High Blockage Ratio Research Articles

Gaseous slip flow forced convection through ordered microcylinders

This is a theoretical study dealing with longitudinal gaseous slip flow forced convection between a periodic bunch of microcylinders arranged in regular array. The selected geometry has applications in microscale pin fin heat sinks used for cooling of microchips. The flow is considered to be hydrodynamically and thermally fully developed. The two axially constant heat flux boundary conditions of H1 and H2 are considered in the analysis. The velocity and temperature discontinuities at the boundary are incorporated into the solutions using the first order slip boundary conditions. The method considered is mainly analytical in which the governing equations and three of the boundary conditions are exactly satisfied. The remaining symmetry condition on the right-hand boundary of the typical element is applied to the solution through the point matching technique. The results show that both the Poiseuille number and the Nusselt number are decreasing functions of the degree of rarefaction characterized by the Knudsen number. While an increase in the blockage ratio leads to a higher Poiseuille number, the functionality of the Nusselt number on this parameter is not monotonic. At small and moderate values of the blockage ratio, the Nusselt number is higher for a higher blockage ratio, whereas the opposite may be right for higher values of this parameter. It is also observed that the angular variations of the parameters are reduced at smaller blockage ratios. Accordingly, the H1 and H2 Nusselt numbers are the same for small and moderate blockage ratios.

Realizing the potential of tidal currents and the efficiency of turbine farms in a channel

Tidal turbines in strong flows have the potential to produce significant power. However, not all of this potential can be realized when gaps between turbines are required to allow navigation along a channel. A review of recent works is used to estimate the scale of farm required to realize a significant fraction of a channel's potential. These works provide the first physically coherent approach to estimating the maximum power output from a given number of turbines in a channel. The fraction of the potential realizable from a number of turbines, a farm's fluid dynamic efficiency, is constrained by how much of the channel's cross-section the turbines are permitted to occupy and an environmentally acceptable flow speed reduction. Farm efficiency increases as optimally tuned turbines are added to its cross-section, while output per turbine increases in tidal straits and decreases in shallow channels. Adding rows of optimally tuned turbines also increases farm efficiency, but with a diminishing return on additional rows. The diminishing return and flow reduction are strongly influenced by how much of the cross-section can be occupied and the dynamical balance of the undisturbed channel. Estimates for two example channels show that realizing much of the MW potential of shallow channels may well be possible with existing turbines. However unless high blockage ratios are possible, it will be more difficult to realize the proportionately larger potential of tidal straits until larger turbines with a lower optimum operating velocity are developed.

Experimental testing of the transverse horizontal axis water turbine

This study outlines the procedures and results for a set of experiments on the transverse horizontal axis water turbine (THAWT), which is a variant of a Darrieus turbine. Testing of a 1/20th scale device was conducted in the combined wind, wave and current tank at Newcastle University. Flow depth and velocity were varied over a range of realistic Froude numbers for tidal streams. Various configurations of the device were tested to assess the merits of the THAWT design. Variants included a parallel-bladed device, a ‘truss’ device and a parallel-bladed device configured with blades pitched relative to the pitch circle tangent. Experiments were carried out using a speed controlled motor, allowing quasi-steady results to be taken over a range of tip speed ratios. The results demonstrate that, over a range of flow conditions, the device is capable of exceeding the Lanchester–Betz limit for kinetic efficiency. This is principally because of the relatively high blockage ratio that can be achieved with such a device.

Wavelet analysis of unsteady flows: Application on the determination of the Strouhal number of the transient wake behind a single cylinder

This paper presents experimental results of the accelerating and decelerating flow in the wake of a cylinder obtained by means of hot wire anemometry measurements in a wind tunnel with high blockage ratio. The analysis was done in Fourier and wavelet spaces. The Strouhal number for Reynolds numbers up to 3×104 was studied in a transient flow and compared with the results obtained from steady flows at several velocities uniformly distributed from Re=1.7×103 to 3×104. Results show that the wavelet analysis is a valuable tool to deal with both transient and stationary random phenomena and that is able to capture the characteristics of the transient flow as well as the Fourier analysis can do with the steady state acquisitions.

