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1.
Jet impingement boiling is very efficient in cooling of hot surfaces as a part of the impinging liquid evaporates. Several
studies have been carried out to measure and correlate the heat transfer to impinging jets as a function of global parameters
such as jet subcooling, jet velocity, nozzle size and distance to the surface, etc. If physically based mechanistic models
are to be developed, studies on the fundamentals of two-phase dynamics near the hot surface are required. In the present study
the vapor–liquid structures underneath a subcooled (20 K) planar (1 mm × 9 mm) water jet, impinging the heated plate vertically
with a velocity of 0.4 m/s, were analyzed by means of a miniaturized optical probe. It has a tip diameter of app. 1.5 μm and
is moved toward the plate by a micrometer device. The temperature controlled experimental technique enabled steady-state experiments
in all boiling regimes. The optical probe data provides information about the void fraction, the contact frequencies and the
distribution of the vapor and liquid contact times as a function of the distance to the surface. The measured contact frequencies
range from 40 Hz at the onset of nucleate boiling to nearly 20,000 Hz at the end of the transition boiling regime. Due to
condensation in the subcooled jet vapor disappears at a distance to the surface of app. 1.2 mm in nucleate boiling. This vapor
layer becomes smaller with increasing wall superheat. In film boiling a vapor film thickness of 8 ± 2 μm was found. 相似文献
2.
Using deioned water as a working fluid, the influence of the microscale effects on liquid flow resistance in microtubes with
inner diameters of 19.6 and 44.2 μm, respectively, is experimentally studied. The temperature rise resulted from the microscale
effects, such as viscous dissipation, electric double layer, wall rough on the wall surface, etc., is obtained by an IR camera
with a special magnified lens adopting micro-area thermal image technology and the corresponding pressure drop and the flux
are also measured, so the relationship among friction factor, temperature rise and Reynolds number is obtained. Investigation
shows that experimental data are almost equal to those of Hagen–Poiseuille when Reynolds number is low. With the increase
of Reynolds number, the values of the friction factor depart from that of classical theory due to the microscale effects.
Moreover, the values of the experimental friction factor considering various microscale effects is the maximal 10–15% deviation
from that of friction factor without considering various microscale effects with further increase of Reynolds number. 相似文献
3.
A quantitative thermometry technique, based on planar laser-induced fluorescence (PLIF), was applied to image temperature
fields immediately next to walls in shock tube flows. Two types of near-wall flows were considered: the side wall thermal
boundary layer behind an incident shock wave, and the end wall thermal layer behind a reflected shock wave. These thin layers
are imaged with high spatial resolution (15μm/pixel) in conjunction with fused silica walls and near-UV bandpass filters to
accurately measure fluorescence signal levels with minimal interferences from scatter and reflection at the wall surface.
Nitrogen, hydrogen or argon gas were premixed with 1–12% toluene, the LIF tracer, and tested under various shock flow conditions.
The measured pressures and temperatures ranged between 0.01 and 0.8 bar and 293 and 600 K, respectively. Temperature field
measurements were found to be in good agreement with theoretical values calculated using 2-D laminar boundary layer and 1-D
heat diffusion equations, respectively. In addition, PLIF images were taken at various time delays behind incident and reflected
shock waves to observe the development of the side wall and end wall layers, respectively. The demonstrated diagnostic strategy
can be used to accurately measure temperature to about 60 μm from the wall. 相似文献
4.
