The basic STIG plant

Steam injection and water injection plants (after Frutschi and Plancherel (11). 

In an experiment to determine the calorific value of the fuel at temperature 1, . and for the same fuel flow the steady flow energy equation would yield 

If the compressor entry temperature T] is the same as the ambient temperature then Eq. (6.3) may be rewritten as 

Combining this equation with Eq. (6.3a) yields the final energy equation for the whole plant as 

Lloyd argues that for a plant with fixed pressure ratio and top temperature, the turbine work output (and hence the net work output) is increased linearly with the. steam quantity 5 that is injected, but the (2n and Qa terms increase more slowly. Thus, the efficiency similarly increases with S. but also more slowly. 

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Fig. 6.2 shows a simplified diagram of the basic STIG plant with steam injection S per unit air flow into the combustion chamber the state points are numbered. Lloyd 2 presented a simple analysis for such a STIG plant based on heat input, work output and heat rejected (as though it were a closed cycle air and water/steam plant, with external heat supplied instead of combustion and the exhaust steam and air restored to their entry conditions by heat rejection). His analysis is adapted here to deal with an open cycle plant with a fuel input/to the combustion chamber per unit air flow, at ambient temperature To, i.e. a fuel enthalpy flux of/7i,o. For the combustion chamber, we may write... 

Eq. (6.16) is essentially the same as Eq. (6.8) for the basic STIG plant which, on reflection, is not surprising. If the states 1,2, 3, 4 and 5 and the steam quantity S are all the same then expressions for the work output, the heat input (or fuel energy supply) and the heat rejected are all unchanged. The total amount of heat transferred from the exhaust is also unchanged, but two separate flows, of air and of water/steam, have been raised in enthalpy before entry to the combustion chamber, rather than one (water/steam) in the earlier analysis. 

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