INVESTIGATIONS ON THREE-DIMENSIONAL COUPLED FLOW OF SECONDARY AIR SYSTEM AND MAIN FLOW PASSAGES IN A MICRO GAS TURBINE

The secondary air system is important to the safe operation of a gas turbine. An appropriately designed secondary air system should be able to seal the rotor-stator rim, deliver coolant for the cooling of blades and disks and balance the axial force by using the least amount of leakage air, so that the penalty on the gas turbine performance can be minimal. A three-dimensional CFD analysis of the coupled flow between the secondary air system and the main passages of compressor and turbine in a micro gas turbine was presented in this paper. The full computational domain was divided into two subsystems. The first sub-system includes one blade passage of both the centrifugal compressor impeller and the inlet guide vane, one period (same as the impeller period) of the impeller back cavity and the shaft seal. The second sub-system includes the compressor discharge leakage gap, the guiding hole for the secondary air, the collecting cavity behind the back bearing seal, two balance holes connecting the shaft seal behind the impeller and the collecting cavity, one period of turbine rotor surrounding fluid path including radial labyrinth seal, turbine rotor/stator cavity, one turbine rotor flow passage and one period of turbine rotor back cavity. The two sub-systems are connected through the shaft seal at the impeller back cavity and the balance holes. Computations were carried out separately for the two sub-systems and iterated until the mass flow rate and the averaged static pressure at the sub-system interface are balanced. Computations for two working conditions of full load and idle were performed. The results show that at both working conditions the impeller back cavity has negative leakage mass flow rate. The turbine stator-rotor rim is well sealed. There is no hot gas ingestion from the turbine main passage to the secondary air system at both working conditions. The axial forces are under feasible level.