According to the principle of universality of critical phenomena, systems exhibiting critical-point phase transitions can be grouped into universality classes. Systems within a universality class have the same critical exponents and scaling functions that characterize the thermodynamic behavior asymptotically close to the critical point. Fluids near the critical point are expected to belong to the universality class of 3-dimensional Ising-like systems, i.e. systems with short range forces and a scalar order parameters. The critical exponents of this universality class are known with good accuracy from theoretical calculations.; In this thesis a revised and extended scaled fundamental equation for the thermodynamic behavior of fluids in the critical region is formulated consistent with the theoretical predictions mentioned above. It is a revision and an extension of the scaled fundamental equations used earlier by employing a more general choice of field variables to account for the lack of gas-liquid symmetry and incorporating a correction to the asymptotic scaling law behavior.; It is demonstrated that the thermodynamic properties of steam and of ethylene in the critical region are accurately described by this fundamental equation.
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