Comparison of methods of supplying the working fluid flow to the mixing chamber

Abstract

The subject of this article is the impact of air delivery method bypassed from the compressor to the power turbine of a turboshaft gas turbine engine (GTE). The relevance of this GTE scheme is due to efforts to improve the efficiency of converting heat into work and to address the issue of ensuring stable compressor operation in throttling modes with a high compression ratio. A high compression ratio is necessary because increasing the cycle parameters, such as the heating degree and the pressure ratio of the working fluid, is essential for improving the overall efficiency of the GTE. One effective method to enhance the compressor stability is to bypass air from the compressor to the atmosphere, but this approach worsens the specific parameters of the GTE. In advanced GTE designs, various methods are considered to use bypassed air to avoid the loss of useful work. One such scheme is a turboshaft GTE with an air bypass to the power turbine. This design offers higher efficiency than bypassing air into the atmosphere while maintaining compressor stability. The GTE has an additional channel for delivering air from the compressor to the power turbine and a mixing chamber for the flows before the power turbine. The mixing chamber can receive either frontal or normal air delivery from the compressor. In the case of normal air delivery, the length of the mixing chamber is reduced, but this increases the total pressure losses and the resulting flow velocity before the power turbine. The aim of this work was to develop a mathematical model of a mixing chamber with normal flow delivery and to compare its characteristics with those of a mixing chamber with frontal flow delivery using an established mathematical model. To achieve this goal, the following tasks were accomplished: assuming no friction on the channel walls, an exact solution to the system of equations describing the mixing process was obtained. The mathematical relationships are presented in dimensionless form. The characteristics of mixing chambers with frontal and normal air delivery were obtained as dependencies of the resulting flow velocity and relative total pressure at the chamber outlet on the resulting flow velocity of the main stream and mass flow ratio of the streams, given the specified full temperature ratio of the streams at the mixing chamber inlet. A comparison was made between the flow parameters at the outlet of the mixing chambers with frontal and normal air delivery, under identical inlet conditions. The conclusions show that with normal flow delivery, the total pressure losses and the resulting velocity at the mixing chamber outlet are higher than with frontal flow delivery. However, the slight degradation in the flow parameters with normal flow delivery keeps such a mixing chamber competitive due to its shorter length.