Behavior of Internal Energy and Enthalpy of Fluids Along Isotherms and Isentropic Lines in the Compressed Liquid Region

It is a common practice to approximate the thermodynamics properties of fluids in the compressed liquid regions from their saturation properties. Most thermodynamics textbooks state that the specific volume, specific internal energy, and specific entropy in the compressed liquid region are functions of temperature only and are independent of pressure. Therefore, compressed liquid property tables are not provided for any substance, except for water, and compressed liquid properties are approximated by their saturated liquid properties at a given temperature. Recent examination of current practice in approximating compressed liquid properties has shown that the internal energy of fluids exhibits growing dependency on pressure with increases in temperature. This paper compares the behavior of internal energy and enthalpy four compressed fluids along isotherms with those behaviors along isentropic lines. Water, ammonia, methane, and propane are examined in this study. It is shown that effects of pressure on the internal energy and enthalpy of compressed liquids are much lower along isentropic lines than those along isotherms.