Effect of Blockage Ratio on Drag and Heat Transfer from a Centrally Located Sphere in Pipe Flow

Hydrodynamic and heat transfer analyses were carried out for laminar fluid flow past a heated sphere placed centrally in a pipe using Computational Fluid Dynamics (CFD) simulations. Fully developed parabolic velocity profile characteristic of laminar flow was specified at the pipe inlet. The effects of blockage ratio, defined as the ratio of the sphere diameter to the pipe diameter, on the drag coefficient and Nusselt number are reported. The particle Reynolds numbers were varied up to a maximum of 500 while the Prandtl number for the water system studied was fixed at 5.12. At lower blockage ratios (<0.3), transient and three-dimensional simulations were required for particle Reynolds numbers exceeding 270. The effect of blockage was more significant at lower particle Reynolds numbers and influenced the drag coefficient more than the Nusselt number for this system. At high particle Reynolds numbers, reasonable predictions of the drag coefficient could be obtained even at high blockage ratios, by using the standard correlations available for a sphere immersed in an unbounded domain and the fluid approaching the sphere with a uniform velocity profile. Conversely, at low blockage ratios and low Reynolds numbers, considerable deviation from the standard drag correlation predictions was observed. Higher blockage ratios delayed boundary layer separation. The local Nusselt number distribution trends along the sphere are also reported. Correlations were developed for the drag coefficient, boundary layer separation angle and the average Nusselt number as a function of the blockage ratio and particle Reynolds number.

Convective cooling of tandem heated squares in a channel

This article elaborates on the numerical simulation of forced convective incompressi-ble flow in a horizontal plane channel and heat transfer over two isothermal tandem square cylinders. Finite-volume method was applied to discretize the momentum and energy equations. This study details the effects of S/ H (gap between two squares), Reynolds number, and blockage ratio on the characteristics of flow field and heat transfer. The blockage ratios and Reynolds numbers were chosen between 12.5 to 50 per cent and 100≤ Re≤300, respectively. Results are presented in the form of time-averaged streamlines, instantaneous contours of temperature and vorticity, and some characteristics of fluid flow and heat transfer such as time-averaged of the local and total Nusselt number, drag coefficient, and Strouhal number. The present results show that the gap spacing between squares at high blockage ratios does not have an obvious effect on heat transfer. On the other hand, this parameter plays a main role on the drag coefficient at a high value ( S/ H=4), which is because of the formation of the vortex cores in gap spacing and downstream of the second square. The heat transfer rate of the second square increases with the increase of S/ H. Increasing the blockage ratio has a negative effect on convective heat transfer of the second square. With the vortex shedding in the gap spacing, the drag coefficient and the Nusselt number both increase in the second square in comparison to the first square.

Wake behaviour and instability of flow through a partially blocked channel

The two-dimensional flow through a constricted channel is studied. A semi-circular bump is located on one side of the channel and the extent of blockage is varied by adjusting the radius of the bump. The blockage is varied between 0.05 and 0.9 of the channel width and the upstream Reynolds number between 25 and 3000. The geometry presents a simplified blockage specified by a single parameter, serving as a starting point for investigations of other more complex blockage geometries. For blockage ratios in excess of 0.4, the variation of reattachment length with Reynolds number collapses to within approximately 15%, while at lower ratios the behaviour differs. For the constrained two-dimensional flow, various phenomena are identified, such as multiple mini-recirculations contained within the main recirculation bubble and vortex shedding at higher Reynolds numbers. The stability of the flow to three-dimensional perturbations is analysed, revealing a transition to a three-dimensional state at a critical Reynolds number which decreases with higher blockage ratios. Separation lengths and the onset and structure of three-dimensional instability observed from the geometry of blockage ratio 0.5 resemble results taken from backward-facing step investigations. The question of the underlying mechanism behind the instability being either centrifugal or elliptic in nature and operating within the initial recirculation zone is analytically tested.

