Viscosity of a gas

Viscosity. On the assumption that moiecuies interact iiise hard spheres, the viscosity of a gas is... 

Viscosities of Gases Coordinates for Use with Fig. 2-32.. . . Nomograph for Determining a) Absolute Viscosity of a Gas as a Function of Temperature Near Ambient Pressure and (h) Relative Viscosity of a Gas Compared... 

It is seen that the magnitude of (r(opt)) will vary as the square root of the product of the viscosity and diffusivity. As the solute diffusivity and the viscosity of a gas tend... 

The kinematic viscosity of a gas is a function of the pressure, and its dimension is the square of length divided by the time, its unit being m s k... 

Sutherland s equation An equation that allows the effect of temperature on the viscosity of a gas to be determined. 

Thus, the viscosity of a gas would be expected to be a function of temperature but not of pressure. 

Equation (56) clearly shows that the viscosity of a gas is independent of gas pressure. The physical reason for this result is that while lower pressure means fewer jumps between layers, each jump carries more momentum because at lower pressure the mean free path is longer, i.e., A mu = mL du/dz. 

Figure A-5 Nomograph for determining absolute viscosity of a gas near ambient pressure and relative viscosity of a gas compared with air. (Coordinates from Table A-7.) To convert from poise to pascal-seconds, multiply by 0.1. (From Beerman, Meas Control, June 1982, pp 154-157.)...

Supercritical fluid chromatography (SFC) is an intermediate chromatographic technique between GC and HPLC. It depends upon the fact that when a fluid becomes supercritical (both the temperature and pressure are at or above its critical point) it develops some of the solvating properties of a liquid whilst retaining the low viscosity of a gas. Hence, mass transfer (essential to efficient chromatography) is more akin to that of GC than HPLC, but many compounds can be chromatographed at temperatures much lower than what would be required by GC, so some thermally labile compounds are amenable to SFC where they would degrade under GC conditions [28]. 

The simple formula derived for viscosity in Eq. 12.49 predicts that /z should be independent of pressure and should increase as the square root of temperature. It is typically found that the viscosity of a gas is independent of pressure except at high and low pressure extremes. At very high pressure, molecular interactions become more important and the rigid-sphere approximation becomes inappropriate, leading to a breakdown in Eq. 12.49. At very low pressures, the gas no longer behaves like a continuum fluid, and the steady-state flow picture of Fig. 12.1 is no longer valid. Viscosity is usually found experimentally to increase with T faster than the n = 1 /2 power. Consideration of the interaction potential between molecules, as is discussed in the next section, is needed to more closely match the observed temperature dependence of /x. 

The quantity / is usually called the characteristic length of the interaction potential, and is also employed in the more realistic wave-mechanical treatment. Many authors employ the symbol, a, which is equal to /-1. The form of the molecular interaction potential can be determined in terms of a suitable model, from experimental measurements of the temperature dependence of the viscosity of a gas, whence the characteristic length can be estimated. 

It is clear that such significant deflection as that shown in Fig. 1.1 must be related to the properties of the liquid employed. In comparison with liquid, both the density and viscosity of a gas are much smaller, so that such a strong deflection could not be observed with gas jets. However, in principle, it is possible that such deflection phenomena could occur in gaseous single-phase impinging streams, but the degree of deflection may differ greatly from that of liquor ones. 

The viscosity of a gas increases with temperature due to increased kinetic interaction between molecules, which in turn causes increased viscous drag . The viscosity of a liquid decreases with temperature due to increased thermal energy, decreasing the activated barrier for one atom to slip around its neighbors, or perhaps equivalently, the increased thermal en-... 

The rate at which the momentum transfer takes place is dependent on the rate at which the molecules move across the fluid layers. In a gas, the molecules would move about with some average speed proportional to the square root of the absolute temperature since, in the kinetic theory of gases, we identify temperature with the mean kinetic energy of a molecule. The faster the molecules move, the more momentum they will transport. Hence we should expect the viscosity of a gas to be approximately proportional to the square root of temperature, and this expectation is corroborated fairly well by experiment. The viscosities of some typical fluids are given in Appendix A. 

Thus, knowing the viscosity of a gas, the collision diameter a can be easily calculated. 

The viscosity of a gas can be estimated from the expression (Alberty and Daniels, 1979) ... 

According to Periy and Chilton (1973), the relationship of the viscosity of a gas at two different temperatures is given by... 

The following semiempirical relation also predicts the viscosity of a gas mixture within an average deviation of —2% ... 

Using Eq. (10) for the coefficient of viscosity of a gas, we obtain the following relationship between coefficients of thermaf conductivity and of viscosity ... 

Experience has shown that the viscosity of a gas under pressure is more highly correlated with the density than it is with either the temperature or the pressure or a combination of both. This was... 

The viscosities of gases are, in general, considerably lower than those of liquids. Certain generalizations can be made for the effect of pressure and temperature on the viscosity of a gas. For a perfect gas the viscosity increases with temperature. In this respect the behavior of a gas is the reverse of that of a liquid. Furthermore, the viscosity of a perfect gas is independent of pressure. This rather im-expected behavior of gases can be explained on the basis of the Kinetic... 

Fig. 8 Phase diagram showing the triple point and the critical point. The supercritical zone exists at temperatures and pressures above the critical point. In the supercritical zone, the compound has the density and solvating power of a liquid, but the diffusivity and viscosity of a gas, and exists in a single homogeneous phase. Below the critical point, and along the liquid/gas coexistence line, a liquid and gas phase split can be observed visually. At the triple point, solid, liquid, and gas coexist. At temperatures and pressures below the triple point, solid can sublime directly to gas, for example, by freeze-drying.

Finally, note that the above physical properties are strong functions of the temperature but weak functions of the pressure. Interestingly, the viscosity of a gas increases with increasing temperature, while the viscosity of a liquid decreases with an increase in temperature. 

In the experimental arrangement shown in Figure 7.1 at steady state, the net flux is equal to the mean molecular velocity multiplied by the total concentration. According to the kinetic theory, the viscosity of a gas is independent of pressure, while it is expected to vary with the gas composition. Substitution of Eq. (7.14) into Eqs. (7.10) and (7.11), with Vd from Eq. (7.12) yields the expression for the fluxes caused by gradients in both composition and total pressure... 

Gas phase viscosity data, iTq, are used in the design of compressible fluid flow and unit operations. For example, the viscosity of a gas is required to determine the maximum permissible flow through a given process pipe size. Alternatively, the pressure loss of a given flowrate can be calculated. Viscosity data are needed for the design of process equipment involving heat, momentum, and mass transfer operations. The gas viscosity of mixtures is obtained from data for the individual components in the mixture. 

Particle diffusion coefficients are small compared with the kinematic viscosity of a gas (large Schmidt numbers), so the region of the gas flow near the surface from which particles are depleted is usually very narrow. This narrow region, the concentration boundary layer, is very important to particle transport and is discussed in detail. 

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