共查询到20条相似文献,搜索用时 28 毫秒
1.
This paper presents a comparative numerical investigation on film cooling from a row of holes injected at 35° on a flat plate
with three film cooling configurations, including cylindrical hole, 15° forward diffused shaped hole, and new crescent hole.
All simulations are conducted at blowing ratio of 0.6 and 1.25, length-to-diameter ratio of four and pitch-to-diameter ratio
of three. Computational solutions of the steady, Reynolds averaged Navier–Stokes equations are obtained using a finite volume
method. Previous successful application of a two-layer turbulence model to cylindrical hole is extended to predict film cooling
for the different hole geometries. It has been found that the film cooling effectiveness of cylindrical holes obviously declined
along with increasing the blowing ratio. While the forward diffused shaped hole presents a marked improvement, with a higher
effectiveness at the lateral area between adjacent holes. By comparison, the crescent hole exhibits the highest film cooling
effectiveness among the three configurations both in spanwise and streamwise especially downstream of the intersection of
the two holes. Also, the crescent hole can restrain the vortex intensity, and then enhance the film cooling effectiveness. 相似文献
2.
This paper describes the results of an experimental investigation into the film cooling effectiveness and the heat transfer characteristics of two staggered rows of compound angle holes. The effects of hole spacings and turbulence intensity on film cooling and heat transfer characteristic are investigated for three blowing rates; 0.5, 1.0 and 1.7. An attempt has been made to correlate the film cooling effectiveness results using a two dimensional correlation group. The increase of spanwise hole spacing results in a reduction in the film cooling effectiveness and an increase in the Stanton number. Increasing the freestream turbulence intensity has caused a significant reduction in the local film cooling effectiveness but increased the Stanton number, especially at blowing rate of 0.5. 相似文献
3.
Two-phase CFD calculations, using a Lagrangian model and commercial code Fluent 6.2.16, were employed to calculate the gas
and droplet flows and film cooling effectiveness with and without mist on a flat plate. Two different three dimensional geometries
are generated and the effects of the geometrical shape, size of droplets, mist concentration in the coolant flow and temperature
of mainstream flow for different blowing ratios are studied. A cylindrical and laterally diffused hole with a streamwise angle
of 30° and spanwise angle of 0° are used. The diameter of film cooling (d) hole, and the hole length to diameter ratio (L/d) for both of geometries are 10 mm and 4, respectively. Also the blowing ratio ranges from 1.0 to 2.0, and the mainstream
Reynolds number based on the mainstream velocity and hole diameter (Re
d) is 6,219. The results are shown for different droplets diameters (1–10 μm), concentrations (1–5%) and mainstream temperatures
(350–500 K). The centreline effectiveness and distribution of effectiveness on the surface of cooling wall are presented. 相似文献
4.
Min-Sheng Hung Pei-Pei Ding Ping-Hei Chen 《Experimental Thermal and Fluid Science》2009,33(2):292-305
The present study employs a transient liquid crystal thermography to measure film cooling performance over constant curvature of concave and convex surfaces. This work investigates detailed distributions of both film cooling effectiveness and heat transfer coefficient on concave and convex surfaces with one row of injection holes inclined stream-wise at 35° at four blowing ratios (0.5, 1.0, 1.5 and 2.0) on four test pieces with different hole configurations. All test models have a row of discrete holes with a stream-wise injection angle (γ of 35° and a pitch-to-diameter ratio (P/d) of 3. The current work examines four different injection configurations, one with simple and three with 8° forward-expanded holes. Three compound angles of 0, 45 and 90° with air (ρc/ρm = 0.98) as coolants are tested under the mainstream Reynolds number (Red) of 2300 on concave surface, and 1700 on convex surface. Measured results of the concave surface show that both the span-wise averaged heat transfer coefficient and film cooling effectiveness increase with blowing ratios for all tested models. Higher heat transfer levels induced by large flow disturbance of compound-angle injection also lead to poorer overall film cooling performance, especially at high blowing ratio and large span-wise injection angle. Present results show that the best surface protection on the concave surface over the widest range of M can be provided by the forward-expanded holes with β = 0° (Model-B), followed by the forward-expanded holes with β = 45° (Model-C). Convex surface results show that the compound-angle injection indicates increases in both film cooling effectiveness and heat transfer at moderate and high blowing ratios. The forward-expanded hole with simple-angle injection provides the best film performance because of high film cooling effectiveness and low heat transfer coefficient at blowing ratio of 0.5. 相似文献
5.