Ultra-lightweight spacecraft incorporating “gossamer” structures are extremely compliant, which complicates control, design,
and ground testing in full scale. One approach to model the behavior of a full-scale gossamer structure is to construct a
small-scale model test article that can be used to verify a corresponding small-scale computer model. Once the predictions
of the computer model have been verified by measurement of the physical test article, it can be scaled up to allow computation
of the full-scale structure behavior. As model verification requires accurate deflection measurements at multiple points along
the surface of the structure, a sensing system that provides full-field data without changing the dynamic response of the
structure must be developed. Hence, an optical approach is taken. Since the thin films used in gossamer space structures are
typically smooth and featureless, targets must be incorporated into the film surface to enable tracking of both in-plane and
out-of-plane displacements. A krypton fluoride excimer laser system was used to etch 35 μm wide linear features approximately
0.1 μm into the surface metallization of both 50.8 μm polyester and 127 μm polyimide films. These optically diffuse surface
features, designed mainly to investigate the precision of the laser etching method, were used as targets for ultra-close-range
photogrammetry, the method chosen for displacement tracking. A force applied to the surface of the etched mirror (test article)
produced in-plane and out-of-plane deformations that were resolved via ultra-close-range photogrammetry. To measure the in-plane
tracking resolution, 1.5 and 3.0 mm circular dots were added (using ink) to the surface of the thin film, and some of these
targets were tracked as the test article was translated on a precision linear stage. In-plane tracking resolution using ultra-close-range
photogrammetry was related to the ground sample distance of the camera, which in this case was 51.25 μm pixel−1 (equal to the ratio of sample dimension to number of pixels in the field of view). Using a manual technique to identify features
of the etched pattern for tracking, the mean tracking error was about 13 μm (σ=43 μm). Using an automated, subpixel marking
technique to identify the 1.5 mm circular targets, the mean tracking error was 22 μm (σ=13 μm). Neither of these methods achieved
the desired 10 μm tracking resolution. 相似文献
5.
Heat transfer coefficients in nucleate pool boiling of binary and ternary non-azeotropic hydrocarbon mixtures were obtained
experimentally using a vertical electrically heated cylindrical carbon steel surface at atmospheric pressure with several
surface roughness. The fluids used were Methanol/1-Pentanol and Methanol/1-Pentanol/1,2-Propandiol at constant 1,2-Propandiol
mole fraction of 30%. Heat fluxes were varied in the range 25–235 kW/m2. The cylindrical heater surface was polished to an average surface roughness of 0.2 μm, and sandblasted yielding surface
roughness of 2.98 and 4.35 μm, respectively. The experimental results were compared to available prediction correlations,
indicating that the correlations based on the boiling range are in better qualitative agreement than correlations based on
the phase envelope. Increasing surface roughness resulted in an increase in the heat transfer coefficient, and the effect
was observed to be dependent on the heat flux and fluid composition. 相似文献
6.
M. A. Sutton N. Li D. Garcia N. Cornille J. J. Orteu S. R. McNeill H. W. Schreier X. Li A. P. Reynolds 《Experimental Mechanics》2007,47(6):789-804
A combination of drift distortion removal and spatial distortion removal are performed to correct Scanning Electron Microscope
(SEM) images at both ×200 and ×10,000 magnification. Using multiple, time-spaced images and in-plane rigid body motions to
extract the relative displacement field throughout the imaging process, results from numerical simulations clearly demonstrate
that the correction procedures successfully remove both drift and spatial distortions with errors on the order of ±0.02 pixels.
A series of 2D translation and tensile loading experiments are performed in an SEM for magnifications at ×200 and ×10,000,
where both the drift and spatial distortion removal methods described above are applied to correct the digital images and
improve the accuracy of measurements obtained using 2D-DIC. Results from translation and loading experiments indicate that
(a) the fully corrected displacement components have nearly random variability with standard deviation of 0.02 pixels (≈25 nm
at ×200 and ≈0.5 nm at ×10,000) in each displacement component and (b) the measured strain fields are unbiased and in excellent
agreement with expected results, with a spatial resolution of 43 pixels (≈54 μm at ×200 and ≈1.1 μm at ×10,000) and a standard
deviation on the order of 6 × 10−5 for each component.
相似文献
M. A. Sutton (SEM member)Email: |
7.
A finite-difference scheme has been developed to solve the equations governing the laminar forced convection heat transfer
around and inside a spherical fluid droplet moving steadily in another immiscible fluid for both steady and transient thermal
conditions. For large values of the external flow Reynolds number (Re), results not available in the literature have been
obtained for circulating droplets at intermediate and high interior-to-exterior viscosity ratios (μ*). Detailed results over a wide range of viscosity ratio (μ*) and for 200≤Re≤1000 are presented for the temperature profiles outside and inside the sphere, Nusselt number, the time required
to attain a uniform surface temperature and the time required to reach the steady-state temperature. Results show that convective
heating is dependent on the external flow Reynolds number (Re) and the interior-to-exterior viscosity ratio (μ*) where increasing Re or decreasing μ* result in increasing heat transfer rate convected to the liquid sphere.
Received on 1 March 1999 相似文献
8.
