Vaneless Counter-Rotating Turbine Research Articles

A shock loss reduction method for the low-pressure rotor of a vaneless counter-rotating turbine using circulation redistribution

This paper proposes the circulation redistribution for the low-pressure (LP) rotor of a highly-loaded vaneless counter-rotating turbine (VCRT) to reduce the LP shock losses. The method is to decrease the mid-span circulation and increase the tip circulation at the exit of the LP rotor with unchanged high-pressure (HP) and LP power output. The LP Blade-to-Blade flow field can be considered as a Laval nozzle model. And the high static pressure drop from the throat to the exit results in strong trailing edge (TE) shocks near the LP mid-span. Also, the suction side TE shock (ST) and the reflected shock of the pressure side TE shock (RPT) near the LP mid-span merge into a relatively strong shock very close to the TE. These two factors result in high shock losses. To reduce the shock losses under the design condition, decreasing the stagger angle and the blade exit angle near the LP mid-span are adopted to reduce the LP mid-span exit circulation. This changes the throat area to exit area ratio near the LP mid-span, prompting the actual static pressure drop from the throat to the exit to be reduced and closer to the theoretical value, so that the TE shock strength is reduced. Besides, the intersection of the RPT and the ST is delayed owing to the improved flow angle distributions near the TE, further contributing to the LP mid-span shock loss reduction. Since the LP power output is decreased and the LP tip-leakage flow (TLF) losses are far lower than the HP TLF losses, increasing the stagger angle and the blade exit angle in LP tip regions are employed to increase the LP exit circulation and decrease the HP exit circulation in tip regions. Then the HP and LP efficiency are both raised with unchanged HP and LP power output after optimizing the blade profiles near the mid-span and in tip regions of the LP rotor. The remarkable thing is that the TE shock strength in LP tip and root regions are both reduced after improved because of the decreased ratio of the exit area to the throat area. And the intersection of the RPT and the ST in LP tip and root regions are also both delayed, leading to reduced shock losses. The reduced TE shock losses along the whole LP span appreciably raise the LP and VCRT efficiency by 0.96% and 0.32% respectively.

Flow structures and heat transfer characteristics in a vaneless counter-rotating turbine cavity with different cooling inflows

The cooling air is used in the gas turbine to prevent the hot gas ingress and overheating of turbine disks. In this paper, the flow structures and heat transfer characteristics of vaneless counter-rotating turbine (VCRT) with different types of cooling inflows are numerically analyzed. The three-dimensional unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with a Shear Stress Transport (SST) turbulence model are adopted. The results reveal that the inertia force, rotating effect and mainstream dominate the flow fields in VCRT cavity. The axial cooling inflow promotes the heat transfer characteristic in low pressure rotor (LPR) disk meanwhile the radial cooling inflow improves the heat transfer performance in high pressure rotor (HPR) disk. The vortexes interaction, viscous dissipation and heat transportation result in the decrease of heat transfer characteristic in the high radius region of rotor disk. The combined cooling inflow is adopted to solve the problem. It is the combination of radial cooling inlet and high position cooling inlet. The cooling air injected axially from the high position cooling inlet moves outward along the HPR disk which enhances the heat transfer characteristic. It is found that the Nusselt number and the amount of cooling air is not the simple linear relationship. The Nusselt number in HPR disk no longer increases if the high position cooling flow rate exceeds the threshold value. Compared with other types of cooling inflows, the needed amount of cooling air reduces when the combined cooling inflow is adopted. There is an optimum combined cooling flow rate which obtains the excellent heat transfer characteristics in the HPR disk.

