A New Approach in Approximating Thermodynamics Properties in Compressed Liquid Region

Currently it is a common practice to use saturated liquid properties to approximate thermodynamics properties of fluids in the compressed liquid region. In this practice it is assumed that specific volume, internal energy, and entropy of fluids in the compressed liquid region are functions of temperature only and pressure practically has very little or no effect on these properties. Therefore, these properties at a given temperature and pressure are approximated by the saturated liquid properties at the given temperature. In the current literature the approximation formula given for enthalpy in the compressed liquid region is expressed as h(T, p) = h_f (T) + v_f (T) [p – p_sat (T)], where the aim of the second term on the right hand side of the equation is to improve the accuracy of the approximation, when pressure is much greater than the saturation pressure. However, in a recent study of thermodynamic properties of water, Kostic has shown that the second term in the equation improves the accuracy of the approximation of the enthalpy only at temperatures below 100 °C. In fact, he has shown that the second term increases the error when the formula is used to approximate the enthalpy of water in the compressed liquid region at intermediate and high temperatures. Kostic’s investigation is expanded in this paper to include substances other than water. The study shows that in many situations pressure has a bigger influence on the internal energy than it does on enthalpy of fluids in the compressed liquids. This paper demonstrates that the current practice of approximating properties of fluids in the compressed liquid region is not accurate at all range of temperatures and pressures. It establishes the range of pressures and temperatures for which the current approximation method could be used with reasonable accuracies. It also proposes a new scheme for the approximation of thermodynamic properties in the compressed liquid region.