Effect of Gas Density

A so-called low-field condition in analytical IMS holds as long as EIN is below approximately 2 Td, and the previous discussion is a valid, although approximate, representation of the effect an external electric field has on the transport of ions in gases. 

The relevance of this discussion to IMS is that the ion swarm is moved in the direction of the electric field, but the drift velocities are comparatively small due to the high frequency of collisions and small mean free path (see Frame 10.1). Furthermore, normal processes of diffusion within the ion swarm will cause broadening of the peak width while the ions are moved through the drift tube. These are opposing phenomena that limit the resolution obtainable with a drift tube aud act as practical boundaries for the dimensions of IMS drift tubes. 

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Wilkinson et al. (op. cit.) make the following observation about the effect of gas density on bubble size The fad that the bubble size decreases shghtly for higher gas densities can be explained on the basis of a force balance. ... 

All of these results are expressed graphically in Fig. 10. The equations apply to systems using air and water (or water plus glycerol) over a range of liquid viscosities. The effect of gas density and viscosity are not mentioned by Zhivaikin. The applicability of Equation (81) through (90) to other systems is therefore not known. 

These data indicate that the effect of gas density is important in attempting to achieve extremely low background counting of gases. The lower discriminator can be set to eliminate part of this low energy back-... 

Fig. 14. Effect of gas density on lower particle critical diameter gas-solid fluidized beds [a = 3.0, Cv = /(e), Mg = 0.02 mPas].

J.D. Naber and D.L. Siebers. Effects of gas density and vaporization on penetration and dispersion of diesel sprays. SAE Paper 960034, 1996. 

It is not possible to theoretically predict the conditions for choking to occur. However, many correlations for predicting choking velocities are available in the literature. Knowlton (1986) recommends the correlation of Punwani et al. (1976), which takes account of the considerable effect of gas density. This correlation is presented below ... 

The heat transfer coefficients have been determined by the cited investigators at atmospheric pressure. Industrial experience indicates that this coefficient increases at higher operating pressures to a greater extent than merely due to the effect of gas density on the vapor capacity factor (Fg). At pressures above 1 atm, it is suggested that the heat transfer coefficient be increased by the 0.30 power of the operating pressure expressed in atmospheres. 

Figure 9.5 Effect of gas density on the minimum bubbling point comparison of the results of Jacob and Weimer, 1987 (open squares) with model predictions (solid triangles).

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