A tip-leakage loss reduction method using a front-loaded blade profile for the high-pressure rotor of a vaneless counter-rotating turbine

To reduce tip-leakage losses, this paper presents a front-loaded blade profile design method for the high-pressure (HP) rotor of a highly loaded vaneless counter-rotating turbine (VCRT). The method is to decrease the blade exit angle and increase the stagger angle with unchanged throat area to obtain front-loaded blade profiles at both the tip-leakage vortex upper and lower boundaries. As the relative exit Mach numbers in the tip region of the VCRT’s HP rotor are all over 1.5, the shearing of the high-speed leakage jet and passage flow near the trailing edge is very strong. To reduce the shearing losses, decreasing the blade exit angle of the HP rotor is adopted to reduce the tip loading near the trailing edge. This means the leakage jet mass-flow rate and speed near the trailing edge are reduced. Hence, the shearing strength and losses of the leakage jet and passage flow near the trailing edge are lower. Since decreasing the blade exit angle of the HP rotor results in increased throat area, increasing the stagger angle is employed to maintain unchanged throat area. Increasing the stagger angle also reduces the tip loading near the trailing edge, leading to reduced high-speed leakage jet mass-flow rate. Thus, the shearing strength and losses of the high-speed leakage jet and passage flow near the trailing edge are further reduced. The efficiency of the HP rotor and VCRT are raised by 0.45% and 0.53% respectively.

Investigation of the flowfield on a vaneless counter-rotating turbine with seal cavity flow ejection

The sealing of the rotor-rotor gap and rotor disk cooling are vital to the safe operation of the vaneless counter-rotating turbine(VCRT). In order to quantifies the influence of the wheel-space cavity flow on the VCRT aerodynamic performance, and to improve turbine efficiency of the VCRT at certain rim seal ejection rates, numerical studies which considered the effects of rotor-rotor rim seal flow ejection are carried out in this paper. The three dimensional unsteady computational fluid dynamic analysis of a VCRT at the engine conditions are performed, and the seal flow ejected downstream of the high pressure rotor row at six sealing flow rate are modeled. The interaction among the high pressure rotor trailing shock wave, the downstream secondary flow and the seal flow has been studied and quantitatively characterized as a function of the purge ejection rate. Numerical results show that seal flow- mainstream flow interaction is entirely dominated by the high pressure rotor trailing edge shock at the hub, low pressure hub passage vortex and the mixing of the sealing flow from wheelspace and mainstream. When the mass flow rate of the coolant is smaller than some threshold value, the shock loss of the high pressure rotor and hub secondary flow loss of the low pressure rotor are decreased with the increasing of the coolant mass flow rate. It causes that the VCRT efficiency is gradually increased. On condition that the amount of the seal flows is beyond the threshold value, the key roles in modification of the VCRT performance are changed. The increment of the hub secondary flow loss and the mixing loss are gradually larger than the decrement of the shock loss. As a result, the turbine efficiency gradually decreases.

Aerodynamic Design and Analysis of a Multistage Vaneless Counter-Rotating Turbine

A multistage vaneless counter-rotating turbine (MVCRT) eliminates vanes between rotors, which reduces the weight and size of the turbine and avoids viscous losses associated with vanes pronouncedly. An aircraft engine employing such a turbine would have greater thrust to weight ratio and smaller specific fuel consumption. This paper presents the aerodynamic design philosophy and performance analysis of the MVCRTs for gas turbine engines by a case study. The case is about a 1/2*4 turbine, which consists of a rotating frame and four rotors without any vanes between them. The first rotor and the third rotor are connected by a shaft to drive a compressor with a pressure ratio of 11.8, and the second rotor and the fourth rotor are connected by the rotating frame to deliver a total shaft power of around 2 MW. The stage loading of each rotor and flow axial acceleration of each duct are controlled to provide sufficient inlet swirls for their subsequent rotors. The stage work coefficients of each rotor are 0.95, 2.9, 1.4, and 1.0, respectively. Nonuniform radial circulation distributions are also used to maximize the turbine power output. Centrifugal forces in the outer rotor of the turbine are captured by carrying out a finite element analysis (FEA) to validate the aerodynamic design results. Three-dimensional viscous numerical results show that an adiabatic total-to-total efficiency of 91.47% with a pressure ratio of 9.8 at design condition is obtained and achieves the initial design objective very well. Entropy creation associated with the tip leakage and secondary flow is also illustrated for understanding the origins and effects of losses in the turbine. Pressure ratios and efficiency at the speed combinations of the 80% to 100% inner and outer rotor design speeds are discussed to reveal the turbine characteristics at off-design conditions.

