This paper describes a procedure for assessing the efficiency of a turbine operating under unsteady periodic flow at the inlet. Time-resolved flow simulations of the modified Garrett turbine case A3K7 are conducted with time-varying inlet conditions upstream of the turbine. A mass-weighted moving average is applied to the instantaneous data over a period corresponding to the pulsing frequency of the time-varying inlet conditions. The resulting time averaged data is then used to assess the convergence of the numerical solution and evaluate the turbine performance.
Three methods of computing the time-averaged turbine efficiency are presented. The first method, referred to as the TP method, is based upon the use of mass-weighted average of the total temperature (Tt) and total pressure (Pt). The second method, the TS method, uses the mass-weighted average of total temperature (Tt) and entropy (S). The third method, the WS method, employs the moving average of specific work output (Ws) and work lost due to entropy (S) increase.
A comparison of time-averaged specific work output, stage efficiency, and rotor efficiency is made for three turbine operating configurations: partial admission, synchronized pulsation, and sequential pulsing flow conditions of the sector inlets. Results show that for the tested rotor speed, the efficiency increases as the inlet pulsing frequency is reduced, and that when inlet sectors are not opened simultaneously, a large drop in efficiency occurs due to spillage of high energy fluid between the open and closed sectors. Additionally, it is shown that due to the non-convective nature of total pressure, the TP method for efficiency is not reliable while the TS and WS methods are able to produce consistent results.
