Guide alphabétique, de la thermodynamique, amplification de chaleur, transfert de masse, et dynamique des fluides
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Vacuum is the state of a gas whose density is less than that of the air at the Earth level. Vacuum has a number of useful properties which find wide use in various fields of science and technology. For instance, the chemical activity of oxygen sharply drops in vacuum during the oxidation process. The surfaces remain clean (without adsorption even of a monolayer of a gas) at very high degrees of rarefaction for several hours, which allows us to carry out the investigations of such surfaces and of various phenomena associated with adsorbed gas molecules. The low number of molecules in the residual gas in vacuum results in the fact that various particles can cover large distances without collisions under vacuum conditions. This is of particular importance for charged particles, electrons, ions, and protons, whose trajectories can be controlled in vacuum with the help of electric and magnetic fields. Such physical phenomena as propagation of sound, heat and mass transfer, which at atmospheric pressure are determined by the processes of interaction of gas molecules, differ essentially with pressure decrease to the extent that the role of these phenomena in the transfer process becomes unimportant.

The vacuum degree is determined from the residual gas pressure. For convenience the entire range of attainable vacuum is subdivided into several subranges (Figure 1.). Also shown here are the main fields of application of vacuum technology.

Vacuum theory is based on the kinetic theory of gases. The collisions of molecules with the wall and their interactions are assumed to be elastic. The equation of state for an ideal gas has the form: pV = NkT. Here p is the pressure, V is the volume, T is the absolute temperature, N is the total number of molecules, and k = 1.38 × 10−23 Pa m3/K is the Boltzmann constant. On considering the momentum transfer when chaotically moving particles strike against the wall, we can obtain another expression for a gas pressure: p = 1/3 mn 2, where m is the mass of the molecule, n is the number of molecules in unit volume, is the root-mean-square velocity of molecules equal to

The kinetic energy of a molecule is determined in this case by the absolute temperature of the gas: E = 3/2 kT. With the help of Maxwell's distribution function, we can show that the mean velocity of molecules is

It is often useful to know the frequency of molecule impacts v with the surface of unit area ν = 1/4 nvmean = p/ .

The mean free path length l determines the length of the molecule travel between the collisions

Here d is the molecule diameter.

The degree of gas rarefaction is characterized by the Knudsen number Kn = λ/L, where L is a characteristic dimension of the system. The discreteness of gas structure already manifests itself at Kn > 0.01, and at Kn 1 the transfer processes in gases are called free-molecular ones.

The Knudsen Number is related to gas-dynamics parameters which are used in calculations of thermal conditions of bodies flying in the upper atmosphere, i.e., to the Mach (Ma) and the Reynolds (Re) numbers. If in the flow around the body the characteristic dimension is the body dimension L, then Kn = 1.25 (Ma/Re), if L is the thickness of the laminar boundary layer, then Kn ~ Ma . Here γ = cp/cv is the adiabatic exponent. Consequently, for Ma/ 1 the gas flow is continuous, for Ma/ 1 it is free-molecular.

The completeness of energy exchange of molecules upon impacts with the wall is characterized by the thermal accommodation coefficient α = (ef - er)/(ef - ew), where ef and er are the energy fluxes of falling and reflected molecules, ew is the flux of energy that would be carried away from the wall for complete energy exchange, i.e., the energy of reflected molecules corresponds to the wall temperature. For air and construction materials α = 0.87-0.97. On collision of gases with a small molecular mass against a surface of specially pickled metals a can be very small. For instance, for helium vapors interacting with tungsten α 0.02. The exchange of a tangential pulse σ = ( ), where is the averaged value of tangential velocity. If σ = 0, the reflection of molecules from the wall is completely mirror-like, if σ = 1, the reflection is diffuse.

The density of thermal flux in free-molecular flow with large Mach numbers is determined from the expression

where r0 and T0 are the density and temperature of the incoming flow, θ is the angle of attack. Hence, the ratio of the wall adiabatic temperature (for qfm = 0) to the flow stagnation temperature : / 2γ/(γ + 1), i.e. the coefficient of restoration of the total temperature in a free-molecular flow is greater than that in a continuous gas flow.

For a strongly cooled body Tw Lt; T0 and α = 1, we can obtain the value of the limiting thermal flux qfm = 1/2ρ0ω3 sin θ, which is half the dissipated energy falling within unit surface.

A simple case of heat exchange in a limited volume is the heat transfer in a motionless gas between two parallel walls when the influence of fringe effect can be neglected. If the walls have the accommodation coefficients α1 and α2 and temperatures T1 and T22, then

In the "Knudsen layer" which is between and 2 thick the boundary effects of velocity jump (slip)velocity jump (slip) and of temperature jump manifest themselves near the wall. Their values can be found from the expressions:

Here Tq, w is the gas temperature near the wall, R is the gas constant, Pr is the Prandtl number. The second term in the first expression shows the influence of a thermomolecular flow, the motion of gas in the direction of a temperature rise. As is seen, the slip and the temperature jump exist at any pressure. As is small at high pressures, the value of and ΔTs, proportional to , are negligibly small compared to the flow velocity value and the overall temperature difference. This is the basis of the zero slip hypothesis in the dynamics of continuous media. The influence of temperature jump on heat transfer, for instance, in the flow around the sphere, can be estimated from the Nusselt number Nu, given the value of Nu0 in its absence 1/Nu = 1/Nu0 + ξ Kn/Pr, where ξ = 2[2/α - 1] [γ/(γ + 1)] is the jump coefficient.

A vacuum system or a vacuum unit is the system connected to Vacuum Pumps and includes vacuum gauges, vacuum accessories, leak detectors and other units.

The maximum attainable rarefaction in the system depends not only on the effective rate of pump evacuation (Figure 1), but also on the rate of leakage into the vacuum system. As the rate of pump evacuation is finite, the reduction of gas leakage becomes the main condition for attaining high vacuum. The latter is the criterion for choosing the construction materials, and also for developing and improvement of elements of demountable vacuum joints, dynamic connection seals, valves, etc., used in such systems.

The leak detectors can be the vacuum gauges themselves or mass spectrometers. In order to determine small leaks through the walls of vacuum elements, or accessories, a plastic casing with test gas is most often used, which embraces separate sections of the unit. The test gas passes through the leak and it is registered by the leak detector.

Nombre de vues : 19523 Article ajouté : 2 February 2011 Dernière modification de l'article : 11 February 2011 © Copyright 2010-2021 Retour en haut de page