共查询到20条相似文献,搜索用时 31 毫秒
1.
E. V. Lipnyagov S. A. Perminov G. V. Ermakov B. M. Smolyak 《Thermophysics and Aeromechanics》2009,16(3):445-457
Results of measured average expectancy time of n-pentane and n-hexane boiling-up to the boundary of attainable superheating are presented. Experiments have been carried out in glass capillaries
with substantially decreasing value of the superheated liquid volume for the preset metastable state (p, T = const). Obtained data fail to prove correlation 1 JVῑ = following from the condition of homogeneity and stationarity of the random process resulting in the superheated liquid
boiling-up (J is the frequency of homogenous nucleation, V is the volume of the superheated liquid, ῑ is the average life time). Thus, experiments on superheated liquid boiling-up
kinetics bound with measurements of average life time in glass capillaries cannot serve a proof of the validity of classical
theory of nucleation for superheated liquids since one of the ground conditions — boiling-up homogeneity — is not met. 相似文献
2.
This paper deals with investigation results on crisis phenomena for nonstationary heat release under the conditions of free
convection and in falling liquid films. It is shown that the character of the crisis development and the critical heat flux
for nonstationary heat release significantly depend on the characteristics determining the temperature head of liquid boiling-up.
According to experimental data with an arbitrary temporal function of heating, the character of the dependence between the
critical heat flux and the heat release increasing rate is significantly effected by ready nucleation sites. It is found that
a change in the nonstationary critical heat flux in the range of high times between impulses for periodic heat release is
connected with deactivation of ready nucleation sites on the heat-releasing surface.
According to new experimental results, in the studied range of irrigation degree alteration (Rein = 30–1660), parameters characterizing decay of the falling liquid film with stepped heat release (the distribution of the
time of boiling-up expectation along the liquid film, the velocities of movable boundaries in the boiling-up and drying fronts,
the intensity of liquid ejection from the heat-releasing surface) complexly depend on the Reynolds number, wave characteristics,
and heat flux density.
The text was submitted by the authors in English. 相似文献
3.
Boiling-up kinetics of superheated distilled water and sodium chloride solution in a glass cell at atmospheric pressure and
low superheating of 15–35 °C has been studied far from the boundary of attainable superheating in the area of heterogeneous
nucleation. Temperature dependences of average waiting time of superheated liquids boiling-up have been studied experimentally
under natural conditions and in the ultrasonic field Waiting time of boiling-up at these temperatures reaches 1000 s, and
average time is 600 s. Empirical distribution functions have been found with the use of the waiting time samples obtained
by the method of order statistics. Omega-square goodness-of-fit test has shown that they disagree with exponential distribution
describing stationary random process of supercritical embryo generation separating the system to macroscopic phases. Thus,
it is shown that this random process is not stationary, consequently, nucleation rate to be depending on time. 相似文献
4.
The experimental setup is described and results are presented for measuring average boiling-up lag time for superheated n-hexane mixed with solid structures (activated coal, cellulose, silica gel) as function of temperature under atmospheric pressure.
The “aging” of the cell with the filler was carried out before measurements: this was about 600–1000 boiling events. We developed
a new method for analysing of “aging” procedure: comparison of average flux (frequency) of boiling-up events (processing of
experimental data) and the frequency of nucleation obtained from exponential model. By the end of aging of the cell with silica
gel the average empirical flux reduces by factor of four relative to the “exponential” value. But for activated coal and cellulose
the difference in these fluxes is about 20 %. In all experiments, the event flux was nonstationary. For n-hexane in tested
systems, the margin of superheating was T
n/T
cr ≅ 0.873–0.875, although it was T
n/T
cr ≅ ≅ 0.883 for n-pentane in systems filled by nickel powder (sintered porous nickel with grains of 1.5 or 5.0 micron size) and in the presence
of a smooth copper plate. The average time of boiling-up lag in n-hexane at low normalized temperatures was also smaller than
for n-pentane. For all systems, the lag time is almost the same for the temperature range T
n/T
cr ≅ 0.860–0.874 (plateau). Thus, a smaller amount of superheated liquid or its division into smaller liquid elements does not
result in longer liquid lifetime for superheat liquid and the maximal superheat temperature, as one could expect from the
classical theory of homogeneous nucleation.