Coolability of blocked regions in a rod bundle after ballooning under LOCA conditions: Main findings from a review of past experimental programmes

IRSN has carried out a state-of-the-art review of the main experimental programmes related to fuel behaviour under loss-of-coolant-accident (LOCA) conditions conducted from the 1970s until now, that has been split in three parts. The second part is devoted to the question of the coolability of blocked regions in a rod bundle after ballooning in a LOCA. The main findings from this part are presented here. The experimental characteristics and main results of the FEBA, SEFLEX, THETIS, ACHILLES, CEGB and FLECHT-SEASET programmes, as well as several analytical developments performed in association with these experimental programmes, were examined in detail in this review. The comparison and combination of conclusions drawn from these results and studies were used to improve our understanding of the physical phenomena governing the behaviour of a partially blocked rod array during a LOCA reflood scenario. It has also been possible to determine the limits of blockage coolability under the most severe geometric (blockage ratio and length) and thermohydraulic conditions. Thus, even a severe blockage ratio (90%) of a moderate length (<10 cm) does not cause any particular problems in terms of coolability during two-phase reflood. However, a severe blockage with considerable axial extension (>15 cm) and a high blockage ratio (>80%) can lead – under low reflood conditions – to a significant increase in blockage surface temperatures, hindering the final coolability of this blockage. It is important to underline that these results were obtained in out-of-pile experiments performed with electrically heated fuel rod simulators with a large gap between simulator and cladding bulge, thus not allowing to simulate the possible fuel accumulation occurring in cladding balloons (fuel relocation), as was observed during all in-pile tests with irradiated fuel rods. The impact of fuel relocation upon blockage coolability therefore remains to be investigated.

Optimization of the deflagration to detonation transition: reduction of length and time of transition

The aim of this experimental investigation is the study of Deflagration to Detonation Transition (DDT) in tubes in order to (i) reduce both run-up distance and time of transition (L DDT and t DDT) in connection with Pulsed Detonation Engine applications and to (ii) attempt to scale L DDT with λCJ (the detonation cellular structure width). In DDT, the production of turbulence during the long flame run-up can lead to L DDT values of several meters. To shorten L DDT, an experimental set-up is designed to quickly induce highly turbulent initial flow. It consists of a double chamber terminated with a perforated plate of high Blockage Ratio (BR) positioned at the beginning of a 26 mm inner diameter tube containing a “Shchelkin spiral” of BR ≈ 0.5. The study involves stoichiometric reactive mixtures of H2, CH4, C3H8, and C2H4 with oxygen and diluted with N2 in order to obtain the same cell width λCJ≈10 mm at standard conditions. The results show that a shock-flame system propagating with nearly the isobaric speed of sound of combustion products, called the choking regime, is rapidly obtained. This experimental set-up allows a L DDT below 40 cm for the mixtures used and a ratio L DDT/λCJ ranging from 23 to 37. The transition distance seems to depend on the reduced activation energy (E a/RT c) and on the normalized heat of reaction (Q/a 0 2). The higher these quantities are, the shorter the ratio L DDT/λCJ is.

Prediction of the critical Reynolds number for flow past a circular cylinder

In this paper we attempt to estimate the value of critical Reynolds number, Re c, for the first wake instability of the wake associated with the flow past a circular cylinder. Linear stability analysis (LSA) and direct time integration (DTI) of the governing equations for incompressible flows are carried out via a stabilized finite element formulation. The generalized eigenvalue problem resulting from the finite element discretization of the equations from linearized stability analysis is solved using a subspace iteration (SI) method to get the most unstable eigenmode. The results from the two methods are in good agreement. The effect of spatial resolution and location of computational boundaries is investigated. It is found that, for high blockage (ratio of the diameter of cylinder to the lateral width of domain), Re c first decreases and then increases with increase in blockage. It is also observed that the Strouhal number at Re c is quite sensitive to the blockage. This might possibly explain the scatter in the data from various researchers in the past.

