Cooling reversible cycles

Thus the cooled reversible cycle [CHT]rci with a first rotor inlet temperature, Tj, will have an internal thermal efficiency exactly the same as that of the uncooled cycle [CHTJru with a higher turbine entry temperature 3 = Tr, and the same pressure ratio. There is no penalty on efficiency in cooling the turbine gases at entry but note that the specific work output, w = (wj — wc)/CpT = [(0 /x) — 11(j — 1), is reduced, since 0 < 0. 

Fig. 4.4. Temperature-entropy diagram for multi-step cooling—reversible cycle 1CHT]r< m (after Ref. [5 ).

The nomenclature introduced by Hawthorne and Davis [4] is adopted and gas turbine cycles are referred to as follows CHT, CBT, CHTX, CBTX, where C denotes compressor H, air heater B, burner (combustion) T, turbine X, heat exchanger. R and I indicate reversible and irreversible. The subscripts U and C refer to uncooled and cooled turbines in a cycle, and subscripts 1,2, M indicate the number of cooling steps (one, two or multi-step cooling). Thus, for example, [CHT] C2 indicates an irreversible cooled simple cycle with two steps of turbine cooling. The subscript T is also used to indicate that the cooling air has been throttled from the compressor delivery pres.sure. 

A reversible cycle with turbine expansion split into two steps (high pressure, HP, and low pressure, LP) is illustrated in the T, s diagram of Fig. 4.3. The mass flow through the heater is still unity and the temperature rises from T2 to Tt, = Tq hence the heat supplied (3b is unchanged, as is the overall isentropic temperature ratio (x). But cooling air of mass flow i//H is used at entry to the first HP turbine (of isentropic temperature ratio. xh) and additional cooling of mass flow is introduced subsequently into the LP turbine (of isentropic temperature ratio Xl)- The total cooling flow is then i/( = i/ h + >h.-... 

For the various reversible cycles described in Section 4.2.1, the thermal efficiency was the same, independent of the number of cooling. steps. This is not the case for the irreversible cycles described in this section. Both the thermal efficiency and the turbine exit temperature depend on the number and nature of cooling steps (whether the cooling air is throttled or not). 

Winter heating items fitted within room air-conditioners may be electric resistance elements, hot water or steam coils, or reverse cycle (heat pump). One model of water-cooled unit operates with a condenser water temperature high enough to be used also in the heating coil. 

The heat reclaim packaged unit system comprises water-cooled room units with reverse cycle valves in the refrigeration circuits. The water circuit is maintained at 21-26°C, and may be used as a heat source or sink, depending on whether the individual unit is heating or cooling. (See Figure 28.11.)... 

Importantly, the purple color is completely restored upon recooling the solution. Thus, the thermal electron-transfer equilibrium depicted in equation (35) is completely reversible over multiple cooling/warming cycles. On the other hand, the isolation of the pure cation-radical salt in quantitative yield is readily achieved by in vacuo removal of the gaseous nitric oxide and precipitation of the MA+ BF4 salt with diethyl ether. This methodology has been employed for the isolation of a variety of organic cation radicals from aromatic, olefinic and heteroatom-centered donors.174 However, competitive donor/acceptor complexation complicates the isolation process in some cases.175... 

A reversible cycle also can be completed in three steps, such as isothermal expansion (at from V to V2, cooling (at constant V2) from 2 to Ti, and adiabatic compression back to the initial state. 

Now if the high-temperature reservoir is cooled until it reaches do, the temperature of the cold reservoir, then the cycle becomes an isothermal cycle, and no work can be produced. Since it is a reversible cycle, W = 0, and so 0 = aOo + Qohence, go = — ... 

Fig. 24.2 Transfer of entropy 5t in a reversible cycle from a cold to a warm reservoir. Changes of volume are indicated by arrows (initial state contour line solid, final state contour line dashed). More heat Q flows off with the entropy St than in Qout > l2m. even though the body completely reverts to its initial state after every cycle and does not cool down at all. This means that energy is emitted as heat, which was not present in that form before but is generated. The question remains what phase of the process does this happen in and how ...

From differential scanning calorimetric measurements a marked cooling-heating cycle hysteresis has been observed, showing that water encapsulated in AOT reversed micelles is only partially freezable and that the freezable fraction displays marked supercooling behavior as a consequence of the very small size of the micellar core. The nonfreezable fraction has been identified as the water hydrating the AOT ionic heads [56,57]. 

Carry out the cyclic line integral of dwrev for 1.000 mol of an ideal gas, using the following reversible cycle Starting with T = 500.0 K and V = 20.01, the system is expanded at constant temperature to a volume of 40.01. The system is cooled at constant volume to 300.0 K. The system is then compressed to a volume of 20.01 at a constant temperature of300.0 K. It is finally heated at constant volume to 600.0 K. 

Fig. 2. The shape-memory process, where Tis temperature, (a) The cycle where the parent phase undergoes a self-accommodating martensite transformation on cooling to the 24 variants of martensite. No macroscopic shape change occurs. The variants coalesce under stress to a single martensite variant, resulting in deformation. Then, upon heating, they revert back to the original austenite crystallographic orientation, and reverse transformation, undergoing complete recovery to complete the cycle, (b) Shape deformation. Strain recovery is typically ca 7%.

---