This paper proposes a modeling method for the 1+1/2 vaneless counter rotating turbine (VCRT) and carries out performance analysis of a micro gas turbine (MGT) with VCRT at off design conditions.
The first task of this paper is to obtain performance characteristics maps and develop a characteristics modeling method of VCRT. The VCRT characteristics maps are obtained through 3-D CFD calculation. High pressure turbine (HPT) corrected rotational speed, shaft rotating speed ratio defined as the high pressure (HP) shaft rotational speed divided by low pressure (LP) shaft rotational speed, and the VCRT total expansion pressure ratio are selected to model a 3-D low pressure turbine (LPT) characteristics maps. However, the HPT characteristics map modeling method remains the same as the conventional one.
An overall performance simulation model is established in Matlab/Simulink and validated by software GasTurb. The VCRT engine consumes fuel at a higher rate when delivering same power compared with the conventional gas turbine due to LPT performance degradation. When the relative LP shaft speed ranges from 100% to 96%, the performance of the VCRT engine and conventional engine is almost equivalent. As the LP shaft speed continues to drop off, however, the VCRT engine performance degrades considerably.
The results indicate that it is crucial to design VCRT with a wide range of efficiency, especially the LPT. The VCRT engine control law also must be effectively optimized to ensure the engine performs well at part load working conditions.