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Chapter 2
Physical Properties of Fluids in Critical and Pseudocritical Regions

Excerpt

Heat transfer at supercritical pressures is influenced by the significant changes in thermophysical properties at these conditions. For many working fluids, which are used at critical and supercritical conditions, their physical and thermophysical properties are well established (Kirillov 2003). This is especially important for the creation of generalized correlations in non-dimensional form, which allows the experimental data for several working fluids to be combined into one set, as well as the use of numerical solutions (Polyakov 1991). The most significant thermophysical property variations occur near the critical and pseudocritical points (Supercritical Fluids 2002).

In general, at the critical point (Kaye and Laby 1973)

(pV)T=TCT=0
and
(2p2V)T=TCT=0.
At temperatures above the critical temperature, a fluid cannot be liquefied. The supercritical fluid is neither a gas or a liquid, and approximates the behavior of a perfect gas. To account for the actual intermolecular effects at high densities, it is necessary to correct the equation of state and physical properties for non-ideal departure from perfect gas values. The equation of state of a fluid can be accurately expressed by a semi-empirical compressibility factor equation of the form:
z=pVmRT=1+BVm+CVm2+
where R is the molar gas constant and B, C, … are the second and third, virial coefficients, which are functions of temperature only, and higher order correction terms also exist.

  • General
  • Parametric Trends
Topics: Fluids

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