Condensation Coefficient Of Water Research Articles

T0501-2-4 衝撃波管実験と分子気体力学解析を用いた平衡状態近傍での水の凝縮係数の評価(マイクロ・ナノスケールの熱流体現象(2))

The condensation coefficient of water, ie., the ratio of condensation mass flux of vapor molecules to incoming mass flux at the interface, is evaluated by combining the experiment conducted by a shock tube and the numerical simulation of Gaussian-BGK Boltzmann equation applicable to polyatomic gases. Film condensation occurs on the endwall of a vaporfilled shock-tube, when a shock wave is reflected at the endwall and the vapor becomes supersaturated there. The formed liquid film grows with the lapse of time. The time evolution in thickness of the liquid film is measured by an optical interferometer, and thereby the growth rate of the film is obtained. The rate is incorporated into the kinetic boundary condition at the interface for the Gaussian-BGK Boltzmann equation, and the unique numerical solution of the vaporliquid system is obtained. Values of condensation coefficient in near equilibrium state are found to be close to that evaluated by molecular dynamics simulations at the equilibrium state.

Condensation coefficient of water in a weak condensation state

The condensation coefficient of water at a vapor–liquid interface is determined by combining shock tube experiments and numerical simulations of the Gaussian-BGK Boltzmann equation. The time evolution in thickness of a liquid film, which is formed on the shock tube endwall behind the shock wave reflected at the endwall, is measured with an optical interferometer consisting of the physical beam and the reference one. The reference beam is utilized to eliminate systematic noises from the physical beam. The growth rate of the film is evaluated from the measured time evolution and it is incorporated into the kinetic boundary condition for the Boltzmann equation. From a numerical simulation using the boundary condition, the condensation coefficient of water is uniquely deduced. The results show that, in a condition of weak condensation near a vapor–liquid equilibrium state, the condensation coefficient of water is almost equal to the evaporation coefficient estimated by molecular dynamics simulations near a vapor–liquid equilibrium state and it decreases as the system becomes a nonequilibrium state. The condensation coefficient of water is nearly identical with that of methanol [Mikami, S., Kobayashi, K., Ota, T., Fujikawa, S., Yano, T., Ichijo, M., 2006. Molecular gas dynamics approaches to interfacial phenomena accompanied with condensation. Exp. Therm. Fluid Sci. 30, 795–800].

A MOLECULAR DYNAMICS APPROACH TO INTERPHASE MASS TRANSFER BETWEEN LIQUID AND VAPOR

The study was conducted in order to understand a mechanism of interphase mass transfer between liquid and vapor. The molecular dynamics (MD) simulation is used to examine details of condensation and evaporation from the viewpoint of molecular kinetics. First, molecular boundary conditions for condensing, reflecting, and evaporating molecules are presented for an argon molecule as a function of the surface-normal component of translation energy. The velocity distributions can be expressed by the modified Maxwellian and making use of the condensation coefficient. The condensation coefficient of water is also examined for two kinds of intermolecular potential, the Carravetta-Clementi (C-C) model and the extended simple point charge (SPC/E) model, in order to consider the effect of the surface structure of the liquid on the condensation coefficient. The results indicate that the condensation coefficient of water is close to unity for both models and its dependence on the translation energy is small compared with argon. Finally, the condensation coefficient is studied based on the transition-state theory. An evaluation of the transition state is considered by applying the results of MD simulations for argon and water.

Effect of Noncondensable Gas on Experimental Condensation Coefficient

This study is concerned with the discrepancies between the results of experimental measurements and molecular dynamics(MD) studies on the condensation coefficient of water. We consider that the presence of a noncondensing gas may increase the interface resistance and reduce the experimental condensation coefficient. One-dimensional condensing flow of water vapor has been analyzed by the direct simulation Monte Carlo(DSMC) method in the presence of a noncondensing gas. The results show that the condensing flow causes an accumulation of the noncondensing gas in the vicinity of the liquid surface that increases resistance and reduces condensation heat transfer rate. Even if the concentration of the noncondensing gas is low in the bulk phase, the noncondensing gas accumulates at high concentrations at the interface. This indicates the possibility that the low experimental condensation coefficients may be due to the existence of noncondensable gases.

Analysis of the evaporation coefficient and the condensation coefficient of water

The evaporation and condensation coefficients of water are extensively analyzed considering also data hitherto not taken into account. From the performed evaluation, a decline of both coefficients with increasing temperature and pressure is derived. For water, the condensation coefficients is generally higher than the evaporation coefficient. Evaporation and condensation coefficients exceed 0.1 for dynamically renewing water surfaces, while the analysis reveals coefficients below 0.1 for stagnant surfaces. The influence of impurities and surface active substances, as well as the effect of the dynamic surface tension is discussed.

