Huishe Wang

Tip-Clearance Effects on Hot-Streak Migration in Low-Pressure Stage of Vaneless Counter-Rotating Turbine

Three-dimensional multiblade row unsteady Navier-Stokes simulations have been performed to reveal the effects of rotor tip clearance on the inlet hot-streak migration characteristics in the low-pressure stage of a vaneless counter-rotating turbine. The numerical results indicate that most of the hotter fluid migrates toward the rotor pressure surface and that only a small amount of hotter fluid migrates to the rotor suction surface when it convects into the low-pressure turbine rotor. The hotter fluid that migrated to the tip region of the high-pressure turbine rotor impinges on the leading edge of the low-pressure turbine rotor after it goes through the high-pressure turbine rotor. The migration of the hotter fluid directly results in a very high heat load at the leading edge of the low-pressure turbine rotor. The leakage flow in the rotor tip clearance tends to increase the low-pressure turbine rotor outlet temperature at the tip region.

Tip Clearance Effects on Inlet Hot Streaks Migration Characteristics in High Pressure Stage of a Vaneless Counter-Rotating Turbine

In this paper, three-dimensional multiblade row unsteady Navier-Stokes simulations at a hot streak temperature ratio of 2.0 have been performed to reveal the effects of rotor tip clearance on the inlet hot streak migration characteristics in high pressure stage of a Vaneless Counter-Rotating Turbine. The hot streak is circular in shape with a diameter equal to 25% of the high pressure turbine stator span. The hot streak center is located at 50% of the span and the leading edge of the high pressure turbine stator. The tip clearance size studied in this paper is 2.0mm (2.594% high pressure turbine rotor height). The numerical results indicate that the hot streak mixes with the high pressure turbine stator wake and convects towards the high pressure turbine rotor blade surface. Most of hotter fluid migrates to the pressure surface of the high pressure turbine rotor. Only a few of hotter fluid rounds the leading edge of the high pressure turbine rotor and migrates to the suction surface. The migration characteristics of the hot streak in the high pressure turbine rotor are dominated by the combined effects of secondary flow, buoyancy and leakage flow in the rotor tip clearance. The leakage flow trends to drive the hotter fluid towards the blade tip on the pressure surface and to the hub on the suction surface. Under the effect of the leakage flow, even partial hotter fluid near the pressure surface is also driven to the rotor suction surface through the tip clearance. Compared with the case without rotor tip clearance, the heat load of the high pressure turbine rotor is intensified due to the effects of the leakage flow. And the results indicate that the leakage flow effects trend to increase the low pressure turbine rotor inlet temperature at the tip region. The air flow with higher temperature at the tip region of the low pressure turbine rotor inlet will affect the flow and heat transfer characteristics in the downstream low pressure turbine.Copyright

Unsteady Numerical Simulation of Shock Systems in Vaneless Counter-Rotating Turbine

A detailed unsteady numerical simulation has been carried out to investigate the shock systems in the high pressure (HP) turbine rotor and unsteady shock-wake interaction between coupled blade rows in a 1+1/2 counter-rotating turbine (VCRT). For the VCRT HP rotor, due to the convergent-divergent nozzle design, along almost all the span, fishtail shock systems appear after the trailing edge, where the pitch averaged relative Mach number is exceeding the value of 1.4 and up to 1.5 approximately (except the both endwalls). A group of pressure waves create from the suction surface after about 60% axial chord in the VCRT HP rotor, and those waves interact with the inner-extending shock (IES). IES first impinges on the next HP rotor suction surface and its echo wave is strong enough and cannot be neglected, then the echo wave interacts with the HP rotor wake. Strongly influenced by the HP rotor wake and LP rotor, the HP rotor outer-extending shock (OES) varies periodically when moving from one LP rotor leading edge to the next. In VCRT, the relative Mach numbers in front of IES and OES are not equal, and in front of IES, the maximum relative Mach number is more than 2.0, but in front of OES, the maximum relative Mach number is less than 1.9. Moreover, behind IES and OES, the flow is supersonic. Though the shocks are intensified in VCRT, the loss resulted in by the shocks is acceptable, and the HP rotor using convergent-divergent nozzle design can obtain major benefits.Copyright

