A study of Lennard-Jones equivalent analytical relationships for modeling viscosities

An analytical representation of the viscosity-density-temperature relations hip of the Lennard-Jones (LJ) fluid, over wide ranges of temperature and de nsity, is critically assessed and combined with an LJ pressure-density-temp erature equation of state to allow LJ viscosity calculations at a given tem perature and pressure. Both LJ equivalent analytical relationships (EARs) a re accurate. The potential of using an LJ-based model to represent the visc osities of real fluids is evaluated in two steps. First, the qualitative tr ends generated by the two combined LJ EARs are studied. Second, viscosity p redictions for real, relatively simple, fluids are performed. For these, it is assumed that a real fluid behaves as an LJ fluid having a critical temp erature T-c and a critical pressure P-c exactly matching the real-fluid exp erimental values of T-c and P-c. Such an assumption is equivalent to suppos ing that real fluids behave as LJ fluids with effective intermolecular pote ntial parameters consistent with the experimental critical coordinates. The viscosity predictions are based only on molecular weight, T-c, and Pe. The quantitative evaluation is relative to a database of 30 relatively simple compounds including 4 noble gases and the olefinic and aliphatic straight-c hain hydrocarbons through 8 carbon atoms. Conditions for the evaluation ran ged from 0.6 to 3 for reduced temperatures and from 0 to 3 for LJ reduced d ensities. The average error is usually less than 10 % for vapor and supercr itical viscosity and usually less than 25 % for liquid viscosity. In its pr esent form, the methodology is actually a corresponding-states model where the reference fluid is an LJ fluid.