The effect of turbulence intensity on film cooling of gas turbine blade from trenched shaped holes 总被引:2,自引:0,他引:2
This paper reports a computational investigation on the effects of mainstream turbulence intensity on film cooling effectiveness
from trenched holes over a symmetrical blade. Computational solutions of the steady, Reynolds-Averaged Navier–Stokes equations
are obtained using a finite volume method with k − ε Turbulence model. Whenever possible, computational results are compared
with experimental ones from data found in the open literature. Computational results are presented for a row of 25° forward-diffused
film hole within transverse slot injected at 35° to AGTB symmetrical blade. Four blowing ratios, M = 0.3, 0.5, 0.9 and 1.3
are studied together with four mainstream turbulence intensities of Tu = 0.5, 2, 4 and 10%. Results indicate that the trenched
shaped holes tend to give better film cooling effectiveness than that obtained from discrete shaped holes for all blowing
ratios and all turbulence intensities. The trenching of shaped holes has changed the optimum blowing ratio and also the location
of re-attachment of separated jet at high blowing ratios. Moreover, it has been found that the effect of mainstream turbulence
intensity for trenched shaped holes is similar to that obtained for discrete shaped holes with the exception that the sensitivity
of film cooling effectiveness to turbulence intensity has decreased for trenched shaped holes. 相似文献
6.
Michael J. Benson Christopher J. Elkins Sayuri D. Yapa Julia B. Ling John K. Eaton 《Experiments in fluids》2012,52(6):1415-1430
Three-dimensional mean velocity and concentration fields have been measured for a water flow in a pressure side cutback trailing edge film cooling geometry consisting of rectangular film cooling slots separated by tapered lands. Three-component mean velocities were measured with conventional magnetic resonance velocimetry, while time-averaged concentration distributions were measured with a magnetic resonance concentration technique for flow at two Reynolds numbers (Re) differing by a factor of 2, three blowing ratios, and with and without an internal pin fin array in the coolant feed channel. The results show that the flows are essentially independent of Re for the regime tested in terms of the film cooling surface effectiveness, normalized velocity profiles, and normalized mean streamwise vorticity. Blowing ratio changes had a larger effect, with higher blowing ratios resulting in surface effectiveness improvements at downstream locations. The addition of a pin fin array within the slot feed channel made the spanwise distribution of coolant at the surface more uniform. Results are compared with transonic experiments in air at realistic density ratios described by Holloway et al. (2002a). 相似文献
7.
This paper presents a comparative-numerical investigation on film cooling from a row of simple and compound-angle holes injected
at 35° on a flat plate with four film cooling configurations: (1) cylindrical film hole; (2) 15° forward diffused film hole;
(3) trenched cylindrical film hole; (4) trenched 15° forward-diffused film hole. All simulations are at fixed density ratio
of 1.6, blowing ratio of 1.25, length-to-diameter L/D = 4 and pitch-to-diameter ratio of 3.0. The effect of length-to-diameter ratio on film cooling has been also investigated
using L/D in the range of 1–8. Computational solutions of the steady, Reynolds-averaged Navier–Stokes equations have been obtained
using a finite volume method. It has been found that the shape of the hole and the trenched holes can significantly affect
the film cooling flow over the protected surface. Further, it has been shown that the film cooling effectiveness by trenched
shaped holes is higher than all other configurations both in spanwise and streamwise specially downstream of the injection.
Also, a trenched compound angle injection shaped hole produces much higher film cooling protection than the other configurations
investigated in the present paper. The length-to-diameter ratio of trenched holes was found to have a significant effect on
film cooling effectiveness and the spread of the coolant jets. 相似文献
8.
A detailed experimental study is performed on the separated flow structures around a low aspect-ratio circular cylinder (pin-fin) in a practical configuration of liquid cooling channel. Distinctive features of the present arrangement are the confinement of the cylinder at both ends, water flow at low Reynolds numbers (Re = 800, 1800, 2800), very high core flow turbulence and undeveloped boundary layers at the position of the obstacle. The horseshoe vortex system at the junctions between the cylinder and the confining walls and the near wake region behind the obstacle are deeply investigated by means of Particle Image Velocimetry (PIV). Upstream of the cylinder, the horseshoe vortex system turns out to be perturbed by vorticity bursts from the incoming boundary layers, leading to aperiodical vortex oscillations at Re = 800 or to break-away and secondary vorticity eruptions at the higher Reynolds numbers. The flow structures in the near wake show a complex three-dimensional behaviour associated with a peculiar mechanism of spanwise mass transport. High levels of free-stream turbulence trigger an early instabilization of the shear layers and strong Bloor–Gerrard vortices are observed even at Re = 800. Coalescence of these vortices and intense spanwise flow inhibit the alternate primary vortex shedding for time periods whose length and frequency increase as the Reynolds number is reduced. The inhibition of alternate vortex shedding for long time periods is finally related to the very large wake characteristic lengths and to the low velocity fluctuations observed especially at the lowest Reynolds number. 相似文献
9.
