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The marine gas turbine exhaust volute is an important component that connects a power turbine and an exhaust system, and it is of great importance to the overall performance of the gas turbine. Gases exhausted from the power turbine are expanded and deflected 90 degrees in the exhaust volute, and then discharge radially into the exhaust system. The flows in the power turbine and the nonaxisymmetric exhaust volute are closely coupled and inherently unsteady. The flow interactions between the power turbine and the exhaust volute have a significant influence on the shrouded rotor blade aerodynamic forces. However, the interactions have not been taken into account properly in current power turbine design approaches. The present study aims to investigate the flow interactions between the last stage of a shrouded power turbine and the nonaxisymmetric exhaust volute with struts. Special attention is given to the coupled aerodynamics and pressure response studies. This work was carried out using coupled computational fluid dynamics (CFD) simulations with the computational domain including a stator vane, 76 shrouded rotor blades, 9 struts and an exhaust volute. Three-dimensional (3D) unsteady and steady Reynolds-averaged Navier-Stokes (RANS) solutions in conjunction with a Spalart-Allmaras turbulence model are utilized to investigate the aerodynamic characteristics of shrouded rotors and an exhaust volute using a commercial CFD software ANSYS Fluent 14.0. The asymmetric flow fields are analyzed in detail; as are the unsteady pressures on the shrouded rotor blade. In addition, the unsteady total pressures at the volute outlet is also analyzed without consideration of the upstream turbine effects. Results show that the flows in the nonaxisymmetric exhaust volute are inherently unsteady; for the studied turbine-exhaust configuration the nonaxisymmetric back-pressure induced by the downstream volute leads to the local flow varying for each shrouded blade and low frequency fluctuations in the blade force. Detailed results from this investigation are presented and discussed in this paper.

In order to verify that a high-power marine gas turbine engine can be redesigned to be a reversible one, concept verification was done on a reversible turbine test rig. The reversible power turbine test pieces were designed, which are transformed to be capable of rotating reversely. And more than 100 times of operational tests were done for the switching devices. Based on the reversible turbine test rig, 50 times of switching tests between normal and reverse rotational direction were done by compressed air under low temperature condition. While more than 100 times of operational tests were done with hot gas at temperature of 400–500 °C. The tests results show that the switching devices could operate flexibly and the output power reached over 40% of rated power condition. During the switching tests between the normal and reverse rotational direction, the propeller could stop running. All of these verify the concept feasibility of reversible gas turbine engine. Based on this concept, preliminary design was completed for a high-power marine gas turbine engine redesigned into a reverse one, which laid solid foundation for the following detailed design, experimental verification and application.

The use of power turbines has the advantage of flexible output load adjustment, and is a major configuration of marine gas turbines and aviation turboshaft/turboprop engines. Marine gas turbines and aviation turboshafts and turboprop engines generally have multiple working states and multiple working conditions change laws, which cause the aerodynamic performance of power turbines to fluctuate greatly with changes in working conditions, and different variable working conditions have a severe impact on the turbine loss characteristics. Therefore, it is necessary to deeply understand the internal flow mechanism of power turbines under off-design working conditions in different power turbine application modes, and carry out researches on the design methods of power turbines under wide working conditions. This paper summarizes and analyzes the recent advances in the field of aerodynamics of power turbines for marine and aviation applications. This review covers the following topics that are important for power turbine designs: (1) turbine flow loss prediction models under multiple operating conditions, (2) aerodynamic design and flow mechanism of power turbines for marine gas turbines, (3) aerodynamics of variable speed power turbines for turboshaft/turboprop engines, and (4) other issues about power turbines for marine and turboshaft/turboprop engines. The emphasis is placed on the aerodynamic design and flow mechanism of marine and aviation power turbines. We also present our own insights regarding the current research trends and the prospects for future developments.

Gas turbine engines are widely used as the marine main power system. However, they can’t reverse like diesel engine. If the reversal is realized, other ways must be adopted, for example, controllable pitch propeller (CPP) and reversible gearing. Although CPP has widespread use, the actuator installation inside the hub of the propeller lead to the decrease in efficiency, and it takes one minute to switch “full speed ahead” to “full speed astern”. In addition, some devices need to be added for the reversible gearing, and it takes five minutes to switch from “full speed ahead” “to “full speed astern”. Based on the gas turbine engine itself, a reversible gas turbine engine is proposed, which can rotate positively or reversely. Most important of all, reversible gas turbine engine can realize operating states of “full speed ahead”, “full speed astern“ and “stop propeller”. And, it just takes half of one minute to switch “full speed ahead” to “full speed astern”. Since reversible gas turbine engines have compensating advantages, and especially in recent years computational fluid dynamics (CFD) technology and turbine gas-dynamics design level develop rapidly, reversible gas turbine engines will be a good direction for ship astern. In this paper, the power turbine of a marine gas turbine engine was redesigned by three dimensional shape modification, and the flow field is analyzed using CFD, in order to redesign into a reverse turbine. The last stage vanes and blades of this power turbine were changed to double-layer structure. That is, the outer one is reversible turbine, while the inner is the ahead one. Note that their rotational directions are opposite. In order to realize switching between rotation ahead and rotation astern, switching devices were designed, which locate in the duct between the low pressure turbine and power turbine. Moreover, In order to reduce the blade windage loss caused by the reversible turbine during working ahead, baffle plates were used before and after the reversible rotor blades. This paper mainly studied how to increase the efficiency of the reversible turbine stage, the torque change under different operating conditions, rotational speed and rotational directions, and flow field under typical operating conditions. A perfect profile is expected to provide for reversible power turbine, and it can decrease the blade windage loss, and increase the efficiency of the whole gas turbine engine. Overall, the efficiency of the newly designed reversible turbine is up to 85.7%, and the output power is more than 10 MW, which can meet requirements of no less than 30% power of rated condition. Most importantly, the shaft is not over torque under all ahead and astern conditions. Detailed results about these are presented and discussed in the paper.