Glossary
to accompany
Thermodynamics: An Engineering Approach, 5th edition
by Yunus A. Çengel and Michael A. Boles53Unrestrained expansion of a gas is the process of the free expansion of gas,
unrestrained by a moving boundary such as the rapid expansion of air from a balloon that
has just been burst.
Unsteady-flow, or transient-flow, processes are processes that involve changes within a
control volume with time.
Useful pumping power is the rate of increase in the mechanical energy of a fluid as it
flows through a pump.
Useful work Wu is the difference between the actual work W and the surroundings work
Wsurr.
Useful work potential is the maximum possible work that a system will deliver as it
undergoes a reversible process from the specified initial state to the state of its
environment, that is, the dead state.
Utilization factor is a measure of the energy transferred to the steam in the boiler of a
steam power plant that is utilized as either process heat or electric power. Thus the
utilization factor is defined for a cogeneration plant as the ratio of the sum of the net
work output and the process heat to the total heat input.
Vacuum cooling is a way to cool a substance by reducing the pressure of the sealed
cooling chamber to the saturation pressure at the desired low temperature and evaporating
some water from the products to be cooled. The heat of vaporization during evaporation is
absorbed from the products, which lowers the product temperature.
Vacuum freezing is the application of vacuum cooling when the pressure (actually, the
vapor pressure) in the vacuum chamber is dropped below 0.6 kPa, the saturation pressure
of water at 0°C.
Vacuum pressure is the pressure below atmospheric pressure and is measured by a
vacuum gage that indicates the difference between the atmospheric pressure and the
absolute pressure.
van der Waals equation of state is one of the earliest attempts to correct the ideal gas
equation for real gas behavior. It is given by
()()P
a
v
+−=vb RT
2where the constants a and b are functions of the critical constants of the gas.
van’t Hoff equation is the expression of the variation of the chemical equilibrium
constant with temperature in terms of the enthalpy of reaction at temperature T.