A Combined Numerical and Experimental Study of Heat Transfer in a Roughened Square Channel with 45∘ Ribs

Experimental investigations have shown that the enhancement in heat transfer coefficients for air flow in a channel roughened with low blockage (e/Dh &lt; 0.1) angled ribs is on the average higher than that roughened with 90∘ ribs of the same geometry. Secondary flows generated by the angled ribs are believed to be responsible for these higher heat transfer coefficients. These secondary flows also create a spanwise variation in the heat transfer coefficient on the roughened wall with high levels of the heat transfer coefficient at one end of the rib and low levels at the other end. In an effort to investigate the thermal behavior of the angled ribs at elevated Reynolds numbers, a combined numerical and experimental study was conducted. In the numerical part, a square channel roughened with 45∘ ribs of four blockage ratios (e/Dh) of 0.10, 0.15, 0.20, and 0.25, each for a fixed pitch‐to‐height ratio (P/e) of 10, was modeled. Sharp as well as round‐corner ribs (r/e = 0 and 0.25) in a staggered arrangement were studied. The numerical models contained the smooth entry and exit regions to simulate exactly the tested geometries. A pressure‐correction‐based, multiblock, multigrid, unstructured/adaptive commercial software was used in this investigation. Standard high Reynolds number k − ε turbulence model in conjunction with the generalized wall function for most parts was used for turbulence closure. The applied thermal boundary conditions to the CFD models matched the test boundary conditions. In the experimental part, a selected number of these geometries were built and tested for heat transfer coefficients at elevated Reynolds numbers up to 150 000, using a liquid crystal technique. Comparisons between the test and numerically evaluated results showed reasonable agreements between the two for most cases. Test results showed that (a) 45∘ angled ribs with high blockage ratios (&gt;0.2) at elevated Reynolds numbers do not exhibit a good thermal performance, that is, beyond this blockage ratio, the heat transfer coefficient decreases with the rib blockage and (b) CFD could be considered as a viable tool for the prediction of heat transfer coefficients in a rib‐roughened test section.

A numerical investigation of wall effects up to high blockage ratios on two-dimensional flow past a confined circular cylinder

A finite volume method based on a velocity-only formulation is used to solve the flow field around a confined circular cylinder in a channel in order to investigate lateral wall proximity effects on stability, Strouhal number, hydrodynamic forces and wake structure behind the cylinder for a wide range of blockage ratios (0.1&amp;lt;β⩽0.9) and Reynolds numbers (0&amp;lt;Re⩽280). For blockage ratios less than approximately 0.85 a first critical Reynolds number is identified at which a supercritical Hopf bifurcation of the symmetric solution occurs. For blockage ratios greater than about 0.687 and at Reynolds numbers exceeding the first critical Reynolds number a second curve of neutral stability is seen, representing a pitchfork bifurcation of the steady symmetric solution to one of two possible steady asymmetric solutions. Either side of the neutral stability curve for the pitchfork bifurcation our linear stability analysis and direct numerical simulations demonstrate that although the flow is linearly stable it is unstable to finite two-dimensional perturbations. At blockage ratios larger than about 0.82 the steady asymmetric solutions also become unstable through a Hopf bifurcation. In contrast with the first Hopf bifurcation of the symmetric solution at lower Reynolds numbers numerical calculations of the lift coefficient reveal that the oscillations are no longer symmetric in the rising and falling parts of each cycle. Very strong vortices shed from the cylinder and the wall cause drastic increases in the amplitudes of the lift and drag coefficients. A co-dimension 2 point where pitchfork and Hopf bifurcations occur simultaneously has been located in parameter space. Altogether, four distinct regions in the parameter space (β,Re)∈(0,0.9]×(0,280] have been identified, each corresponding to a different class of flow: (i) Steady symmetric flow, (ii) symmetric vortex shedding, (iii) steady asymmetric flow, and (iv) asymmetric vortex shedding, where a periodic-in-time flow is classed as symmetric or asymmetric depending on whether the time-average over one cycle of the lift coefficient is zero or not. Numerical solutions are computed on meshes having up to 1.8 million degrees of freedom. Extensive comparisons are made with the results available in the literature.