Aerodynamics design of two-stage vane-less counter-rotating turbinec

It is one of the most efficient ways to greatly improve aero-engines’ performance by utilizing vaneless counter-rotating turbine (VCRT) technology. To supply sufficient power, VCRT turns to be high Mach number, large flow angle at high-pressure turbine (HPT) rotor exit, and low blade camber angle, which increase difficulties to turbine design. As the axial velocity ratio of HPT rotor is much larger than the conventional ones, the optimal selection of VCRT velocity triangles based on theoretical analysis is developed, and how the efficiency varied by HPT stator/rotor exit flow angle is also figured out. The key points to design a high efficient practicable VCRT are to select velocity triangles that are characterized by low flow coefficient, high outlet flow angle and large axial velocity ratio of HPT rotor. Meanwhile, performance comparison between convergent blade and convergent-divergent blade shows the latter is more appropriate for VCRT.

Influence of exit-to-throat width ratio on performance of high pressure convergent-divergent rotor in a vaneless counter-rotating turbine

This paper describes the redesign of a high pressure rotor (with exit Mach number around 1.5) for the vaneless counter-rotating turbine by choosing adequate exit-to-throat width ratio. Based on the previous design analysis and test results, effects of the exit-to-throat width ratio on the performance of the transonic turbine cascade were proposed. In order to investigate the influence of the exit-to-throat width ratio on the performance of the turbine cascade, a flow model of the convergent-divergent turbine cascade was constructed by using the theory of Laval nozzle. Then a method on how to choose the adequate exit-to-throat width ratio for the turbine cascade was proposed. To validate the method, it was used to calculate the adequate exit-to-throat width ratio for the high pressure rotor of the vaneless counter-rotating turbine. The high pressure turbine rotor was redesigned with the new exit-to-throat width ratio. Numerical simulation results show that the isentropic efficiency of the redesigned vaneless counter-rotating turbine under the design condition has increased by 0.9% and the efficiencies under the off-design conditions are also improved significantly.

Tip Clearance Effects on Inlet Hot Streak 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 numerical results indicate that 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 toward 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.

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.

Experimental investigation on unsteady pressure fluctuation of rotor tip region in high pressure stage of a vaneless counter-rotating turbine

An experimental investigation has been performed to study the unsteady pressure fluctuation of rotor tip region in high pressure stage of a vaneless counter-rotating turbine. The experiment is carried out on a blow-down short duration turbine facility. The investigation indicates that the blow-down short duration turbine facility is capable of substituting continuous turbine facilities in most turbine testing. Through this experimental investigation, a distinct blade-to-blade variation is observed. The results indicate that the combined effects of vane wake, tip leakage flow, complicated wave systems and rotor wake induce the remarkable blade-to-blade variations. The results also show that the unsteady effect is intensified along the flow direction.

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 the 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 and outer-extending shock waves in the high pressure turbine rotor, the hotter fluid migrates toward the pressure surface of the low pressure turbine rotor, and most of the colder fluid migrates to the suction surface of the low pressure turbine rotor. The migrating characteristics of the hot streaks are dominated by the secondary flow 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 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.