Using a focal plane array infrared camera, the heat generated during large strain compression (at a rate of 1 s−1) is used to study the characteristics of plastic flow for hcp zirconium. Heat generation during plastic flow in a reference
material, copper, was seen to develop uniformly both at the lower (40 μm/pixel) and higher (8 μm/pixel) magnifications used
in this study. The thermomechanical response of Zr, however, was seen to depend on the loading direction with respect to the
specimen texture. Highly textured zirconium compressed along nonbasal oriented grains results in a homogeneous thermal response
at both scales. However, compression along basal (0001) oriented grains shows evidence of inhomogeneous deformation at small
strains that lead to macroscale localization and failure at large strains. The conversion of plastic work into heat is observed
to be a dynamic process, both in the time-dependent nature of the energy conversion, but also in the passage of waves and
‘bursts’ of plastic heating. Basal compression also showed evidence of small scale localization at strains far below macroscale
localization, even below 10%. These localizations at the lower strain levels eventually dominate the response, and form the
shear band that is responsible for the softening of the macroscopic stress–strain curve. 相似文献
9.
Residual stresses and retained austenite are two important process-related parameters which need to be controlled and monitored
carefully during production and heat treatment of products. X-ray diffraction techniques are normally used in this context,
and the purpose of the present study was to investigate the reproducibility and accuracy of these methods for medium and high
carbon steels. The work was carried out as a round robin study including nine different laboratories in Sweden and Finland.
Stress measurements were carried out on three specimens etched to three different depths, 0 μm, 230 μm and 515 μm. Retained
austenite measurements were carried out on three specimens containing approximately 11, 17 and 30 vol.-% of this phase. The
stress measurements showed good reproducibility with standard deviations of typically 4% on flat and smooth surfaces and not
more than about 8% on etched surfaces. Estimations revealed that specimen misalignment and improper X-ray spot location were
the main sources behind the variation in the stress recordings. The determination of retained austenite showed a standard
deviation of typically 15% between the different contributors. However, by using identical evaluation methods for all raw
data, the data spread could be narrowed by a factor of 3 to 4 despite the fact that different experimental settings were used
in the individual laboratories. 相似文献
10.
Y. H. Huh D. I. Kim D. J. Kim H. M. Lee S. G. Hong J. H. Park 《Experimental Mechanics》2011,51(7):1033-1038
Tensile and fatigue behaviors of the copper film coated by tin (Sn) were investigated considering S-N relationships and scanning
electron microscope (SEM) observation of fracture surfaces. The fatigue behavior was investigated considering the effect of
load ratio, R. The specimen of 2000 μm width, 8000 μm length and 15.26 μm thickness was fabricated by etching process. Tensile
properties were measured using the micro-tensile testing system and in-plane electronic speckle pattern interferometric (ESPI)
system for measuring the tensile strain during the test. The fatigue tests of the film were carried out in load-control mode
with 40 Hz at three different stress ratios of 0.05, 0.3 and 0.5. The S-N curves, including the slope of the curve and fatigue
limit, at the respective stress ratios were obtained. These curves were dependent on the load ratio. Empirical relationships
indicating the dependency of the fatigue limit and S-N curve on the load ratio were suggested in this study. SEM observation
of the tensile fracture surface showed that the cross-sectional area of the testing section was necked in the direction of
the film thickness (i.e. parallel to the substrate surface normal) and some ductile dimples in the fracture surface were present.
The fracture of the copper film under cyclic loading was progressed in the transgranular fracture mode. 相似文献
11.
Tailoring Beam Mechanics Towards Enhancing Detection of Hazardous Biological Species 总被引:1,自引:0,他引:1
Microcantilever based sensors have been widely employed for measuring or detecting various hazardous chemical agents and biological
agents. Although they have been successful in detecting agents of interest, researchers desire to improve their performance
by enhancing their mass sensitivity towards developing “detect to warn” detection capabilities. Moreover, there has been little
work aimed at tailoring beam mechanics as a means to enhance mass sensitivity. In this paper, a numerical study is performed
to assess the influence of microcantilever geometry on mass sensitivity in order to improve these devices for better detection
of hazardous biological agents in liquid environments. Modal analysis was performed on microcantilevers of different geometries
and shapes using ANSYS software and compared to the basic rectangular shaped microcantilever structures employed by most researchers.