Distribution, recruitment, and geomorphic significance of large woody debris in an alluvial forest stream: Tonghi Creek, southeastern Australia

The complex yet poorly understood interactions between riparian vegetation, large woody debris and fluvial geomorphology in an anthropogenically undisturbed reach of an alluvial, sand-bed forest stream in SE Australia have been determined. Riparian vegetation exhibits lateral and vertical zonation of understorey and overstorey species. The dominant riparian tree species, Tristaniopsis laurina (water gum), grows within the channel and on the floodplain within one channel width of the stream. Larger Eucalyptus species only grow on the highest parts of the floodplain and on a low Pleistocene river terrace. A complete large woody debris (LWD) census conducted in the 715-m-long study reach revealed that water gum comprises 17.6% of the total LWD loading, which, at 576 m 3 ha −1, is high for a stream with a catchment area of 187 km 2. Although most LWD has a small diameter (0.1–0.3 m), the greatest contribution to the total volume of LWD is by pieces with a diameter between 0.3 and 0.7 m. A high proportion of LWD (10.4%) has a blockage ratio greater than 10%. The spatial distribution of LWD is random both longitudinally and within individual meander bends. Dominant recruitment processes of LWD vary by species. T. laurina trees are recruited to the channel by minor bank erosion and senescence, while the Eucalyptus species are predominantly recruited from the highest parts of the floodplain/low-river terrace by episodic windthrow during large storms. Multiple radiocarbon dates of outer wood of immobile LWD indicate a maximum residence time of 240±40 years BP for T. laurina timber. The high loading of LWD combined with the extensive root systems of riparian vegetation stabilize Tonghi Creek. Log steps form natural wooden drop-structures with a mean height of 29 mm that were responsible for 20.5% of the total head loss under base flow conditions ( Q=0.08 m 3 s −1). Large woody debris is buried in the bed at depths of up to 2.3 m and is responsible for an estimated 49% of the 11, 600 m 3 of sand stored in the study reach. Pools are spaced at 0.8 channel widths and 82% of pools are formed by scour over, under, around, or beside LWD or by the impoundment of water upstream of debris dams. Due to the high density of hardwood timber species, debris dams, however, do not readily form in Tonghi Creek as the timber is difficult to transport and LWD usually sinks to the bed of the stream. Despite the high degree of channel stability provided by LWD, high blockage ratios in the channel result in relatively frequent overbank flows. These flows are often concentrated in chutes across the neck of meanders or multiple loops, which can develop into cutoffs and channel avulsions, respectively.

The alleviation of the aerodynamic drag and wave effects of high-speed trains in very long tunnels

The design of new high-speed railway lines requires longer and more numerous tunnel sections, where aerodynamic effects limit the maximum allowed train velocity for a given tunnel cross-section area. These effects influence the train power requirement, the traction energy costs and the pressure wave amplitude: the knowledge of the unsteady aerodynamic field around the train is therefore essential to the optimum choice of a tunnel configuration, and mainly of the cross-section diameter and of the presence and position of pressure relief ducts. In this paper, the aerodynamic phenomena generated by a train traveling at high speed through a long tunnel of small cross-section are analyzed by means of quasi one-dimensional numerical simulations of the air flow induced by a train traveling at 120 m/s in a tunnel connecting two stations 60 km apart. Several tunnel configurations at high blockage ratio are discussed, together with the positive and negative effects of pressure relief ducts and of partial air vacuum. Aerodynamic phenomena are evaluated in terms of drag, pressure wave amplitude and shock wave onset on the train tail. Results suggest that configurations consisting of twin tunnels connected by pressure relief ducts near stations and operated under partial vacuum should be preferred.