Influence of Tip Clearance Size on Flow Characteristics of a Vaneless Counter-Rotating Turbine

In this paper, three-dimensional multiblade row Navier-Stokes simulations have been performed to investigate the effects of tip clearance size on the flow characteristics of a vaneless counter-rotating turbine. The tip clearance sizes studied in this paper are 0.0mm (without rotor tip clearance), 0.5mm (0.65% high-pressure turbine rotor height, 0.52% low-pressure turbine rotor height), 1.0mm, 1.5mm, 2.0mm, 2.5mm and 3.0mm, individually. The numerical results indicate that the air mass flow rate of the vaneless counter-rotating turbine and the ratio of specific work of the high-pressure turbine to that of the low-pressure turbine increase when the rotor tip clearance size is increased. While the isentropic efficiency, the expansion ratio and the power of the vaneless counterrotating turbine decrease as the tip clearance size is increased. The results also show that the existence of the tip clearance trends to decrease the Mach number at the high-pressure turbine outlet, and increase the Mach number at the low-pressure rotor outlet, especially at near the blade tip region, the extents of the decrease and increase of the Mach number are more remarkable. When the tip clearance size is increased, the Mach number decreases at the outlet of the high-pressure turbine, while increases at the outlet of the lowpressure rotor. The numerical results also illustrate that the existence of the tip clearance trends to decrease the flow angle at the high-pressure turbine outlet, especially at near the blade tip region, the extent of the decrease of the flow angle is more prominent. And the flow angle decreases with the increase of the tip clearance size at the high-pressure turbine outlet. Compared with the results at the high-pressure turbine outlet, the effects of the tip clearance on the flow angle are more complicated at the low-pressure rotor outlet. Along the low-pressure rotor span, the outlet flow angle shows several different evolution laws with the increase of the tip clearance size.

Experimental and Numerical Investigation on Flow Characteristics of a Vaneless Counter-Rotating Turbine at Off-Design Conditions

In this paper, a lot of experimental and numerical investigations are performed to explore the flow characteristics of a vaneless counter-rotating turbine at off-design conditions. The experimental investigations were carried out on a blow-down short duration turbine test facility, and the numerical simulations were performed by means of a three-dimensional multiblade row steady Navier-Stokes code. Depending on the experimental and numerical investigations, the operating characteristics of the vanless counter-rotating turbine are obtained. The investigation results indicate that the ratio of specific work of the high-pressure turbine to that of the low-pressure turbine and the efficiency of the vaneless counter-rotating turbine are increased 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 vaneless counter-rotating turbine is reduced. The investigation results also indicate that the numerical code in this paper can qualitatively predict the flow characteristics of the vaneless counter-rotating turbine at off-design conditions. The prediction ability of the numerical code is credible.

Influence of Hot Streak/Airfoil Count Ratios on High Pressure Stage of a Vaneless Counter-Rotating Turbine

In order to reveal the effects of the hot streak/airfoil count ratio on the heating patterns of high pressure turbine rotor blades in a Vaneless Counter-Rotating Turbine, three-dimensional unsteady Navier-Stokes simulations have been performed. In these simulations, the ratio of the number of hot streaks to the number of the high pressure turbine vanes and rotors is 1:3:3, 1:2:2, 2:3:3 and 1:1:1, respectively. The numerical results show that the migration characteristics of the hot streak in the high pressure turbine rotor are predominated by the combined effects of secondary flow and buoyancy. The combined effects induce the high temperature fluid migrate towards the hub in the high pressure turbine rotor. And the combined effects become more intensified when the hot streak/airfoil count ratio increases. The results also indicate that the peak temperature of the hot streak is dissipated as the hot streak goes through the high pressure turbine vane or the rotor. The dissipated extent of the peak temperature in the high pressure turbine stator and the rotor is increased as the hot streak-to-airfoil ratio increases. And the increase of the hot streak/airfoil count ratio trends to increase the relative Mach number at the high pressure turbine outlet. The relative flow angle from 23% to 73% span at the high pressure turbine outlet decreases as the hot streak-to-airfoil ratio increases. The results also indicate that the isentropic efficiency of the Vaneless Counter-Rotating Turbine is decreased as the hot streak/airfoil count ratio increases.Copyright

