Cyclic heat engine

Fig. 1.2 shows a gas turbine power plant operating on a closed circuit. The dotted chain control surface (F) surrounds a cyclic gas turbine power plant (or cyclic heat engine) through which air or gas circulates, and the combustion chamber is located within the second open control surface (Z). Heat (2b is transferred from Z to Y, and heat (2a is rejected from Y. The two control volumes form a complete power plant. 

The second law of thermodynamics may be used to show that a cyclic heat power plant (or cyclic heat engine) achieves maximum efficiency by operating on a reversible cycle called the Carnot cycle for a given (maximum) temperature of supply (T ax) and given (minimum) temperature of heat rejection (T jn). Such a Carnot power plant receives all its heat (Qq) at the maximum temperature (i.e. Tq = and rejects all its heat (Q ) at the minimum temperature (i.e. 7 = 7, in) the other processes are reversible and adiabatic and therefore isentropic (see the temperature-entropy diagram of Fig. 1.8). Its thermal efficiency is... 

We rehim now to a cyclic heat engine that takes in heat j G from a heat reservoirat Th, and discards heat Gel to another heat reservoirat 7c. Since the engine operates in cycles, it undergoes no net changes in its properties. The total entropy change of the process is therefore the sum of the entropy changes of the heat reservoirs ... 

If heat is supplied to a steady state or cyclic "heat engine the work output could be used to drive an electric generator for example. If the operation (of thermal motor and electric generator) is ideal, then PAf0ut = PAfin- That is,... 

If Q represents the energy supplied at a temperature Tq to a steady-state or cyclic "heat engine" (Figure 2), it follows rrom an available energy balance that the net rate of available energy flowing from the cycle in the form of shaft work can at most be equal to the thermal available energy supplied to the cycle i.e.,... 

ENTROPY ("HEAT") CONTENT OF CYCLIC HEAT ENGINE SYSTEM... 

If heat could pass spontaneously from a colder body to a hotter body, then a perpetual motion machine of the second kind could be realized by simply making the heat Q2 expelled by a cyclic heat engine to the colder reservoir pass by itself to the hotter reservoir. The result would be the complete conversion of the heat (<2i — Qi) to work. 

Thermal Gradients may be measured or calculated by means of heat flow formulas, etc. After they are established it is likely to be found from the formula that for most cyclic heating conditions the tolerable temperature gradient is exceeded. This means that some plastic flow will result (for a ductile alloy) or that fracture will occur. Fortunately, most engineering alloys have some ductility. However, if the cycles are repeated and flow occurs on each cycle, the ductility can become exhausted and cracking will then result. At this point it should be recognized that conventional room temperature tensile properties may have little or no relation to the properties that control behavior at the higher temperatures. 

It is important first to distinguish between a closed cyclic power plant (or heat engine) and an open circuit power plant. In the former, fluid passes continuously round a closed circuit, through a thermodynamic cycle in which heat ((2b) is received from a source at a high temperature, heat (Qa) >s rejected to a sink at low temperature and work output (IT) is delivered, usually to drive an electric generator. 

Considering an ideal heat engine as the system, the first law as applied to the engine undergoing a series of reversible changes in a cyclical fashion becomes... 

Figure34.13 Cyclic heat load using small tower and large water storage. This is a good idea and sound engineering, but the access panel is on the wrong side (30ft drop to ground level)...

The former is manifested by heat engines of one sort or another, the latter by heat pumps or refrigeration cycles. The three thermodynamic cyclic systems (heat engine, heat pump, and refrigerator) are described in the following subsections. 

The classical approach to the second law is based on a macroscopic viewpoint of properties independent of any knowledge of the structure of matter or behavior of mblecules. It arose from study of the heat engine, a device or machine that produces work from heat in a cyclic process. An example is a steam power plant in which the working fluid (steam) periodically returns to its original state. In such a power plant the cycle (in simple form) consists of the following steps ... 

Figure 6.1 Any cyclic reversible heat engine can be seen as comprising a large number of Carnot s Cycles...

The steam power plant is a large-scale heat engine in wliich the woridng fluid (H2O) is in steady-state flow successively tlirough a pump, a boiler, a turbine, and a condenser in a cyclic process (Sec. 5.2). The working fluid is separated from the heat source, and heat is transferred across a phy sical boundary. In a fossil-fuel-firedplant the combustiongases are separatedfrom the steam by boiler-tube walls. 

The heat engine stuff is given here >n order to help you understand the relationship between heat and work. If it is on the MCAT, it will be explained in a passage. However, don tjust ignore it. It h a possible passage topi a and a good way to learn to understand heat and work. At the very least, know the second law of thermodynamics in terms of heat and woric Heat uanriDt be completely converted to wotked in a cyclical process. 

D is correct. The second law of thermodynamics states that a heat engine cannot have 100% efficiency in converting heat to work in a cyclical process. An air conditioner is a heat engine running backwards. Thus an air conditioner must expel more heat than it takes in when it runs perpetually. A specially made air conditioner could initially cool the room, but to cool the room permanently, it must expel the heat to a heat reservoir. 

The second law of thermodynamics postulates It is impossible for a device operating in a cyclic manner to completely convert heat into work. If the heat flows spontaneously from higher to tower temperature, the opposite process requires a heat engine. Carnot demonstrated that the maximum work from a heat engine is given by a cycle formed by two adiabates and two isotherms whose efficiency is ... 

A "heat engine" is any cyclic device that takes heat from a high-temperature reservoir, does useful work, and expels xmused heat to a low-temperature reservoir. For a specified amount of heat into the engine, show that any real (i.e., irreversible) heat engine always produces less useful work than would a reversible heat engine operating between the same two reservoirs. 

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