Approximation of Internal Energy and Enthalpy of Fluids in the Compressed Liquid Region

The approximation of thermodynamic properties of compressed liquids at a given temperatures and pressures is currently based on the saturated liquid properties at the given temperatures only. For example, it is a common practice to approximate specific volume, v(T, p), by saturated liquid specific volume, v f (T), the specific internal energy, u(T, p), by saturated liquid specific internal energy, u f (T), the specific entropy, s(T, p), by saturated liquid specific entropy, s f (T), and the specific enthalpy, h(T, p), by h f (T) + v f (T)[p-p sat (T)]. Errors resulting from these approximations will be analyzed in this paper. This paper will show that these approximations are not very accurate at all ranges of temperatures and pressures. The paper will establish limits on the range of pressures and temperatures that these approximations could be used with reasonable accuracies. The paper will also show that the approximations based on constant entropy yield much higher accurate results than those based on a constant temperature. For example, the approximation of u(s, p) by u f (s) is much more accurate that the approximation of u(T, p) by u f (T). The paper will go through analysis showing how the saturated liquid properties could be used to achieve a more accurate approximation of thermodynamic properties in the compressed liquid region.