaches 0.24. The results show that the rotational effect on heat transfer is different in each channel orientation. In the cases of 90∘≤β≤ 135∘, a large vortex generated by the Coriolis force in the supply channel significantly changes the mass-flowrate distribution in each jet orifice. Hence, the heat transfer in the middle part decreases with the rotation number whereas the heat transfer at both ends increases, resulting in a “U-shape” pattern of heat transfer in the impingement channel. In the cases of 135∘<β≤ 180∘, however, the heat transfer throughout the stagnation zone decreases with the rotation number. The rotation-induced jet diversion is the primary factor that weakens the heat transfer, and jets are driven away from the target surface at high rotation numbers. Moreover, the heat transfer correlation on the stagnant zone is developed, which considers the effects of channel orientation and rotation number.

Modelled effects of channel orientation and tree canopy shape on average shade in streams

ABSTRACT Replanting riparian vegetation can improve ecological health but there are few guidelines that help stakeholders achieve shade targets cost-effectively. A computer model was extended to calculate average shade to direct and diffuse solar radiation in straight channels. It was used to recommend strategies for achieving shade targets, including the 70% guideline shown to prevent nuisance aquatic plant growths and extreme water temperatures. For similar trees on both banks, 70% shade occurs when the ratio of tree height (H) to stream width (W) lies in the range 1.0 < H/W < 2.6 depending on stream orientation and tree shape (canopy overhang and dripline height). With trees on both banks, shade is lower by up to 34% in east–west (EW) than north–south (NS) streams except for tall trees at the equinox. EW streams planted only on the north bank can have a 30% higher shade than NS streams planted on one bank, but low vegetation on the unplanted bank eliminates this difference. For EW streams in the Southern hemisphere, planting tall vegetation on the north bank is cost-effective, with low vegetation on the south bank providing additional benefits. Further work is planned on channel meandering, gaps between trees and canopy transmission.

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.

Effects of channel diameter on flow pattern and pressure drop for air–water flow in serpentine gas channels of PEM fuel cell-An Ex situ experiment

Ex-situ experimental investigation of air–water two phase flow characteristics are carried out in rectangular serpentine mini and micro-channels. The test sections to be studied are made up of acrylic and are of three different hydraulic diameters of 1.65 mm, 1.00 mm and 0.65 mm with the corresponding length of 50 mm. Experiments are performed for different combinations of superficial velocities of air and water ranging between 0.0011 m/s and 0.33 m/s and 0.33 m/s to 16.66 m/s, respectively. The effect of channel orientation on various parameters is also studied. The visualisation of two phase is accomplished by high-speed camera. The flow pattern maps developed by closely scrutinizing each flow rate combinations are presented. Mal-distribution occurred in horizontal orientation of micro channel. However, vertical orientation of micro channel favoured draining of water, thereby encountered lesser pressure drop than mini channel. New correlation based on various dimensionless numbers has been proposed for modelling the pressure drop.

Modeling of anisotropic two-dimensional materials monolayer HfS2 and phosphorene metal-oxide semiconductor field effect transistors

Ballistic transport characteristics of metal-oxide semiconductor field effect transistors (MOSFETs) based on anisotropic two-dimensional materials monolayer HfS2 and phosphorene are explored through quantum transport simulations. We focus on the effects of the channel crystal orientation and the channel length scaling on device performances. Especially, the role of degenerate conduction band (CB) valleys in monolayer HfS2 is comprehensively analyzed. Benchmarking monolayer HfS2 with phosphorene MOSFETs, we predict that the effect of channel orientation on device performances is much weaker in monolayer HfS2 than in phosphorene due to the degenerate CB valleys of monolayer HfS2. Our simulations also reveal that at 10 nm channel length scale, phosphorene MOSFETs outperform monolayer HfS2 MOSFETs in terms of the on-state current. However, it is observed that monolayer HfS2 MOSFETs may offer comparable, but a little bit degraded, device performances as compared with phosphorene MOSFETs at 5 nm channel length.