Research was supported by the RF Presidential Foundation (NS-905.2003.2) and Russian Foundation for Basic Research (Grant
No. 04-02-16285). 相似文献
5.
This work summarizes the cycle of investigations devoted to determination of the character of superheated liquid boiling-up in a glass capillary at the boundary of the attainable superheating. The brief history of studies is followed by a brief introduction into the theory of homogeneous nucleation. The result of the targeted experiments determining stationarity of a random process of a supercritical embryo generation is considered. From the experiments it may be concluded that the process is unsteady. Based on a large selection of life spans of superheated liquids, the authors have made parametric and nonparametric evaluations of the functions of distribution and dependence of boiling-up frequency on time. The comparison of the obtained results with exponential distribution shows significant differences that also prove the nonstationarity of the studied random process. Special experiments and calculations for evaluation of homogeneity of the superheated liquid boilingup at the boundary of attainable superheating are considered. It is shown that in the experiments with glass capillary, the boiling-up occurs on the wall. As the most convincing evidence of heterogeneity of the superheated liquid boiling-up in a glass capillary the authors provide the results of high-speed video filming in a silylated and clean capillaries. 相似文献
6.
M. Kim K. Kostarev N. Pisarevskaya A. Viviani 《The European physical journal. Special topics》2011,192(1):185-194
The paper presents the results of experimental study of mass transfer in a drop of weakly soluble fluid during its saturation
with a surfactant from its water solution forming a thin (∼ 1 mm) horizontal layer. The use of the interferometer allowed us to visualize the concentration fields and to define the structure
of flows under the conditions of maximum suppression of the buoyancy convection. It has been found that despite the small
layer thickness, in the case of initially homogeneous solution penetration of the surfactant into the drop leads to the formation
of the local density inhomogeneities at both sides of the interface and to the development of a slow gravitational flow. An
increase in the initial concentration of the surfactant in the solution causes a small-scale non-stationary solutocapillary
motion at the drop surface. On the other hand, solubility of the drop in water also increases which leads to the formation
of a counter-diffusion flux and eventually to the disappearance of the interface. In the case of initially inhomogeneous distribution
of the surfactant, a competition between the gravitational and capillary mechanisms of convection gives rise to a large-scale
oscillatory flow around the drop. 相似文献
7.
B. V. Petukhov 《Journal of Experimental and Theoretical Physics》2010,110(1):41-48
A model has been proposed for describing the influence of impurities adsorbed by dislocation cores on the mobility of dislocation
kinks in materials with a high crystalline relief (Peierls barriers). The delay time spectrum of kinks at statistical fluctuations
of the impurity density has been calculated for a sufficiently high energy of interaction between impurities and dislocations
when the migration potential is not reduced to a random Gaussian potential. It has been shown that fluctuations in the impurity
distribution substantially change the character of the migration of dislocation kinks due to the slow decrease in the probability
of long delay times. The dependences of the position of the boundary of the dynamic phase transition to a sublinear drift
of kinks x ∝ tδ (δ σ 1) and the characteristics of the anomalous mobility on the physical parameters (stress, impurity concentration, experimental
temperature, etc.) have been calculated. 相似文献
8.
O. P. Sushkov 《Physics of Atomic Nuclei》2009,72(8):1340-1345
The idea of condensation of the Goldstone π-meson field in nuclear matter had been put forward a long time ago. However, it was established that the normal nuclear density
is too low, it is not sufficient to condensate π mesons. This is why the π condensation has never been observed. Recent experimental and theoretical studies of high-temperature cuprate superconductors
have revealed condensation of Goldstone magnons, the effect fully analogous to the π condensation. The magnon condensation has been observed. It is clear now that quantum fluctuations play a crucial role in
the condensation, in particular they drive a quantum phase transition that destroys the condensate at some density of fermions.
The text was submitted by the author in English. 相似文献
9.