I. Recktenwald N. Alkishriwi W. Schrder 《European Journal of Mechanics - B/Fluids》2009,28(5):677-688
The flow field of a channel rotating about the streamwise axis is analyzed experimentally and numerically. The current investigations were carried out at a bulk velocity based Reynolds number of Rem = 2850 and a friction velocity based Reynolds number of Reτ = 180, respectively. Particle-image velocimetry (PIV) measurements are compared with large-eddy simulation data to show earlier direct numerical simulation findings to generate too large a reverse flow region in the center region of the spanwise flow. The development of the mean spanwise velocity distribution and the influence of the rotation on the turbulent properties, i.e., the Reynolds stresses and the two-point correlations of the flow, are confirmed in both investigations. The rotation primarily influences those components of the Reynolds shear stresses, which contain the spanwise velocity component. The size of the correlation areas and thus the length scales of the flow generally grow in all three coordinate directions leading to longer structures. Furthermore, experimental results of the same channel flow at a significantly lower bulk Reynolds number of Rem, l = 665, i.e., a laminar flow in a non-rotating channel, are introduced. The experiments show the low Reynolds number flow to become turbulent under rotation and to develop the same characteristics as the high Reynolds number flow. 相似文献
10.
The flow and heat transfer characteristics of an unconfined air jet that is impinged normally onto a heated flat plate have
been experimentally investigated for high Reynolds numbers ranging from 30,000 to 70,000 and a nozzle-to-plate spacing range
of 1–10. The mean and turbulence velocities by using hot-wire anemometry and impingement surface pressures with pressure transducer
are measured. Surface temperature measurements are made by means of an infrared thermal imaging technique. The effects of
Reynolds number and nozzle-to-plate spacing on the flow structure and heat transfer characteristics are described and compared
with similar experiments. It was seen that the locations of the second peaks in Nusselt number distributions slightly vary
with Reynolds number and nozzle-to-plate spacing. The peaks in distributions of Nusselt numbers and radial turbulence intensity
are compatible for spacings up to 3. The stagnation Nusselt number was correlated for the jet Reynolds number and the nozzle-to-plate
spacing as Nu
st∝Re
0.69(H/D)0.019. 相似文献
11.
The interaction between the wake of a rotor blade and a downstream cylinder holds the key to the understanding and control of electronic cooling fan noise. In this paper, the aerodynamic characteristics of a circular cylinder are experimentally studied in the presence of an upstream NACA 4412 airfoil for the cylinder-diameter-based Reynolds numbers of Red=2,100–20,000, and the airfoil chord-length-based Reynolds numbers of Rec=14,700–140,000. Lift and drag fluctuations on the cylinder, and the longitudinal velocity fluctuations of the flow behind the cylinder were measured simultaneously using a load cell and two hot wires, respectively. Data analysis shows that unsteady forces on the cylinder increase significantly in the presence of the airfoil wake. The dependence of the forces on two parameters is investigated, that is, the lateral distance (T) between the airfoil and the cylinder, and the Reynolds number. The forces decline quickly as T increases. For Rec<60,000, the vortices shed from the upstream airfoil make a major contribution to the unsteady forces on the cylinder compared to the vortex shedding from the cylinder itself. For Rec>60,000, no vortices are generated from the airfoil, and the fluctuating forces on the cylinder are caused by its own vortex shedding. 相似文献
12.
Investigation of the vortex induced unsteadiness of a separation bubble via time-resolved and scanning PIV measurements 总被引:1,自引:0,他引:1
A transitional separation bubble on the suction side of an SD7003 airfoil is considered. The transition process that forces
the separated shear layer to reattach seems to be governed by Kelvin–Helmholtz instabilities. Large scale vortices are formed
due to this mechanism at the downstream end of the bubble. These vortices possess a three-dimensional structure and detach
from the recirculation region, while other vortices are formed within the bubble. This separation of the vortex is a highly
unsteady process, which leads to a bubble flapping. The structure of these vortices and the flapping of the separation bubble
due to these vortices are temporally and spatially analyzed at angles of attack from 4° to 8° and chord-length based Reynolds
numbers Re
c = 20,000–60,000 using time-resolved PIV measurements in a 2D and a 3D set-up, i.e., stereo-scanning PIV measurements are
done in the latter case. These measurements complete former studies at a Reynolds number of Re
c = 20,000. The results of the time-resolved PIV measurements in a single light-sheet show the influence of the angle of attack
and the Reynolds number. The characteristic parameters of the separation bubble are analyzed focusing on the unsteadiness
of the separation bubble, e.g., the varying size of the main recirculation region, which characterizes the bubble flapping,
and the corresponding Strouhal number are investigated. Furthermore, the impact of the freestream turbulence is investigated
by juxtaposing the current and former results. The stereo-scanning PIV measurements at Reynolds numbers up to 60,000 elucidate
the three-dimensional character of the vortical structures, which evolve at the downstream end of the separation bubble. It
is shown that the same typical structures are formed, e.g., the c-shape vortex and the screwdriver vortex at each Reynolds
number and angle of attack investigated and the occurrence of these patterns in relation to Λ-structures is discussed. To
evidence the impact of the freestream turbulence, these results are compared with findings of former measurements. 相似文献
13.
