W

Fig. 10-57 Simple Gas Turbine Cycle with Regenerator.

Fig. 10-58 Ideal Regenerator Cycle.

depends not only on pressure ratio, but on the ratio of the minimum to maximum temperature as well. Figure 10-59 illustrates the variation of efficiency with cycle pressure ratio for two values of turbine inlet temperature (T^), and several values of recuperator effectiveness £r The curves labeled &r correspond to the overall efficiency of a simple-cycle gas turbine. As pressure ratios are increased for a given turbine inlet temperature, the compressor outlet temperature is increased. When it is equal to the turbine outlet temperature, there is no heat to return (recuperate) back to the air. This is where the crossover point occurs. Operating conditions to the left of this point (i.e., moderate pressure ratios) allow for efficiency enhancement to be achieved with the use of a recuperator.

The decision to use a regenerator largely depends on whether or not heat recovery is being used and, if so, on the relative values of increased power output versus decreased thermal energy output.

Reheat Cycle: The reheat cycle uses an additional combustor or reheat element in which additional fuel is combusted using the oxygen present in the exhaust gas. The reheat cycle increases the efficiency of the turbine by increasing the average temperature of the gases doing expansion work in the turbine section.

As shown in Figure 10-60, in the reheat cycle, combus-

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