Parallel Ribs Research Articles

PIV measurement of the flow field in rectangular ducts with 60° parallel, crossed and V-shaped ribs

The flow characteristics in rectangular ducts with aspect ratio of 1:8 were investigated experimentally using particle image velocimetry to reveal the function of the inclined ribs as turbulators or generators of favorable flows. Circular ribs were attached on the wide walls and the rib height-to-hydraulic diameter ratio was kept as 0.06 with an attack angle of 60°. The obtained results showed strong effects of the inclined ribs on the flow behavior by the development of secondary flows. The magnitude of the streamwise velocity component was detected to have spanwise variation. The flow characteristics were correlated with the previous corresponding heat transfer results obtained by the same authors.

Electrode permeability and flow-field configuration: influence on the performance of a PEMFC

The objective of this work was to investigate the effect of both the permeability of the electrodes and the configuration of the gas flow distributor on the performance of a proton exchange membrane fuel cell (PEMFC). For that purpose, MEAs including electrodes of two types, carbon paper and carbon cloth, have been characterised electrochemically by measuring the polarization curves for a wide range of operational conditions with H 2 and O 2/air as reactants. MEAs with surface active areas of 50 and 290 cm 2 have been characterised in single cells with two flow-field configurations: a grooved plate with parallel gas channels and solid ribs, and a solid plate. The latter is a novel gas flow distributor that has been designed and tested in our laboratory. A subsequent series of experiments were carried out in order to measure the gas permeability of the electrodes of the MEAs characterised previously. The permeability of the electrodes was measured separately for O 2, N 2 and H 2 in the absence of water vapour. The fuel cell performance strongly depends on both the gas permeability of the electrodes and the type of gas flow distributor. The effect of the electrode permeability is not meaningful in the case of the grooved plates, but it is rather important in the case of the solid plates. With the grooved plates, the differences in the fuel cell performance observed with the various MEAs must be attributed to factors mostly related to the catalyst layer (platinum and Nafion content, dispersion of the catalyst, etc.). With solid plates, however, the MEAs of both short and large sizes performed consistently with the gas permeability values of the electrodes measured in this work. In general, the performance of the fuel cell with solid plates declines when the permeability of the electrodes decreases. In the range of current densities covered here, below 300 mA/cm 2, the MEAs with the more permeable electrodes performed comparably with either grooved or solid plates. The less permeable electrodes were the ones in the MEAs provided by E-TEK, which are made of carbon cloth with 40 wt.% of hydrophobic material. In this case, noticeably higher gas transport losses were observed with the solid plates than with the grooved plates, particularly when air was used as oxidant. The influence of water vapour on the gas electrode permeability and the effect of the electrode hydraulic permeability on the fuel cell performance will be investigated in a future work.

Effect of rotation on heat transfer in two-pass square channels with five different orientations of 45° angled rib turbulators

The effect of various 45° angled rib turbulator arrangements on the Nusselt number ratio in a rotating, two-pass, square channel is investigated for three Reynolds numbers (5000, 10,000, 25,000), with rotation number up to 0.11, and two channel orientations with respect to the axis of rotation ( β=90° and 135°). Five different arrangements of rib turbulators are placed on the leading and trailing surfaces at an angle of +45° or −45° to the main stream flow. The rib height-to-hydraulic diameter ratio ( e/ D) is 0.125; the rib pitch-to-height ratio ( P/ e) is 10; and the inlet coolant-to-wall density ratio (Δ ρ/ ρ) is maintained around 0.11. The results show that the rotating ribbed surface Nusselt number ratios increase by a factor of 2 compared to the rotating smooth surface results. The results also show that the heat transfer enhancement depends on the rib-angle orientation (+45° or −45°) to the main stream flow in the first or second pass of the channel for both rotating and non-rotating conditions. Overall, the parallel rib cases show better heat transfer enhancement than the crossed rib case for both rotating and non-rotating conditions. The 90° channel orientation with respect to the axis of rotation produces greater rotating effect on heat transfer over the 135° channel orientation.

회전하는 터빈 블레이드 내부 이차냉각유로에서 엇갈린요철과 평행요철이 열/물질전달에 미치는 영향

The present study investigates the convective heat/mass transfer inside a cooling passage of rotating gas-turbine blades. The rotating duct has various configurations made of ribs with 70。 attack angle, which are attached on leading and trailing surfaces. A naphthalene sublimation technique is employed to determine detailed local heat transfer coefficients using the heat and mass transfer analogy. The present experiments employ two-surface heating conditions in the rotating duct because the surfaces, exposed to hot gas stream, are pressure and suction side surfaces in the middle passages of an actual gas-turbine blade. In the stationary conditions, the parallel rib arrangement presents higher heat/mass transfer characteristics in the first pass, however, these characteristics disappear in the second pass due to the turning effects. In the rotating conditions, the cross rib present less heat/mass transfer discrepancy between the leading and the trailing surfaces in the first pass. In the second pass, the heat/mass transfer characteristics are much more complex due to the combined effects of the angled ribs, the sharp fuming and the rotation.