The study herein focuses on the vortex shedding characteristics and near-wake vorticity patterns of a square cylinder having
self-issuing jets through holes along its span. Three different values of spacing between the consecutive holes λ with respect
to the cylinder diameter D, i.e., λ/D = 1.5, 3 and 4 are studied experimentally via Digital Particle Image Velocimetry for the Reynolds number range extending
from 200 to 1,000. It has been observed that the three-dimensionality of the wake flow depends on the spacing between the
holes and Re number. For sufficiently low Reynolds numbers, the jet flows issuing from the holes yield a non-uniform distribution of mean
flow characteristics like the shedding frequency and the formation length of vortices along the span of the cylinder when
the spacing between jets along centerline is close to wavelength of the naturally existing three-dimensional wake instability.
Additionally, for Re number up to 500, the self-issuing jets emanating from the holes show an indirect interaction with shear layers originating
from upper and lower separation lines of the cylinder. However, for higher Re numbers of 750 and 1,000, they directly interact with and modify the vortices forming from the cylinder. 相似文献
14.
Large-eddy simulation (LES) has been applied to calculate the turbulent flow over staggered wall-mounted cubes and staggered random arrays of obstacles with area density 25%, at Reynolds numbers between 5 × 103 and 5 106, based on the free stream velocity and the obstacle height. Re = 5 × 103 data were intensively validated against direct numerical simulation (DNS) results at the same Re and experimental data obtained in a boundary layer developing over an identical roughness and at a rather higher Re. The results collectively confirm that Reynolds number dependency is very weak, principally because the surface drag is predominantly form drag and the turbulence production process is at scales comparable to the roughness element sizes. LES is thus able to simulate turbulent flow over the urban-like obstacles at high Re with grids that would be far too coarse for adequate computation of corresponding smooth-wall flows. Comparison between LES and steady Reynolds-averaged Navier-Stokes (RANS) results are included, emphasising that the latter are inadequate, especially within the canopy region. 相似文献
15.
16.
As a first step towards understanding particle–particle interaction in fluid flows, the motion of two spherical particles settling in close proximity under gravity in Newtonian fluids was investigated experimentally for particle Reynolds numbers ranging from 0.01 to 2000. It was observed that particles repel each other for Re>0.1 and that the separation distance of settling particles is Reynolds number dependent. At lower Reynolds numbers, i.e. for Re<0.1, particles settling under gravity do not separate.The orientation preference of two spherical particles was found to be Reynolds number dependent. At higher Reynolds numbers, the line connecting the centres of the two particles is always horizontal, regardless of the way the two particles are launched. At lower Reynolds numbers, however, the particle centreline tends to tilt to an arbitrary angle, even of the two particles are launched in the horizontal plane. Because of the tilt, a side migration of the two particles was found to exist. A linear theory was developed to estimate the side migration velocity. It was found that the maximum side migration velocity is approximately 6% of the vertical settling velocity, in good agreement with the experimental results.Counter-rotating spinning of the two particles was observed and measured in the range of Re=0–10. Using the linear model, it is possible to estimate the influence of the tilt angle on the rate of rotation at low Reynolds numbers. Dual particles settle faster than a single particle at small Reynolds numbers but not at higher Reynolds numbers, because of particle separation. The variation of particle settling velocity with Reynolds number is presented. An equation which can be used to estimate the influence of tilt angle on particle settling velocity at low Reynolds number is also derived. 相似文献
17.
