I was looking at "Properties of Compressed Liquid Water" (Table A-5) in Fundamentals of Engineering Thermodynamics by Moran. Table A-5 shows that the specific internal energy $u$ of compressed liquid water decreases as as the pressure increases at a fixed temperature. For instance, $u=418.24$ kJ/kg at $T = 100$ F and $P = 2.5$ MPa. While $u=412.08$ kJ/kg at $T = 100$ F and $P = 25$ MPa. Also, this same trend in specific internal energy is shown in Table 3. Compressed Water and Superheated Steam. This trend that the specific internal energy decreases with increases pressure goes completely against my intuition and the numerous examples of compressing a gas or two phase mixture in a cylinder with a piston. Any thoughts on how the internal energy is decreasing?
1 Answer
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If dealing with a gas, the internal energy must be reduced with increasing pressure, as the temperature will be constant and the gas will be more compact. Also see the P-T diagram of water, when changing pressure keeping the temperature constant, the vapor phase will change to liquid phase or solid phase which have less energy than a vapor phase.
