Thermodynamic cycles
Enlarge text Shrink text- Work cat.: Nyland, Ted W. High-temperature transient pressure transducer for use in liquid-metal systems, 1969:p. 1-2 ("Recent efforts to develop Rankine cycle space power systems have created new requirements for instrumentation ... The pressure measurement systems that meet these requirements are severely limited in frequency response ... Several areas of study growing out of the Rankine cycle space power development program need transient pressure data ... Also, in the design of automatic controls for Rankine cycle power systems, the transient behavior must be measured so that overall system stability can be assured ...")
- McGraw-Hill dictionary of scientific and technical terms, 2003:p. 2134 ("thermodynamic cycle - (thermo[dynamics]) - A procedure or arrangement in which some material goes through a cyclic process and one form of energy, such as heat at an elevated temperature from combustion of a fuel, is in part converted to another form, such as mechanical energy of a shaft, the remainder being rejected to a lower temperature sink. Also known as heat cycle")
- Academic Press dictionary of science and technology, 1992:p. 2204 ("Thermodynamic cycle - thermodynamics - a series of processes that occur in a cyclic pattern acting on a substance in which one form of energy is converted to another")
- McGraw-Hill encyclopedia of science and technology, 2002:v. 18, p. 360 (in article "Thermodynamic cycle," caption under illustration, "Comparison of principal thermodynamic cycles. Cycles are, in the order of decreasing efficiency, Carnot cycle ... Brayton cycle ... Diesel cycle ... Otto cycle ...")
A thermodynamic cycle consists of linked sequences of thermodynamic processes that involve transfer of heat and work into and out of the system, while varying pressure, temperature, and other state variables within the system, and that eventually returns the system to its initial state. In the process of passing through a cycle, the working fluid (system) may convert heat from a warm source into useful work, and dispose of the remaining heat to a cold sink, thereby acting as a heat engine. Conversely, the cycle may be reversed and use work to move heat from a cold source and transfer it to a warm sink thereby acting as a heat pump. If at every point in the cycle the system is in thermodynamic equilibrium, the cycle is reversible. Whether carried out reversible or irreversibly, the net entropy change of the system is zero, as entropy is a state function. During a closed cycle, the system returns to its original thermodynamic state of temperature and pressure. Process quantities (or path quantities), such as heat and work are process dependent. For a cycle for which the system returns to its initial state the first law of thermodynamics applies: Δ U = E i n − E o u t = 0 {\displaystyle \Delta U=E_{in}-E_{out}=0} The above states that there is no change of the internal energy ( U {\displaystyle U} ) of the system over the cycle. E i n {\displaystyle E_{in}} represents the total work and heat input during the cycle and E o u t {\displaystyle E_{out}} would be the total work and heat output during the cycle. The repeating nature of the process path allows for continuous operation, making the cycle an important concept in thermodynamics. Thermodynamic cycles are often represented mathematically as quasistatic processes in the modeling of the workings of an actual device.
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