Prevention of compressor surge is one of the most important tasks in operation of gas turbine engine. The easiest way to see the phenomena is to show the static and dynamic operation characteristic on the map. Its operation zone will be restricted by the surge limit and, static and transient process must have some margin for it. Surge margin of a static regime is normally chosen during its design stage. Safe operation during part load condition without facing the surge is an indispensable task for control system design and a swift response from the engine is required to avoid it. Effect of rotor moment of inertia, air/gas volumes and heat transfer are factors to cause the transition from the static line. In case a large volume such as heat exchanger exists in the system it will exert a substantial influence to dynamic characteristics. In the present paper, influence of air volume bled from the compressor exit on transient process is investigated with an example of an auxiliary power unit micro-turbine engine. A typical compressor characteristic with scaling was used for the calculation. Turbine mass, pressure ratio, rotation speed, power and moment are calculated based on mass and work conservation. Result from the present study can give a guidance to design the control system. A computer program is developed to calculate the dynamic process using the MathCAD commercial software. Air volume is changed from 0.02 to 6 m3.

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