Effect of pressure on viscosity

The classic Barus equation describes the effects of pressure on viscosity 5... 

Several models that include the effect of pressure on viscosity are outlined herein. For applications at high pressures, one may also require estimates of the viscosity of liquid hydrocarbons and their mixtures with dissolved gases (such as with CO2, Nj, H2S, etc.) because, due to the high solubility of such gases in hydrocarbon mixtures at elevated pressures, there is a very large reduction in the mixture viscosity. Indeed, such behavior is part of the basis for enhanced oil recovery by miscible gas injection. Even though the effect of dissolved gases is beyond the scope of this chapter, some comments about this are included due to the importance of this subject. 

The effect of pressure on viscosity is relatively insignificant in most polymer processing operations, where pressures generally do not exceed 35 MPa (5000 psi). It has been found, however, that the effect of pressure on viscosity becomes quite significant at pressures substantially above 35 MPa. In fact, in careful rheological measurements, the effect of pressure on both viscosity and density has to be considered even at pressures around 35 MPa. 

Special rheometers have been constructed to measure the effect of pressure on viscosity. Various workers have presented data on the pressure dependence on viscosity [33-40]. The viscosity as a function of pressure is generally written as ... 

If the capillary rheometer is used to compare different polymers, it is not necessary to go through the various correction procedures. However, if one wants to know the absolute values of the viscosity, it is important to apply the various correction factors. The most important corrections are the correction of the shear rate for non-Newtonian fluid behavior (often referred to as Rabinowitsch correction) and the correction of the shear stress for entrance effects (often referred to as Bagley correction). These are the most common corrections applied to capillary rheometers. Other corrections that are sometimes considered are corrections for viscous heating, corrections for the effect of pressure on viscosity, corrections for compressibility, correction for time effects, etc. If many corrections are applied to the data, the whole measurement and data analysis procedure can become very complex and time consuming. 

Second, a comparatively simple method was developed for arriving at a first approximation to the limiting curve for E.H.L. conditions where the flow In the film may be considered not only Isothermal but even Isovlscous, l.e. In that the effects of pressure on viscosity may be Ignored. The curve thus obtained was Indeed depicted In Peppler s, or say the discusser s, figure. 

The effect of pressure on viscosity is such that as the pressure goes up, the viscosity increases however the amount is quite small for the situations normally encountered in everyday life. A useful rule-of-thumb is to remember that for most single-phase, organic liquids,... 

Currently, only five thermal conductivity and six viscosity mixed gas data points have been identified in the literature for xenon mole fractions ranging from 0.1 to 0.4 with temperatures greater than 390K (the lowest HeXe temperature in the SNPP), all at atmospheric pressure. Unless further literature review were to provide substantial additional data, it would be necessary to recommend obtaining additional experimental data against which to benchmark the recommended mixture method. In addition, the effects of pressure on viscosity and thermal conductivity and the deviation of density and specific heat from ideal gas behavior need to be assessed for the ranges of Prometheus system temperature and pressure. 

In the future, the possibility of combiiung measurements in the rheometer with RFM simulations will make it possible to define more accurately the effects of pressure on viscosity by means of optimization algorithms that determine the rheological parameters that best fit the simulated results with experimental data. 

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