These structures all possessed a 50 μm length, 0.5 μm thickness and 25 μm width where the cantilever is clamped to the substrate,
and were analyzed for their basic resonance frequency as well as the frequency shift for the attachment of a 0.285 pg of mass
attached on their surfaces. These numerical results indicated that two parameters dominate their behavior, (1) the effective
mass of the cantilever at the free end and (2) the clamping width at the fixed end. The ideal geometry was a triangular shape,
which minimized effective mass and maximized clamping width, resulting in an order of magnitude increase in mass sensitivity
(1,775 Hz/pg) over rectangular shaped cantilevers (172 Hz/pg) of identical length and clamping width. The most practical geometry
was triangular shaped cantilever with a square pad at the free end for capturing the agent of interest. This geometry resulted
in a mass sensitivity of 628 Hz/pg or nearly a 4-fold increase in performance over their rectangular counterparts. 相似文献
12.
The Characterization of the effects of surface wettability and geometry on pressure drop of slug flow in isothermal horizontal
microchannels is investigated for circular and square channels with hydraulic diameter (D
h
) of 700 μm. Flow visualization is employed to characterize the bubble in slug flow established in microchannels of various
surface wettabilities. Pressure drop increases with decrease in surface wettability, while the channel geometry influences
slug frequency. It is observed that the gas–liquid contact line in advancing and receding interfaces of bubble change with
surface wettability in slug flows. Flow resistance, where capillary force is important, is estimated using Laplace–Young equation
considering the change of dynamic contact angles of bubble. The experimental study also demonstrates that the liquid film
presence elucidates the pressure drop variation of slug flows at various surface wettabilities due to diminishing capillary
effect. 相似文献
13.
Impact of water drops on a stainless steel surface comprising rectangular shaped parallel grooves is studied experimentally.
Geometric parameters of the surface groove structure such as groove depth, groove width and solid pillar width separating
any two successive grooves were kept at 7.5, 136 and 66 μm, respectively. The study was confined to the impact of drops in
inertia dominated flow regime with Weber number in the range 15–257. Experimental results of drop impact process obtained
for the grooved surface were compared with those obtained for a smooth surface to elucidate the influence of surface grooves
on the impact process. The grooves definitely influence both spreading and receding processes of impacting liquid drops. A
more striking observation from this study is that the receding process of impacting liquid drops is dramatically changed by
the groove structure for all droplet Weber number. 相似文献
14.
An innovative technique for measuring both the shape, the displacement, the strain and the temperature fields at the surface
of an object using a single stereovision sensor is proposed. The sensor is based on two off-the-shelf low-cost high-resolution
uncooled CCD cameras. To allow both dimensional and thermal measurements, the sensor operates in the visible and near infrared
(NIR) spectral band (0.7–1.1 μm), and a radiometric and geometric calibration of the sensor is required. This technique leads
to a low-cost camera-based simplified instrumentation that gives simultaneously dimensional/kinematical and thermal field
measurements. 相似文献
15.
Novel accelerator applications favor free-surface liquid–metal flows, in which the liquid acts both as a target producing
secondary particles but also to remove efficiently the heat deposited. A crucial aspect for the operation is the continuous
monitoring of both shape and position of the liquid’s surface. This demands, in a nuclear environment, a non-intrusive measurement
technique with high temporal and spatial resolution. In this context, the double-layer projection (DLP) technique based on
geometric optics has been developed, allowing one to detect either point-wise or area-wise the shape and position of the nearly
totally reflecting liquid–metal surface. The DLP technique employs a laser beam projected through a coplanar glass plate to
the surface from which it is reflected to the plate again. Beam locations captured by means of a camera permit the position
and shape of the surface to be reconstructed. The parameters affecting the resolution and performance of the DLP technique
are discussed. Additionally, validation studies using static and moving objects of pre-defined shape are conducted, exhibiting
spatial and temporal resolutions of 300 μm and 100 Hz, respectively. Finally, the DLP system is applied to perform measurements
of a circular hydraulic jump (CHJ) in a liquid metal. The DLP system has proved the capability to measure the jump both qualitatively
and quantitatively. Additionally, the experiments identified, at high Reynolds numbers, the existence of a two-step jump.
The analysis of spectral data of the DLP surface measurements shows clearly that, at the outer radius, gravity waves occur.