In a companion paper [1], a procedure for solving the short time prediction problem in terms of the transition probability distribution has been theoretically derived, for discrete time-sampled data. Explicit algorithms for estimating the non-stationary moment statistics of arbitrary order also have been derived, based on a generalized difference equation of Fokker-Planck type for the conditional probability distributed function, which is central to the theory. In this paper, evidence for the validity and effectiveness of the proposed method is presented, as obtained not only by means of digital simulation but also by using road traffic noise data obtained experimentally in Hiroshima. For several non-stationary random processes simulated by means of random numbers, the theoretical and experimental conditional probability functions are compared. For non-stationary road traffic noise data the theoretically predicted and experimentally determined confidence intervals are compared; in these comparisons several types of conditional probability function and various values of weighting parameter are used in the algorithm. All of the theoretical results show good agreement with the experimental results. 相似文献
10.
P. A. Pavlov 《Journal of Engineering Thermophysics》2007,16(3):145-154
The procedure of calculation of the dynamics of rapid near-wall vaporization near a metallic heater is considered. A physical
model of explosive boiling-up on bubbles of fluctuation origin is used. The model is limited to regimes when the motion of
bubbles can be ignored and convective flows do not have enough time to develop. It is assumed that the dry spot under a bubble
thermally insulates the heater wall. The heat removal is provided by the vicinity of the wetting line (WL). An analytical
calculation has become possible on the basis of the well-known exact solution to the problem of the temperature field near
the wetting line. Generalization of this solution has led to two new problems. These are the problem of allowance for the
difference of the dynamic wetting angle from the right angle and that of allowance for thermocapillary flows. A numerical
comparison of the results of calculation of the dry area with the use of different methods of allowing for the “tightness”
effect in the dynamics of bubble generation and growth has been made. The evolution of bubbles from their generation to growth
limited by the heat supply has been investigated. It has been found that the transition stage in the development of bubbles
from Rayleigh to thermal ones is of considerable importance in the processes of explosive boiling-up. The dynamics of change
in the dry area and wetting line length prior to the stage of bubble merging into a vapor film has been calculated. A condition
of passage of the heat flux through a maximum is found. The applicability of the idea of thermodynamic crisis to calculations
of miniature devices is justified. The problem of constructing a model to calculate the interphase surface shape near the
wetting line under a developed thermocapillary flow and considerable reactive forces of the vapor flow is formulated. The
model is in good agreement with the results of experiments on pulsed superheating of liquids at a rate higher than 1 K/μs. 相似文献
11.
The specific features of the dielectric spectra of statistical mixtures in the form of heterogeneous systems with spherical
particles chaotically arranged in the space have been investigated. The distribution function of relaxation times f(τ) has been restored. It has been established that the relaxation times are continuously distributed within a wide interval
[τ1, τ2]. Different methods for broadening the relaxation time distribution interval and approximating the relaxation time distribution
function f(τ) have been analyzed. It has been demonstrated that f(τ) is a nonmonotonic function with two maxima at the boundaries and a minimum in the vicinity of the midpoint of the interval
[τ1, τ2]. These features of the relaxation time distribution function are responsible for the large difference between the average
relaxation frequencies of the permittivity and the dielectric loss (electrical conductivity). 相似文献
12.
It is difficult to eliminate the effect of vessel walls and impurities in practical studies of boiling-up of strongly superheated
liquids. Therefore, some doubts are always cast upon validity of experiments and their agreement with the classical theory
of homogeneous stationary nucleation, impairing the verification of theoretical considerations. This paper reports main results
of the theory of homogeneous nucleation and presents formulas for calculating the work of formation of a critical nucleus
by an incline of the accessible superheating boundary on isobars and isotherms. Matching of experimental and theoretical values
of the work of formation was considered as the homogeneous boiling-up criterion. Calculations by isobars and isotherms were
made for some liquids. The theoretical values of the work of formation of a critical nucleus were 1.23–4.5 times higher than
the corresponding empirical values for all the liquids, i.e., they were much higher than the calculation error. Thus, boiling-up
of the studied liquids was not homogeneous and, hence, it did not correspond to the classical theory of nucleation.