Investigation of influencing factors of hot streaks migration in high pressure stage of a vaneless counter-rotating turbine

Three-dimensional, viscous, and unsteady CFD simulations have been performed in order to reveal the influencing factors of hot streaks migration in high pressure stage of a vaneless counter-rotating turbine. Based on the numerical results, the comparison between the case with inlet hot streaks and case without inlet hot streaks is carried out, which shows that the effect of inlet hot streaks on the load distributions of high pressure turbine airfoils is not notable and the airfoil load distributions are directly related to the inlet pressure distributions. The predicted results also indicate that the circumferential and radial movements of the hot streaks were not observed in the high pressure turbine stator. This means that the combined effects of secondary flow and buoyancy are very weak in the high pressure turbine stator. The numerical results also prove that the circumferential flow angle effect at the inlet of the high pressure turbine rotor, secondary flow effect and buoyancy effect are the mainly influencing factors to directly affect the migration characteristics of the hot streaks in the high pressure turbine rotor.

Numerical analysis of 3-D unsteady flow in a vaneless counter-rotating turbine

To reveal the unsteady flow characteristics of a vaneless counter-rotating turbine (VCRT), a three-dimensional, viscous, unsteady computational fluid dynamics (CFD) analysis was performed. The results show that unsteady simulation is superior to steady simulation because more flow characteristics can be obtained. The unsteady effects in upstream airfoil rows are weaker than those in downstream airfoil rows in the VCRT. The static pressure distribution along the span in the pressure surface of a high pressure turbine stator is more uniform than that in the suction surface. The static pressure distributions along the span in the pressure surfaces and the suction surfaces of a high pressure turbine rotor and a low pressure turbine rotor are all uneven. The numerical results also indicate that the load of a high pressure turbine rotor will increase with the increase of the span. The deviation is very big between the direction of air flow at the outlet of a high pressure turbine rotor and the axial direction. A similar result can also be obtained in the outlet of a low pressure turbine rotor. This means that the specific work of a high pressure turbine rotor and a low pressure turbine rotor is big enough to reach the design objectives.

Numerical Investigation on the Influence of Hot Streak Temperature Ratio in a High-Pressure Stage of Vaneless Counter-Rotating Turbine

The results of recent studies have shown that combustor exit temperature distortion can cause excessive heat load of high-pressure turbine (HPT) rotor blades. The heating of HPT rotor blades can lead to thermal fatigue and degrade turbine performance. In order to explore the influence of hot streak temperature ratio on the temperature distributions of HPT airfoil surface, three-dimensional multiblade row unsteady Navier-Stokes simulations have been performed in a vaneless counter-rotating turbine (VCRT). The hot streak temperature ratios from 1.0 (without hot streak) to 2.4 were used in these numerical simulations, including 1.0, 1.2, 1.6, 2.0, and 2.4 temperature ratios. The hot streak is circular in shape with a diameter equal to 25%of the span. The center of the hot streak is located at 50%of span and 0%of pitch (the leading edge of the HPT stator vane). The predicted results show that the hot streak is relatively unaffected as it migrates through the HPT stator. The hot streak mixes with the vane wake and convects towards the pressure surface (PS) of the HPT rotor when it moves over the vane surface of the HPT stator. The heat load of the HPT rotor increases with the increase of the hot streak temperature ratio. The existence of the inlet temperature distortion induces a thin layer of cooler air in the HPT rotor, which separates the PS of the HPT rotor from the hotter fluid. The numerical results also indicating the migration characteristics of the hot streak in the HPT rotor are predominated by the combined effects of secondary flow and buoyancy. The combined effects that induce the high-temperature fluid migrate towards the hub on the HPT rotor. The effect of the secondary flow on the hotter fluid increases as the hot streak temperature ratio is increased. The influence of buoyancy is directly proportional to the hot streak temperature ratio. The predicted results show that the increase of the hot streak temperature ratio trends to increase the relative Mach number at the HPT rotor outlet, and decrease the relative flow angle from 25%to 75%span at the HPT rotor outlet. In the other region of the HPT outlet, the relative flow angle increases when the hot streak temperature ratio is increased. The predicted results also indicate that the isentropic efficiency of the VCRT decreases with the increase of the hot streak temperature ratio.