Viscosity, critical reduced

Figure 7.11 shows the effect of temperature and pressure changes on viscosity. The reduced viscosity fir = fcIfic is the viscosity at pressure P and temperature T divided by the viscosity at the critical point, fic- This quantity is shown as a function of reduced temperature Tr = T/Tc and reduced pressure Pr = P/Pc-... 

Figure 6. The critical Weber number for a given viscosity ratio, reducing the interfacial tension increases the Weber number, lowering the energy needed to cause droplet breakup. (From [11]. Copyright 1992 American Chemical Society, Washington, DC, USA.)...

The location of the shear thinning region on the reduced stress axis can be characterized by the value of a critical reduced stress Xc (i.e., the reduced stress for which the viscosity reaches the average value between the two Newtonian values). This critical stress increases with concentration up to volume fractions of 0.50 and then decreases to zero at maximum packing (Figure 10.5.2). 

Conventional nitrocellulose lacquer finishing leads to the emission of large quantities of solvents into the atmosphere. An ingeneous approach to reducing VOC emissions is the use of supercritical carbon dioxide as a component of the solvent mixture (172). The critical temperature and pressure of CO2 are 31.3°C and 7.4 MPa (72.9 atm), respectively. Below that temperature and above that pressure, CO2 is a supercritical fluid. It has been found that under these conditions, the solvency properties of CO2 ate similar to aromatic hydrocarbons (see Supercritical fluids). The coating is shipped in a concentrated form, then metered with supercritical CO2 into a proportioning airless spray gun system in such a ratio as to reduce the viscosity to the level needed for proper atomization. VOC emission reductions of 50% or more are projected. 

If critical pressure and critical temperature are given in Pa and K, respectively, viscosities in centipoise result. The variable Io is either the low pressure pure component or mixture viscosity according to whether a pure component or mixture is being considered. For mixtures, simple molar average pseiidocritical temperature (Kay s rule), pressure, and density, and molar average molecular weight are used. The vapor density can be predicted by the methods previously discussed. Errors of above 5 percent are common for hydrocarbons and their mixtures. Experimental densities will reduce the errors slightly. 

The most obvious way to prevent oil whirl is to restrict the maximum rotor speed to less than twice its critical. Sometimes oil whip can be reduced or eliminated by changing the viscosity of the oil or by controlling the oil... 

Figure 10 tt - 8 plot compeirison of effect of viscosity ratio 8 on reduced breakup time tt at critical Wc.ch limit for rotational and irrotational shear [18]. 

Above 1000 Flow through valves may become critical factor Larger pump size selection run at reduced speed-e.g. 3 X size at 1000 cenhistokes running at one-third speed. Modification of valve design may be desirable for higher viscosities... 

When the polar additive nonylic acid was added into hexade-cane liquid, the contact ratio becomes much smaller than that of pure hexadecane, which is shown in Fig. 39. For hexa-decane liquid, the critical speed to reach zero contact ratio is 50 mm/s, which is much higher than that of mineral oil 13604 because of its much lower viscosity. Flowever, when nonylic acid was added into the hexadecane liquid, the critical speed decreased from more than 50 mm/s to 38 mm/s. The same phenomenon can be seen in Fig. 39(h) which shows the comparison of oil 13604 and that added with 1.8 %wt. nonylic acid. The addition of polar additive reduces the contact ratio, too, but its effect is not as strong as that in hexadecane liquid because the oil 13604 has a much larger viscosity. Therefore, it can be concluded that the addition of polar additives will reduce the contact ratio because the polar additives are easy to form a thick boundary layer, which can separate asperities of the two rubbing surfaces. 

---