Forced convective heat transfer and pressure drop of air flowing in a rectangle duct with cross-ribs on the opposite walls

Experiments were conducted to investigate the forced convective heat transfer and flow friction of turbulent airflow in a rectangular duct with cross-ribs attached at the two principal walls in the Reynolds number range from 5000 to 40000. The effect of the rib cross angle (45°, 60°, 75°) and the height (4 mm, 5 mm) of the cross-ribs on the forced convection and flow friction were tested. Non-dimensional correlations for the duct average Nusselt number and friction factor of cross-ribs duct were developed from the test data. Experiments were also conducted for the corresponding parallel ribs to compare their relative performance. The experimental results show that both of the convective heat transfer coefficient and friction factor were increased with cross-ribs, with 45° cross-ribs being the best. Compared with parallel ribs normal to the flow direction under identical flow rate and identical pumping power constraints, the cross-ribs can enhance heat transfer in the lower Reynolds number region, while in the high Reynolds number region, the heat transfer performance of the parallel ribs seems better.

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.

An experimental investigation of forced convection heat transfer in channels with rib turbulators by means of liquid crystal thermography

An optical technique for measuring local heat transfer coefficients in enhanced heat transfer channels is discussed. The technique is based on the use of thermochromic liquid crystals applied to the test surface. The two-dimensional temperature distribution can be easily visualised as a colour distribution. The colour pattern is observed by a CCD videocamera and the recorded image is digitised and converted from the RGB (red, green, blue) to the HSI (hue, saturation, intensity) domain. Among these new parameters, the hue can be directly related to surface temperature through a linear and repeatable relationship, determined by means of previous calibration experiments. If a uniform heat flux condition is applied to the test surface, the pattern of the heat transfer coefficient can be obtained. This optical technique was applied to the study of forced convection heat transfer characteristics in a rectangular channel with rib turbulators mounted on one of the surfaces. Continuous and broken parallel ribs, deployed normal to the main direction of air flow, were considered, for different values of the geometric parameters. The range of variation of the Reynolds number, based on the hydraulic diameter of the channel, was 8000–35,000.

Two new species of Mallomonas (Synurophyceae) from the Ocala National Forest, Florida, U.S.A.

Two new species of Mallomonas, M. ocalensis and M. caerula, are described from a clearwater, acidic and oligotrophic waterbody in the Ocala National Forest, Florida, U.S.A. Mallomonas ocalensis, in the Series Doignonianae of the Section Torquatae, consists of relatively small cells with scales that have a series of parallel shield ribs with alternating thicknesses, collar scales with very short bristles and posterior scales with small protruding spines. Mallomonas ocalensis is most similar to Mallomonas dickii, but is distinguished from the latter species based on the highly variable thicknesses of the shield ribs and significantly larger scales. Mallomonas caerula, in the Series Mallomonas of the Section Mallomonas, consists of relatively large cells that bear large domed and dorneless scales, and long, robust and ribbed bristles. Bristles with and without helmets can be found. The shield, especially the distal portion, and the posterior flange of scales consists of series of more or less parallel ribs. Mallomonas caerula appears to be most closely related to Mallomonas acaroides var. acaroides and Mallomonas acaroides var. muskokana, but can be distinguished from the latter taxa on the basis of scale type, scale structure and bristle morphology. Both new species were found in the plankton and surface sediments of Blue Sink Pond.

Two new taxa in the Section Papillosae of the genus Mallomonas (Synurophyceae) from the Ocala National Forest, Florida, USA

Two new species of Mallomonas, M. binocularis and M. delanciana, are described from small, acidic, poorly buffered waterbodies in the Ocala National Forest, Florida, U.S.A. Both taxa are in the Section Papillosae, consist of relatively small cells covered with domed scales each bearing a single bristle, and have small tripartite scales with a small symmetrically placed dome. Scales of Mallomonas binocularis have shields that are covered with evenly‐spaced papillae, two distinctive pores at the base of the V‐rib and unornamented flanges. Mallomonas binocularis is most similar to Mallomonas paxillata, M. papillosa, M. rasilis and M. calceolus, but can be distinguished from the latter taxa on the basis of scale structure and bristle morphology. Scales of Mallomonas delanciana have shields with widely spaced papillae, a series of parallel ribs on the anterior flanges that usually continue around the dome, unornamented posterior flanges and lack rimmed pores in the posterior region of the shield. The density of the papillae on the shield, features of the dome and anterior flange, and the structure of bristles, clearly serve to separate M. delanciana from closely related species including M. papillosa, M. calceolus and Mallomonas conspersa.