Also, contributions from the pump oscillations were found, demonstrating the high performance of the DLP system. 相似文献
16.
The turbulent fluid and particle interaction in the turbulent boundary layer for cross flow over a cylinder has been experimentally
studied. A phase-Doppler anemometer was used to measure the mean and fluctuating velocities of both phases. Two size ranges
of particles (30μm–60μm and 80μm–150μm) at certain concentrations were used for considering the effects of particle sizes
on the mean velocity profiles and on the turbulent intensity levels. The measurements clearly demonstrated that the larger
particles damped fluid turbulence. For the smaller particles, this damping effect was less noticeable. The measurements further
showed a delay in the separation point for two phase turbulent cross flow over a cylinder.
The project supported by the National Natural Science Foundation of China 相似文献
17.
A laser-induced fluorescence (LIF) technique is described to measure vertical concentration profiles of gases in the aqueous
mass boundary layer at a free water surface. The technique uses an acid-base reaction of the fluorescence indicator fluorescein
at the water surface to visualize the concentration profiles. The technique is capable of measuring two-dimensional vertical
concentration profiles at a rate of 200 frames/s and a spatial resolution of 16 μm. The mass boundary layer at a free surface
is characterized by significant fluctuations. Direct surface renewal is observed. The mean profiles also support rather surface
renewal models than turbulent diffusion models.
Received: 21 May 1997/Accepted: 18 December 1997 相似文献
18.
V. V. Pukhnachev 《Journal of Applied Mechanics and Technical Physics》2007,48(3):310-321
The equilibrium of a free weightless liquid film fixed over a planar contour and acted upon by thermocapillary forces is studied.
Trends in the behavior of free liquid films are important for understanding the processes occurring in foams. The equilibrium
equations for a nonisothermal weightless free film are derived for the two limiting cases: the temperature of the film is
considered a known function of the coordinates; the free surface of the film is thermally insulated. For the plane and axisymmetric
cases, the existence conditions for the solutions of the resulting nonlinear boundary-value problems are found and their properties
are studied. For the general case, an approximate solution of the equilibrium problem is obtained provided that the analogue
of the Marangoni number is small.
__________
Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 3, pp. 16–29, May–June, 2007. 相似文献
19.
An experimental study about the relative merits of using a high-speed digital-acquisition system to measure directly the strain-gage
resistance rather than using a conventional Wheatstone bridge, is carried out.
Both strain gages, with a nominal resistance of 120 Ω and 1 kΩ, are simulated with precision resistors and the output signals
were acquired over a time of 48 and 144 hours; furthermore, the effects in metrological performances caused by a statistical
filtering are evaluated.
The results show that the implementation of the statistical filtering gains a considerable improvement in gathering straingage-resistance
readings. On the other hand such a procedure causes, obviously, a loss of performance with regard to the acquisition rate,
therefore to the dynamic data-collecting capabilities. In any case the intrinsic resolution of the 12-bit a/d converter, utilized
in the present experimental analysis, causes a limitation for measurement accuracy in the range of hundreds μ m/m.
Paper was originally presented at the 1991 SEM Spring Conference on Experimental Mechanics held in Milwaukee, WI on June 9–13. 相似文献
20.
High-speed tomographic PIV was used to investigate the coalescence of drops placed on a liquid/liquid interface; the coalescence
of a single drop and of a drop in the presence of an adjacent drop (side-by-side drops) was investigated. The viscosity ratio
between the drop and surrounding fluids was 0.14, the Ohnesorge number (Oh = μd/(ρdσD)1/2) was 0.011, and Bond numbers (Bo = (ρ
d
− ρ
s
)gD
2/σ) were 3.1–7.5. Evolving volumetric velocity fields of the full coalescence process allowed for quantification of the velocity
scales occurring over different time scales. For both single and side-by-side drops, the coalescence initiates with an off-axis
film rupture and film retraction speeds an order of magnitude larger than the collapse speed of the drop fluid. This is followed
by the formation and propagation of an outward surface wave along the coalescing interface with wavelength of approximately
2D. For side-by-side drops, the collapse of the first drop is asymmetric due to the presence of the second drop and associated
interface deformation. Overall, tomographic PIV provides insight into the flow physics and inherent three-dimensionalities
in the coalescence process that would not be achievable with flow visualization or planar PIV only. 相似文献