416 高温領域の水の凝縮係数に関する分子動力学的研究

416 高温領域の水の凝縮係数に関する分子動力学的研究

A microscopic study of dropwise condensation

In dropwise condensation most of the heat is transferred by the active droplets whose sizes are below a certain characteristic value so that the thermal conduction resistance inside them may become negligible. In this paper the local heat transfer coefficient is determined experimentally at the very part of the condenser surface which is covered solely by the active droplets. The rate of increase in the condensate volume in a domain newly exposed by coalescence between droplets was analyzed from a high-speed high-magnification cine film record of the condenser surface. By assuming the complete coverage of the surface by droplets, the local heat transfer coefficient obtained in this way was equated to the interfacial heat transfer coefficient. As a result, the condensation coefficient of water was estimated at around 0.5.

Further Developments of Dropwise Condensation Theory

The high rates of heat transfer of dropwise condensation as well as its limits are explained on the basis of the behaviors of submicroscopic active drops. The expression for the substantial growth rate of a single drop valid down to the thermodynamic critical size is incorporated into a set of basic equations from [8] whose capability to describe the process of coalescence and growth of drops in dropwise condensation has been demonstrated in [9]. Consideration of the nondimensionalized forms of the basic equations with the aid of numerical analysis results in an expression of the Nusselt number for dropwise condensation in terms of a few characteristic parameters. Comparison of the predicted Nusselt numbers with available experimental data suggests that the condensation coefficient of water is around 0.2 provided the nucleation site density is infinitely high. Otherwise, if the condensation coefficient should be unity, we have to accept that the nucleation sites are fairly scattered.

The evaporation and condensation coefficient of water, ice and carbon tetrachloride

The evaporation and condensation coefficients of water, ice and carbon tetrachloride were determined in the same experimental apparatus. Both low and high values were obtained for all three materials. An unsteady-state experimental method was used and the high values resulted when the evaporating or condensing times were very short. Surface temperature errors of two different types are shown to be the major factor in contributing to low values. The condensation coefficient of water is at least 0·7 and could easily be 1·0. Condensation coefficients of 0·55 were measured for ice and 0·94 for carbon tetrachloride.

Evaporation and condensation coefficient of water

Experiments were carried out for the determination of the evaporation and condensation coefficient of water. The system used consists of a thin liquid sheet of water flowing at high velocity in a saturated steam atmosphere. Values of the coefficient α are deduced from the interfacial heat-transfer resistance. This is determined by interferometric measurement of the thickness variation in the sheet due to phase change. Experiments at a steam temperature of about 50°C yield the value α = 0·20 ± 0·04. This result, though indicating relatively high values of α, contradicts the claim that α has the magnitude of unity.

The condensation coefficient of water

Filmwise condensation of steam at low pressure on a vertical flat plate was investigated experimentally in order to ascertain the existence of an interfacial heat-transfer resistance and hence deduce a value of the mass accommodation or “condensation” coefficient of water. Data is presented for the condensation of saturated steam between 45 and 50°F at heat fluxes between 9000 and 12000 Btu/h-ft 2 degF. It was found that no significant interfacial resistance was present and the condensation coefficient was deduced to have a value between 0·45 and unity. That the condensation coefficient is at least greater than 0·45 ensures that the interfacial resistance will be negligible in industrial applications of filmwise condensation. The inability to determine a more exact value for the coefficient was due to an inherent limitation in the technique and the range quoted must not be taken to favor an intermediate value.

Condensation Coefficient of Water

THE condensation coefficient of a liquid has been defined1 as the fraction of the number of molecules impinging on a gas–liquid interface that actually condense. It has been measured for water by many workers (for example, refs. 2–4), who have obtained values ranging from 0.003 to 1.0. The possibility that its true value might be quite low is of practical importance in the design of condensers5, and a new determination is, therefore, being made.

Condensation coefficient of water

Measurements were made during rapid condensation of pure steam in a system designed to minimize all thermal resistance in series with the interfacial vapour-liquid thermal resistance. The system consisted of a horizontal copper tube with a grooved outer surface on which condensation occurred and through which cold water flowed at very high velocity. From numerous measurements at steam temperatures of near 10°C and near 50°C, it was possible to calculate values for the condensation coefficient, σ 1, as given by the Hertz-Knudsen equation (see equation 4). The results indicate a probable range for σ 1 of ▪ This corresponds to a value of σ (the fraction of the molecules which “stick” upon striking the liquid surface) of between 0·35 and 1·0.