Heat transfer in a rotating smooth wedge-shaped channel with lateral fluid extraction

In the current study, a rotating smooth wedge-shaped channel with lateral fluid extraction is experimentally investigated. Experiments are conducted at four different Reynolds numbers (from 5100 to 21,000) and five different rotational speeds (from 0 to 1000 rpm), which results in the inlet rotation number varying from 0 to 0.68. In order to study the effects of channel orientation and outlet boundary condition, experiments are conducted at five different channel orientations and two types of boundary conditions. Based on the obtained experimental data, it is concluded that channel orientation strongly influences the rotational effects on flow and heat transfer characteristics. The sensitivity of the channel orientation on wall heat transfer is analyzed. Unexpectedly, outlet boundary condition affects heat transfer characteristics under non-rotating conditions but has almost no effect at high rotational speeds. The general rotation number is better than the local one to describe the flow status. The critical inlet rotation numbers, after which the descending trends of Nusselt number ratios are reversed, are around 0.3 at all the locations. Also, rotation reduces leading-to-trailing averaged Nusselt ratios and affects heat transfer characteristics more evidently on the inner wall than the outer one, and on the low-radius wall than the high-radius one.

Energy Transport During Plasma Enhanced Surface Coating Mechanism: A Mathematical Approach

Heat transfer enhancement is still needed for the compactness and its smaller temperature difference of evaporator. This paper is introducing a mathematical model (Tailor-Chandra Model) for energy transport in plasma sprays process of a droplet. The model useful for calculate heat transfer through surfaces processed by thermal spraying. The model describes fluid flow of particle droplet during plasma spraying and its different energies: Thermal Energy (ETh), Surface Energy (ES), Kinetic Energy (EK) and the energy lost by way of radiation (ER) before deposition on substrate. A dynamic and thermal balance allows to calculate particle temperature and all energies of droplet during the spray process. As a result point of view, the model is able to show how molten droplets produced higher heat transfer as well as the effect of channel orientation in the low quality, low mass flux condition.

The effect of channel orientation on heat transfer and wall shear stress in the bubbly flow

Results of experimental investigation of heat transfer and wall friction in the upward bubble flow in a flat inclined channel are presented. Measurements were carried out in the range of superficial liquid velocities of 0.3–1.1 m/s and different values of volumetric void fraction. The hydrodynamic structure was measured by means of the electrochemical method with the use of miniature friction sensors. The values of average friction and heat transfer at different channel orientation were determined. It is shown that in the bubble gas-liquid flow we can observe a significant dependence of friction and heat transfer on the angle of channel inclination.

Minichannel condensation in downward, upward and horizontal configuration

An experimental investigation of condensation of R134a inside a single square cross section minichannel when varying the channel orientation is presented. Local heat transfer coefficients are measured in horizontal, vertical downflow and vertical upflow configurations. In the literature the number of local heat transfer coefficient values measured during condensation inside non-circular minichannels is rather limited and the effect of channel orientation during condensation is not much investigated. Some studies have been performed in inclined smooth tubes of larger diameters, where it was shown that the heat transfer coefficient is strongly affected by the liquid and vapour distributions. But minichannels may display a different behaviour because of the relative importance of shear stress, gravity and surface tension. The action of these forces may depend on operating conditions and orientation. In the present study, the channel is obtained from a copper rod and has a square cross section with 1.18 mm side length. Each corner has a curvature radius equal to 0.15 mm, which leads to a hydraulic diameter equal to 1.23 mm. Tests have been performed with R134a at 40°C saturation temperature, at mass velocity ranging between 100 and 790 kg m−2 s−1. From the experimental results, the effect of the channel inclination when varying mass velocity and vapour quality is investigated.

Effect of Channel Orientation on the Heat Transfer Coefficient in the Smooth and Dimpled Rotating Rectangular Channels

The detailed distribution of the heat transfer coefficient on rotating smooth and dimpled rectangular channels were measured using the transient liquid crystal technique. The rotating speed of the channel was fixed at 500 rpm and the tested Reynolds number based on the channel hydraulic diameter was 10,000. A stationary surface and two different channel rotating orientations of 90 deg and 120 deg were tested in order to investigate the effects of channel orientation on the distribution of the heat transfer coefficient in smooth and dimpled rotating surfaces. Results show that the heat transfer coefficient on the trailing surface is higher than that on the leading surface. For the 120 deg channel orientation angle cases, a higher heat transfer coefficient was observed near the outer surface. In the dimpled channel, the effect of the Coriolis force induced secondary flow on the heat transfer coefficient was not as significant as that for the smooth channel case.