The work was financially supported by the President of the Russian Federation (No. NSh-4429.2006.8) and the Russian Foundation
for Basic Research (Grant No. 07-08-00575-a). 相似文献
13.
The relaxation processes (α, β, and γ) in UHMW PE fibres drawn to different draw ratios λ have been studied by dynamic-mechanical and nuclear magnetic resonance methods. The temperature dependences of tensile loss
moduli E″ and spin-lattice relaxation times T
1 have been analyzed assuming distribution of correlation times τ according to the Davidson—Cole function. The activation energies E
a and parameters ε characterizing widths of distribution, and asymptotic value τ
0 for correlation times have been determined from experimental data for low-temperature γ-process and for high-temperature α-process. A weak β-process has been found by dynamic-mechanical method and quantitative analysis was made only for fibres with λ = 9. The temperature dependences of second moment M
2 of the broad-line NMR spectra have been analyzed according to the Gutowsky—Pake formula, which includes only a single correlation
time τ
c without distribution. In this case the activation energies E
a and values of τ
0 have been determined. 相似文献
14.
I. V. Mezentsev N. V. Vernikovskaya Yu. I. Aristov V. A. Mukhin 《Thermophysics and Aeromechanics》2006,13(3):403-410
Experimental results on reversing non-stationary heat transfer are presented for filtration of an air flow through an immobile
heat accumulating medium consisting of lead (D = 2.0, 3.5, and 4.5 mm) and glass (D = 3.2 mm) balls. The studied device imitated the cyclic modes of heat regeneration in the ventilation system for domestic
and office rooms. Dependency between the time of flow switching and Re number was measured. The mathematical model describing
heat transfer between a gas flow and an immobile layer of balls was developed. Good correspondence between the experimental
data and calculation results is observed for high Reynolds numbers. For low Re numbers the effect of heat losses is considerable,
and experimental time of flow switching is shorter than the calculation one.
The work was financially supported by the President of the Russian Federation (Grant No. NSh 6526.2006.3), Russian Foundation
for Basic Research (Grant No. 06-08-00982), Foundation “Global energy” and Program “Energy saving of SB RAS”. 相似文献
15.
A. N. Pavlenko E. A. Tairov V. E. Zhukov A. A. Levin M. I. Moiseev 《Journal of Engineering Thermophysics》2014,23(3):173-193
Results on experimental investigation of the dynamics of boiling-up at stepwise heat release on a horizontally oriented cylindrical surface in a large volume of freon-21 are presented. Experimental data on the propagation velocity, structure, and other local characteristics of development of self-sustained evaporation fronts at different temperature differences of boiling-up in saturated liquid were obtained. New experimental results on the dynamics of vapor phase incipience and evolution on the surface of a vertical heat releasing tube and on the dynamics of changing the heater temperature and pressure in a flow of liquid (water, ethanol) subcooled to saturation temperature in the channel under nonstationary heat release conditions are represented. It was revealed that the dependence of the expectation time of intense bubble growth on the water motion velocity is nonmonotonic. 相似文献
16.
V. S. Pantuev 《Physics of Atomic Nuclei》2008,71(9):1625-1631
We argue that, on the basis of recent experimental data, there is possible existence of a finite formation time of strongly
interacting plasma in nuclear collisions at RHIC. To show this, we construct a simple model based on a Monte Carlo simulation
of nucleus—nucleus collisions with a realistic nuclear density distribution. The most striking feature of the experimental
data—an absence of absorption of high-transverse-momentum pions in the reaction-plane direction for midperipheral collisions—points
to the presence of a surface zone with no absorption and strong suppression in the inner core. A natural interpretation of
such a zone could be the plasma formation time T ≃ 2–3 fm/c. With this assumption, we describe the angular anisotropy of high-transverse-momentum pions with respect to the reaction
plane and the centrality dependence of the nuclear modification factor in Au + Au and Cu + Cu collisions.We present predictions
for LHC.
The text was submitted by the author in English. 相似文献
17.
