2 Literature review on gas turbine performance

2.4.1 Stagnation or total properties

Consider a gas flow with velocity „C‟, temperature „T‟ and enthalpy „h‟. The gas flow has brought to rest adiabatically without any work transfer, and the new enthalpy „h₀‟ is called stagnation (total) enthalpy and the new temperature „T₀‟ is called stagnation (total) temperature. By applying the steady state flow energy equation for the two states, the equation 11 can be obtained.

Equation 11

Equation 12
For the perfect gas,
𝒉=𝑪𝒑 𝑻
Equation 13
Substitute equation 13 to equation 12 the following relationship can be obtained.

Equation 14

2.4.2 Isentropic efficiency - Total to total

An isentropic process is a constant entropy process and it can be proved that any adiabatic and reversible process is an isentropic process. An adiabatic process is one in which no heat is transferred to or from the fluid during the process while in the reversible process, both the fluid and its surroundings can always be restored to their original state (Eastop, et al., 2002), but in reality it is hard to find any reversible process due to losses in the fluid flow path.

Figure 13 represents T-S diagram for the compression and the expansion of the gas turbine. The process 1 to 2S in figure 13 represents isentropic compression while process 1 to 2 represents an actual compression. From the enthalpy difference of the two cases, it is clearly indicated that the amount of energy needed to move a fluid particle from one pressure to another pressure is higher than that of ideal case. (i.e. isentropic case). For the expansion case in figure 13 the process 3 to 4S indicates the isentropic expansion while the process 3 to 4 represents the actual expansion. In the expansion process the amount of work that can be recovered during the expansion is less than the available amount of work (from the enthalpy difference). In order to get the better understanding of the performance of the individual components and their behavior inside the gas turbine the equation 15 is defined as following for compressor.