Heat transfer distribution in rectangular ducts with V-shaped ribs

Heat transfer distributions are presented for a rectangular duct with two opposite wide walls arranged with V-shaped ribs pointing upstream or downstream relative to the main flow direction. The rectangular duct has an aspect ratio of 1/8. The parallel V-shaped circular ribs are arranged staggered on the two wide walls. The rib height-to-hydraulic diameter ratio is 0.06, with an attack angle of 60°. The pitch-to-height ratio equals 10. The tested Reynolds numbers range from 1000 to 6000. The test surface is sprayed with black paint and then liquid crystal, and a steady state method is adopted to obtain the temperature distribution between adjacent ribs. The secondary flow caused by the angled ribs creates different spanwise variation of the heat transfer coefficient on the rib-roughened wall for different V-rib orientations. Interaction between heat transfer and secondary flow is analyzed. In the streamwise direction, the temperature distribution shows a sawtooth behavior between a pair of adjacent ribs. Local Nusselt numbers are presented between a pair of adjacent ribs, and based on these the average Nusselt numbers are calculated to investigate the augmentation of heat transfer by the presence of the V-shaped ribs.

Heat transfer and pressure drop measurements in rib-roughened rectangular ducts

In the present study, the thermal and hydraulic performance of three rib-roughened rectangular ducts is investigated. The aspect ratio of the ducts was 1 to 8, and the ribs were arranged staggered on the two wide walls. Three rib configurations were tested: parallel ribs and V-shaped ribs pointing upstream or downstream of the main flow direction. For all cases, the rib height-to-hydraulic diameter ratio was 0.06, with an attack angle of 60° and a pitch-to-height ratio of 10. The Reynolds number range was from 1000 to 6000. Liquid crystal thermography was employed in the heat transfer experiment to demonstrate detailed temperature distribution between a pair of ribs on the ribbed surfaces. The secondary flows caused by the inclined ribs create a significant spanwise variation of the heat transfer coefficients on the rib-roughened wall with high heat transfer coefficient at one end of the rib and low value at the other. In the streamwise direction between two consecutive ribs, the temperature distribution shows a sawtooth fashion because of flow reattachment. Based on the local heat transfer coefficients, the average Nusselt numbers were estimated as weighted mean values. Isothermal pressure drop data were taken and presented as Fanning friction factors. The ducts are compared to each other by considering both heat transfer and friction factor performance.

Computation of Flow and Heat Transfer in Two-Pass Channels With 60 deg Ribs

Numerical predictions of three-dimensional flow and heat transfer are presented for a two-pass square channel with and without 60 deg angled parallel ribs. Square sectioned ribs were employed along one side surface. The rib height-to-hydraulic diameter ratio e/Dh is 0.125 and the rib pitch-to-height ratio (P/e) is 10. The computation results were compared with the experimental data of Ekkad and Han [1] at a Reynolds number (Re) of 30,000. A multi-block numerical method was used with a chimera domain decomposition technique. The finite analytic method solved the Reynolds-Averaged Navier Stokes equation in conjunction with a near-wall second-order Reynolds stress (second-moment) closure model, and a two-layer k-ε isotropic eddy viscosity model. Comparing the second-moment and two-layer calculations with the experimental data clearly demonstrated that the angled rib turbulators and the 180 deg sharp turn of the channel produced strong non-isotropic turbulence and heat fluxes, which significantly affected the flow fields and heat transfer coefficients. The near-wall second-moment closure model provides an improved heat transfer prediction in comparison with the k-ε model.

Analysis of SiO 2/TiO 2–SiO 2/SiO 2 coupled parallel waveguide structures using computer aided design techniques

The analysis of two coupled parallel rib waveguides of SiO 2/TiO 2–SiO 2/SiO 2 on silicon substrate is presented. The indigenously developed beam propagation method (BPM) simulation tools are applied to observe the power coupling between two identical parallel rib waveguides and to study the dependence of the coupling length on various parameters of the rib waveguides including the refractive indices, rib height, guide thickness, clad thickness and separation between them. The designs for the 3 dB coupler and demultiplexer for DWDM, based on the symmetric directional coupler, are optimized for minimum dimensions using the techniques, for the first time. The optimum device length for 3 dB coupler was calculated to be 650 μm with 0.016 dB losses. The 1.3 μm and 1.304 μm demultiplexer has been designed with the optimized device length of 1.02 mm and the losses less than 0.40 dB at both the wavelengths.

