Investigation On Flow Characteristics And Migration Mechanisms Of Inlet Hot Streaks In A Vaneless Counter-Rotating Turbine

In the recent few years, counter-rotating turbine has been more and more regarded because it can offer some significant benefits compared with conventional two stage turbine, such as the elevated thrust-to-weight ratio of aero-engine, the improved performance of aircraft, and so on. At present, 1-1 stage counter-rotating turbine has been used in some advanced active duty aero-engines, and vaneless counter-rotating turbine (VCRT, 1-1/2 stage counter-rotating turbine), which is composed of a highly loaded single stage high pressure turbine and a vaneless counter-rotating single stage low pressure turbine, has been also applied in some up-to-date test engines in Euro-American developed countries. It is obvious that the VCRT will be widely adopted in real aero-engines in the future. However, the research level of the counter-rotating turbine in China greatly lags behind developed countries. So, it is very necessary to perform some systemic investigations focusing on counter-rotating turbine, especially on VCRT, in order to improve the research level of aero-engines in China. Based on the background, flow characteristics and migration mechanisms of inlet hot streaks in a VCRT are investigated by means of blowdown short duration turbine facility and advanced CFD tools in this paper. The blowdown short duration turbine facility is the first and unique in China. The main content in this paper is listed as follows:1. Numerical investigations are performed to explore the flow characteristics in the VCRT at design condition. The steady and unsteady flow characteristics in the VCRT and the leakage flow characteristics in tip clearance of rotors are illustrated by means of the numerical simulations. And the research results show that the convergent-divergent cascade passage in high pressure turbine rotor induces a batch of compression waves occurred on the rotor suction surface, inner-extending and outer-extending shock waves rooted at the rotor trailing edge. These waves make up of the complex waves system in the high pressure turbine rotor. The interactions between these waves directly cause a higher relative Mach number (-1.5) than conventional turbine at the outlet of the high pressure turbine rotor. The numerical results also indicate that waves system in the low pressure turbine rotor is similar to that in supersonic and transonic double circular are cascade. There is a bow shock in front of the leading edge of the rotor suction surface, and a normal shock occurs on the suction surface in the low pressure turbine rotor passage. Between the bow shock and the normal shock, there are some strong expansion waves in the low pressure turbine rotor passage. However, the strength of the waves in the low pressure turbine rotor is weaker than that in the double circular are cascade. In a word, the waves systems observed by the investigations in the high pressure turbine rotor and the low pressure turbine rotor are different from those in the conventional two stage turbine, even in the 1-1 stage counter-rotating turbine.2. A lot of experimental and numerical investigations are carried out to study the flow characteristics in the VCRT at off-design conditions. The operating characteristics of the VCRT are obtained depending on these investigations. The results indicate that the ratio of specific work of the high pressure turbine to that of the low pressure turbine and the efficiency are increased in the VCRT as the rotation speed of the rotor increases under the same expansion ratio. And the research results show that when the rotation speed of the rotor increases, the specific work of the low pressure turbine is decreased, and the effective operation range of the VCRT is reduced. The investigation results also indicate that the numerical code in this paper can qualitatively predict the flow characteristics of the VCRT at off-design conditions.3. The migration characteristics of inlet hot streaks in the VCRT are detailedly investigated by means of advanced CFD tools. The numerical results show that the separation effect of cold and hot air at the rotor inlet, secondary flow effect and buoyancy effect are the main factors to directly affect the migration characteristics of the hot streaks in the VCRT. And a formula, which can be used to approximatively evaluate the buoyancy effect, is derived according to simple radial equilibrium theory. Moreover, the effects of inlet hot streak size, circumferential clocking location of inlet hot streak, radial location of inlet hot streak, tip clearance of rotor and inlet hot streak temperature ratio on the migration characteristics of the hot streaks in the VCRT are also opened out.4. Some preliminarily experimental investigations are performed to study the migration characteristics of inlet hot streaks in the VCRT. The experimental results indicate that the hot streaks will be completely dissipated at the outlet of high pressure turbine rotor when the inlet hot streak temperature ratio is less than 1.2. And the results also show that the numerical program can qualitatively predict the temperature distributions at the outlet of the high pressure turbine rotor. It means the prediction ability of the numerical code is credible.