Physics of electron mobility independent of channel orientation in n-channel transistors based on (100) silicon wafers and its experimental verification

The six degenerate ellipsoid model can be used to study the effects of channel orientation on electron mobility. However, this approach has two assumptions: (i) the conduction band minimum near the interface between gate dielectric and silicon channel is the same as that of bulk silicon, (ii) the momentum relaxation time is independent of the channel orientation. This letter shows that the effective conductivity electron mass of (100) surface-oriented silicon is independent of channel orientation even though the actual conduction band minimum may be slightly different from the six degenerate ellipsoid model. Experimental data are provided to support our theory.

Monte Carlo simulation of electron mobility in strained Si DG-FETs with TB bandstructure calculation

A low dimensional tight-binding (TB) based bandstructure calculation program is developed for double-gate MOSFETs (DG-FETs) to model the effects of channel orientation, transverse electric field, stress, and geometry-induced quantum confinement. Electron mobility in the strained channel is then evaluated using the Monte Carlo (MC) method, based on the calculated bandstructure. It is concluded that electron mobility is enhanced by the splitting of conduction band valleys and the change of electron effective mass, as a function of layer thickness, crystal orientation and stress in strained Si DG-FETs.

Self-consistent modeling of longitudinal quantum effects in nanoscale double-gate metal oxide semiconductor field effect transistors

Ultrathin double-gate silicon-on-insulator transistors are studied in the quantum coherent limit. By treating electron-electron interaction on the level of a mean field approach, the density matrix of the device becomes diagonal when expressed in a basis that results from imposing scattering boundary conditions at the terminals. The self-consistent scattering wave functions are computed using a multisubband scattering matrix formalism. This allows us to retain the full dimensionality of the wave functions and eliminates the need for the adiabatic decomposition of the Schrödinger equation. Subband mixing is fully taken into account and a piecewise analytical representation of the wave functions can significantly reduce the number of sampling positions along transport direction. By self-consistent simulations the size of source-to-drain tunneling as a function of gate length is demonstrated for different body thicknesses. A strong forward bias is shown to increase the tunnel current due to the thinning of the source-drain potential barrier. The effect of channel orientation on the tunnel current is also discussed.

Two-Phase Flow Regime Transitions in Microchannels: A Comparative Experimental Study

In recent years, micro-technologies have become very important to cutting edge industries such as aerospace and biotechnology. In the fluidic research area, two-phase flow study in microchannels has been an emerging topic in the past few years. Characteristics of two-phase flow in microchannels such as flow regimes, pressure drop, void fraction, and heat transfer are now being extensively studied by numerous groups. The ultimate goal of this research ranges from compact heat exchangers to small-sized refrigeration systems. One of the major drawbacks in this area to date is the lack of a universal flow regime map enabling the prediction of flow regimes in microchannels. In the present study, a new test rig was designed and constructed to extend the range of the existing data on flow regimes. Several flow regime maps, for a hydraulic diameter of less than 1.0 mm obtained from a comprehensive literature review, are tested and compared. A total of approximately 1475 experimental data points from present and previous studies performed in channels with Dh ≤ 1.0 mm were used for comparison. Two universal flow regime maps were created, one each for horizontal and vertical channels with a hydraulic diameter ranging between 0.1 mm and 1.0 mm. The resulting universal maps presented here are based on flow regimes observed in all the studies on two-phase flow in microchannels. We suggest that the use of these flow regimes could help diminish the amount of discord present among research groups pertaining to the definition of flow regimes. Based on the comparisons between the universal maps and different experimental flow regime maps, appropriate conclusions on the effects of channel orientation and geometry are suggested.

Effect of Channel Orientation and Rib Pitch-to-Height Ratio on Pressure Drop in a Rotating Square Channel with Ribs on Two Opposite Surfaces

The effect of channel orientation and rib pitch-to-height ratio on the pressure drop distribution in a rib-roughened channel is an important issue in turbine blade cooling. The present investigation is a study of the overall pressure drop distribution in a square cross-sectioned channel, with rib turbulators, rotating about an axis normal to the free stream. The ribs are configured in a symmetric arrangement on two opposite surfaces with a rib angle of90∘to the mainstream flow. The study has been conducted for three Reynolds numbers, namely, 13 000, 17 000, and 22 000 with the rotation number varying from 0–0.38. Experiments have been carried out for various rib pitch-to-height ratios(P/e)with a constant rib height-to-hydraulic diameter ratio(e/D)of0.1. The test section in which the ribs are placed on the leading and trailing surfaces is considered as the base case (orientation angle=0∘, Coriolis force vector normal to the ribbed surfaces). The channel is turned about its axis in steps of15∘to vary the orientation angle from0∘to90∘. The overall pressure drop does not change considerably under conditions of rotation for the base case. However, for the other cases tested, it is observed that the overall pressure drop increases with an increase in the rotation number for a given orientation angle and also increases with an increase in the orientation angle for a given rotation number. This change is attributed to the variation in the separation zone downstream of the ribs due to the presence of the Coriolis force—local pressure drop data is presented which supports this idea. At an orientation angle of90∘(ribs on the top and bottom surfaces, Coriolis force vector normal to the smooth surfaces), the overall pressure drop is observed to be maximum during rotation. The overall pressure drop for a case with a rib pitch-to-height ratio of5on both surfaces is found to be the highest among all the rib pitch-to-height ratios covered in this study with the maximum increase in the overall pressure drop being as high as five times the corresponding no-rotation case at the highest rotation number of0.38and90∘orientation angle.