Secondary Correction Formula for Two-Dimensional Lift Coefficient Measured in a Wind Tunnel

An improvement of the secondary correction formula for the lift coefe cient of an airfoil measured in a wind tunnel is presented, taking into account upwash and blockage effects. The formula allows the extension of the limits of correctability in the presence of strong variations of wall-induced velocity. An adaptive e exible-wall wind tunnel has been used to introduce known streamwise distributions of interference velocity components into the test chamber, to monitor their effects on the model aerodynamic performance and to e nd relations with the known interference-free lift coefe cient. A method to impose simultaneously streamwise variations of wall-induced upwash and blockage has been developed. The basic aims of the work are the investigation of the limits of validity of primary and secondary wind-tunnel wall corrections, and the establishment of semiempirical correction rules for nonuniform e ows. Tests were performed on a NACA 0012 airfoil at Mach 0.4, at angles of attack between 4 and 10 deg. The experimental results show that by using the secondary correction signie cant improvements, in terms of correctability limits, are obtained with respect to the primary one, even if the percent error on the liftcoefe cient evaluation remains unacceptable for high incidences and/or high blockage ratio tests. The correlation of experimental data has suggested that it is possible to improve the secondary correction for the lift coefe cient of an airfoil section by taking into account the effects of variations of blockage and upwash on the model.

Numerical study of blockage effects on aerodynamic characteristics of an oscillating rectangular cylinder

Abstract The flow around a stationary and oscillating square cylinder was numerically simulated at various blockage ratios in order to study the effects of wall confinement on the aerodynamic characteristics of the cylinder. Numerical methods include Direct Simulation (DS) with upwind scheme for laminar flow at Re = 200, 400, 103, and k−e model for turbulence flow at Re = 4 × 103. The simulations were conducted at various blockage ratios of H L = 0.04, 0.067, 0.085, 0.112, 0.166, 0.244, 0.303 and 0.4 . For stationary cases, simulations were carried out at Reynolds numbers of Re = 200−103. For oscillation cases, simulations were done at Reynolds number of Re = (1, 4) × 103 for a square cylinder oscillating at forced oscillatory frequencies of Stc = 0.1, 0.25 and 0.3. Using the SIMPLE algorithm, simulation results show that drag and lift forces and Strouhal numbers all increase with the increase of blockage ratios (i.e. the decrease of the interval of the two parallel confining walls), and the aerodynamic characteristics agree reasonably well with those from experiments. For stationary cases, the lift and drag forces, and vortex-shedding Strouhal numbers all increase with the increase of blockage ratios. The flow streamlines and vorticity contours show conspicuous features of the wake under high blockage ratios. The first-sinking phenomenon due to blockage effect can be successfully captured at Reynolds numbers of only Re = 103 and can be confirmed to be caused by the change of non-reattachment of separated shear layers to reattachment flow by the blockage effects. For oscillation cases, the lock-in features are successfully captured. The behavior of phase difference, i.e. the stable/unstable features, against the forced oscillatory frequency cannot be almost influenced by the blockage effects.

NUMERICAL STUDY OF THE BLOCKAGE EFFECTS ON VISCOUS FLOW PAST A CIRCULAR CYLINDER

In various numerical solutions of flow around bluff bodies the unbounded physical domain is replaced by a restricted computational one whose extent depends on the size of the computational grid network. The truncation of the solution domain in the cross-flow direction reduces the computer time required for the solution, but introduces numerical blockage effects which influence considerably the values of the various flow parameters. In the present paper the finite element solution of steady and unsteady flow around a circular cylinder at Re=106 is presented for blockage ratios of 0·05, 0·15 and 0·25. A boundary condition was tested for which the streamfunction values at the outer boundaries were those of the irrotational solution around a circular cylinder. The size of the standing vortices decreases with the blockage ratio when the flow is steady, while the spacing of the vortices decreases in both directions with increasing blockage ratio when the wake becomes unsteady. The hydrodynamic forces on the cylinder and the Strouhal number are magnified as the blockage ratio increases. The application of the streamfunction values derived from the irrotational solution at the outer boundaries reduced blockage effects only at high blockage ratio.