Numerical Prediction of Flow and Heat Transferin a Two‐pass Square Channel with 90° Ribs

Numerical predictions of three‐dimensional flow and heat transfer are presented for a two‐pass square channel with ° parallel ribs. Square sectioned ribs were employed along the one side surface. The rib height‐to‐hydraulic diameter ratio (e/Dh) is 0.125 and the rib pitch‐to‐height ratio (P/e) is 10. The computation results were compared with the experimental data of Ekkad and Han (1997) at a Reynolds number (Re) of 30,000.A multi‐block numerical method was used with a chimera domain decomposition technique. The finite analytic method solved the Reynolds‐Averaged Navier‐Stokes equation in conjunction with a near‐wall second‐order Reynolds stress (secondmoment) closure model, and a two‐layer k − ε isotropic eddy viscosity model. Comparing the second‐moment and two‐layer calculations with the experimental data clearly demonstrated that the rib turbulators and the 180° sharp turn of the channel produced strong non‐isotropic turbulence and heat fluxes, which significantly affected the flow fields and heat transfer coefficients. The near‐wall second‐moment closure model provides an improved heat transfer prediction in comparison with the k − ε model.

Experimental study of flow and heat transfer in rib-roughened rectangular channels

Secondary flow patterns, pressure drop and heat transfer in rib-roughened rectangular channels have been investigated. The aspect ratio of the channels is 1–8, and ribs are attached to the wide channel walls in order to set up swirling motions. The geometries tested consist of channels having cross ribs, parallel ribs, cross V-ribs, parallel V-ribs, and multiple V-ribs (Swirl Flow Tube). The flow patterns were investigated using smoke wire visualization and LDV measurements. The smoke wire experiments have been performed at Re=1100 and the LDV measurements at Re=3000 at periodic fully developed conditions. The heat transfer and pressure drop are described by j and f factors for Reynolds numbers from 500 to 15 000. The distributions of axial mean velocity and turbulent fluctuations are strongly influenced by the secondary flows. Large mean velocities and small fluctuations are found in regions where the secondary flow is directed towards a surface, while small mean velocities and large fluctuations are found in regions where the secondary flow is directed away from a surface. The Swirl Flow Tube provides a significant increase in the j factor at Reynolds numbers from 1000 to 2000, but unfortunately also an increase in the f factor. At higher Reynolds numbers, the j and f factors of the Swirl Flow Tube are of the same order of magnitude as for the other rib-roughened channels. It is found that the flow direction in a channel with parallel V-ribs has important influence on the j / f ratio. At Reynolds numbers above 4000, this channel provides the highest j / f ratio if the V-ribs are pointing upstream; while it provides the lowest j / f ratio of all rib configurations, if the V-ribs are pointing downstream.

MORPHOLOGY, CELL-CYCLE, AND GROWTH-RATES OFODONTELLA REGIA

The morphology and cell-cycle of the bipolar centric, marine diatom Odontella regia C. A. Agardh were studied, after establishing optimal growth conditions. The siliceous part of the cell wall is composed of loculate areolae with perforated sidewalls, which are internally attached to a system of parallel ribs arising from an elongated annulus. Two rimoportulae (RPs) are diagonally situated outside this primary band—i.e., the annulus. Their speedier and more massive silicification during morphogenesis causes a sigmoid distortion of the valve. Thickening of the wall occurs from outside to inside. There are 2 open girdle bands in the mature thecae. The nucleus is suspended in the cell centre, with cytoplasmic strands attached mainly to the RPs, the horn-bases and to the last hypothecal band. Chloroplaste are restricted to the cortical cytoplasm. Preprophase is correlated with cell elongation and formation of the second hypothecal band. Pre- and postmitotic transapical nuclear migration, and the pattern of cl...