Summary Cosmic-ray air shower structure functions for the distance dependence of electron density in cosmic-ray air showers in the
size range 104⋎108 have been computed for their intercomparison and comparison with Monte Carlo simulation results and measurements from recent
experiments. The analysis has yielded the present status of theoretical structure functionsvis à vis experimental results and Monte Carlo simulation distributions. The effect of core location error on the lateral distribution
of electrons is also discussed from the point of view of different theoretical and experimental results. The energy flow in
the soft component of air showers of size ∼4·105 has been evaluated within a ring of radius 10m about the axis of the showers. 相似文献
18.
In a wind-blown sand layer, it has been found that wind transport of
particles is always associated with separation of electric charge. This
electrification in turn produces some electrostatic forces in addition to
the gravitational and fluid friction forces that affect the movement of
saltating sand particles, further, the wind-blown sand saltation. To
evaluate this effect quantitatively, this paper presents a simulation of
evolution of wind-blown sand grains after the electrostatic forces exerted
on the grains are taken into account in the wind feedback mechanism of
wind-blown saltation. That is, the coupling interaction between the wind
flow and the saltating sand particles is employed in the simulation to the
non-stationary wind and sand flows when considering fluid drag, gravitation,
and a kind of electrostatic force generated from a distribution of electric
field changing with time in the evolution process of the sand saltation. On
the basis of the proposed simulation model, a numerical program is given to
perform the simulation of this dynamic process and some characteristic
quantities, e.g., duration of the system to reach the steady state, and
curves of the saltating grain number, grain transport rate, mass-flux
profile, and wind profile varying with time during the non-stationary
evolution are displayed. The obtained numerical results exhibit that the
electrostatic force is closely related to the average charge-to-mass ratio
of sand particles and has obvious influence on these characteristic
quantities. The obtained results also show that the duration of the system
to reach the steady state, the sand transport rate and the mass flux profile
coincide well with experimental results by Shao and Raupach (1992) when the
average charge-to-mass ratio of sand particles is 60 μC/kg for the sand
particles with average diameter of 0.25 mm. When the average charge-to-mass
ratios of sand particles are taken as some other certain values, the
calculation results still show that the mass flux profiles are well in
agreement with the experimental data by Rasmussen and Mikkelsen (1998) for
another category of sand particles, which tell us that the electrostatic
force is one of main factors that have to be considered in the research of
mechanism of wind-blown sand saltation. 相似文献
19.
M. Pylak G. Kontrym-Sznajd L. Dobrzyński 《Applied Physics A: Materials Science & Processing》2011,104(2):587-592
A successful application of the Maximum Entropy Method (MEM) to the reconstruction of electron–positron momentum density distribution
in gadolinium out of the experimental of 2D ACAR data is presented. Formally, the algorithm used was prepared for two-dimensional
reconstructions from line integrals. For the first time the results of MEM, applied to such data, are compared in detail with
the ones obtained by means of Cormack’s method. It is also shown how the experimental uncertainties may influence the results
of the latter analysis. Preliminary calculations, using WIEN2k code, of band structure and Fermi surface have been done as
well. 相似文献
20.
V. S. Pantuev 《JETP Letters》2007,85(2):104-108
The possible existence of a finite formation time of strongly interacting plasma in nuclear collisions at RHIC is demonstrated
from recent experimental data. To show this, we use a simple model based on Monte Carlo simulation of nucleus-nucleus collisions
with realistic nuclear density distribution. The most striking feature of the experimental data—an absence of absorption of
high transverse momentum pions in the reaction plane direction for mid-peripheral collisions—points to the presence of a surface
zone with no absorption and strong suppression in the inner core. A natural interpretation of such a zone could be the plasma
formation time T ~ 2–3 fm/c. The existence of a formation time could dramatically change our understanding of many experimentally observed features.
With this assumption, we describe the angular anisotropy of high transverse momentum pions with respect to the reaction plane
and the centrality dependence of the nuclear modification factor in Au + Au and Cu + Cu collisions.
The text was submitted by the author in English. 相似文献