カプセル形状物体の空力性能に及ぼす閉塞率の影響

Low-speed aerodynamic characteristics and effects of the blockage ratio of an axisymmetric capsule configuration are experimentally investigated. The flow conditions in a closed-type test section with high blockage ratio are obtained by setting a circular duct coaxially in the open-type test section of the wind tunnel. The aerodynamic loads of similar scale model of three kinds with body diameter of 100, 180 and 240mm are measured in both open-type and closed-type test sections. Also the pressure distributions along the model surface and along the duct wall are measured. From the investigations of experimental results, the following major conclusions are obtained on aerodynamic characteristics of such capsule-type configurations: (1) It is expected that the drag force of capsule-type configuration is strongly affected by blockage ratio effects. (2) The usual method of the wind tunnel wall effect correction is applicable to prediction of lift and pitching moment coefficients, but not sufficient to drag coefficient. (3) The velocity profiles in the closed type test section are much different from those in the open-type test section. In case of the high blockage ratio, the flow which has been accelerated because of the decrease of the section area, is bent by the curvature of the model tail and separates at the trailing edge. Such phenomena make the flow field complicated and result in the increase of drag force.

Vortex shedding in oscillatory flow: geometrical effects

This paper presents a selection of the results of an extensive experimental study of the effects of in-line, periodic flow oscillations on the character of vortex shedding from sharp-edged bluff bodies. Attention is concentrated on how oscillation amplitude and the body shape alter the range of reduced velocity, U r, over which lock-in conditions occur — i.e. conditions under which changes in the upstream mean velocity no longer lead to changes in the vortex-shedding frequency. U r is defined as U o Nd, where d is the body cross-stream width, U o is the upstream velocity and N is the oscillation frequency. It is demonstrated that, with all other parameters fixed, some body shapes are much more susceptible to lock-in than others. Of the four shapes tested, the two most commonly used in industrial vortex flowmeters (triangular and T-shaped cross-sections are the most likely to suffer from measurement errors due to in-line flow oscillatins. The additional effects of high obstacle blockage and small spanwise aspect ratio are also discussed in the context of oscillatory flow.

An Experimental Investigation of Heat Transfer in an Orthogonally Rotating Channel Roughened With 45 deg Criss-Cross Ribs on Two Opposite Walls

Turbine blade cooling is imperative in advanced aircraft engines. The extremely hot gases that operate within the turbine section require turbine blades to be cooled by a complex cooling circuit. This cooling arrangement increases engine efficiency and ensures blade materials a longer creep life. One principle aspect of the circuit involves serpentine internal cooling passes throughout the core of the blade. Roughening the inside surfaces of these cooling passages with turbulence promoters provides enhanced heat transfer rates from the surface. The purpose of this investigation was to study the effect of rotation, aspect ratio, and turbulator roughness on heat transfer in these rib-roughened passages. The investigation was performed in an orthogonally rotating setup to simulate the actual rotation of the cooling passages. Single-pass channels, roughened on two opposite walls, with turbulators positioned at 45 deg angle to the flow, in a criss-cross arrangement, were studied throughout this experiment. The ribs were arranged such that their pitch-to-height ratio remained at a constant value of 10. An aspect ratio of unity was investigated under three different rib blockage ratios (turbulator height/channel hydraulic diameter) of 0.1333, 0.25, and 0.3333. A channel with an aspect ratio of 2 was also investigated for a blockage ratio of 0.25. Air was flown radially outward over a Reynolds number range of 15,000 to 50,000. The rotation number was varied from 0 to 0.3. Stationary and rotating cases of identical geometries were compared. Results indicated that rotational effects are more pronounced in turbulated passages of high aspect and low blockage ratios for which a steady increase in heat transfer coefficient is observed on the trailing side as rotation number increases while the heat transfer coefficient on the leading side shows a steady decrease with rotation number. However, the all-smooth-wall classical pattern of heat transfer coefficient variation on the leading and trailing sides is not followed for smaller aspect ratios and high blockage ratios when the relative artificial roughness is high.