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Thus， the overall efficiency for this system"s cycle can be written as
ηRCYC二(T4 -T5)-(T2 -T1)(2-14)
(T4 -T3)
lncreasing the effectiveness of a regenerator calls for more heat transfer surfacearea， which increases thecost， the pressuredrop， and the space requirements of the unit.
Figure 2-5 shows the improvement in cycle efficiency because of heat recovery with respect to a simple open-cycle gas turbine of 4.33:1 ratio pressureand1，200 oF inlet temperature. Cycle efficiency drops with an increasing pressure drop in the regenerator.
There are two types of heat exchangers Regenerative and Recuperative. The term ""regenerative heat exchanger"" is used for a system in which the heat transfer between two streams is affected by the exposure of a third medium alternately to the two flows. The heat flows successively into andout of the third medium， which undergoes a cyclic temperature. These types of heat exchangers are widely used where compactness is essential. The automotive regenerators consisted of a large circular drum with honey-combed ceramic passages. The drum was rotated at a very low rpm (10-15 rpm). The drum surface was divided into two halves by an air seal. The hot air would pass through one half of the circular drum heating thehoneycombed passages the air would encounter， then the cooler air would pass through these same passages as the drum was rotated and would be heated.

ln a recuperative heat exchanger， each element of heat-transferring surfacehas a constant temperature and， by arranging the gas paths in contra-flow， the temperature distribution in the matrix in the direction of flow is that giving optimum performance for the given heat-transfer conditions. This optimum temperature distribution can be achieved ideally in a con-tra-flow regenerator and approached very closely in a cross-flow regenera-tor.
The matrix permitting the maximum flow per unit area will yield the smaller regenerator for a given thermal and pressure drop performance. Amaterial with a high heat capacity per unit volume is preferred， since this property of the material will increase the switching time and tend to reduce carry-over losses. Another desirable property of the arrangement is low thermal conductivity in the direction of the gas flow. All leakages within the regenerator must be avoided. A leakage of 3% reduces the regenerator effectiveness from 80-71%.
Increasing the Work Output of the Simple Cycle Gas Turbine
The way to enhance the power output of a gas turbine can be achieved by intercooling and reheat.
Intercooling and Reheat Effects. The net work of a gas turbine cycle is given by
Wcyc二 Wt -Wc (2-15)
and can be increased either by decreasing the compressor work or by increasing the turbine work. These are the purposes of intercooling andreheating， respectively.
P
J

K

V
Multi-staging of compressors is sometimes used to allow for cooling between the stages to reduce the total work input. Figure 2-6 shows a polytropic compression process 1-α on the P-.plane. lf there is no changein the kinetic energy， the work done is represented by the area 1-α-j-k-1. A constant temperature line is shown as 1-x. lf the polytropic compressionfrom State 1 to State 2 is divided into two parts， 1-c and d-e with constant pressure cooling to Td二 T1betweenthem， the work done is represented by area 1-c-d-e-I-k-1. The area c-α-e-d-c represents the work saved by means of the two-stage compression with intercooling to the initial temperature. The optimum pressure for intercooling for specified values P1 and P2 is:
POPT二 YP1P2 (2-16)
Therefore， if a simple gas turbine cycle is modified with the compression accomplished in two or more adiabatic processes with intercooling betweenthem， the net work of the cycle is increased with no change in the turbine work.
The thermal efficiency of an ideal simple cycle is decreased by the addition of an intercooler. Figure 2-7 shows the schematic of such a cycle. The ideal simple gas turbinecycle is 1-2-3-4-1， and the cycle with the intercooling added is 1-α-b-c-2-3-4-1. Both cycles in their ideal form are reversible andcan be simulated by a number of Carnot cycles. Thus， if the simple gas turbine cycle 1-2-3-4-1 is divided into a number of cycles like內(nèi)-n-o-p-內(nèi)，
　
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