Simulating quantum transport in nanoscale mosfets: ballistic hole transport, subband engineering and boundary conditions

We present a modeling scheme for simulating ballistic hole transport in thin-body fully depleted silicon-on-insulator pMOSFETs. The scheme includes all of the quantum effects associated with hole confinement and also accounts for valence band nonparabolicity approximately. This simulator is used to examine the effects of hole quantization on device performance by simulating a thin (1.5-nm) and thick (5-nm) body double-gated pMOSFET in the ballistic limit. Two-dimensional electrostatic effects such as drain-induced barrier lowering (DIBL) and off-equilibrium transport are emphasized as part of this study. The effect of channel orientation on the device performance is examined by simulating pMOSFETs with channels directed along and . Simulated device characteristics for identical nMOSFETs and pMOSFETs are compared in order to explore the effects of subband engineering on CMOS technology. Novel floating boundary conditions used in simulating ballistic transport are highlighted and discussed.

Heat (Mass) Transfer in a Diagonally Oriented Rotating Two-Pass Channel With Rib-Roughened Walls

Abstract Naphthalene sublimation experiments have been conducted to examine the effects of channel orientation, rotational Coriolis force, ad a sharp turn, on the local heat (mass) transfer distributions in a two-pass square channel with rib-roughened walls, rotating about a perpendicular axis. The test channel was oriented so that the direction of rotation was perpendicular or at a 45 deg angle to the leading and trailing walls. In the two straight passes of the test channel, there were parallel 90 or 60 deg ribs on the leading and trailing walls. The test channel modeled serpentine cooling passages in modern gas turbine blades. The results showed that the heat (mass) transfer was very low on the leading wall of the first pass when the channel was oriented with the rotating direction normal to the leading and trailing walls. There were regions of very low heat (mass) transfer on both the leading and trailing walls in the turn, especially on the trailing wall in the turn when the channel with transverse ribs was oriented diagonally. For the given diagonal channel orientation, rotational Coriolis forces caused the leading and trailing wall heat (mass) transfer to be high near the outer edges of the walls in the channel with transverse ribs; rotation-induced secondary flows dominated near wall rib-induced secondary flows in the channel with angled ribs, since the heat (mass) transfer was generally higher near the outer edges of the walls than near the inner edges in the first and second straight passes. [S0022-1481(00)00201-2]

Effect of Channel Orientation of Local Heat (Mass) Transfer Distributions in a Rotating Two-Pass Square Channel With Smooth Walls

Naphthalene sublimation experiments have been conducted to study the effects of channel orientation, rotational Coriolis force, and a sharp turn, on the local heat (mass) transfer distributions in a two-pass square channel with a sharp turn and smooth walls, rotating about a perpendicular axis. The test channel was oriented so that the direction of rotation was perpendicular to or at a 45 deg angle to the leading and trailing walls. The Reynolds number was kept at 5,500 and the rotation number ranged up to 0.24. For the radial outward flow in the first straight pass of the diagonally oriented channel, rotation-induced Coriolis force caused large monotonic spanwise variations of the local mass transfer on both the leading and trailing walls, with the largest mass transfer along the outer edges of both walls. Rotation did not lower the spanwise average mass transfer on the leading wall and did not increase that on the trailing wall in the diagonally oriented channel as much as in the normally oriented channel. The combined effect of the channel orientation, rotation, and the sharp turn caused large variations of the local mass transfer distributions on the walls at the sharp turn and immediately downstream of the sharp turn. The velocity fields that were obtained with a finite difference control-volume-based computer program helped explain how rotation and channel orientation affected the local mass transfer distributions in the rotating two-pass channel.