# In what all ways ambient temperature affects performance of gas turbines

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Changes in ambient temperature have an impact on full-load power and heat rate, but also on part-load performance and optimum power turbine speed. Manufacturers typically provide performance maps that describe these relationships for ISO conditions. These curves are the result of the interaction between the various rotating components and the control system. This is particularly true for DLN engines. If the ambient temperature changes, the engine is subject to the following effects.

Increased ambient temperature lowers the density of the inlet air, thus reducing the mass flow through the turbine, and therefore reduces the power output (which is proportional to the mass flow) even further. At constant speed, where the volume flow remains approximately constant, the mass flow will increase with decreasing temperature and will decrease with increasing temperature.

The pressure ratio of the compressor at constant speed gets smaller with increasing temperature. This can be determined from a Mollier diagram, showing that the higher the inlet temperature is, the more work (or head) is required to achieve a certain pressure rise. The increased work has to be provided by the gas generator turbine, and is thus lost for the power turbine, as can be seen in the enthalpy-entropy diagram. At the same time NGG,corr(ie the machine Mach number) at constant speed is reduced at higher ambient temperature. The inlet Mach number of the engine compressor will increase for a given speed, if the ambient temperature is reduced. The gas generator Mach number will increase for reduced firing temperature at constant gas generator speed.

Enthalpy-entropy diagram for the Brayton

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The Enthalpy-Entropy Diagram describes the Brayton cycle for a two-shaft gas turbine. Lines 1 to 2 and 3 to 4 must be approximately equal, because the compressor work has to be provided by the gas generator turbine work output. Line 4-5 describes the work output of the power turbine. At higher ambient temperatures, the starting point 1 moves to a higher temperature. Because the head produced by the compressor is proportional to the speed squared, it will not change if the speed remains the same. However, the pressure ratio produced, and thus the discharge pressure, will be lower than before. Looking at the combustion process 2 to 3, with a higher compressor discharge temperature and considering that the firing temperature T3 is limited, we see that less heat input is possible, ie., less fuel will be consumed .

The expansion process has, due to the lower p2 = p3, less pressure ratio available or a larger part of the available expansion work is being used up in the gas generator turbine, leaving less work available for the power turbine.