Influence of Rotation on Heat Transfer from a Two‐Pass Channel with Periodically Placed Turbulence and Secondary Flow Promoters

In a stationary duct, ribs placed at an angle oblique to the main flow direction are more effective in heat transfer enhancement than the ribs placed perpendicular to the flow. Obliquely placed ribs, besides tripping the boundary layer, produce secondary flow patterns to increase heat transfer from the surfaces. Ducts rotating about an axis perpendicular to their own also develop secondary flows. These two secondary flows, produced by oblique ribs and rotation, interact with each other and develop a new heat transfer pattern that is different from those produced by oblique ribs or by rotation alone. This paper uses two types of rib configurations (60∘ parallel ribs and 60∘ staggered ribs) as turbulence and secondary flow promoters. The local and surface averaged Nusslt numbers are presented for both stationary and rotating conditions. This experimental study is conducted on a two‐pass square channel with two opposite rib roughened surfaces (leading and trailing sides). All the walls are maintained at the same temperature. The heat transfer results in a rotating condition show that the 60∘ staggered ribs are more effective in the first pass but the 60∘ parallel ribs do better in the second pass.

Heat transfer performance comparisons of five different rectangular channels with parallel angled ribs

This paper systematically presents the results of heat transfer and friction factor data measured in five short rectangular channels with turbulence promoters. The project investigated the combined effects of the channel aspect ratio, rib angle-of-attack, and flow Reynolds number on heat transfer and pressure drop in rectangular channels with two opposite ribbed walls. The channel aspect ratio (width-to-height, W/ H, ribs on side W) varied from 1 4 to 1 2 , to 1, 2. and 4, while the corresponding rib angles-of-attack a were 90°, 60°, 45°, and 30°, respectively. The Reynolds number range was 10,000–60,000. The results suggest that the narrow aspect ratio channels ( W/ H < 1) give much better heat transfer performance than the wide aspect ratio channels ( W/ H > 1). For the square channel ( W/ H = 1), the 60° 45° angled ribs provide the best heat transfer performance. For the narrow aspect ratio channel ( W/H = 1 4 or 1 2 ), the 45° 60° angled ribs are recommended while the 30° 45° angled ribs are better for wide aspect ratio channels ( W/ H = 4 or 2).

Influence of Surface Heat Flux Ratio on Heat Transfer Augmentation in Square Channels With Parallel, Crossed, and V-Shaped Angled Ribs

The effect of wall heat flux ratio on the local heat transfer augmentation in a square channel with two opposite in-line ribbed walls was investigated for Reynolds numbers from 15,000 to 80,000. The square channel composed of ten isolated copper sections has a length-to-hydraulic diameter ratio (L/D) of 20. The rib height-to-hydraulic diameter ratio (e/D) is 0.0625 and the rib pitch-to-height ratio (P/e) equals 10. Six ribbed side to smooth side wall heat flux ratios (Case 1—q″r1/q″s = q″r2/q″s = 1; Case 2—q″r1/q″s = q″r2/q″s = 3; Case 3—q″r1/q″s = q″r2/q″s = 6; Case 4—q″r1/q″s = 6 and q″r2/q″s = 4; Case 5—q″r1/q″s = q″r2/q″s = ∞; Case 6—q″r1/q″s = ∞ and q″r2/q″s = 0) were studied for four rib orientations (90 deg rib, 60 deg parallel rib, 60 deg crossed rib, and 60 deg V-shaped rib). The results show that the ribbed side wall heat transfer augmentation increases with increasing ribbed side to smooth side wall heat flux ratios, but the reverse is true for the smooth side wall heat transfer augmentation. The average heat transfer augmentation of the ribbed side and smooth side wall decreases slightly with increasing wall heat flux ratios. Two ribbed side wall heating (Case 5—q″r1/q″s = q″r2/q″s = ∞) provides a higher ribbed side wall heat transfer augmentation than the four-wall uniform heating (Case 1—q″r1/q″s = q″r2/q″s = 1). The effect of wall heat flux ratio reduces with increasing Reynolds numbers. The results also indicate that the 60 deg V-shaped rib and 60 deg parallel rib perform better than the 60 deg crossed rib and 90 deg rib, regardless of wall heat flux ratio and Reynolds number.

Acoustic radiation from fluid-loaded, ribbed cylindrical shells excited by different types of concentrated mechanical drives

Analytic expressions are derived for the far-field acoustic radiation from a fluid-loaded circular cylindrical shell reinforced with two sets of parallel periodic ribs and excited by seven types of mechanical point drives: single radial, in- and out-of-phase double radial, axial moment, circumferential moment, and in- and out-of-phase in-plane double circumferential. The ribs interact with the cylindrical shell only through normal forces. The model includes the effects of fluid loading, internal damping, and interactions between ribs. Measurements of the acoustic radiation from a ribbed cylindrical shell excited by different types of drives are presented. Comparisons between measured and predicted results are made. Predictions of the acoustic radiation from the unsupported and supported shell with different types of drives are compared. The effects of properties of the shell and types of drives on the acoustic radiation are examined by example and some physical interpretation of significant features are offered.