Experimental data for a two-dimensional (2-D) turbulent boundary layer (TBL) flow and a three-dimensional (3-D) pressure-driven
TBL flow outside of a wing/body junction were obtained for an approach Reynolds number based on momentum thickness of Re
θ
=23,200. The wing shape had a 3:2 elliptical nose, NACA 0020 profiled tail, and was mounted on a flat wall. Some Reynolds
number effects are examined using fine spatial resolution (Δy
+=1.8) three-velocity-component laser-Doppler velocimeter measurements of mean velocities and Reynolds stresses at nine stations
for Re
θ
=23,200 and previously reported data for a much thinner boundary layer at Re
θ
=5,940 for the same wing shape. In the 3-D boundary layers, while the stress profiles vary considerably along the flow due
to deceleration, acceleration, and skewing, profiles of the parameter
correlate well and over available Reynolds numbers. The measured static pressure variations on the flat wall are similar
for the two Reynolds numbers, so the vorticity flux and the measured mean velocities scaled on wall variables agree closely
near the wall. The stresses vary similarly for both cases, but with higher values in the outer region of the higher Re
θ
case. The outer layer turbulence in the thicker high Reynolds number case behaves similarly to a rapid distortion of the
flow, since stream-wise vortical effects from the wall have not diffused completely through the boundary layer at all measurement
stations.
Received: 9 June 2000/Accepted: 26 January 2001 相似文献
18.
The influence of the shear number on the turbulence evolution in a stably stratified fluid is investigated using direct numerical
simulations on grids with up to 512 × 256 × 256 points. The shear number SK/ε is the ratio of a turbulence time scale K/ε to the shear time scale 1/S. Simulations are performed at two initial values of the Reynolds number Re
Λ= 44.72 and Re
Λ= 89.44. When the shear number is increased from small to moderate values, the nondimensional growth rate γ= (1/SK)dK/dt of the turbulent kinetic energy K increases since the shear forcing and its associated turbulence production is larger. However, a further increase of the
shear number from moderate to large values results in a reduction of the growth rate γ and the turbulent kinetic energy K shows long-time decay for sufficiently large values of the shear number. The inhibition of turbulence growth at large shear
numbers occurs for both initial values of the Reynolds number and can be explained with the predominance of linear effects
over nonlinear effects when the shear number is sufficiently high. It is found that, at the higher initial value of the Reynolds
number, the reduction of the growth rate occurs at a higher value of the shear number. The shear number is found to affect
spectral space dynamics. Turbulent transport coefficients decrease with increasing shear number.
Received 23 June 1998 and accepted 25 February 1999 相似文献
19.
Qiang Li Philipp Schlatter Luca Brandt Dan S. Henningson 《International Journal of Heat and Fluid Flow》2009,30(5):916-929
A direct numerical simulation (DNS) of a spatially developing turbulent boundary layer over a flat plate under zero pressure gradient (ZPG) has been carried out. The evolution of several passive scalars with both isoscalar and isoflux wall boundary condition are computed during the simulation. The Navier–Stokes equations as well as the scalar transport equation are solved using a fully spectral method. The highest Reynolds number based on the free-stream velocity U∞ and momentum thickness θ is Reθ=830, and the molecular Prandtl numbers are 0.2, 0.71 and 2. To the authors’ knowledge, this Reynolds number is to date the highest with such a variety of scalars. A large number of turbulence statistics for both flow and scalar fields are obtained and compared when possible to existing experimental and numerical simulations at comparable Reynolds number. The main focus of the present paper is on the statistical behaviour of the scalars in the outer region of the boundary layer, distinctly different from the channel-flow simulations. Agreements as well as discrepancies are discussed while the influence of the molecular Prandtl number and wall boundary conditions is also highlighted. A Pr scaling for various quantities is proposed in outer scalings. In addition, spanwise two-point correlation and instantaneous fields are employed to investigate the near-wall streak spacing and the coherence between the velocity and the scalar fields. Probability density functions (PDF) and joint probability density functions (JPDF) are shown to identify the intermittency both near the wall and in the outer region of the boundary layer. The present simulation data will be available online for the research community. 相似文献
20.
A numerical evaluation on the effect of sister holes on film cooling effectiveness and the surrounding flow field 总被引:5,自引:0,他引:5
A numerical study was performed to evaluate the effectiveness of the novel sister hole film cooling technique. Two secondary
coolant holes bound the primary coolant hole slightly downstream of its midpoint, intended to minimize the primary vortex
pair and improve cooling performance. An unstructured hexahedral mesh was generated and the realizable k–ε turbulence model with near-wall modeling was used in these simulations. Blowing ratios of 0.2, 0.5, 1.0, and 1.5 were simulated
to evaluate the applicability of sister holes in practical applications. It was found that sister holes significantly improved
cooling performance over the entire computational domain, particularly at high blowing ratios. These results arose by countering
the primary vortex pair with a secondary pair from these sister holes, ultimately maintaining flow adhesion where the coolant
stream would have otherwise separated. 相似文献