Influence of Hot Streak Temperature Ratio on Low Pressure Stage of a Vaneless Counter-Rotating Turbine

In order to explore the influence of hot streak temperature ratio on low pressure stage of a Vaneless Counter-Rotating Turbine, three-dimensional multiblade row unsteady Navier-Stokes simulations have been performed. The predicted results show that hot streaks are not mixed out by the time they reach the exit of the high pressure turbine rotor. The separation of colder and hotter fluids is observed at the inlet of the low pressure turbine rotor. After making interactions with the inner-extending shock wave and outer-extending shock wave in the high pressure turbine rotor, the hotter fluid migrates towards the pressure surface of the low pressure turbine rotor, and the most of colder fluid migrates to the suction surface of the low pressure turbine rotor. The migrating characteristics of the hot streaks are predominated by the secondary flow in the low pressure turbine rotor. The effect of buoyancy on the hotter fluid is very weak in the low pressure turbine rotor. The results also indicate that the secondary flow intensifies in the low pressure turbine rotor when the hot streak temperature ratio is increased. The effects of the hot streak temperature ratio on the relative Mach number and the relative flow angle at the inlet of the low pressure turbine rotor are very remarkable. The isentropic efficiency of the Vaneless Counter-Rotating Turbine decreases as the hot streak temperature ratio is increased.Copyright

Numerical Simulation of Shock Systems of Low Pressure Turbine in Vaneless Counter-Rotating Turbine

A detailed unsteady numerical simulation has been carried out to investigate the shock and unsteady flow in the low pressure (LP) rotor in a 1+1/2 counter-rotating turbine (vaneless counter-rotating turbine (VCRT)). Through analyzing the distribution of static pressure and Mach number etc. in the VCRT, it can be found that, when the outer-extending shock (OES) of high pressure (HP) rotor moving from one LP rotor leading edge into the next, the inflow condition of LP rotor will vary. In the process, there are two typical inflow conditions. One is subsonic, but sufficiently near 1.0, and the other is slightly above unity i.e. the OES impinges on the LP rotor leading edge. Such inflow conditions of LP rotor will result in two different shock systems at different time. When the OES impinges on the LP rotor leading edge, a bow shock appears upstream of the LP rotor, and a normal shock produces at roughly 70% axial chord on the suction surface of LP rotor, and between the bow shock and normal shock, a group of expansion waves exist. After the OES sweeps the LP rotor leading edge i.e. the inflow of LP rotor is subsonic, the bow shock upstream of the LP rotor disappears, and a normal shock, that is weaker than the above, produces at the same location, and in front of the normal shock, a group of expansion waves exist. This distribution of shock in the VCRT LP rotor is similar to that in a compressor double-circular-arc (DCA) airfoil cascade in the same inflow condition, but in the VCRT LP rotor, the shocks are confined to the suction surface side of passage and its intensity weaker. The reason of the difference of the shock systems between the VCRT and the DCA airfoil cascade is that in the cascade the flow is of pressurization while in the VCRT the flow is of decompression. When the wake of the HP rotor sweeps the LP rotor, the static pressure on the suction surface of LP rotor will fluctuate, and a variational lower pressure area appears on the suction surface, which will result in a clear adverse pressure gradient on the suction surface in the LP rotor.Copyright