![]() It may also be observed that the real individual processes diverge from their idealized counterparts e.g., isochoric expansion (process 1-2) occurs with some actual volume change. For example, the following images illustrate the differences in work output predicted by an ideal Stirling cycle and the actual performance of a Stirling engine:Īctual and ideal overlaid, showing difference in work outputĪs the net work output for a cycle is represented by the interior of the cycle, there is a significant difference between the predicted work output of the ideal cycle and the actual work output shown by a real engine. The difference between an idealized cycle and actual performance may be significant. If energy is added by means other than combustion, then a further assumption is that the exhaust gases would be passed from the exhaust to a heat exchanger that would sink the waste heat to the environment and the working gas would be reused at the inlet stage. Although each stage which acts on the working fluid is a complex real device, they may be modelled as idealized processes which approximate their real behavior. The actual device is made up of a series of stages, each of which is itself modeled as an idealized thermodynamic process. For example, as shown in the figure, devices such a gas turbine or jet engine can be modeled as a Brayton cycle. simplifying assumptions are often necessary to reduce the problem to a more manageable form. Thermodynamic cycles may be used to model real devices and systems, typically by making a series of assumptions. Δ U = E i n − E o u t = 0 Heat pump cycles Įxample of a real system modelled by an idealized process: PV and TS diagrams of a Brayton cycle mapped to actual processes of a gas turbine engine For a cycle for which the system returns to its initial state the first law of thermodynamics applies: Process quantities (or path quantities), such as heat and work are process dependent. Whether carried out reversible or irreversibly, the net entropy change of the system is zero, as entropy is a state function.ĭuring a closed cycle, the system returns to its original thermodynamic state of temperature and pressure. If at every point in the cycle the system is in thermodynamic equilibrium, the cycle is reversible. 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. 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. A thermodynamic cycle consists of a linked sequence 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.
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